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Landscape-Level Effects of Forest on Pollinators and Fruit Set of Guava (Psidium guajava L.) in Orchards across Southern Thailand

Landscape-Level Effects of Forest on Pollinators and Fruit Set of Guava (Psidium guajava L.) in... diversity Article Landscape-Level E ects of Forest on Pollinators and Fruit Set of Guava (Psidium guajava L.) in Orchards across Southern Thailand 1 , 2 3 4 Katrine Hansen *, Tuanjit Sritongchuay , Sara Bumrungsri , Benno I. Simmons , 5 1 , Niels Strange and Bo Dalsgaard * Center for Macroecology, Evolution and Climate, GLOBE Institute, University of Copenhagen, 2100 Copenhagen Ø, Denmark Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Mengla 666303, China; t.sritongchuay@gmail.com Department of Biology, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand; sara.b@psu.ac.th Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Penryn TR10 9FE, UK; benno.simmons@gmail.com Department of Food and Resource Economics & Center for Macroecology, Evolution and Climate, University of Copenhagen, Rolighedsvej 23, 1958 Copenhagen, Denmark; nst@ifro.ku.dk * Correspondence: Katrinekvj@gmail.com (K.H.); bo.dalsgaard@sund.ku.dk (B.D.); Tel.: +45-225-290-17 (K.H.); +45-423-245-53 (B.D.) Received: 29 May 2020; Accepted: 23 June 2020; Published: 25 June 2020 Abstract: Pollination by wild pollinators is a key ecosystem service threatened by anthropogenic-induced land-use change. The proximity to natural habitat has previously been shown to positively affect pollinator communities and improve crop yield and quality but empirical evidence is limited from most parts of the World. Here, across six farms in Southern Thailand, we investigated the significance of landscape-level effects of natural habitat (proportion of and distance to evergreen forest) on both visitation rate and richness of pollinators as well as fruit set of guava (Psidium guajava L.), a local economically-important crop in the tropics. Overall, the most abundant pollinator was the Asian honey bee Apis cerana (39% of all visits) and different species of stingless bees (37%). We found that pollinator richness was unrelated to the proportion and distance to evergreen forest, however, the proportion of forest within a 1, 5 and 10 km radius had a significant positive impact on visitation rate of wild pollinators. Still, neither the various forest parameters nor pollinator visitation rate showed a significant impact on fruit set of guava, perhaps because guava self-pollinates. This illustrates that landscape-level degradation of natural habitat may negatively impact pollinator communities without diminishing the crop yield of the farmers. Keywords: crop pollination; ecosystem services; land-use; plant-pollinator interactions; pollination 1. Introduction Flowering plants rely on successful pollination for sexual reproduction. Although many plants may be able to self-pollinate or use abiotic agents such as wind to transport their pollen, the majority of plants use animal pollinators as vectors to transport pollen between plant individuals [1,2]. This makes plant-pollinator interactions one of the most important ecological functions for sustaining our ecosystems [3]. Notably, animal pollination plays a crucial role in warm and humid tropical ecosystems where an estimated average of 94% of all flowering plant species are thought to be animal pollinated [1,2,4]. A large variety of crops benefit from being pollinated by animals; thus, pollination is also a key ecosystem service that benefits us humans [5,6]. Ecient pollination is known to contribute Diversity 2020, 12, 259; doi:10.3390/d12060259 www.mdpi.com/journal/diversity Diversity 2020, 12, 259 2 of 13 significantly to both yield and quality in the production of close to 75% of the most important food crops worldwide [5,7,8]. As pollinator contribution to yield di ers greatly between plant species and we consume heavily certain crops that are not pollinator-dependent, this corresponds to around 5–8% of the global crop production [5]. As for wild plants, crop plants grown in tropical areas are more dependent on animal pollination than crop plants used in temperate regions [8,9]. Globally, there is an increasing concern that the ecosystem services provided by wild pollinators are threatened by anthropogenic-induced global change [3,8,10]. Pollinator extinctions and declines have been observed in both Europe and North America [11–13], whereas we are lacking solid data for wild pollinators in most other parts of the World, such as tropical South America, Africa and Asia [5,7,11,13]. Some of the most important anthropogenic-induced drivers of change in pollinator communities are climate and land-use change, including deforestation and conversion to agriculture [5,10]. Land-use change from natural habitat to agricultural land has increased by 25% since the 1960s and most of this expansion has happened in the developing world, such as the tropical forests of South-east Asia [10,14]. Therefore, it is crucial to better understand how landscape-level e ects impact pollinators and their services [10,15,16]. The proximity to natural habitat has been shown to positively a ect pollinator communities and improve crop yield or quality [8,15,17–22]. In other words, farmers growing pollinator-dependent crops close to natural habitat have been observed to have a greater crop production than farmers far from natural habitat. Land-use changes and habitat degradation thus seem to not only harm nature but also farm productivity. Managed honey bees are utilized worldwide to meet the need of pollination in large orchards or degraded landscapes. The most frequently-used species are the European honey bee Apis mellifera L. and the Asian honey bee Apis cerana L. both from the family Apidae [7,23,24]. Although managed honey bees benefit crop yield, wild bee species often pollinate crops more eciently and improve cross-pollination more so than by managed honey bees [23]. Thus, we cannot simply substitute wild pollinators for managed honey bees. Furthermore, land-use changes will most likely have a negative e ect on ecological resilience of pollinator communities underlining the need to better understand the impact of land-use change on wild pollinators [25]. The guava tree (Psidium guajava L.) belonging to the Myrtaceae family is thought to be native to tropical Mexico and Central America [26–28] but today it is widespread and locally an economically-important crop produced in many tropical and subtropical landscapes around the world [26,29]. Guava is considered part of the so-called “minor tropical fruit” production together with fruits such as mango and mangosteen. Minor tropical fruits are mostly grown in smaller households and traded regionally and in smaller volumes [29,30] but the production and trade are gaining more importance globally, as many of the minor fruits are recognized as a contribution to a healthier diet [29]. In Thailand, for instance, besides being important for the economy of local households, it has been estimated that minor fruits account for almost 7 per cent of agricultural earnings [29]. The guava is an important minor tropical fruits in the food industry, especially in the production of juice, fruit bars or other dehydrated food products [31]. In tropical areas, production of guava fruits can be induced year around by pruning the branches, making guava trees suitable for continuous fruit production and, thus, an excellent crop to conduct pollination experiments and to examine land-use impacts on fruit production [26]. Although a few pollination experiments have been conducted on guava, it still remains uncertain whether guava is mainly cross-pollinated by insects or self-pollinated [26,27,31,32]. Furthermore, despite its importance, we know little about how land-use impact the pollination and fruit set of guava. Therefore, in this study we aim to test the e ect of natural forest in the surrounding landscape on pollinator communities and the pollination of guava in orchards across Southern Thailand. To examine this, we estimated pollinator richness and visitation rate and performed pollination experiments (open vs. closed pollination treatment) in six guava orchards surrounded by di erent amounts of natural forest. We also measured the morphology of guava flowers and conducted pollination experiments to describe the overall dependency of guava on pollinators. We then tested the following hypotheses—(1) pollinator richness and visitation rate increase close to evergreen forest and with a larger proportion of Diversity 2020, 12, 259 3 of 13 evergreen forest in the surrounding landscape and (2) fruit set of guava trees increases on farms with a higher richness and visitation rate of pollinators and a larger proportion of (and/or closer distance to) evergreen forest in the surrounding landscape. 2. Materials and Methods 2.1. Study Site and Species (Psidium Guajava) Guava production is common in Southern Thailand for both private and commercial purposes [33]. Here, we focus on plantations growing guava as a monoculture for commercial sale. All guava trees studied were of the local variety “Kimju guava” [34]. The study took place in Southern Thailand from November 2019 until March 2020. Six guava plantations with varying degrees of surrounding evergreen forest were chosen. The plantations were located in the provinces Phattalung and Songkhla 0  0  0  0 (6 54 to 7 52 N and 99 53 to 100 36 E; Table 1). In these provinces, it takes between four and five months from flowering to harvest of a ripe guava. The reference land-use map was obtained from the Land Development Department of Thailand (LDD) [35]. The LDD provided the shape file data which has scale of 1:25,000 digitized from satellite images resolution of 2.5 m. Evergreen forest is defined as “areas dominated by trees generally 5–40 m tall and 20% of total vegetation cover. More than 75% of the tree species maintain their leaves all year. Canopy is never without green foliage.” The evergreen forest fraction was calculated within a 1, 5 and 10 km radius for each site using ARCGIS 10.3. The studied orchards ranged in size from 0.4 ha to 2.8 ha all with guava trees older than two years. All study sites were chosen based on having similar management practices such as being monocultures, using pruning to initiate flowering and using limited amount of pesticides. In the Phattalung and Songkhla provinces, we were unable to locate more than six farms fulfilling these criteria. Table 1. Study site overview. General information about study sites (location, size, number and age of guava tress) and forest parameters (Distance to closest evergreen forest patch and % evergreen forest within a radius of 1, 5 and 10 km from the study site). Study Sites Information Evergreen Forest Parameters Size No. Age Trees Distance 1 km 5 km 10 km Site Latitude Longitude (ha) Trees (Years) (m) (%) (%) (%) 1 6.989000 100.616611 1.6 1500 4–5 371 15.63 12.96 7.27 2 7.035883 100.280283 2.8 1000 2–4 752 0.00 0.00 0.00 3 7.048433 100.449133 2.3 839 5 5475 0 0 3.30 4 6.908079 100.502339 0.4 500 2 770 2.33 2.85 5.09 5 7.555000 100.130433 1.6 998 3 8676 3.47 0.16 0.39 6 7.873817 99.885733 0.5 216 3.5 7070 0 0 2.21 2.2. Floral Traits and Pollination Experiments To characterize guava flowers, we observed—number of flowers in an inflorescence, number of petals and color of stigma. We also measured the floral corolla width and the length of anthers using a digital caliper. To characterize the floral reward, we counted the number of anthers per flower and used micropipettes to investigate whether the guava flower produced nectar (observed from 7.00 to 11.00 AM). To gain knowledge of the reproductive system of guava, we conducted a series of pollination experiments—(1) open pollination (OP); flowers were left open allowing unhindered animal visitation (control treatment), (2) wind-pollination (WP); mature buds were bagged before opening with a coarse-meshed gauze (0.8 mm), allowing wind carried pollen through but otherwise left untouched, (3) cross-pollination (CP); mature buds with cracked calyx were bagged before opening with a spunbond nonwoven bag (which is a highly e ective barrier to pollen grains) and were hand-pollinated the following morning with pollen from flowers from a di erent tree, (4) self-pollination within the same Diversity 2020, 12, 259 4 of 13 tree (SP); mature buds with cracked calyx were bagged before opening with a spunbond nonwoven bag and were hand-pollinated the following morning with pollen from a di erent flower on the same tree, (5) self-pollination within the same flower (SF); mature buds with cracked calyx were bagged before opening with a spunbond nonwoven bag and the following morning each flower were hand-pollinated with pollen from its own anthers (i.e., within the same flower). Hand-pollination was done by using a cotton bud to transfer pollen grains to the stigma and using anthers dapped directly onto the stigma to ensure sucient pollen deposit. This was done between 7 and 9 AM. After applying one of the four treatments, excluding the control treatment (OP), flowers were bagged again and bags were left on for a week to ensure the stigma was no longer receptive when bags were removed. The initial fruit set was counted three weeks after flowering, when fruits were approximately 2 cm in diameter. Fruit set was calculated as the proportion of flowers developing into an initial fruit for each treatment and each tree. All pollination experiments were conducted at only one study site (Site 6, Table 1). Each treatment included five mature buds per tree in ten healthy trees located approximately ten meters from the edge (i.e., 50 flowers per treatment). Although some trees were used for more than one treatment, due to a lack of suitable trees with a sucient amount of mature buds at the study site, we were unable to conduct all treatments at the same trees as it ideally should have been [18,36]. This resulted in some treatments being paired on the same trees, while other treatments were conducted on other trees. We thus did not perform any statistical analysis to examine whether pollinations experiments di ered in fruit set but use the treatments to qualitatively assess the ability of guava to produce fruits under di erent pollination scenarios. 2.3. Pollinator Visitation and Richness On each study site, we estimated pollinator visitation rate and species richness shortly after anthesis from 7.30 to 9.00 AM. For all sites, visitation rate and richness measurements were conducted within one day. At each site, we chose ten healthy trees located approximately ten meters from the edge of the orchards and with a minimum of one flower. Pollinator visitation was monitored for 15 min per tree (resulting in 150 min of visitation per study site) followed by a 5 min observation period on the same tree to estimate pollinator richness (resulting in 50 min of richness per study site). In both visitation and richness observations, the number of individuals visiting the flowers were noted together with the number of visits. For the visitation rate, we did not catch the visiting animals but registered the visiting species. When it was not possible to identify the visiting animal to species-level, we instead registered the taxonomic group, that is, whether it was a bee, stingless bee, carpenter bee, wasp, fly, beetle or ant. One or two flowers were observed at each tree and a registration of visits was made every time a visiting animal touched the reproductive parts of an observed flower. Additionally, we recorded every time a new individual touch the reproductive parts of the flowers, thus we estimated both the total number of pollinator individuals and total number of pollinator visits observed within 15 min. Average visitation rate of all pollinators on each study site was calculated as average number of visits per flower per tree, then averaged across all ten trees yielding one number per site (visits/flower/15 min). Average visitation rate was also calculated for di erent sub-groups for each study site—Wild pollinators (all pollinators without the domesticated honey bee A. cerana); All bee species; Wild bee species (all bee species without A. cerana). Many farmers set up empty boxes within or close by the study fields as nesting spot for A. cerana. Thus, A. cerana is often semi-managed and not dependent on forest as nesting area, which is why we exclude A. cerana from the ‘wild pollinator ’ and ‘wild bee’ subgroups. For the species richness estimation, species were either identified in the field or collected and identified by a taxonomist to the lowest possible taxonomic level. The calculations on estimated richness were based on the number of individuals observed during visitation (15 min) and individuals caught or observed during the 5 min of richness estimation. Individuals were caught in the field using an insect net or using a clear plastic bag gently placed over the insect. The insects were quickly Diversity 2020, 12, 259 5 of 13 transferred to a microtube containing 99% ethanol and pinned shortly after returning from the field. A list of all species is found in Table A1, Appendix A. 2.4. Pollinator Exclusion Experiment At each study site, ten healthy trees approximately ten meters from the edge of the orchard were chosen in the same area as trees used for monitoring visitation rate and pollinator richness. On each tree we chose ten mature flower buds randomly and subjected to one of two treatments—pollinator exclusion (covered in a 0.8 mm coarse-meshed gauze allowing the flower to be pollinated by wind but not by pollinators) and open pollination (control treatment), allowing us to investigate the di erence in fruit set with and without pollinators. Bags were left on for 5–10 days to ensure the stigma was no longer receptive. The initial fruit set was counted three weeks after flowering when initial fruits were approximately 2 cm in diameter. Fruit set of each treatment was calculated as the proportion of flowers developing into an initial fruit for each tree. The additional fruit set with pollinators access was calculated as the di erence in fruit set between the two treatments (bagged and open) for each tree. This method controls for variations between plants (which could be caused by di erence in water supply, microclimate, management history and soil chemistry) presenting only the di erence of fruit set with and without pollinators [17]. For each study site, an average of the additional fruit set with pollinators was calculated across the ten trees and used for later analyses. We note that the experiments without pollinators in a few cases resulted in higher fruit set than when pollinators had access to the flowers. We in these cases converted the di erences in fruit set to zero (and not negative values), as we doubt pollinators have a negative influence on fruit set but that these negative values were caused by stochasticity. However, in the below regression analyses we also conducted the analyses using negative values, qualitatively obtaining the same results (not shown). 2.5. Statistical Analyses For each pollination experiment (OP, WP, CP, SP and SF), we calculated the mean fruit set. Since some treatments were paired and others were not (see Section 2.3), no formal statistical test were performed to examine whether the treatments differed in mean fruit set. To test the e ect of the di erent forest parameters on pollinator visitation rate, linear regressions were made between all forest parameters (proportion of evergreen forest within 1, 5 and 10 km radius as well as distance to nearest forest patch) and the di erent visitation rates (all pollinators, wild pollinators, all bee species, wild bee species). The estimated pollinator richness at each study site was calculated using the vegan package version 2.5–6 in R [37] and also regressed against all forest parameters. Finally, we regressed the average additional fruit set with pollinators against the forest parameters. Since data is an average of proportions, we may assume the six averages follow a normal distribution following the central limit theorem (CLT) [38]. To test if the residuals were normally distributed, a QQ plot for the residuals were performed for each regression using the stats package in R [39]. For all regressions, we report the R values and the significance level of the regressions (Table 2). Regressions between di erent visitation rates and the proportion of evergreen forest within the three radiuses were plotted with the corresponding confidence interval using ggplot2 in R [40]. All analyses were conducted using R 3.6.3 [39]. Diversity 2020, 12, 259 6 of 13 Table 2. Linear regression models of pollinator visitation rates (VR) (visits/flower/15 min), richness and additional fruit set with pollinators as a function of four measures of evergreen forest e ect: % of evergreen forest in the surrounding landscape within di erent distances to the study site (1, 5 and 10 km) and the distance to nearest forest patch from each study site (in meter). For each model, we also report the coecient of determination (R ) and the regression coecients with standard error. NS p > 0.05, * p < 0.05, ** p < 0.01. Dependent Independent R Regression Coecients Visitation rate Forest NS 1000 m (%) 0.51 0.42 (0.21) 5000 m (%) 0.68 0.57 (0.20) * All NS 10,000 m (%) 0.47 0.88 (0.47) 4 4 NS Distance (m) 0.48 6.710 (3.510 ) 1000 m (%) 0.79 0.69 (0.18) * 5000 m (%) 0.91 0.87 (0.14) ** Wild pollinators 10,000 m (%) 0.72 1.43 (0.45) * 4 4 NS Distance (m) 0.53 9.410 (4.410 ) NS 1000 m (%) 0.52 0.44 (0.21) 5000 m (%) 0.67 0.59 (0.21) * All bees NS 10,000 m (%) 0.45 0.90 (0.50) 4 4 NS Distance (m) 0.34 6.010 (4.210 ) 1000 m (%) 0.86 0.71 (0.14) ** 5000 m (%) 0.97 0.90 (0.08) ** Wild bees 10,000 m (%) 0.74 1.45 (0.42) * 4 4 NS Distance (m) 0.45 8.610 (4.710 ) NS 1000 m (%) 0.27 0.35 (0.29) NS 5000 m (%) 0.31 0.44 (0.33) Estimated richness NS 10,000 m (%) 0.37 0.88 (0.58) 4 4 NS Distance (m) 0.34 6.510 (4.510 ) NS 1000 m (%) 0.39 0.005 (0.003) NS 5000 m (%) 0.24 Additional initial 0.004 (0.004) NS fruit set 10,000 m (%) 0.20 0.007 (0.007) 5 6 NS Distance (m) 0.35 4.410 (5.710 ) VR and Richness NS VR All 0.0001 0.0001 (0.006) NS Additional initial VR Wild pollinators 0.1141 0.0032 (0.004) NS fruit set VR All bees 0.0069 0.0010 (0.006) NS VR Wild bees 0.1589 0.0038 (0.004) NS Richness 0.0765 0.0031 (0.005) 3. Results 3.1. Floral Morphology Guava trees have 1–3 flowers in each inflorescence (Figure 1). Floral corolla width was measured to be 47.63 4.17 mm (mean SD; n = 10) and consist of 5–6 white and scentless petals. Flowers are hermaphrodite and contain one light-green stigma and an estimated 621.50 62.06 (mean SD; n = 10) anthers with an average max length of 15.33 1.56 mm (mean SD; n = 7). Flowers have little or no nectar (no nectar was detected in the field in this study) but we observed large amounts of pollen on each anther. Diversity 2020, 12, 259 7 of 14 Diversity 2020, 12, 259 7 of 14 Diversity 2020, 12, 259 7 of 13 Figure 1. Illustration of guava flowers at different stages (bud, cracked bud and open flower) and the most important pollinators: (A) Three species of stingless bee, from left; Heterogona itama, Tetragonula laeviceps and Geniotrigona thoracica. (B) Asian honey bee Apis cerana. (C) Carpenter bee Xylocopa (Koptortosoma) aestuans. Figure 1. Illustration of guava flowers at different stages (bud, cracked bud and open flower) and the most Figure 1. Illustration of guava flowers at different stages (bud, cracked bud and open flower) and the important pollinators: (A) Three species of stingless bee, from left; Heterogona itama, Tetragonula laeviceps and most important pollinators: (A) Three species of stingless bee, from left; Heterogona itama, Tetragonula 3.2. Pollination Experiments Geniotrigona thoracica. (B) Asian honey bee Apis cerana. (C) Carpenter bee Xylocopa (Koptortosoma) aestuans. laeviceps and Geniotrigona thoracica. (B) Asian honey bee Apis cerana. (C) Carpenter bee Xylocopa P. guajava set fruit in all pollinator experiments. Initial fruit set ranged from 69% in open 3.2. (Koptor Pollination tosomExperiments a) aestuans. pollination (OP), 70% in pollination by wind alone (WP), 76% in cross pollination (CP) to 78% and P. guajava set fruit in all pollinator experiments. Initial fruit set ranged from 69% in open 3.2. Pollination Experiments 96% in self-pollination within same flower (SF) and same tree (SP), respectably (Figure 2). pollination (OP), 70% in pollination by wind alone (WP), 76% in cross pollination (CP) to 78% and 96% P. guajava set fruit in all pollinator experiments. Initial fruit set ranged from 69% in open in self-pollination within same flower (SF) and same tree (SP), respectably (Figure 2). pollination (OP), 70% in pollination by wind alone (WP), 76% in cross pollination (CP) to 78% and 96% in self-pollination within same flower (SF) and same tree (SP), respectably (Figure 2). Figure 2. Initial fruit set (% fruit set 3 weeks after blooming) of Psidium guajava L. of five pollination Figure 2. Initial fruit set (% fruit set 3 weeks after blooming) of Psidium guajava L. of five pollination treatments. The treatments were conducted on ten trees on one farm (Farm 6, see Table 1), showing the average fruit set of five flower buds pr. tree (a total of 50 flower buds pr. treatment). OP, Open treatments. The treatments were conducted on ten trees on one farm (Farm 6, see Table 1), showing Figure 2. Initial fruit set (% fruit set 3 weeks after blooming) of Psidium guajava L. of five pollination pollination (n = 20 trees); WP, Wind pollination (n = 10); CP, Cross pollination between plants (n = 10); the average fruit set of five flower buds pr. tree (a total of 50 flower buds pr. treatment). OP, Open treatments. The treatments were conducted on ten trees on one farm (Farm 6, see Table 1), showing SP, Self-pollination within same tree (n = 10); SF, Self-pollination within same flower (n = 10). pollination (n = 20 trees); WP, Wind pollination (n = 10); CP, Cross pollination between plants (n = 10); the average fruit set of five flower buds pr. tree (a total of 50 flower buds pr. treatment). OP, Open pollination (n = 20 trees); WP, Wind pollination (n = 10); CP, Cross pollination between plants (n = 10); SP, Self-pollination within same tree (n = 10); SF, Self-pollination within same flower (n = 10). SP, Self-pollination within same tree (n = 10); SF, Self-pollination within same flower (n = 10). Diversity 2020, 12, 259 8 of 14 Diversity 2020, 12, 259 8 of 13 3.3. Pollinator Visitation Rate and Richness in Relation to Forest 3.3. Pollinator Visitation Rate and Richness in Relation to Forest In total, 26 pollinating species and 498 individuals were observed visiting guava flowers within In total, 26 pollinating species and 498 individuals were observed visiting guava flowers within the total observation time of 1200 min for all sites (pooling visitation and richness periods). Bees were the total observation time of 1200 min for all sites (pooling visitation and richness periods). Bees were the dominant taxonomic group representing 89% of the observed individuals, 39% being A. cerana the dominant taxonomic group representing 89% of the observed individuals, 39% being A. cerana and and 37% various species of stingless bees. Carpenter bees constituted 4%, other bee species 5%, flies 37% various species of stingless bees. Carpenter bees constituted 4%, other bee species 5%, flies 8%, 8%, beetles 2% and ants 1% of all observed individuals. One wasp was observed touching a flower beetles during the enti 2% and ants re ti1% me period. A comp of all observed individuals. lete species l One ist ca wasp n be v was iewobserved ed in Tabtouching le A1, Apa pflower endix A during . The the entire time period. A complete species list can be viewed in Table A1, Appendix A. The average average visitation rate of pollinators was closely related to the proportion of evergreen forest in the visitation surroundin rate g of landsc pollinators ape witwas hin a closely 1, 5 and related 10 km to rad theiu pr soportion (Table 2,of Fiever gure gr 3een ). The v forest isitin ati the on r surr ate o ounding f wild pollinators and wild bees had the strongest association to forest (72% < R < 97%), whereas all landscape within a 1, 5 and 10 km radius (Table 2, Figure 3). The visitation rate of wild pollinators pollinators and all bees including managed honey bees were less strongly associated to forest (34% < and wild bees had the strongest association to forest (72% < R < 97%), whereas all pollinators and 2 2 R < 68%). Pollinator richness was not associated to the proportion of evergreen forest (Table 2). all bees including managed honey bees were less strongly associated to forest (34% < R < 68%). Distance to forest showed no significant impact on either visitation rate or richness of pollinators Pollinator richness was not associated to the proportion of evergreen forest (Table 2). Distance to forest (Table 2). showed no significant impact on either visitation rate or richness of pollinators (Table 2). Figure 3. Visitation rate (visits/flower/15 min) in relation to the percentage of evergreen forest within a 1, 5 and 10 km radius from each study site: red circle = 1 km, green triangle = 5 km and blue square = 10 km Figure 3. Visitation rate (visits/flower/15 min) in relation to the percentage of evergreen forest within and for four measures of visitation rate: (A) Visitation rate of all pollinators, (B) Visitation rate of wild a 1, 5 and 10 km radius from each study site: red circle = 1 km, green triangle = 5 km and blue square pollinators, (C) Visitation rate of bees and (D) Visitation rate of wild bees. All regressions are printed = 10 km and for four measures of visitation rate: (A) Visitation rate of all pollinators, (B) Visitation with 95% corresponding confidence intervals. Solid lines indicate significant associations (p < 0.05), rate of wild pollinators, (C) Visitation rate of bees and (D) Visitation rate of wild bees. All regressions whereas dashed lines indicated non-significant relationships (p > 0.05). For statistics, see Table 2. are printed with 95% corresponding confidence intervals. Solid lines indicate significant associations (p < 0.05), whereas dashed lines indicated non-significant relationships (p > 0.05). For statistics, see 3.4. Fruit Set in Relation to Pollinators and Forest Table 2. An average of the additional fruit set of guava with pollinators were calculated for each study site. This additional fruit set with pollinators ranged from 0% to 12% and had a mean SD of 4.67 4.50%. 3.4. Fruit Set in Relation to Pollinators and Forest Landscape e ects had no significant impact on additional fruit set with pollinators (p > 0.05). Likewise, An average of the additional fruit set of guava with pollinators were calculated for each study pollinator visitation rates and richness were not associated to fruit set (p > 0.05; Table 2). site. This additional fruit set with pollinators ranged from 0% to 12% and had a mean ± SD of 4.67 ± 4.50%. Landscape effects had no significant impact on additional fruit set with pollinators (p > 0.05). Likewise, pollinator visitation rates and richness were not associated to fruit set (p > 0.05; Table 2). Diversity 2020, 12, 259 9 of 13 4. Discussion In guava orchards in Southern Thailand, we found a significant correlation between the proportion of evergreen forest in the surrounding landscape and the visitation rate of insect pollinators on guava flowers (Table 2; Figure 3). On the other hand and contrary to similar studies on other crops, the proximity of forest showed no significant relationship with visitation rate within the guava orchards [8,17,18,22]. Moreover, neither pollinator richness nor fruit set were associated to the distance to forest nor the amount of forest in the surrounding landscape. These results indicate that although deforestation negatively impact pollinator communities, this does not necessarily cause a lower fruit set of crops on farmland embedded within degraded landscapes. The reason for this lack of e ect of deforestation on guava fruit set is underscored by the finding that additional fruit set of guava due to pollinators (i.e., open vs. closed pollination) showed no relationship with either visitation rate or with richness of pollinators. Results also showed that wind pollination and self-pollination of guava flowers seem to have a greater influence on fruit set than originally anticipated (Figure 2). It has been unclear from previous studies if guava is mainly pollinated by insects or self-pollinating but self-pollination within the same flower was believed to be uncommon due to the stigma being higher than the central anthers [26,32,34,41]. Our results show that self-pollination may be the most common pollination strategy in guava, more so than previously thought, which may be the reason fruit set was not related to landscape-level amount of forest and insect visitation. Moreover, fruit set is not only dependent on pollinators, other factors such as local management and seasonality (wet or dry season) might also have a large impact on fruit set [42]. Some local farmers were reporting that insucient or excess water supply may cause fruit abortion of guava [43], which may have obscured any potential e ect of the amount of forest in the surrounding landscape on pollination of guava. The landscape-level e ects of forest had stronger associations with visitation rates of wild pollinators and wild bees (both without A. cerana) than visitation rates including A. cerana (Table 2). This could be due to the fact that many farms provided nesting-boxes in or close by the orchard to attract A. cerana, diminishing the relevance of forest fragments as nesting areas. Pollination of wild bee species has proved to increase both yield and quality in di erent crops, making wild bees just as or even more important, than managed bees such as A. cerana [23]. Moreover, dependence on a single species for pollination can prove to be risky, if this species is declining due to parasites, pathogens or other factors [23,44]. For instance, large fluctuations in the abundance of the managed honey bee A. mellifera has been observed on co ee farms in Costa Rica [18] and in America a major threat is the colony collapse disorder (CCD), a still unsolved mystery of empty beehives [45]. Thus, pollination solely depending on pollinators from managed beehives might be less resilient to external factors such as disease [44]. Therefore, long-term it would be preferable to rely on a diverse fauna of wild pollinators with high redundancy of pollination services, making it important to sustain the forest left in southern Thailand. The proportion of evergreen forest sites was, in general, relatively low for all study sites (<16%). Instead the sites were surrounded by high proportions of other agricultural areas or, mostly, rubber plantations that accounted for 37 to 95% of the surrounding landscape. Natural habitat such as forest is known to have a positive impact on pollination visitation rate, mainly because many bee species make nests in forest areas [17,18,20–22]. Most frequent pollinating species in this study were bees (89%) and, specifically, 37% of all visiting pollinators were stingless bees (Figure 1). Tropical forest can act as a reservoir for stingless bees for pollinating crops, given that the orchard is within the bee’s forage range [21]. The forage range of stingless bees are believed to be closely related to their body size, ranging between 500 and 2000 m [21]. Only three out of six study sites were this close to evergreen forest and stingless bees were observed in all of them (Table 1, Table A1 Appendix A). In the three study sites further away from evergreen forest (5475 to 8675 m), no stingless bees were observed, only visits from A. cerana and two species of large carpenter bees—Xylocopa (Mesotrichia) latipes cf. (Drury, 1773) and Xylocopa (Koptortosoma) aestuans (Latreille, 1802) both member of the family Apidae. Diversity 2020, 12, 259 10 of 13 Members of the Xylocopa genus have been known to forage over greater distances allowing them to search for resources far away from their nesting area [46]. At the guava orchards, Xylocopa bees were observed to forage by making short visits to individual flowers and visiting numerous flowers in one feeding bout, whereas stingless bees often visited only one or few flowers within the same guava plants. It is therefore likely that Xylocopa bees are more e ective pollinators of guava flowers than are stingless bees, which may also partly explain why overall pollinator visitation rates and landscape characteristics do not predict fruit set in our study [43]. We note that the proportion of forest and visitation rate showed a significant positive relationship not only at short distance but even at a 5 and 10 km radius from the study site, that is, much further distances than most bees and other insect pollinators are travelling to forage. This may indicate that larger patches of forest fragments even beyond the forage range of pollinators is indirectly beneficial for sustaining pollinator abundances. It is well established that habitat loss and fragmentation are some of the main drivers of the ongoing global pollinator decline, having a negative e ect on species richness and abundance, with larger suitable habitat being important for sustaining pollinator robustness [15,47]. Areas with intermediate levels of disturbance, such as tree gardens, plantations, smaller forest fragments and undeveloped urban areas in the tropics might be able to function as micro-habitats and support pollinating species [15,47,48]. A theory might be that larger forest fragments work as a ‘source’ and support the abundance of a species in areas closer to the guava fields. Taken together, the findings in this study underlines the importance of natural habitat to sustain pollinator communities but illustrates that crop fruit set does not necessarily diminish on farmland surrounded by little natural habitat. We note that all our study sites were in fairly degraded landscapes (<16% forest cover). As for biodiversity in general [49,50], it is possible that native pollinator richness and their services would have been higher in landscapes with higher cover of natural habitat. Future studies should thus aim at including farms embedded within more pristine landscapes. Landscape degradation continues to occur at an alarming rate, especially in many tropical regions, making it crucial that we invest in understating its impact on plants, pollinators and their interactions. Thus, more research is needed to better understand pollinator-crop relationships and the importance of natural habitat to sustain pollinator services and crop production in the future. Author Contributions: Conceptualization, K.H., T.S., S.B., B.I.S., N.S. and B.D.; Data curation, K.H. and T.S.; Formal analysis, K.H. and T.S.; Funding acquisition, K.H.; Investigation, K.H. and T.S.; Methodology, K.H., T.S., N.S. and B.D.; Project administration, K.H., T.S., S.B., N.S. and B.D.; Resources, K.H., S.B. and B.D.; Supervision, T.S., S.B., N.S. and B.D.; Visualization, K.H.; Writing—Original draft, K.H.; Writing—Review & editing, T.S., S.B., B.I.S., N.S. and B.D. All authors have read and agreed to the published version of the manuscript. Funding: The fieldwork in this project was funded by; SCIENCE grants: Henrik Tofte Jacobsen s Grant, 15000 DKK, William Demant Fonden, 8500 DKK and Knud Højgaards Fond, 13000 DKK. B.I.S was supported by a Royal Commission for the Exhibition of 1851 Research Fellowship. Acknowledgments: We thank all the guava farmers in Songkhla and Phattalung that allowed us to conduct fieldwork in their orchards. We are grateful for the help and assistance with identifying my bee specimens provided by Natapot Warrit and his student from the bee lab at the Chulalongkorn University, Bangkok. This project could not have been done without the hard work, dedication and translation provided by field assistant Piyaporn Suksai. A big thanks go to her and her family for their help and for providing accommodation during fieldwork in Phattalung. Also thanks to Helle Sørensen for statistical advice. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results. Diversity 2020, 12, 259 11 of 13 Appendix A Table A1. Complete list of all species observed in the six guava orchards. Identified down to lowest possible taxonomic level. Individuals Species Study Sites Observed (Total) Bees (Anthophila) Anthophila sp. 1 3 4 Nomia (Acunomia) strigata 5 4 Braunsapis cupulifera cf. 7 4 Ceratina (ceratinidia) nigrolateralis 10 4 Apis Apis cerana 196 1, 2, 3, 5, 6 Apis dorcata 1 1 Apis florana 2 1 Carpenter beess Xylocopa (Koptortosoma) aestuans 14 2, 3, 4 (Xylocopa) Xylocopa (Mesotrichia) latipes cf. 4 3 Geniotrigona thoracica 14 1 Heterotrigona itama 145 1, 4 Tetragonula (laeviceps gr.) 32 1, 2, 4 Stingless Tetragonula (pagdeni gr.) 1 1 bees(Meliponines) Tetragonula sp. 6 4 Meliponines sp. 1 1 Wasps (Vespidae) Vespa Anis 1 1 Beetles (Coleoptera) Coleoptera sp. 1 2 5 Coleoptera sp. 2 7 5 Coleoptera sp. 3 1 5 Flies (Diptera) Diptera sp. 1 19 4 Diptera sp. 2 2 4 Diptera sp. 3 3 4 Diptera sp. 4 15 2 Ants (Formicidae) Formicidae sp. 1 4 5 Formicidae sp. 2 2 4 Formicidae sp. 3 1 5 References 1. Rech, A.R.; Dalsgaard, B.; Sandel, B.; Sonne, J.; Svenning, J.C.; Holmes, N.; Ollerton, J. The macroecology of animal versus wind pollination: Ecological factors are more important than historical climate stability. Plant Ecol. Divers. 2016, 9, 253–262. [CrossRef] 2. Ollerton, J.; Winfree, R.; Tarrant, S. How many flowering plants are pollinated by animals? Oikos 2011, 120, 321–326. [CrossRef] 3. Ollerton, J. Pollinator diversity: Distribution, ecological function, and conservation. Annu. Rev. Ecol. Evol. Syst. 2017, 48, 353–376. [CrossRef] 4. Bawa, K. Plant-pollinator interactions in tropical rain forests. Annu. Rev. Ecol. Evol. 1990, 21, 399–422. [CrossRef] 5. Potts, S.G.; Imperatriz-Fonseca, V.; Ngo, H.; Biesmeijer, J.C.; Breeze, T.; Dicks, L.; Garibaldi, L.; Settele, J.; Vanbergen, A.J.; Aizen, M.A.; et al. Summary for Policymakers of the Assessment Report of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) on Pollinators, Pollination and Food Production; Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services: Bonn, Germany, 2016; ISBN 978-92-807-3568-0. 6. Daily, G.C. Nature’s Services; Island Press: Washington, DC, USA, 1997; Volume 3. 7. Allen-Wardell, G.; Bernhardt, P.; Bitner, R.; Burquez, A.; Buchmann, S.; Cane, J.; Cox, P.A.; Dalton, V.; Feinsinger, P.; Ingram, M.; et al. The potential consequences of pollinator declines on the conservation of biodiversity and stability of food crop yields. Conserv. Biol. 1998, 12, 8–17. [CrossRef] Diversity 2020, 12, 259 12 of 13 8. Klein, A.M.; Vaissière, B.E.; Cane, J.H.; Ste an-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Tscharntke, T. Importance of pollinators in changing landscapes for world crops. Proc. R. Soc. B Biol. Sci. 2007, 274, 303–313. [CrossRef] [PubMed] 9. Roubik, D.W. Pollination of Cultivated Plants in the Tropics; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 1995; ISBN 9251036594. 10. Dalsgaard, B. Land-Use and climate impacts on plant–pollinator interactions and pollination services. Diversity 2020, 12, 168. [CrossRef] 11. Biesmeijer, J.C.; Roberts, S.P.M.; Reemer, M.; Ohlemüller, R.; Edwards, M.; Peeters, T.; Scha ers, A.P.; Potts, S.G.; Kleukers, R.; Thomas, C.D.; et al. Parallel declines in pollinators and insect-pollinated plants in Britain and the Netherlands. Science 2006, 313, 351–354. [CrossRef] 12. Ollerton, J.; Erenler, H.; Edwards, M.; Crockett, R. Extinctions of aculeate pollinators in Britain and the role of large-scale agricultural changes. Science 2014, 346, 1360–1362. [CrossRef] 13. Hallmann, C.A.; Sorg, M.; Jongejans, E.; Siepel, H.; Hofland, N.; Schwan, H.; Stenmans, W.; Müller, A.; Sumser, H.; Hörren, T.; et al. More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS ONE 2017, 12, e0185809. [CrossRef] 14. Laurance, W.F. Forest destruction in tropical Asia. Curr. Sci. 2007, 93, 1544–1550. 15. Sritongchuay, T.; Hughes, A.C.; Memmott, J.; Bumrungsri, S. Forest proximity and lowland mosaic increase robustness of tropical pollination networks in mixed fruit orchards. Landsc. Urban Plan. 2019, 192, 103646. [CrossRef] 16. Büchert Lindberg, A.; Mogens Olesen, J. The fragility of extreme specialization: Passiflora mixta and its pollinating hummingbird Ensifera ensifera. J. Trop. Ecol. 2001, 17, 323–329. [CrossRef] 17. Ricketts, T.H.; Daily, G.C.; Ehrlich, P.R.; Michener, C.D. Economic value of tropical forest to co ee production. Proc. Natl. Acad. Sci. USA 2004, 101, 12579–12582. [CrossRef] [PubMed] 18. Ricketts, T.H. Tropical forest fragments enhance pollinator activity in nearby co ee crops. Conserv. Biol. 2004, 18, 1262–1271. [CrossRef] 19. Rathcke, B.J.; Jules, E.S. Habitat fragmentation and plant pollinator interactions. Curr. Sci. 1993, 65, 273–277. 20. Garibaldi, L.A.; Ste an-Dewenter, I.; Kremen, C.; Morales, J.M.; Bommarco, R.; Cunningham, S.A.; Carvalheiro, L.G.; Chaco , N.P.; Dudenhö er, J.H.; Greenleaf, S.S.; et al. Stability of pollination services decreases with isolation from natural areas despite honey bee visits. Ecology 2011, 14, 1062–1072. [CrossRef] 21. Blanche, K.R.; Ludwig, J.A.; Cunningham, S.A. Proximity to rainforest enhances pollination and fruit set in orchards. J. Appl. Ecol. 2006, 43, 1182–1187. [CrossRef] 22. Ricketts, T.H.; Regetz, J.; Steffan-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Bogdanski, A.; Gemmill-Herren, B.; Greenleaf, S.S.; Klein, A.M.; Mayfield, M.M.; et al. Landscape effects on crop pollination services: Are there general patterns? Ecol. Lett. 2008, 11, 499–515. [CrossRef] 23. Garibaldi, L.A.; Ste an-Dewenter, I.; Winfree, R.; Aizen, M.A.; Bommarco, R.; Cunningham, S.A.; Kremen, C.; Carvalheiro, L.G.; Harder, L.D.; Afik, O.; et al. Wild pollinators enhance fruit set of crops regardless of honey bee abundance. Science 2013, 340, 1608–1611. [CrossRef] 24. Aizen, M.A.; Garibaldi, L.A.; Cunningham, S.A.; Klein, A.M. How much does agriculture depend on pollinators? Lessons from long-term trends in crop production. Ann. Bot. 2009, 103, 1579–1588. [CrossRef] [PubMed] 25. Tylianakis, J.M.; Didham, R.K.; Bascompte, J.; Wardle, D.A. Global change and species interactions in terrestrial ecosystems. Ecol. Lett. 2008, 11, 1351–1363. [CrossRef] [PubMed] 26. Pommer, C.V.; Murakami, K.R.N. Breeding guava (Psidium guajava L.). In Breeding Plantation Tree Crops: Tropical Species; Jain, S.M., Priyadarshan, P.M., Eds.; Springer: New York, NY, USA, 2009; pp. 83–120. ISBN 9780387712017. 27. Salazar, D.M.; Melgarejo, P.; Martínez, R.; Martínez, J.J.; Hernández, F.; Burguera, M. Phenological stages of the guava tree (Psidium guajava L.). Sci. Hortic. 2006, 108, 157–161. [CrossRef] 28. Morton, J.F.; Dowling, C.F. Fruits of Warm Climates, Pomegranate; JF Morton: Miami, FL, USA, 1987; ISBN 0-9610184-1-0. 29. Altendorf, S. Minor Tropical Fruits: Mainstreaming a Niche Market; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 2018; Available online: http://www.fao.org/fileadmin/templates/est/ COMM_MARKETS_MONITORING/Tropical_Fruits/Documents/Minor_Tropical_Fruits_FoodOutlook_1_ 2018.pdf (accessed on 13 May 2020). Diversity 2020, 12, 259 13 of 13 30. Food and Agriculture Organization of the United Nations (FAO). Food Outlook: Biannual Report on Global Food Markets; Global Information and Early Warning System on Food and Agriculture: Rome, Italy, 2017; Available online: http://www.fao.org/3/a-i7343e.pdf (accessed on 13 May 2020). 31. Kadam, D.M.; Kaushik, P.; Kumar, R. Evaluation of guava products quality. Int. J. Food Sci. Nutr. Eng. 2012, 2012, 7–11. [CrossRef] 32. Singh, R.; Sehgal, O. Studies on the blossom-biology of Psidium guajava L. (Guava) II. pollen studies, stigmatal receptivity, pollination and fruit-set. Indian J. Hortic. 1968, 25, 52–59. 33. Thanarut, C.; Ko, J.-C. Guava, wax apple, sugar apple and pineapple productions in Thailand. Acta Hortic. 2017, 1166, 7–14. [CrossRef] 34. Mitra, S.K.; Thingreingam Irenaeus, K.S. Guava cultivars of the world. Acta Hortic. 2018, 1205, 905–910. [CrossRef] 35. Land Development Department. GIS Data on Land Use Resources. Available online: http://www.ldd.go.th/ www/lek_web/web.jsp?id=18907 (accessed on 29 May 2020). 36. Klein, A.-M.; Ste an-Dewenter, I.; Tscharntke, T. Bee pollination and fruit set of co ea arabica and C. canephora (Rubiaceae). Am. J. Bot. 2003, 90, 153–157. [CrossRef] 37. Oksanen, J.; Blanchet, F.G.; Friendly, M.; Kindt, R.; Legendre, P.; McGlinn, D.; Minchin, P.R.; O’Hara, R.B.; Simpson, G.L.; Solymos, P.; et al. Package ‘vegan’Community Ecology Package. R Package Version 2.5-6. 2019. Available online: https://cran.r-project.org/web/packages/vegan/vegan.pdf (accessed on 12 May 2020). 38. Ekstrøm, C.T.; Sørensen, H. Introduction to Statistical Data Analysis for the Life Sciences; CRC Press: Boca Raton, FL, USA, 2014; ISBN 9781439894866. 39. R Core Team. R: A language and environment for Statistical Computing; R Foundation for Statistical Computing: Vienna, Austria, 2020. 40. Hadley, W. Ggplot2: Elegant Graphics for Data Analysis; Springer: Houston, TA, USA, 2016. 41. Alves, J.E.; Freitas, B.M. Foraging behavior and pollination eciency of five bee species on guava (Psidiumguajava L.) flowers. Rev. Ciência Agronômica 2008, 37, 216. 42. Tangtorwongsakul, P.; Warrit, N.; Gale, G.A. E ects of landscape cover and local habitat characteristics on visiting bees in tropical orchards. Agric. For. Entomol. 2018, 20, 28–40. [CrossRef] 43. Hansen, K. (University of Copenhagen, Copenhagen, Denmark). Personal Communication, 2020. 44. Winfree, R.; Williams, N.M.; Dusho , J.; Kremen, C. Native bees provide insurance against ongoing honey bee losses. Ecol. Lett. 2007, 10, 1105–1113. [CrossRef] [PubMed] 45. Stokstad, E. The case of the empty hives. Science 2007, 316, 970–972. [CrossRef] [PubMed] 46. Zurbuchen, A.; Landert, L.; Klaiber, J.; Müller, A.; Hein, S.; Dorn, S. Maximum foraging ranges in solitary bees: Only few individuals have the capability to cover long foraging distances. Biol. Conserv. 2010, 143, 669–676. [CrossRef] 47. Potts, S.G.; Biesmeijer, J.C.; Kremen, C.; Neumann, P.; Schweiger, O.; Kunin, W.E. Global pollinator declines: Trends, impacts and drivers. Trends Ecol. Evol. 2010, 25, 345–353. [CrossRef] 48. Nascimento, V.T.; Agostini, K.; Souza, C.S.; Maruyama, P.K. Tropical urban areas support highly diverse plant-pollinator interactions: An assessment from Brazil. Landsc. Urban Plan. 2020, 198, 103801. [CrossRef] 49. Mariana Morais, V.; Cristina, B.; Leandro Reverberi, T.; Érica, H.; Pedro Ferreira, D.; Wesley Rodrigues, S.; Paulo Roberto, G.; Jean Paul, M. Predicting the non-linear collapse of plant–frugivore networks due to habitat loss. Ecography 2019, 42, 1765–1776. [CrossRef] 50. Melo, I.; Ochoa-Quintero, J.M.; Oliveira Roque, F.; Dalsgaard, B. A review of threshold responses of birds to landscape changes across the world. J. Field Ornithol. 2018, 89, 303–314. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Diversity Multidisciplinary Digital Publishing Institute

Landscape-Level Effects of Forest on Pollinators and Fruit Set of Guava (Psidium guajava L.) in Orchards across Southern Thailand

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diversity Article Landscape-Level E ects of Forest on Pollinators and Fruit Set of Guava (Psidium guajava L.) in Orchards across Southern Thailand 1 , 2 3 4 Katrine Hansen *, Tuanjit Sritongchuay , Sara Bumrungsri , Benno I. Simmons , 5 1 , Niels Strange and Bo Dalsgaard * Center for Macroecology, Evolution and Climate, GLOBE Institute, University of Copenhagen, 2100 Copenhagen Ø, Denmark Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Mengla 666303, China; t.sritongchuay@gmail.com Department of Biology, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand; sara.b@psu.ac.th Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Penryn TR10 9FE, UK; benno.simmons@gmail.com Department of Food and Resource Economics & Center for Macroecology, Evolution and Climate, University of Copenhagen, Rolighedsvej 23, 1958 Copenhagen, Denmark; nst@ifro.ku.dk * Correspondence: Katrinekvj@gmail.com (K.H.); bo.dalsgaard@sund.ku.dk (B.D.); Tel.: +45-225-290-17 (K.H.); +45-423-245-53 (B.D.) Received: 29 May 2020; Accepted: 23 June 2020; Published: 25 June 2020 Abstract: Pollination by wild pollinators is a key ecosystem service threatened by anthropogenic-induced land-use change. The proximity to natural habitat has previously been shown to positively affect pollinator communities and improve crop yield and quality but empirical evidence is limited from most parts of the World. Here, across six farms in Southern Thailand, we investigated the significance of landscape-level effects of natural habitat (proportion of and distance to evergreen forest) on both visitation rate and richness of pollinators as well as fruit set of guava (Psidium guajava L.), a local economically-important crop in the tropics. Overall, the most abundant pollinator was the Asian honey bee Apis cerana (39% of all visits) and different species of stingless bees (37%). We found that pollinator richness was unrelated to the proportion and distance to evergreen forest, however, the proportion of forest within a 1, 5 and 10 km radius had a significant positive impact on visitation rate of wild pollinators. Still, neither the various forest parameters nor pollinator visitation rate showed a significant impact on fruit set of guava, perhaps because guava self-pollinates. This illustrates that landscape-level degradation of natural habitat may negatively impact pollinator communities without diminishing the crop yield of the farmers. Keywords: crop pollination; ecosystem services; land-use; plant-pollinator interactions; pollination 1. Introduction Flowering plants rely on successful pollination for sexual reproduction. Although many plants may be able to self-pollinate or use abiotic agents such as wind to transport their pollen, the majority of plants use animal pollinators as vectors to transport pollen between plant individuals [1,2]. This makes plant-pollinator interactions one of the most important ecological functions for sustaining our ecosystems [3]. Notably, animal pollination plays a crucial role in warm and humid tropical ecosystems where an estimated average of 94% of all flowering plant species are thought to be animal pollinated [1,2,4]. A large variety of crops benefit from being pollinated by animals; thus, pollination is also a key ecosystem service that benefits us humans [5,6]. Ecient pollination is known to contribute Diversity 2020, 12, 259; doi:10.3390/d12060259 www.mdpi.com/journal/diversity Diversity 2020, 12, 259 2 of 13 significantly to both yield and quality in the production of close to 75% of the most important food crops worldwide [5,7,8]. As pollinator contribution to yield di ers greatly between plant species and we consume heavily certain crops that are not pollinator-dependent, this corresponds to around 5–8% of the global crop production [5]. As for wild plants, crop plants grown in tropical areas are more dependent on animal pollination than crop plants used in temperate regions [8,9]. Globally, there is an increasing concern that the ecosystem services provided by wild pollinators are threatened by anthropogenic-induced global change [3,8,10]. Pollinator extinctions and declines have been observed in both Europe and North America [11–13], whereas we are lacking solid data for wild pollinators in most other parts of the World, such as tropical South America, Africa and Asia [5,7,11,13]. Some of the most important anthropogenic-induced drivers of change in pollinator communities are climate and land-use change, including deforestation and conversion to agriculture [5,10]. Land-use change from natural habitat to agricultural land has increased by 25% since the 1960s and most of this expansion has happened in the developing world, such as the tropical forests of South-east Asia [10,14]. Therefore, it is crucial to better understand how landscape-level e ects impact pollinators and their services [10,15,16]. The proximity to natural habitat has been shown to positively a ect pollinator communities and improve crop yield or quality [8,15,17–22]. In other words, farmers growing pollinator-dependent crops close to natural habitat have been observed to have a greater crop production than farmers far from natural habitat. Land-use changes and habitat degradation thus seem to not only harm nature but also farm productivity. Managed honey bees are utilized worldwide to meet the need of pollination in large orchards or degraded landscapes. The most frequently-used species are the European honey bee Apis mellifera L. and the Asian honey bee Apis cerana L. both from the family Apidae [7,23,24]. Although managed honey bees benefit crop yield, wild bee species often pollinate crops more eciently and improve cross-pollination more so than by managed honey bees [23]. Thus, we cannot simply substitute wild pollinators for managed honey bees. Furthermore, land-use changes will most likely have a negative e ect on ecological resilience of pollinator communities underlining the need to better understand the impact of land-use change on wild pollinators [25]. The guava tree (Psidium guajava L.) belonging to the Myrtaceae family is thought to be native to tropical Mexico and Central America [26–28] but today it is widespread and locally an economically-important crop produced in many tropical and subtropical landscapes around the world [26,29]. Guava is considered part of the so-called “minor tropical fruit” production together with fruits such as mango and mangosteen. Minor tropical fruits are mostly grown in smaller households and traded regionally and in smaller volumes [29,30] but the production and trade are gaining more importance globally, as many of the minor fruits are recognized as a contribution to a healthier diet [29]. In Thailand, for instance, besides being important for the economy of local households, it has been estimated that minor fruits account for almost 7 per cent of agricultural earnings [29]. The guava is an important minor tropical fruits in the food industry, especially in the production of juice, fruit bars or other dehydrated food products [31]. In tropical areas, production of guava fruits can be induced year around by pruning the branches, making guava trees suitable for continuous fruit production and, thus, an excellent crop to conduct pollination experiments and to examine land-use impacts on fruit production [26]. Although a few pollination experiments have been conducted on guava, it still remains uncertain whether guava is mainly cross-pollinated by insects or self-pollinated [26,27,31,32]. Furthermore, despite its importance, we know little about how land-use impact the pollination and fruit set of guava. Therefore, in this study we aim to test the e ect of natural forest in the surrounding landscape on pollinator communities and the pollination of guava in orchards across Southern Thailand. To examine this, we estimated pollinator richness and visitation rate and performed pollination experiments (open vs. closed pollination treatment) in six guava orchards surrounded by di erent amounts of natural forest. We also measured the morphology of guava flowers and conducted pollination experiments to describe the overall dependency of guava on pollinators. We then tested the following hypotheses—(1) pollinator richness and visitation rate increase close to evergreen forest and with a larger proportion of Diversity 2020, 12, 259 3 of 13 evergreen forest in the surrounding landscape and (2) fruit set of guava trees increases on farms with a higher richness and visitation rate of pollinators and a larger proportion of (and/or closer distance to) evergreen forest in the surrounding landscape. 2. Materials and Methods 2.1. Study Site and Species (Psidium Guajava) Guava production is common in Southern Thailand for both private and commercial purposes [33]. Here, we focus on plantations growing guava as a monoculture for commercial sale. All guava trees studied were of the local variety “Kimju guava” [34]. The study took place in Southern Thailand from November 2019 until March 2020. Six guava plantations with varying degrees of surrounding evergreen forest were chosen. The plantations were located in the provinces Phattalung and Songkhla 0  0  0  0 (6 54 to 7 52 N and 99 53 to 100 36 E; Table 1). In these provinces, it takes between four and five months from flowering to harvest of a ripe guava. The reference land-use map was obtained from the Land Development Department of Thailand (LDD) [35]. The LDD provided the shape file data which has scale of 1:25,000 digitized from satellite images resolution of 2.5 m. Evergreen forest is defined as “areas dominated by trees generally 5–40 m tall and 20% of total vegetation cover. More than 75% of the tree species maintain their leaves all year. Canopy is never without green foliage.” The evergreen forest fraction was calculated within a 1, 5 and 10 km radius for each site using ARCGIS 10.3. The studied orchards ranged in size from 0.4 ha to 2.8 ha all with guava trees older than two years. All study sites were chosen based on having similar management practices such as being monocultures, using pruning to initiate flowering and using limited amount of pesticides. In the Phattalung and Songkhla provinces, we were unable to locate more than six farms fulfilling these criteria. Table 1. Study site overview. General information about study sites (location, size, number and age of guava tress) and forest parameters (Distance to closest evergreen forest patch and % evergreen forest within a radius of 1, 5 and 10 km from the study site). Study Sites Information Evergreen Forest Parameters Size No. Age Trees Distance 1 km 5 km 10 km Site Latitude Longitude (ha) Trees (Years) (m) (%) (%) (%) 1 6.989000 100.616611 1.6 1500 4–5 371 15.63 12.96 7.27 2 7.035883 100.280283 2.8 1000 2–4 752 0.00 0.00 0.00 3 7.048433 100.449133 2.3 839 5 5475 0 0 3.30 4 6.908079 100.502339 0.4 500 2 770 2.33 2.85 5.09 5 7.555000 100.130433 1.6 998 3 8676 3.47 0.16 0.39 6 7.873817 99.885733 0.5 216 3.5 7070 0 0 2.21 2.2. Floral Traits and Pollination Experiments To characterize guava flowers, we observed—number of flowers in an inflorescence, number of petals and color of stigma. We also measured the floral corolla width and the length of anthers using a digital caliper. To characterize the floral reward, we counted the number of anthers per flower and used micropipettes to investigate whether the guava flower produced nectar (observed from 7.00 to 11.00 AM). To gain knowledge of the reproductive system of guava, we conducted a series of pollination experiments—(1) open pollination (OP); flowers were left open allowing unhindered animal visitation (control treatment), (2) wind-pollination (WP); mature buds were bagged before opening with a coarse-meshed gauze (0.8 mm), allowing wind carried pollen through but otherwise left untouched, (3) cross-pollination (CP); mature buds with cracked calyx were bagged before opening with a spunbond nonwoven bag (which is a highly e ective barrier to pollen grains) and were hand-pollinated the following morning with pollen from flowers from a di erent tree, (4) self-pollination within the same Diversity 2020, 12, 259 4 of 13 tree (SP); mature buds with cracked calyx were bagged before opening with a spunbond nonwoven bag and were hand-pollinated the following morning with pollen from a di erent flower on the same tree, (5) self-pollination within the same flower (SF); mature buds with cracked calyx were bagged before opening with a spunbond nonwoven bag and the following morning each flower were hand-pollinated with pollen from its own anthers (i.e., within the same flower). Hand-pollination was done by using a cotton bud to transfer pollen grains to the stigma and using anthers dapped directly onto the stigma to ensure sucient pollen deposit. This was done between 7 and 9 AM. After applying one of the four treatments, excluding the control treatment (OP), flowers were bagged again and bags were left on for a week to ensure the stigma was no longer receptive when bags were removed. The initial fruit set was counted three weeks after flowering, when fruits were approximately 2 cm in diameter. Fruit set was calculated as the proportion of flowers developing into an initial fruit for each treatment and each tree. All pollination experiments were conducted at only one study site (Site 6, Table 1). Each treatment included five mature buds per tree in ten healthy trees located approximately ten meters from the edge (i.e., 50 flowers per treatment). Although some trees were used for more than one treatment, due to a lack of suitable trees with a sucient amount of mature buds at the study site, we were unable to conduct all treatments at the same trees as it ideally should have been [18,36]. This resulted in some treatments being paired on the same trees, while other treatments were conducted on other trees. We thus did not perform any statistical analysis to examine whether pollinations experiments di ered in fruit set but use the treatments to qualitatively assess the ability of guava to produce fruits under di erent pollination scenarios. 2.3. Pollinator Visitation and Richness On each study site, we estimated pollinator visitation rate and species richness shortly after anthesis from 7.30 to 9.00 AM. For all sites, visitation rate and richness measurements were conducted within one day. At each site, we chose ten healthy trees located approximately ten meters from the edge of the orchards and with a minimum of one flower. Pollinator visitation was monitored for 15 min per tree (resulting in 150 min of visitation per study site) followed by a 5 min observation period on the same tree to estimate pollinator richness (resulting in 50 min of richness per study site). In both visitation and richness observations, the number of individuals visiting the flowers were noted together with the number of visits. For the visitation rate, we did not catch the visiting animals but registered the visiting species. When it was not possible to identify the visiting animal to species-level, we instead registered the taxonomic group, that is, whether it was a bee, stingless bee, carpenter bee, wasp, fly, beetle or ant. One or two flowers were observed at each tree and a registration of visits was made every time a visiting animal touched the reproductive parts of an observed flower. Additionally, we recorded every time a new individual touch the reproductive parts of the flowers, thus we estimated both the total number of pollinator individuals and total number of pollinator visits observed within 15 min. Average visitation rate of all pollinators on each study site was calculated as average number of visits per flower per tree, then averaged across all ten trees yielding one number per site (visits/flower/15 min). Average visitation rate was also calculated for di erent sub-groups for each study site—Wild pollinators (all pollinators without the domesticated honey bee A. cerana); All bee species; Wild bee species (all bee species without A. cerana). Many farmers set up empty boxes within or close by the study fields as nesting spot for A. cerana. Thus, A. cerana is often semi-managed and not dependent on forest as nesting area, which is why we exclude A. cerana from the ‘wild pollinator ’ and ‘wild bee’ subgroups. For the species richness estimation, species were either identified in the field or collected and identified by a taxonomist to the lowest possible taxonomic level. The calculations on estimated richness were based on the number of individuals observed during visitation (15 min) and individuals caught or observed during the 5 min of richness estimation. Individuals were caught in the field using an insect net or using a clear plastic bag gently placed over the insect. The insects were quickly Diversity 2020, 12, 259 5 of 13 transferred to a microtube containing 99% ethanol and pinned shortly after returning from the field. A list of all species is found in Table A1, Appendix A. 2.4. Pollinator Exclusion Experiment At each study site, ten healthy trees approximately ten meters from the edge of the orchard were chosen in the same area as trees used for monitoring visitation rate and pollinator richness. On each tree we chose ten mature flower buds randomly and subjected to one of two treatments—pollinator exclusion (covered in a 0.8 mm coarse-meshed gauze allowing the flower to be pollinated by wind but not by pollinators) and open pollination (control treatment), allowing us to investigate the di erence in fruit set with and without pollinators. Bags were left on for 5–10 days to ensure the stigma was no longer receptive. The initial fruit set was counted three weeks after flowering when initial fruits were approximately 2 cm in diameter. Fruit set of each treatment was calculated as the proportion of flowers developing into an initial fruit for each tree. The additional fruit set with pollinators access was calculated as the di erence in fruit set between the two treatments (bagged and open) for each tree. This method controls for variations between plants (which could be caused by di erence in water supply, microclimate, management history and soil chemistry) presenting only the di erence of fruit set with and without pollinators [17]. For each study site, an average of the additional fruit set with pollinators was calculated across the ten trees and used for later analyses. We note that the experiments without pollinators in a few cases resulted in higher fruit set than when pollinators had access to the flowers. We in these cases converted the di erences in fruit set to zero (and not negative values), as we doubt pollinators have a negative influence on fruit set but that these negative values were caused by stochasticity. However, in the below regression analyses we also conducted the analyses using negative values, qualitatively obtaining the same results (not shown). 2.5. Statistical Analyses For each pollination experiment (OP, WP, CP, SP and SF), we calculated the mean fruit set. Since some treatments were paired and others were not (see Section 2.3), no formal statistical test were performed to examine whether the treatments differed in mean fruit set. To test the e ect of the di erent forest parameters on pollinator visitation rate, linear regressions were made between all forest parameters (proportion of evergreen forest within 1, 5 and 10 km radius as well as distance to nearest forest patch) and the di erent visitation rates (all pollinators, wild pollinators, all bee species, wild bee species). The estimated pollinator richness at each study site was calculated using the vegan package version 2.5–6 in R [37] and also regressed against all forest parameters. Finally, we regressed the average additional fruit set with pollinators against the forest parameters. Since data is an average of proportions, we may assume the six averages follow a normal distribution following the central limit theorem (CLT) [38]. To test if the residuals were normally distributed, a QQ plot for the residuals were performed for each regression using the stats package in R [39]. For all regressions, we report the R values and the significance level of the regressions (Table 2). Regressions between di erent visitation rates and the proportion of evergreen forest within the three radiuses were plotted with the corresponding confidence interval using ggplot2 in R [40]. All analyses were conducted using R 3.6.3 [39]. Diversity 2020, 12, 259 6 of 13 Table 2. Linear regression models of pollinator visitation rates (VR) (visits/flower/15 min), richness and additional fruit set with pollinators as a function of four measures of evergreen forest e ect: % of evergreen forest in the surrounding landscape within di erent distances to the study site (1, 5 and 10 km) and the distance to nearest forest patch from each study site (in meter). For each model, we also report the coecient of determination (R ) and the regression coecients with standard error. NS p > 0.05, * p < 0.05, ** p < 0.01. Dependent Independent R Regression Coecients Visitation rate Forest NS 1000 m (%) 0.51 0.42 (0.21) 5000 m (%) 0.68 0.57 (0.20) * All NS 10,000 m (%) 0.47 0.88 (0.47) 4 4 NS Distance (m) 0.48 6.710 (3.510 ) 1000 m (%) 0.79 0.69 (0.18) * 5000 m (%) 0.91 0.87 (0.14) ** Wild pollinators 10,000 m (%) 0.72 1.43 (0.45) * 4 4 NS Distance (m) 0.53 9.410 (4.410 ) NS 1000 m (%) 0.52 0.44 (0.21) 5000 m (%) 0.67 0.59 (0.21) * All bees NS 10,000 m (%) 0.45 0.90 (0.50) 4 4 NS Distance (m) 0.34 6.010 (4.210 ) 1000 m (%) 0.86 0.71 (0.14) ** 5000 m (%) 0.97 0.90 (0.08) ** Wild bees 10,000 m (%) 0.74 1.45 (0.42) * 4 4 NS Distance (m) 0.45 8.610 (4.710 ) NS 1000 m (%) 0.27 0.35 (0.29) NS 5000 m (%) 0.31 0.44 (0.33) Estimated richness NS 10,000 m (%) 0.37 0.88 (0.58) 4 4 NS Distance (m) 0.34 6.510 (4.510 ) NS 1000 m (%) 0.39 0.005 (0.003) NS 5000 m (%) 0.24 Additional initial 0.004 (0.004) NS fruit set 10,000 m (%) 0.20 0.007 (0.007) 5 6 NS Distance (m) 0.35 4.410 (5.710 ) VR and Richness NS VR All 0.0001 0.0001 (0.006) NS Additional initial VR Wild pollinators 0.1141 0.0032 (0.004) NS fruit set VR All bees 0.0069 0.0010 (0.006) NS VR Wild bees 0.1589 0.0038 (0.004) NS Richness 0.0765 0.0031 (0.005) 3. Results 3.1. Floral Morphology Guava trees have 1–3 flowers in each inflorescence (Figure 1). Floral corolla width was measured to be 47.63 4.17 mm (mean SD; n = 10) and consist of 5–6 white and scentless petals. Flowers are hermaphrodite and contain one light-green stigma and an estimated 621.50 62.06 (mean SD; n = 10) anthers with an average max length of 15.33 1.56 mm (mean SD; n = 7). Flowers have little or no nectar (no nectar was detected in the field in this study) but we observed large amounts of pollen on each anther. Diversity 2020, 12, 259 7 of 14 Diversity 2020, 12, 259 7 of 14 Diversity 2020, 12, 259 7 of 13 Figure 1. Illustration of guava flowers at different stages (bud, cracked bud and open flower) and the most important pollinators: (A) Three species of stingless bee, from left; Heterogona itama, Tetragonula laeviceps and Geniotrigona thoracica. (B) Asian honey bee Apis cerana. (C) Carpenter bee Xylocopa (Koptortosoma) aestuans. Figure 1. Illustration of guava flowers at different stages (bud, cracked bud and open flower) and the most Figure 1. Illustration of guava flowers at different stages (bud, cracked bud and open flower) and the important pollinators: (A) Three species of stingless bee, from left; Heterogona itama, Tetragonula laeviceps and most important pollinators: (A) Three species of stingless bee, from left; Heterogona itama, Tetragonula 3.2. Pollination Experiments Geniotrigona thoracica. (B) Asian honey bee Apis cerana. (C) Carpenter bee Xylocopa (Koptortosoma) aestuans. laeviceps and Geniotrigona thoracica. (B) Asian honey bee Apis cerana. (C) Carpenter bee Xylocopa P. guajava set fruit in all pollinator experiments. Initial fruit set ranged from 69% in open 3.2. (Koptor Pollination tosomExperiments a) aestuans. pollination (OP), 70% in pollination by wind alone (WP), 76% in cross pollination (CP) to 78% and P. guajava set fruit in all pollinator experiments. Initial fruit set ranged from 69% in open 3.2. Pollination Experiments 96% in self-pollination within same flower (SF) and same tree (SP), respectably (Figure 2). pollination (OP), 70% in pollination by wind alone (WP), 76% in cross pollination (CP) to 78% and 96% P. guajava set fruit in all pollinator experiments. Initial fruit set ranged from 69% in open in self-pollination within same flower (SF) and same tree (SP), respectably (Figure 2). pollination (OP), 70% in pollination by wind alone (WP), 76% in cross pollination (CP) to 78% and 96% in self-pollination within same flower (SF) and same tree (SP), respectably (Figure 2). Figure 2. Initial fruit set (% fruit set 3 weeks after blooming) of Psidium guajava L. of five pollination Figure 2. Initial fruit set (% fruit set 3 weeks after blooming) of Psidium guajava L. of five pollination treatments. The treatments were conducted on ten trees on one farm (Farm 6, see Table 1), showing the average fruit set of five flower buds pr. tree (a total of 50 flower buds pr. treatment). OP, Open treatments. The treatments were conducted on ten trees on one farm (Farm 6, see Table 1), showing Figure 2. Initial fruit set (% fruit set 3 weeks after blooming) of Psidium guajava L. of five pollination pollination (n = 20 trees); WP, Wind pollination (n = 10); CP, Cross pollination between plants (n = 10); the average fruit set of five flower buds pr. tree (a total of 50 flower buds pr. treatment). OP, Open treatments. The treatments were conducted on ten trees on one farm (Farm 6, see Table 1), showing SP, Self-pollination within same tree (n = 10); SF, Self-pollination within same flower (n = 10). pollination (n = 20 trees); WP, Wind pollination (n = 10); CP, Cross pollination between plants (n = 10); the average fruit set of five flower buds pr. tree (a total of 50 flower buds pr. treatment). OP, Open pollination (n = 20 trees); WP, Wind pollination (n = 10); CP, Cross pollination between plants (n = 10); SP, Self-pollination within same tree (n = 10); SF, Self-pollination within same flower (n = 10). SP, Self-pollination within same tree (n = 10); SF, Self-pollination within same flower (n = 10). Diversity 2020, 12, 259 8 of 14 Diversity 2020, 12, 259 8 of 13 3.3. Pollinator Visitation Rate and Richness in Relation to Forest 3.3. Pollinator Visitation Rate and Richness in Relation to Forest In total, 26 pollinating species and 498 individuals were observed visiting guava flowers within In total, 26 pollinating species and 498 individuals were observed visiting guava flowers within the total observation time of 1200 min for all sites (pooling visitation and richness periods). Bees were the total observation time of 1200 min for all sites (pooling visitation and richness periods). Bees were the dominant taxonomic group representing 89% of the observed individuals, 39% being A. cerana the dominant taxonomic group representing 89% of the observed individuals, 39% being A. cerana and and 37% various species of stingless bees. Carpenter bees constituted 4%, other bee species 5%, flies 37% various species of stingless bees. Carpenter bees constituted 4%, other bee species 5%, flies 8%, 8%, beetles 2% and ants 1% of all observed individuals. One wasp was observed touching a flower beetles during the enti 2% and ants re ti1% me period. A comp of all observed individuals. lete species l One ist ca wasp n be v was iewobserved ed in Tabtouching le A1, Apa pflower endix A during . The the entire time period. A complete species list can be viewed in Table A1, Appendix A. The average average visitation rate of pollinators was closely related to the proportion of evergreen forest in the visitation surroundin rate g of landsc pollinators ape witwas hin a closely 1, 5 and related 10 km to rad theiu pr soportion (Table 2,of Fiever gure gr 3een ). The v forest isitin ati the on r surr ate o ounding f wild pollinators and wild bees had the strongest association to forest (72% < R < 97%), whereas all landscape within a 1, 5 and 10 km radius (Table 2, Figure 3). The visitation rate of wild pollinators pollinators and all bees including managed honey bees were less strongly associated to forest (34% < and wild bees had the strongest association to forest (72% < R < 97%), whereas all pollinators and 2 2 R < 68%). Pollinator richness was not associated to the proportion of evergreen forest (Table 2). all bees including managed honey bees were less strongly associated to forest (34% < R < 68%). Distance to forest showed no significant impact on either visitation rate or richness of pollinators Pollinator richness was not associated to the proportion of evergreen forest (Table 2). Distance to forest (Table 2). showed no significant impact on either visitation rate or richness of pollinators (Table 2). Figure 3. Visitation rate (visits/flower/15 min) in relation to the percentage of evergreen forest within a 1, 5 and 10 km radius from each study site: red circle = 1 km, green triangle = 5 km and blue square = 10 km Figure 3. Visitation rate (visits/flower/15 min) in relation to the percentage of evergreen forest within and for four measures of visitation rate: (A) Visitation rate of all pollinators, (B) Visitation rate of wild a 1, 5 and 10 km radius from each study site: red circle = 1 km, green triangle = 5 km and blue square pollinators, (C) Visitation rate of bees and (D) Visitation rate of wild bees. All regressions are printed = 10 km and for four measures of visitation rate: (A) Visitation rate of all pollinators, (B) Visitation with 95% corresponding confidence intervals. Solid lines indicate significant associations (p < 0.05), rate of wild pollinators, (C) Visitation rate of bees and (D) Visitation rate of wild bees. All regressions whereas dashed lines indicated non-significant relationships (p > 0.05). For statistics, see Table 2. are printed with 95% corresponding confidence intervals. Solid lines indicate significant associations (p < 0.05), whereas dashed lines indicated non-significant relationships (p > 0.05). For statistics, see 3.4. Fruit Set in Relation to Pollinators and Forest Table 2. An average of the additional fruit set of guava with pollinators were calculated for each study site. This additional fruit set with pollinators ranged from 0% to 12% and had a mean SD of 4.67 4.50%. 3.4. Fruit Set in Relation to Pollinators and Forest Landscape e ects had no significant impact on additional fruit set with pollinators (p > 0.05). Likewise, An average of the additional fruit set of guava with pollinators were calculated for each study pollinator visitation rates and richness were not associated to fruit set (p > 0.05; Table 2). site. This additional fruit set with pollinators ranged from 0% to 12% and had a mean ± SD of 4.67 ± 4.50%. Landscape effects had no significant impact on additional fruit set with pollinators (p > 0.05). Likewise, pollinator visitation rates and richness were not associated to fruit set (p > 0.05; Table 2). Diversity 2020, 12, 259 9 of 13 4. Discussion In guava orchards in Southern Thailand, we found a significant correlation between the proportion of evergreen forest in the surrounding landscape and the visitation rate of insect pollinators on guava flowers (Table 2; Figure 3). On the other hand and contrary to similar studies on other crops, the proximity of forest showed no significant relationship with visitation rate within the guava orchards [8,17,18,22]. Moreover, neither pollinator richness nor fruit set were associated to the distance to forest nor the amount of forest in the surrounding landscape. These results indicate that although deforestation negatively impact pollinator communities, this does not necessarily cause a lower fruit set of crops on farmland embedded within degraded landscapes. The reason for this lack of e ect of deforestation on guava fruit set is underscored by the finding that additional fruit set of guava due to pollinators (i.e., open vs. closed pollination) showed no relationship with either visitation rate or with richness of pollinators. Results also showed that wind pollination and self-pollination of guava flowers seem to have a greater influence on fruit set than originally anticipated (Figure 2). It has been unclear from previous studies if guava is mainly pollinated by insects or self-pollinating but self-pollination within the same flower was believed to be uncommon due to the stigma being higher than the central anthers [26,32,34,41]. Our results show that self-pollination may be the most common pollination strategy in guava, more so than previously thought, which may be the reason fruit set was not related to landscape-level amount of forest and insect visitation. Moreover, fruit set is not only dependent on pollinators, other factors such as local management and seasonality (wet or dry season) might also have a large impact on fruit set [42]. Some local farmers were reporting that insucient or excess water supply may cause fruit abortion of guava [43], which may have obscured any potential e ect of the amount of forest in the surrounding landscape on pollination of guava. The landscape-level e ects of forest had stronger associations with visitation rates of wild pollinators and wild bees (both without A. cerana) than visitation rates including A. cerana (Table 2). This could be due to the fact that many farms provided nesting-boxes in or close by the orchard to attract A. cerana, diminishing the relevance of forest fragments as nesting areas. Pollination of wild bee species has proved to increase both yield and quality in di erent crops, making wild bees just as or even more important, than managed bees such as A. cerana [23]. Moreover, dependence on a single species for pollination can prove to be risky, if this species is declining due to parasites, pathogens or other factors [23,44]. For instance, large fluctuations in the abundance of the managed honey bee A. mellifera has been observed on co ee farms in Costa Rica [18] and in America a major threat is the colony collapse disorder (CCD), a still unsolved mystery of empty beehives [45]. Thus, pollination solely depending on pollinators from managed beehives might be less resilient to external factors such as disease [44]. Therefore, long-term it would be preferable to rely on a diverse fauna of wild pollinators with high redundancy of pollination services, making it important to sustain the forest left in southern Thailand. The proportion of evergreen forest sites was, in general, relatively low for all study sites (<16%). Instead the sites were surrounded by high proportions of other agricultural areas or, mostly, rubber plantations that accounted for 37 to 95% of the surrounding landscape. Natural habitat such as forest is known to have a positive impact on pollination visitation rate, mainly because many bee species make nests in forest areas [17,18,20–22]. Most frequent pollinating species in this study were bees (89%) and, specifically, 37% of all visiting pollinators were stingless bees (Figure 1). Tropical forest can act as a reservoir for stingless bees for pollinating crops, given that the orchard is within the bee’s forage range [21]. The forage range of stingless bees are believed to be closely related to their body size, ranging between 500 and 2000 m [21]. Only three out of six study sites were this close to evergreen forest and stingless bees were observed in all of them (Table 1, Table A1 Appendix A). In the three study sites further away from evergreen forest (5475 to 8675 m), no stingless bees were observed, only visits from A. cerana and two species of large carpenter bees—Xylocopa (Mesotrichia) latipes cf. (Drury, 1773) and Xylocopa (Koptortosoma) aestuans (Latreille, 1802) both member of the family Apidae. Diversity 2020, 12, 259 10 of 13 Members of the Xylocopa genus have been known to forage over greater distances allowing them to search for resources far away from their nesting area [46]. At the guava orchards, Xylocopa bees were observed to forage by making short visits to individual flowers and visiting numerous flowers in one feeding bout, whereas stingless bees often visited only one or few flowers within the same guava plants. It is therefore likely that Xylocopa bees are more e ective pollinators of guava flowers than are stingless bees, which may also partly explain why overall pollinator visitation rates and landscape characteristics do not predict fruit set in our study [43]. We note that the proportion of forest and visitation rate showed a significant positive relationship not only at short distance but even at a 5 and 10 km radius from the study site, that is, much further distances than most bees and other insect pollinators are travelling to forage. This may indicate that larger patches of forest fragments even beyond the forage range of pollinators is indirectly beneficial for sustaining pollinator abundances. It is well established that habitat loss and fragmentation are some of the main drivers of the ongoing global pollinator decline, having a negative e ect on species richness and abundance, with larger suitable habitat being important for sustaining pollinator robustness [15,47]. Areas with intermediate levels of disturbance, such as tree gardens, plantations, smaller forest fragments and undeveloped urban areas in the tropics might be able to function as micro-habitats and support pollinating species [15,47,48]. A theory might be that larger forest fragments work as a ‘source’ and support the abundance of a species in areas closer to the guava fields. Taken together, the findings in this study underlines the importance of natural habitat to sustain pollinator communities but illustrates that crop fruit set does not necessarily diminish on farmland surrounded by little natural habitat. We note that all our study sites were in fairly degraded landscapes (<16% forest cover). As for biodiversity in general [49,50], it is possible that native pollinator richness and their services would have been higher in landscapes with higher cover of natural habitat. Future studies should thus aim at including farms embedded within more pristine landscapes. Landscape degradation continues to occur at an alarming rate, especially in many tropical regions, making it crucial that we invest in understating its impact on plants, pollinators and their interactions. Thus, more research is needed to better understand pollinator-crop relationships and the importance of natural habitat to sustain pollinator services and crop production in the future. Author Contributions: Conceptualization, K.H., T.S., S.B., B.I.S., N.S. and B.D.; Data curation, K.H. and T.S.; Formal analysis, K.H. and T.S.; Funding acquisition, K.H.; Investigation, K.H. and T.S.; Methodology, K.H., T.S., N.S. and B.D.; Project administration, K.H., T.S., S.B., N.S. and B.D.; Resources, K.H., S.B. and B.D.; Supervision, T.S., S.B., N.S. and B.D.; Visualization, K.H.; Writing—Original draft, K.H.; Writing—Review & editing, T.S., S.B., B.I.S., N.S. and B.D. All authors have read and agreed to the published version of the manuscript. Funding: The fieldwork in this project was funded by; SCIENCE grants: Henrik Tofte Jacobsen s Grant, 15000 DKK, William Demant Fonden, 8500 DKK and Knud Højgaards Fond, 13000 DKK. B.I.S was supported by a Royal Commission for the Exhibition of 1851 Research Fellowship. Acknowledgments: We thank all the guava farmers in Songkhla and Phattalung that allowed us to conduct fieldwork in their orchards. We are grateful for the help and assistance with identifying my bee specimens provided by Natapot Warrit and his student from the bee lab at the Chulalongkorn University, Bangkok. This project could not have been done without the hard work, dedication and translation provided by field assistant Piyaporn Suksai. A big thanks go to her and her family for their help and for providing accommodation during fieldwork in Phattalung. Also thanks to Helle Sørensen for statistical advice. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript or in the decision to publish the results. Diversity 2020, 12, 259 11 of 13 Appendix A Table A1. Complete list of all species observed in the six guava orchards. Identified down to lowest possible taxonomic level. Individuals Species Study Sites Observed (Total) Bees (Anthophila) Anthophila sp. 1 3 4 Nomia (Acunomia) strigata 5 4 Braunsapis cupulifera cf. 7 4 Ceratina (ceratinidia) nigrolateralis 10 4 Apis Apis cerana 196 1, 2, 3, 5, 6 Apis dorcata 1 1 Apis florana 2 1 Carpenter beess Xylocopa (Koptortosoma) aestuans 14 2, 3, 4 (Xylocopa) Xylocopa (Mesotrichia) latipes cf. 4 3 Geniotrigona thoracica 14 1 Heterotrigona itama 145 1, 4 Tetragonula (laeviceps gr.) 32 1, 2, 4 Stingless Tetragonula (pagdeni gr.) 1 1 bees(Meliponines) Tetragonula sp. 6 4 Meliponines sp. 1 1 Wasps (Vespidae) Vespa Anis 1 1 Beetles (Coleoptera) Coleoptera sp. 1 2 5 Coleoptera sp. 2 7 5 Coleoptera sp. 3 1 5 Flies (Diptera) Diptera sp. 1 19 4 Diptera sp. 2 2 4 Diptera sp. 3 3 4 Diptera sp. 4 15 2 Ants (Formicidae) Formicidae sp. 1 4 5 Formicidae sp. 2 2 4 Formicidae sp. 3 1 5 References 1. Rech, A.R.; Dalsgaard, B.; Sandel, B.; Sonne, J.; Svenning, J.C.; Holmes, N.; Ollerton, J. The macroecology of animal versus wind pollination: Ecological factors are more important than historical climate stability. Plant Ecol. Divers. 2016, 9, 253–262. [CrossRef] 2. Ollerton, J.; Winfree, R.; Tarrant, S. How many flowering plants are pollinated by animals? Oikos 2011, 120, 321–326. [CrossRef] 3. Ollerton, J. Pollinator diversity: Distribution, ecological function, and conservation. Annu. Rev. Ecol. Evol. Syst. 2017, 48, 353–376. [CrossRef] 4. Bawa, K. Plant-pollinator interactions in tropical rain forests. Annu. Rev. Ecol. Evol. 1990, 21, 399–422. [CrossRef] 5. Potts, S.G.; Imperatriz-Fonseca, V.; Ngo, H.; Biesmeijer, J.C.; Breeze, T.; Dicks, L.; Garibaldi, L.; Settele, J.; Vanbergen, A.J.; Aizen, M.A.; et al. Summary for Policymakers of the Assessment Report of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) on Pollinators, Pollination and Food Production; Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services: Bonn, Germany, 2016; ISBN 978-92-807-3568-0. 6. Daily, G.C. Nature’s Services; Island Press: Washington, DC, USA, 1997; Volume 3. 7. Allen-Wardell, G.; Bernhardt, P.; Bitner, R.; Burquez, A.; Buchmann, S.; Cane, J.; Cox, P.A.; Dalton, V.; Feinsinger, P.; Ingram, M.; et al. The potential consequences of pollinator declines on the conservation of biodiversity and stability of food crop yields. Conserv. Biol. 1998, 12, 8–17. [CrossRef] Diversity 2020, 12, 259 12 of 13 8. Klein, A.M.; Vaissière, B.E.; Cane, J.H.; Ste an-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Tscharntke, T. Importance of pollinators in changing landscapes for world crops. Proc. R. Soc. B Biol. Sci. 2007, 274, 303–313. [CrossRef] [PubMed] 9. Roubik, D.W. Pollination of Cultivated Plants in the Tropics; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 1995; ISBN 9251036594. 10. Dalsgaard, B. Land-Use and climate impacts on plant–pollinator interactions and pollination services. Diversity 2020, 12, 168. [CrossRef] 11. Biesmeijer, J.C.; Roberts, S.P.M.; Reemer, M.; Ohlemüller, R.; Edwards, M.; Peeters, T.; Scha ers, A.P.; Potts, S.G.; Kleukers, R.; Thomas, C.D.; et al. Parallel declines in pollinators and insect-pollinated plants in Britain and the Netherlands. Science 2006, 313, 351–354. [CrossRef] 12. Ollerton, J.; Erenler, H.; Edwards, M.; Crockett, R. Extinctions of aculeate pollinators in Britain and the role of large-scale agricultural changes. Science 2014, 346, 1360–1362. [CrossRef] 13. Hallmann, C.A.; Sorg, M.; Jongejans, E.; Siepel, H.; Hofland, N.; Schwan, H.; Stenmans, W.; Müller, A.; Sumser, H.; Hörren, T.; et al. More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS ONE 2017, 12, e0185809. [CrossRef] 14. Laurance, W.F. Forest destruction in tropical Asia. Curr. Sci. 2007, 93, 1544–1550. 15. Sritongchuay, T.; Hughes, A.C.; Memmott, J.; Bumrungsri, S. Forest proximity and lowland mosaic increase robustness of tropical pollination networks in mixed fruit orchards. Landsc. Urban Plan. 2019, 192, 103646. [CrossRef] 16. Büchert Lindberg, A.; Mogens Olesen, J. The fragility of extreme specialization: Passiflora mixta and its pollinating hummingbird Ensifera ensifera. J. Trop. Ecol. 2001, 17, 323–329. [CrossRef] 17. Ricketts, T.H.; Daily, G.C.; Ehrlich, P.R.; Michener, C.D. Economic value of tropical forest to co ee production. Proc. Natl. Acad. Sci. USA 2004, 101, 12579–12582. [CrossRef] [PubMed] 18. Ricketts, T.H. Tropical forest fragments enhance pollinator activity in nearby co ee crops. Conserv. Biol. 2004, 18, 1262–1271. [CrossRef] 19. Rathcke, B.J.; Jules, E.S. Habitat fragmentation and plant pollinator interactions. Curr. Sci. 1993, 65, 273–277. 20. Garibaldi, L.A.; Ste an-Dewenter, I.; Kremen, C.; Morales, J.M.; Bommarco, R.; Cunningham, S.A.; Carvalheiro, L.G.; Chaco , N.P.; Dudenhö er, J.H.; Greenleaf, S.S.; et al. Stability of pollination services decreases with isolation from natural areas despite honey bee visits. Ecology 2011, 14, 1062–1072. [CrossRef] 21. Blanche, K.R.; Ludwig, J.A.; Cunningham, S.A. Proximity to rainforest enhances pollination and fruit set in orchards. J. Appl. Ecol. 2006, 43, 1182–1187. [CrossRef] 22. Ricketts, T.H.; Regetz, J.; Steffan-Dewenter, I.; Cunningham, S.A.; Kremen, C.; Bogdanski, A.; Gemmill-Herren, B.; Greenleaf, S.S.; Klein, A.M.; Mayfield, M.M.; et al. Landscape effects on crop pollination services: Are there general patterns? Ecol. Lett. 2008, 11, 499–515. [CrossRef] 23. Garibaldi, L.A.; Ste an-Dewenter, I.; Winfree, R.; Aizen, M.A.; Bommarco, R.; Cunningham, S.A.; Kremen, C.; Carvalheiro, L.G.; Harder, L.D.; Afik, O.; et al. Wild pollinators enhance fruit set of crops regardless of honey bee abundance. Science 2013, 340, 1608–1611. [CrossRef] 24. Aizen, M.A.; Garibaldi, L.A.; Cunningham, S.A.; Klein, A.M. How much does agriculture depend on pollinators? Lessons from long-term trends in crop production. Ann. Bot. 2009, 103, 1579–1588. [CrossRef] [PubMed] 25. Tylianakis, J.M.; Didham, R.K.; Bascompte, J.; Wardle, D.A. Global change and species interactions in terrestrial ecosystems. Ecol. Lett. 2008, 11, 1351–1363. [CrossRef] [PubMed] 26. Pommer, C.V.; Murakami, K.R.N. Breeding guava (Psidium guajava L.). In Breeding Plantation Tree Crops: Tropical Species; Jain, S.M., Priyadarshan, P.M., Eds.; Springer: New York, NY, USA, 2009; pp. 83–120. ISBN 9780387712017. 27. Salazar, D.M.; Melgarejo, P.; Martínez, R.; Martínez, J.J.; Hernández, F.; Burguera, M. Phenological stages of the guava tree (Psidium guajava L.). Sci. Hortic. 2006, 108, 157–161. [CrossRef] 28. Morton, J.F.; Dowling, C.F. Fruits of Warm Climates, Pomegranate; JF Morton: Miami, FL, USA, 1987; ISBN 0-9610184-1-0. 29. Altendorf, S. Minor Tropical Fruits: Mainstreaming a Niche Market; Food and Agriculture Organization of the United Nations (FAO): Rome, Italy, 2018; Available online: http://www.fao.org/fileadmin/templates/est/ COMM_MARKETS_MONITORING/Tropical_Fruits/Documents/Minor_Tropical_Fruits_FoodOutlook_1_ 2018.pdf (accessed on 13 May 2020). Diversity 2020, 12, 259 13 of 13 30. Food and Agriculture Organization of the United Nations (FAO). Food Outlook: Biannual Report on Global Food Markets; Global Information and Early Warning System on Food and Agriculture: Rome, Italy, 2017; Available online: http://www.fao.org/3/a-i7343e.pdf (accessed on 13 May 2020). 31. Kadam, D.M.; Kaushik, P.; Kumar, R. Evaluation of guava products quality. Int. J. Food Sci. Nutr. Eng. 2012, 2012, 7–11. [CrossRef] 32. Singh, R.; Sehgal, O. Studies on the blossom-biology of Psidium guajava L. (Guava) II. pollen studies, stigmatal receptivity, pollination and fruit-set. Indian J. Hortic. 1968, 25, 52–59. 33. Thanarut, C.; Ko, J.-C. Guava, wax apple, sugar apple and pineapple productions in Thailand. Acta Hortic. 2017, 1166, 7–14. [CrossRef] 34. Mitra, S.K.; Thingreingam Irenaeus, K.S. Guava cultivars of the world. Acta Hortic. 2018, 1205, 905–910. [CrossRef] 35. Land Development Department. GIS Data on Land Use Resources. Available online: http://www.ldd.go.th/ www/lek_web/web.jsp?id=18907 (accessed on 29 May 2020). 36. Klein, A.-M.; Ste an-Dewenter, I.; Tscharntke, T. Bee pollination and fruit set of co ea arabica and C. canephora (Rubiaceae). Am. J. Bot. 2003, 90, 153–157. [CrossRef] 37. Oksanen, J.; Blanchet, F.G.; Friendly, M.; Kindt, R.; Legendre, P.; McGlinn, D.; Minchin, P.R.; O’Hara, R.B.; Simpson, G.L.; Solymos, P.; et al. Package ‘vegan’Community Ecology Package. R Package Version 2.5-6. 2019. Available online: https://cran.r-project.org/web/packages/vegan/vegan.pdf (accessed on 12 May 2020). 38. Ekstrøm, C.T.; Sørensen, H. Introduction to Statistical Data Analysis for the Life Sciences; CRC Press: Boca Raton, FL, USA, 2014; ISBN 9781439894866. 39. R Core Team. R: A language and environment for Statistical Computing; R Foundation for Statistical Computing: Vienna, Austria, 2020. 40. Hadley, W. Ggplot2: Elegant Graphics for Data Analysis; Springer: Houston, TA, USA, 2016. 41. Alves, J.E.; Freitas, B.M. Foraging behavior and pollination eciency of five bee species on guava (Psidiumguajava L.) flowers. Rev. Ciência Agronômica 2008, 37, 216. 42. Tangtorwongsakul, P.; Warrit, N.; Gale, G.A. E ects of landscape cover and local habitat characteristics on visiting bees in tropical orchards. Agric. For. Entomol. 2018, 20, 28–40. [CrossRef] 43. Hansen, K. (University of Copenhagen, Copenhagen, Denmark). Personal Communication, 2020. 44. Winfree, R.; Williams, N.M.; Dusho , J.; Kremen, C. Native bees provide insurance against ongoing honey bee losses. Ecol. Lett. 2007, 10, 1105–1113. [CrossRef] [PubMed] 45. Stokstad, E. The case of the empty hives. Science 2007, 316, 970–972. [CrossRef] [PubMed] 46. Zurbuchen, A.; Landert, L.; Klaiber, J.; Müller, A.; Hein, S.; Dorn, S. Maximum foraging ranges in solitary bees: Only few individuals have the capability to cover long foraging distances. Biol. Conserv. 2010, 143, 669–676. [CrossRef] 47. Potts, S.G.; Biesmeijer, J.C.; Kremen, C.; Neumann, P.; Schweiger, O.; Kunin, W.E. Global pollinator declines: Trends, impacts and drivers. Trends Ecol. Evol. 2010, 25, 345–353. [CrossRef] 48. Nascimento, V.T.; Agostini, K.; Souza, C.S.; Maruyama, P.K. Tropical urban areas support highly diverse plant-pollinator interactions: An assessment from Brazil. Landsc. Urban Plan. 2020, 198, 103801. [CrossRef] 49. Mariana Morais, V.; Cristina, B.; Leandro Reverberi, T.; Érica, H.; Pedro Ferreira, D.; Wesley Rodrigues, S.; Paulo Roberto, G.; Jean Paul, M. Predicting the non-linear collapse of plant–frugivore networks due to habitat loss. Ecography 2019, 42, 1765–1776. [CrossRef] 50. Melo, I.; Ochoa-Quintero, J.M.; Oliveira Roque, F.; Dalsgaard, B. A review of threshold responses of birds to landscape changes across the world. J. Field Ornithol. 2018, 89, 303–314. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Published: Jun 25, 2020

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