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International Journal of Biodiversity Science, Ecosystem Services & Management, 2013 Vol. 9, No. 2, 90–103, http://dx.doi.org/10.1080/21513732.2013.770800 Biodiversity losses and conservation trade-offs: assessing future urban growth scenarios for a North American trade corridor a, a b a Miguel L. Villarreal *, Laura M. Norman , Kenneth G. Boykin and Cynthia S.A. Wallace a b U.S. Geological Survey, Western Geographic Science Center, Tucson, AZ, USA; Department of Fish, Wildlife, and Conservation Ecology, Center for Applied Spatial Ecology, New Mexico Cooperative Fish and Wildlife Research Unit, New Mexico State University, Las Cruces, NM, USA The Sonoran Desert and Apache Highlands ecoregions of North America are areas of exceptionally high plant and vertebrate biodiversity. However, much of the vertebrate biodiversity is supported by only a few vegetation types with limited distribu- tions, some of which are increasingly threatened by changing land uses. We assessed the impacts of two future urban growth scenarios on biodiversity in a binational watershed in Arizona, USA and Sonora, Mexico. We quantiﬁed and mapped ter- restrial vertebrate species richness using Wildlife Habitat Relation models and validated the results with data from National Park Service (NPS) biological inventories. Future urban growth, based on historical trends, was projected to the year 2050 for (1) a ‘Current Trends’ (CT) scenario and (2) a ‘Megalopolis’ (MEGA) scenario that represented a transnational growth cor- ridor with open-space conservation attributes. Based on CT, 45% of existing riparian woodland (267 of 451species) and 34% of semi-desert grasslands (215 of 451 species) will be lost, whereas in the MEGA scenario, these types would decline by 44% and 24%, respectively. Outcomes of the two models suggest a trade-off at the taxonomic class level: CT would reduce and fragment mammal and herpetofauna habitat, while MEGA would result in loss of avian-rich riparian habitat. Keywords: species richness; urbanization; Wildlife Habitat Relation; habitat model; spatial simulation; urban development Introduction come at the expense of other ecosystem services or adja- cent biological systems, may adversely affect or degrade Biodiversity losses and species extinctions over the next ecosystems that serve more vulnerable segments of human century are expected to be high, driven largely by climate societies, or may have transnational and geopolitical mani- change and land use changes that convert natural vege- festations (Wear and Murray 2004; Gan and McCarl tation to agricultural and urban uses (Sala et al. 2000; 2007; Ewers and Rodrigues 2008). Given these uncertain- Brooks et al. 2002; Dirzo and Raven 2003). Local and ties, conservation plans that aim to preserve biodiversity regional conservation plans that aim to reduce and mini- and ecosystem services and minimize unintended con- mize habitat loss, habitat degradation, and fragmentation sequences will beneﬁt from the results of alternative and promote landscape linkages can help to ameliorate scenarios. the effects of urbanization on species extinctions and Scenarios can provide spatial representations of pos- biodiversity loss (Fischer and Lindenmayer 2007; Pressey sible future urban development and landscape change. et al. 2007). To develop informed plans for the future, Satellite imagery, Geographic Information Systems (GIS), plans that balance the requirements of both humans and the and species niche information can be used to develop esti- ecosystems on which they depend, decision-makers need mates of past, existing, and future species distributions. quantitative assessments describing the potential response These estimates allow us to identify potential changes in of biodiversity and ecosystem services to future land-use spatial and temporal patterns of biodiversity related to decisions. anthropogenic land uses and habitat changes (White et al. The importance and beneﬁts of conservation plan- 1997; Ferrier 2002; Turner et al. 2003; Schumaker et al. ning and other actions aimed at reducing biodiversity loss 2004). These scenarios then give policy-makers, planners, and related ecosystem services (e.g., genetic resources, and land managers a way to envision and communicate the bioprospecting opportunities, primary productivity, and potential effects of land use decisions on people and the pollinators) have been well established in the literature environment (Peterson et al. 2003). Scenario planning does (Olson and Dinerstein 1998; DeFries et al. 2004; Ehrlich not involve predictions or forecasts of a single outcome, and Pringle 2008). However, the realization and imple- rather the intent is to offer a series of plausible situa- mentation of conservation plans may have unintended and tions based on multiple, usually contrasting assumptions unscripted consequences for biodiversity, ecosystem ser- of what the future might bring. Scenario-based planning vices, and human health (Armsworth et al. 2006; Polasky and the development of multiple, alternative scenarios is 2006; Pressey et al. 2007). Conservation beneﬁts may *Corresponding author. Email: firstname.lastname@example.org This work was authored as part of the Contributors’ ofﬁcial duties as Employees of the United States Government and is therefore a work of the United States Government. In accordance with 17 USC. 105, no copyright protection is available for such works under US Law. International Journal of Biodiversity Science, Ecosystem Services & Management 91 becoming increasingly necessary given the high level of (3) How will future urban development, based on uncertainty surrounding the manifestation of future cli- current trends, affect species richness in the mate change, and ecological and societal response to these watershed, and how would the distribution of changes (Millennium Ecosystem Assessment 2005). biodiversity in the watershed differ from current To better understand and predict how these alternative trends if development was constrained to a transna- scenarios of future urban, suburban, and exurban growth tional trade corridor? patterns may affect biodiversity, we compared patterns (4) What trade-offs, in terms of loss and fragmenta- and rates of biodiversity loss related to different scenar- tion of habitat for various species and taxonomic ios of future development. Our research is presented in classes might be experienced under different pat- three steps: (1) quantifying and mapping terrestrial ver- terns of urban growth? tebrate richness using Wildlife Habitat Relation (WHR) models, (2) validating species richness estimates with bio- Materials and methods logical inventory databases, and (3) assessing the potential future changes in patterns and number of species based on Study area contrasting growth scenarios. The southwestern United States has undergone consid- We address the following research questions related to erable physical and biological changes since settlement biodiversity and urban growth in the Santa Cruz Watershed by Euro-Americans, but remains biologically diverse and (SCW): home to a wide array of habitat types that support numer- ous endemic and iconic desert species. Our study area, (1) How many terrestrial vertebrate species (avian, the SCW is a binational watershed located in Arizona, herpetofauna, and mammal) are potentially present USA and Sonora, Mexico (Figure 1). The SCW strad- in the SCW and how are patterns of species rich- dles the Sonoran Desert and Apache Highlands ecoregions, ness distributed over the landscape? resulting in a desert ecotonal environment that contains (2) Are species richness numbers, assessed from both Sonoran and Chihuahuan desert plant communities species lists and models developed from expert and a range of transitional habitats in between (Bailey knowledge, comparable to data from systemat- 1998) (Figure 1). The desert areas are interspersed with ically collected biological inventories from the mountainous ‘sky islands’ that rise out of the deserts up study area? to over 3000 meters in elevation. The ﬂora and fauna Figure 1. Location of the Santa Cruz Watershed relative to surrounding ecoregions and major features of the study area. 92 M.L. Villarreal et al. of sky islands and Sonoran Desert contribute to a bio- nature of the watershed. Patterns of urban development geographically unique environment that is considered one differ across the border at Ambos Nogales, dictated in of the most biologically diverse areas in North America part by geographically external economic forces and policy (Felger and Wilson 1994; Bowers and McLaughlin 1996; decisions (Ingram et al. 1994; Norman et al. 2009). Spector 2002). The sky island mountain ranges of the Natural amenities of the watershed, particularly the Apache Highlands contain a number of vegetation types mild climate, biological diversity, and recreational oppor- that generally follow an elevational gradient and can range tunities, attract tens of thousands of visitors and new from warm desert-mixed cacti or grasslands at the base in-migrants each year to the US portion of the SCW, to mixed conifer and spruce forests on high-elevation and as a result urban areas and exurban development north slopes (Whittaker and Niering 1965). The geo- increased considerably in the late twentieth and early graphic isolation of the sky islands has contributed to small twenty-ﬁrst centuries. Exurban development, in which and genetically isolated populations of plant and animal large, production-oriented cattle ranches are subdivided species that often require special conservation considera- into ‘trophy ranches’, ranchettes, or vacation homes for tion (Koprowski et al. 2005). urbanites, has increased throughout the rural western The Santa Cruz River headwaters are located in the United States (Sheridan 2001; Theobald 2004). The rapid San Rafael Valley in Arizona, a relatively undeveloped rate and sprawling pattern of exurban growth in the SCW grassland comprising cattle ranches, conservation areas, are a cause of concern for conservationist and land man- state parks, and surrounded by national forest lands agers and concerted efforts to engineer more livable and (Figure 1). The river, which ﬂows intermittently over its sustainable landscapes have been devised in recent years. 296 km course, crosses the border into Mexico in the San One such example is the Sonoran Desert Conservation Rafael valley, ﬂows south approximately 20 km to San Plan (SDCP) of Pima County, Arizona, in which vot- Lazaro, Mexico before turning north again to cross the bor- ers approved in 2004 a $174.3 million bond program to der east of Ambos Nogales. Settlement along the Mexican acquire and conserve ranch lands and riparian areas to portion of the Santa Cruz River is sparse, and primary land develop a network of open spaces for species conserva- uses include agriculture in the ﬂoodplain and ranching in tion and other ecosystem services like ﬂood control. The the upland areas. In Arizona, the river remains intermit- Nature Conservancy (TNC) and Arizona State Parks have tent for 20 km until it reaches the Nogales International also been involved in efforts to conserve grassland habi- Wastewater Treatment plant (NIWTP), which treats efﬂu- tats in the headwaters area by purchasing ranch lands and ent from the cites of Nogales Arizona and Sonora, and conservation easements. releases the water into the river channel creating a per- While direct habitat loss due to development and agri- manent ﬂow for approximately 30 km. This reach of the culture is a major conservation concern globally, indi- Santa Cruz River has developed a mature riparian corri- rect habitat loss due to urban, industrial, and agricultural dor that likely matches river conditions when Spanish and groundwater use is an equally important conservation issue Anglo-Americans ﬁrst settled in the area (Mearns 1907; in arid environments. Much of the extensive and species- Bartlett 1965; Logan 2002). Primary land uses along this rich riparian and xeroriparian habitats of the Santa Cruz stretch of the Santa Cruz include urban and commercial River and its tributaries have disappeared due to ground- land uses, golf courses and resorts, ranching, and agri- water overdraft near urban centers and agricultural areas culture. The upland areas are a mixture of private and (Webb et al. 2007; Villarreal et al. 2012). A majority of public lands with varying amounts of livestock ranching the remaining riparian areas in the watershed are either and mining. The Santa Cruz River riparian corridor ter- subsidized by treated efﬂuent from wastewater or occur in minates approximately 10 km north of Tumácacori where areas managed for conservation and/or restoration. These NIWTP water ceases to ﬂow aboveground. The remaining riparian areas are found primarily in ﬂoodplains and chan- river ﬂoodplain is dry through most of the Tucson basin, nels that are not likely to be directly urbanized in part of where groundwater pumping for industrial, commercial, the watershed in the United States due to local ordinance; and residential uses has lowered the water table to a level however, the intensity of adjacent development and related that cannot support large amounts of phreatophytic riparian water use are factors that threaten riparian areas and asso- vegetation. ciated terrestrial and aquatic vertebrate and invertebrate The watershed contains two major urban areas: the communities. Tucson metropolitan area and Ambos Nogales (Nogales, Arizona and Nogales, Sonora). Much of the recent Methodology: data sets (1970–2010) urban development in the watershed occurred Urban change scenarios in four areas: (1) along Interstate 10 in the northwest and southeast of the Tucson metropolitan area, (2) north of The U.S. Geological Survey developed the Santa Cruz the Tucson metropolitan area in Marana and Oro Valley, Watershed Ecosystem Portfolio Model (SCWEPM) to (3) adjacent to the Santa Cruz River along Interstate provide land managers and stakeholders with a deci- 19 from Tucson to Nogales, Arizona, and (4) Ambos sion support tool to assess potential outcomes of climate Nogales (Figure 1). Urbanization patterns (and related change and land use decisions on ecosystem services resource consumption) are complicated by the binational (Norman et al. 2010). The SCWEPM is a GIS-based International Journal of Biodiversity Science, Ecosystem Services & Management 93 multi-criteria decision support web tool that evaluates rela- were aggregated with terrestrial vertebrate species richness tive changes in ecosystem services to land use decisions. maps described in the following section. The SCWEPM is an open-source data warehouse that houses geospatial data sets used to synthesize plausible WHR models future scenarios of change, including climate, population, the economy, resource use, transportation, land use and The Southwest Regional Gap Analysis Project land cover, urbanization, globalization, the nitrogen cycle, (SWReGAP) was a multi-institutional cooperative the water cycle, biological diversity, pollution, and health. effort to map habitat and habitat preferences for terres- A key component of the SCWEPM model is the pat- trial vertebrate species within a 1,373,768 km area of tern and rate of urban development: it is an ecosystem the Southwestern United States that included Arizona, driver with the potential to affect the long-term viabil- Colorado, Nevada, New Mexico, and Utah. SWReGAP ity and stability of ecosystem services in the watershed, species habitat maps were developed using WHR. A WHR ultimately affecting the health of human and ecological model is a textual, mathematical, graphical, or combi- systems. nation statement that predicts where a species is likely Two urban growth scenarios were developed for to exist on a landscape based on its assumed habitat SCWEPM using the fuzzy constrained cellular automata conditions (Boykin et al. 2007). The SWReGAP WHR (CA) model, SLEUTH (Slope, Land use, Exclusion, Urban models were deﬁned using a deductive process that is extent, Transportation, and Hillshade) (Norman et al. based primarily on existing literature and uses vegetation 2012). SLEUTH is a land-use/land-cover change model and other ancillary data (e.g., distance to water bodies, that processes land surface changes that are regulated elevation) as a proxy for vertebrate species distributions by neighborhood rule conditions (Jantz et al. 2010). (Boykin et al. 2007, 2010). The main spatial variable SLEUTH inputs included the following: S, slope from for the WHR models is the SWReGAP land cover and 30 m National Elevation Dataset Digital Elevation (DEM) vegetation data set, which is a seamless map product raster; L, a binational, historical suite of Land Use Land consisting of 125 classes (109 ecological systems and Cover (LULC) data derived from 1979, 1989, 1999, 16 land cover) for all ﬁve states. and 2009 satellite imagery (Villarreal et al. 2011); E, deﬁned exclude areas including conservation areas, pro- Vegetation map tected lands, open water, and ﬂoodplains; U, urban extent layer derived from LULC; T, binational vector transporta- The SWReGAP vegetation map does not include the tion data sets; and H, DEM-derived hillshade (see Norman Sonora, Mexico portion of our study area; therefore, a et al. 2012 for detailed SLEUTH methods). binational vegetation map of the SCW was created for The ‘Current Trends’ (CT) scenario represents poten- the SCWEPM using the same methods employed by the tial urban growth patterns in 2050 based on the con- SWReGAP team (Lowry et al. 2006; Wallace et al. 2011). tinuation of historical trends of land development from The binational SCW vegetation map was modeled after the 1979–2009. The ‘Megalopolis’ (MEGA) scenario was SWReGAP terrestrial vegetation map using NatureServe developed to offer an alternative to the CT that envi- Terrestrial Ecological Systems (TES) units (Wallace et al. sions both continued urban growth and conservation as 2011). This map was developed using the original ﬁeld future priorities for the watershed. The MEGA scenario data collected for SWReGAP, but was supplemented with is biased to promote urban growth around the trinational additional ﬁeld information describing riparian types - bio- CANAMEX freeway (http://www.canamex.org/index.asp), logically important classes that were under- or incorrectly which acts as a conduit for transnational business and mapped in the original SWReGAP data set (Wallace et al. trade. In order to represent this area in the SLEUTH 2011; Villarreal et al. 2012). model, the values for the ‘Exclude’ layer were manipu- lated. The ‘Exclude’ input layer represents a location’s Biodiversity models potential resistance to urbanization by assigning a value (between 0 and 100) that deﬁnes a range of resistance It is difﬁcult to map or predict species richness for large from nonexistent (0) to complete (100). The CANAMEX areas with a high level of detail and accuracy without freeway was buffered by 15 km. (∼9.3 mi.) and assigned acquiring prohibitively large amounts of ﬁeld data. WHR a zero value (free to urbanize), everything outside that models are based on existing ﬁeld data, scientiﬁc literature, buffer zone was assigned a partial weight (50%), meaning and expert knowledge, making them ideal for developing it has 50% likelihood of being excluded from development, generalized species niche models or biodiversity measures. unless it had already been categorized as not available for WHR models have proven to be generally accurate over development, in which case it retained it value of 100 large areas when veriﬁed with biological inventory data (Figure 2). The original excluded layer was used with the (Edwards et al. 1996). We compiled a list of all potential addition of a buffer zone deﬁning this potential MEGA terrestrial vertebrates present in the watershed by extract- Region; this allowed growth to occur outside the zone ing a species list from the SWReGAP WHR database but primarily stimulated growth within (Figure 2). Urban (http://fws-nmcfwru.nmsu.edu/swregap/) based on species cover from the two scenarios developed for the SCWEPM habitat requirements and their relationship to SWReGAP 94 M.L. Villarreal et al. 111° 0′ 0″ W 110° 15′ 0″ W Legend CANAMEX freeway Santa Cruz Watershed Megalopolis Strip Zone International boundary Megalopolis exclude USA prime imagery 0 5 10 miles 0510 km Figure 2. Map of the CANAMEX freeway with a 15-km strip zone on either side, allocated for the Megalopolis urban growth scenario (adapted from Norman et al. 2012). The 0 exclude category is displayed as transparent. vegetation and land use classes mapped in the SCW. then geographically mapped based on the distribution of Because this species list for the SCW was culled from vegetation types in the watershed. Using map algebra, we the greater southwest regional database and was based combined species richness values for the urban classes on some potentially widespread vegetation units that exist from the two growth scenarios with the species rich- throughout the southwest, the actual occurrence of many ness maps described above. Below we discuss the results of the species in the watershed was uncertain. We there- and accuracy of the terrestrial vertebrate list, present the fore cross-validated the list with NPS inventory data from results of the WHR-based biodiversity maps describing four National Park units in the area: Saguaro National total species richness and species richness of three taxo- Park (NP) West, Saguaro NP East, Tumácacori National nomic groups, and analyze the changes to the amount of Historical Park (NHP), and Coronado National Monument habitat and spatial patterns of species richness based on (NM) (Powell et al. 2005, 2006, 2007; Schmidt et al. 2007). the CT and MEGA urban growth scenarios. These NPS inventories were conducted within the full array of vegetation types present in the greater watershed, ranging from ﬂoodplain riparian types (Tumácacori NHP), Results desert grasslands and oak woodlands (Coronado NM), and Species richness, vegetation cover, and distribution of the elevational transition-gradient from Sonoran Desert to biodiversity hotspots mixed conifer (Saguaro NP). The binational SCW vegetation map contained 34 unique Using the species list from the WHR database we cal- vegetation and land use classes (Table 1, Figure 3). The culated a total species richness value and species richness total number of potential terrestrial vertebrates identi- of three taxonomic groups (avian, herpetofauna (reptile ﬁed using SWReGAP WHR models across all vegeta- and amphibian), and mammal) for each pixel of vegetation tion and land cover types is 451, with Desert Riparian and land cover in the SCW. Species richness values were 31° 30′ 0″ N32° 15′ 0″ N International Journal of Biodiversity Science, Ecosystem Services & Management 95 Table 1. A list of vegetation and land cover classes present in the Santa Cruz Watershed, species richness values for each class, and change in area under 2050 Current Trends (CT) and Megalopolis (MEGA) scenarios. 2000 area 2050 CT 2050 MEGA Cover type description Species Mammals Birds Herpetofauna (ha) (ha) (ha) Desert Riparian Woodland and 267 56 171 40 4747 −2153 −2071 Shrubland Desert Riparian Forest 267 56 171 40 435 −78 −77 Desert Lower Montane Riparian 254 54 165 35 4 0 0 Woodland and Shrubland Madrean Pine-Oak Forest and 237 70 125 42 23,064 −365 −315 Woodland Apacherian-Chihuahuan Piedmont 215 75 94 46 110,170 −37,350 −26,161 Semi-Desert Grassland Madrean Pinyon-Juniper Woodland 211 69 106 36 35,326 −727 −637 North American Warm Desert 196 43 114 39 5361 −1276 −1232 Riparian Mesquite Bosque Apacherian-Chihuahuan Mesquite 190 58 88 44 246,102 −76,147 −63,118 Upland Scrub Madrean Encinal 179 67 77 35 151,121 −8763 −6693 Chihuahuan Mixed Desert and 165 72 62 31 38,383 −11,785 −7884 Thorn Scrub Sonoran Palo Verde-Mixed Cacti 159 57 59 43 139,026 −81,988 −75,114 Desert Scrub Madrean Juniper Savanna 156 61 62 33 2143 −10 −8 Agriculture 150 29 112 9 2674 −1606 −1569 Rocky Mountain Ponderosa Pine 147 56 79 12 1922 −51 −51 Woodland Sonora-Mojave 145 62 43 40 9944 −7646 −7255 Creosotebush-white Bursage Desert Scrub Chihuahuan-Sonoran Desert 138 55 54 29 2123 −626 −478 Bottomland and Swale Grassland Madrean Upper Montane 136 49 74 13 3355 −37 −37 Conifer-Oak Forest and Woodland North American Arid West 129 32 91 6 14 −4 −4 Emergent Marsh Mogollon Chaparral 128 49 62 17 923 −200 −175 Chihuahuan Succulent Desert 128 55 45 27 15,287 −1380 −1006 Scrub Colorado Plateau Mixed Bedrock 119 34 62 23 1912 −75 −75 Canyon and Tableland North American Warm Desert 112 41 52 19 208 −141 −132 Wash Sonoran Mid-Elevation Desert 99 36 38 25 5553 −1675 −1467 Scrub Chihuahuan Sandy Plains 98 40 46 12 27 −16 −9 Semi-Desert Grassland North American Warm Desert 96 39 33 24 10,757 −982 −892 Bedrock Cliff and Outcrop Rocky Mountain Aspen Forest and 90 29 59 2 205 −1 −1 Woodland Open Water 76 8 64 4 365 −76 −74 Developed,Medium-High Intensity 60 16 43 1 93,247 240,142 201,499 Chihuahuan Stabilized Coppice 53 24 16 13 10 −7 −7 Dune and Sand Flat Scrub North American Warm Desert 31 13 9 9 1161 −71 −65 Volcanic Rockland North American Warm Desert 21 5 8 8 12 0 0 Pavement Barren Lands, non-speciﬁc 16 11 3 2 8 −4 −4 Recently Mined or Quarried 6 2 3 1 9500 −4799 −4790 Note: The table is sorted by number of species. 96 M.L. Villarreal et al. Figure 3. Binational vegetation map of the Santa Cruz Watershed (left) and the resulting map of total species richness (right) derived from SWReGAP Wildlife Habitat Relation models. Biodiversity hotspots occur in the riparian corridors and the grasslands. Woodland and Shrubland and Desert Riparian Forest sup- Palo Verde-Mixed Cacti Desert Scrub (139,026 ha, 15% of porting the greatest number (267) of species and Recently landscape), found in the northern portion of the watershed Mined or Quarried supporting the fewest (6) (Table 1, around the Tucson Metropolitan area, provides habitat Figure 3). The data indicate a total of 284 bird species, with for 159 species (Tables 1 and 2). It should be noted 171 of these using Desert Riparian Forests, Woodlands, that while Sonoran Palo Verde-Mixed Cacti Desert Scrub and Shrubland. Apacherian-Chihuahuan Piedmont Semi- is not the most biodiverse in terms of terrestrial verte- Desert Grassland supports 75 of 93 total mammal species brates, it contains a number of endemic and iconic Sonoran and 46 of 74 total herpetofauna (amphibian and reptile). plants with considerable conservation value for both Apacherian-Chihuahuan Mesquite Upland Scrub supports biological and cultural reasons. Apacherian-Chihuahuan 44 species of herpetofauna (Table 1, Figure 4). Biodiversity Piedmont Semi-Desert Grasslands, which as mentioned hotspots for each of these taxonomic groups are distributed above had the highest mammal species richness and a in different areas of the watershed: birds in the valley bot- total of 215 species, covered 110,170 ha of the watershed tom riparian woodlands, mammals in the high elevation (12% of the landscape), primarily around the San Rafael grasslands, and herpetofauna in the desert and shrublands Valley. that make up a majority of the natural vegetation in the Three riparian types occupy only a small area of watershed (Figure 4). Mines and quarries and developed the watershed but have the highest species richness val- areas had the lowest species richness. Medium-to-high ues in the watershed: Desert Riparian Woodland and intensity developed areas provide habitat for 60 species: Shrubland (4747 ha; 267 species), Desert Riparian Forest 16 mammals, 43 birds, and 12 herpetofauna. (435 ha; 267 species), and Desert Lower Montane Riparian In 2000, four vegetation types made up a majority of Woodland and Shrubland (4 ha; 254 species). Other the watershed cover, occupying 71% of the total watershed species-rich vegetation types include the mid-elevation area (Table 2). Apacherian-Chihuahuan Mesquite Upland Madrean Pine-Oak Forest and Woodland (23,064 ha; Scrub was the most common cover type in the watershed 237 species) and Madrean Pinyon-Juniper Woodland (246,102 ha, 27% of landscape) (Table 2), and also an (35,326 ha; 211 species), and another bottomland riparian area of relatively high biodiversity (190 species). Madrean type with limited distribution in the SCW, North American Encinal, occurring primarily in the southern portion of Warm Desert Riparian Mesquite Bosque (5361 ha; the watershed in Sonora, and in the Sky Island transi- 196 species). Montane pine, conifer, and aspen types are tional zones, covered 151,121 ha (16.5% of landscape) generally species rich, but do not occupy a large portion with 179 species. Another common cover type, Sonoran of the watershed, are located at high elevation and on NPS International Journal of Biodiversity Science, Ecosystem Services & Management 97 Figure 4. A component breakdown of species richness in the watershed: maps of (A) avian, (B) mammal, and (C) herpetofauna rich- ness. Biodiversity hotspots vary for each of the groups, avian richness is highest in the riparian areas, mammal in the grasslands, and herpetofauna in the desert uplands. or Forest Service lands, and are typically protected from uncomplicated transportation network), it is not surpris- urban development (Table 1). ing that a majority of the projected development for both models occurred along the CANAMEX corridor south of Tucson, and along the I-10 Southeast of Tucson (Figure 5). Model cross-validation In general, the MEGA scenario differed from the CT in Results from the cross-validation of SWReGAP and NPS which it predicted more inﬁll within developed areas of databases indicate that the total number of species tallied the CANAMEX corridor, and less development in the from the GAP database is likely an underestimate of the headwaters area near the US/Mexico border (Figure 5). true species richness. The GAP list contained 84 species In general, habitat conversion was greater under CT not recorded in NPS inventories; however, 74 of these scenario than MEGA, but the difference was sometimes species were birds, mainly wetland and/or neotropical negligible (Table 2; Figure 5). The projected scenar- migrants that occupy habitat not available on NPS lands ios indicate the species habitat with the highest conver- (i.e., ‘open water’ and ‘agriculture’) and were therefore not sion rates are Sonoran Palo Verde-Mixed Cacti Desert detected in NPS inventories but very likely using available Scrub (CT =−59%; MEGA =−54%; Richness = 159), Apacherian-Chihuahuan Mesquite Upland Scrub habitat within the greater watershed. Conversely, the NPS (CT =−31%; MEGA =−26%; Richness = 190), and inventories identiﬁed 94 species not included in the GAP Apacherian-Chihuahuan Piedmont Semi-Desert Grassland database. Of these 94, 56 were conﬁrmed and 38 uncon- ﬁrmed by park. Species present in the NPS inventories but (CT =−34%; MEGA =−24%; Richness = 215) not represented in SWReGAP suggests that the real ﬁgure (Table 2). Sonora-Mojave Creosotebush-white Bursage of species richness for the watershed is closer to 500. Desert Scrub, with a limited distribution, but high herpetofauna diversity (40 species) is expected to decrease the most of all cover types (CT =−77%; MEGA = Potential future biodiversity changes −73%). Sonoran Palo Verde-Mixed Cacti Desert Scrub From 1979 to 2009, the SCW urban areas grew by approx- and Sonora-Mojave Creosotebush-white Bursage Desert imately 60%. Based on these past trends, SLEUTH models Scrub are the most widespread cover types of the Sonoran predicted 240,142 ha of urban development under the CT, Desert Ecoregion, covering 41% and 36% of the total and 201,499 under MEGA (Table 1). Given the inputs and Ecoregion, respectively (Thomas et al. 2010). However, constraints shared by the two models (no public lands, while these types are regionally extensive, a large per- the three-decade growth trends in the watershed, and the centage of their area has low conservation management 98 M.L. Villarreal et al. Table 2. Area and percentage of landscape occupied by vegetation and land cover types in 2000, and landscape changes expected under Current Trends (CT) and Megalopolis (MEGA) scenarios. Area (ha) %Landscape %Landscape %Landscape % change % change Cover type description 2000 2000 CT MEGA CT MEGA Apacherian-Chihuahuan Mesquite 246,102 26.9 18.6 20.0 −30.9 −25.6 Upland Scrub Madrean Encinal 151,121 16.5 15.6 15.8 −5.8 −4.4 Sonoran Palo Verde-Mixed Cacti 139,026 15.2 6.2 7.0 −59.0 −54.0 Desert Scrub Apacherian-Chihuahuan Piedmont 110,170 12.0 8.0 9.2 −33.9 −23.7 Semi-Desert Grassland Developed, Medium-High 93,247 10.2 36.4 32.2 +257.5 +216.1 Intensity Chihuahuan Mixed Desert and 38,383 4.2 2.9 3.3 −30.7 −20.5 Thorn Scrub Madrean Pinyon-Juniper 35,326 3.9 3.8 3.8 −2.1 −1.8 Woodland Madrean Pine-Oak Forest and 23,064 2.5 2.5 2.5 −1.6 −1.4 Woodland Chihuahuan Succulent Desert 15,287 1.7 1.5 1.6 −9.0 −6.6 Scrub North American Warm Desert 10,757 1.2 1.1 1.1 −9.1 −8.3 Bedrock Cliff and Outcrop Sonora-Mojave 9944 1.1 0.3 0.3 −76.9 −73.0 Creosotebush-white Bursage Desert Scrub Recently Mined or Quarried 9500 1.0 0.5 0.5 −50.5 −50.4 Sonoran Mid-Elevation Desert 5553 0.6 0.4 0.4 −30.2 −26.4 Scrub North American Warm Desert 5361 0.6 0.4 0.5 −23.8 −23.0 Riparian Mesquite Bosque Desert Riparian Woodland and 4747 0.5 0.3 0.3 −45.4 −43.6 Shrubland Madrean Upper Montane 3355 0.4 0.4 0.4 −1.1 −1.1 Conifer-Oak Forest and Woodland Agriculture 2674 0.3 0.1 0.1 −60.1 −58.7 Madrean Juniper Savanna 2143 0.2 0.2 0.2 −0.5 −0.4 Chihuahuan-Sonoran Desert 2123 0.2 0.2 0.2 −29.5 −22.5 Bottomland and Swale Grassland Rocky Mountain Ponderosa Pine 1922 0.2 0.2 0.2 −2.7 −2.7 Woodland Colorado Plateau Mixed Bedrock 1912 0.2 0.2 0.2 −3.9 −3.9 Canyon and Tableland North American Warm Desert 1161 0.1 0.1 0.1 −6.1 −5.6 Volcanic Rockland Mogollon Chaparral 923 0.1 0.1 0.1 −21.7 −19.0 Desert Riparian Forest 435 0.0 0.0 0.0 −17.9 −17.7 Open Water 365 0.0 0.0 0.0 −20.7 −20.3 North American Warm Desert 208 0.0 0.0 0.0 −68.1 −63.6 Wash Rocky Mountain Aspen Forest 205 0.0 0.0 0.0 −0.4 −0.4 and Woodland Chihuahuan Sandy Plains 27 0.0 0.0 0.0 −61.0 −32.7 Semi-Desert Grassland North American Arid West 14 0.0 0.0 0.0 −28.7 −28.7 Emergent Marsh North American Warm Desert 12 0.0 0.0 0.0 −3.7 −1.5 Pavement Chihuahuan Stabilized Coppice 10 0.0 0.0 0.0 −71.8 −69.1 Dune and Sand Flat Scrub Barren Lands, non-speciﬁc 8 0.0 0.0 0.0 −48.4 −48.4 Desert Lower Montane Riparian 4 0.0 0.0 0.0 0.0 0.0 Woodland and Shrubland Note: The table is sorted by Area (ha) 2000. International Journal of Biodiversity Science, Ecosystem Services & Management 99 Current trends Megalopolis Scenario difference Developed in both scenarios Current trends only No urban growth Figure 5. Maps depicting changes in mapped species richness distributions under Current Trends and Megalopolis scenarios, for the year 2050. The third map displays agreement and disagreement locations of the two scenarios, where blue depicts areas of projected development shared by both scenarios and yellow for Current Trends only. status and they therefore are potentially threatened by or incidental species, exotic species, urban-dwelling taxa, development and other land uses (Thomas et al. 2010). and taxa extirpated from the southwest region for more Conversion rates of high species richness habitat types than 20 years. The NPS inventories, on the other hand, are variable between models, depending on vegetation type were much more inclusive. Several of the species observed and location. Riparian types, with their limited distribu- by NPS and not included in SWReGAP were noted to tions and high avian species richness values would see con- be rare or uncommon to the park. Additionally, the NPS siderable and comparable growth-related declines under inventories included a few common non-native or domes- both scenarios: Desert Riparian Woodland and Shrubland ticated species such as feral dog (Canis familiaris) and (CT =−45%; MEGA =−44%), Desert Riparian Forest domesticated cattle (Bos taurus). NPS inventories identi- (CT =−18%; MEGA =−18%), North American Warm ﬁed and listed a few vagrant bird species (e.g., Northern Desert Riparian Mesquite Bosque (CT =−24%; MEGA = Parula [Parula Americana]) whose ranges were far out- −23%). These types have both limited distributions and side of the SWReGAP region and therefore not included low conservation management status since a majority of in SWReGAP models. Regardless of these differences, the ﬂoodplain lands in the SCW are privately owned. The the NPS inventories identiﬁed a number of species not Apacherian-Chihuahuan Piedmont Semi-Desert Grassland, included in SWReGAP that frequent the habitats of SCW with high mammal richness, would experience greater con- (and vice versa), and these ground-based inventories, com- version under CT (−34% change) than MEGA (−24% bined with range models developed by SWReGAP increase change). our knowledge the overall biodiversity and species richness of the watershed. Despite the minor inconsistencies between the NPS Discussion and SWReGAP data sets, the total number of species Differences between SWReGAP and NPS inventories can present in the SCW makes it one of the most biodi- be attributed in part to the differences between the intended verse in the southwest region (Boykin et al. 2007, 2012); use of each data set, as well as the methods used to com- however, the raw numbers alone present a misleading pile the information. SWReGAP is a regional database picture of biodiversity due to the small land area con- and species inclusion was based on potential species range taining the habitat types that support the highest diver- maps, while the NPS inventories reﬂect speciﬁc sight- sity. Habitat with highest species richness are riparian ings within park boundaries. SWReGAP methods also types (Desert Riparian Woodland and Shrubland, Desert included a number of taxa ‘exclusion’ rules for vagrant Riparian Forest, and Desert Lower Montane Riparian 100 M.L. Villarreal et al. Woodland and Shrubland), which currently occupy little extracted quickly from the local aquifer (Webb et al. 2007). more than one half of 1% of the total watershed area. The local groundwater required to support high-intensity Under both future scenarios that value is reduced to less population growth along the CANAMEX corridor would than one-third of 1% of the total landscape. The remaining likely decimate existing riparian habitat on the Santa Cruz amount of riparian habitat under the future scenarios is a River and create a real loss of a high biodiversity hotspot generous ﬁgure; SLEUTH-modeled destruction of riparian in the watershed. types is limited to areas replaced by urbanization whereas The two scenarios, one depicting a greater loss of intensiﬁcation of urbanization adjacent to the riparian types grassland habitat and the other a greater loss of riparian, may also compromise them. The projected habitat reduc- present a potential trade-off of biodiversity loss at the tax- tion is, therefore, likely a minimum. The loss of these onomic class level. The Apacherian-Chihuahuan Piedmont rare types would severely reduce the overall biodiversity of Semi-Desert Grasslands had the highest mammal species the watershed and have major implications for watershed richness and Desert Riparian Woodland and Shrubland health through the loss of landscape corridors, loss of pol- and Desert Riparian Forest the highest avian richness, and linator populations, loss of water provisioning services and together both the components of the landscape comprise more. It is important to note that the biodiversity models, major corridor habitat for a majority of the species in the developed using the binational vegetation data from 2000, watershed. The trade-off between riparian habitat loss and are static and do not take into account natural succession, grassland habitat loss with the MEGA versus CT scenar- natural disturbance, climate change, or any type of land ios presents a conundrum for conservationists and urban use change other than urban that could potentially affect planners. Conservation approaches like those utilized in the vegetation between 2000 and 2050. watershed by SDCP and TNC that focus on purchasing and In addition to general biodiversity loss, a num- conserving undeveloped grasslands as habitat and habitat ber of threatened and endangered species would be linkages, may ultimately force more development into the affected by projected habitat reduction. The Yellow-billed CANAMEX corridor and have unintended consequences cuckoo (Coccyzus americanus) and Chiricahua leopard for biodiversity of the watershed. In terms of ecosystem frog (Lithobates [Rana] chiricahuensis), for example, uti- services, the degradation and extirpation of habitat for the lize habitat exclusively within riparian vegetation. The 171 bird species that reside in or visit the SCW riparian major ‘matrix’ vegetation types in the watershed, which areas would result in a loss for the local economy and for are projected to experience the greatest amount of conver- ecosystem services of the region in general. At a local sion and fragmentation (Apacherian-Chihuahuan Mesquite level, bird watching, an activity which often occurs in Upland Scrub, Sonoran Palo Verde-Mixed Cacti Desert species-rich riparian areas, is responsible for a large share Scrub and Madrean Encinal), support a large number of the ecotourism dollars spent in the area (Leones et al. of common, yet ecologically important species, as well 1998; Sekercioglu 2002; U.S. Fish and Wildlife Service as provide habitat for threatened species like the Desert 2009, 2012). Tortoise (Gopherus agassizii) and the Jaguar (Panthera Furthermore, the degradation and loss of Santa Cruz onca), whose northern range includes the highlands of the River riparian habitats would likely have consequences for US–Mexico borderlands. biodiversity at the hemispherical scale; riparian habitat loss in the western United States has already contributed to the contraction of northern breeding ranges for migratory Conservation trade-offs species like the Yellow-billed cuckoo, that migrate annu- ally from South America to the Southwestern United States With both models, we assumed a ﬁxed rate of population (Laymon and Halterman 1987). In addition to ecotourism growth with the distribution and pattern of development dollars and cultural ecosystem services, a biodiverse avi- responding to the constraints imposed on each scenario. fauna are necessary for biocontrol of agricultural and other The CT growth depicts a future where urban and exurban insect pests (e.g., tamarisk beetles), seed dispersal of native development continues at pace, increasingly converting plants, and regulation of population dynamics by apex and fragmenting undeveloped and species-rich grassland species and birds of prey. habitat. The MEGA scenario, where the probability of Conversely, habitat loss and fragmentation of development on lands outside of the CANAMEX corridor grasslands and other species-rich upland types will was reduced by 50%, assumes high-intensity development adversely affect ecosystem function and ecosystem with high human population density inside the corridor. However, because the ﬂoodplain was masked to exclude services if land development progresses under the CT development and the model does not take into account scenario. The large and charismatic species that inhabit groundwater use, the true effect of the MEGA scenario on these areas require considerable patches of unfragmented riparian habitat along the Santa Cruz River is not appar- habitat and may be particularly sensitive to loss of corri- ent in the ﬁnal results. 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International Journal of Biodiversity Science, Ecosystem Services & Management – Taylor & Francis
Published: Jun 1, 2013
Keywords: species richness; urbanization; Wildlife Habitat Relation; habitat model; spatial simulation; urban development
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