Assessing outcomes of urban growth and conservation scenarios on biodiversity in the U.S./Mexico borderlands

Background/Question/Methods

The Sonoran Desert and Apache Highlands ecoregions of North America encompass diverse vegetation communities ranging from desert grasslands to spruce-fir forests, and support exceptionally high biodiversity. However, changing land uses pose an increasing threat to some of the most diverse ecosystems. We assessed the impacts of two future urban growth scenarios on biodiversity in a binational watershed in southern Arizona, USA and northern Sonora, Mexico. We quantified and mapped current terrestrial vertebrate species richness using Wildlife Habitat Relation (WHR) models and a high resolution binational vegetation map, and validated the results with data from local National Park Service biological inventories. Past urban growth patterns were assessed from three decades of land cover data (1979-2009), and projected to the year 2050 using cellular automata models. Future changes to biodiversity were modeled for 2 contrasting scenarios: “Current Trends” development and “Megalopolis” expansion along a transnational growth corridor that would limit development and fragmentation of existing open space.

Results/Conclusions

We found that 451 terrestrial vertebrate species are associated with the current distribution and area of  all vegetation and land cover types, with Desert Riparian Shrubland and Woodland, and Desert Riparian Forest supporting the greatest total species (267), and greatest number of avian (171) species. Apacherian-Chihuahuan Piedmont Semi-Desert Grassland supports 75 of 93 total mammal species and 46 of 74 total herpetofauna.  Based on Current Trends growth trajectories, 45% of existing Desert Riparian Woodland, and 34% of semi-desert grasslands will be lost, whereas, in the Megalopolis scenario, these habitats would decline by 44% and 24% respectively. Differences in growth patterns of the two models suggest a trade-off of biodiversity loss at the taxonomic class level: Current Trends would reduce and fragment more habitat for mammals and herpetofauna, while Megalopolis would likely cause greater loss of avian-rich riparian habitat due to increased residential and industrial groundwater use. The trade-off between riparian and grassland habitat loss with the Megalopolis vs. Current Trends scenarios presents a conundrum for conservationists and urban planners. Conservation approaches that focus on purchasing and conserving undeveloped grasslands as habitat and habitat linkages for mammals may encourage growth of settlements along the binational transportation corridor adjacent to the major riparian corridor.  In doing so, open-space conservation may contribute to degradation and extirpation of remaining habitat for the 171 bird species that reside in or visit the riparian areas.