Growth Scenarios and Conservation Trade-offs in the US-Mexico Borderlands

By Miguel L. Villarreal and Laura M. Norman, United States Geological Survey

Developing and interpreting alternative future land-use scenarios is an effective way to engage communities in local and regional planning and illustrate how rates and patterns of land-use change may affect human well-being and the environment. Along the arid borderlands between the United States and Mexico, many communities have experienced rapid population growth and land use changes in recent decades, but these patterns vary spatially, driven by different cultural, economic and geopolitical forces. For example, colonias, unincorporated communities along the border in Mexico, developed rapidly and somewhat erratically following passage of the North American Free Trade Agreement (NAFTA) in 1994. Just north of the border in the United States, large historic cattle ranches are being subdivided into “ranchettes” and housing developments.

In addition to being home for a growing number of people, the borderlands are a hotspot of biodiversity, which support regionally important economic activities like birding, hunting and ecotourism. Beyond general biodiversity, there are a number of rare and threatened species, like the jaguar (Panthera onca) and neotropical migrant western yellow-billed cuckoo (Coccyzus americanus), that require maintenance of distinct ecological or environmental conditions to support their habitat.

With our research on biodiversity losses and conservation trade-offs, we developed spatially-explicit footprints of future land-use scenarios and examined how they might affect biodiversity patterns in this binational watershed where species habitat is lost directly to land conversion and indirectly by over-extracting groundwater for industrial and residential uses. Scenarios were developed using SLEUTH, a cellular automaton model of urban growth and land use change (which uses Slope, Land use, Exclusion, Urban extent, Transportation and Hillshade as input data).

Our ‘Current Trends’ scenario represents potential urban growth patterns in 2050, based on the continuation of historical trends of land development from 1979–2009. An alternative was ‘Megalopolis,’ which portrays an equivalent amount of urban growth as Current Trends, but spatially focused around an International Trade Corridor (the CANAMEX highway) linking Mexico and the United States. The managed growth aspects of this alternative scenario also complemented a voter approved conservation measurement for county acquisition of open space in southern Arizona (Sonoran Desert Conservation Plan) and the results of community survey data on balancing managed growth with open space conservation (Imagine Greater Tucson).

We assembled biological inventory data from the region, implemented wildlife-habitat models and mapped out the total number of species that may use habitat at any given location. The results indicate that riparian forests and woodlands support the most species overall (267 total terrestrial vertebrates and 171 bird species) and semi-desert grasslands support the most mammal and herpetofauna species. Biodiversity maps were then intersected with the spatial footprints of the 2050 growth scenarios.

The Current Trends scenario showed continued loss of grasslands through the process of ex-urbanization, a process which would effectively fragment important habitat for large, mobile apex predators like the jaguar and mountain lion, as well as a number of important big game species. On the other hand, in the Megalopolis scenario, the high-intensity development along the super corridor, which runs adjacent to the watercourse, would promote the conservation of open space in the larger watershed, yet likely decimate the species-rich riparian zone. Urban and industrial growth along this corridor would result in the loss of important bird habitat, likely overtaxing already compromised groundwater systems, and result in the collapse of riparian habitat as seen near the city of Tucson in the 1950s as the city’s water demands grew (see repeat photographs).

Repeat photographs depict changes to the riparian zone of the Santa Cruz River just south of Tucson, Arizona. The river was once dense with vegetation, but groundwater pumping for agricultural and urban land uses contributed to the loss of vegetation and channel widening. Photographs by R.M. Turner.

The species richness data analyzed for this project indicate the area is one of the most biodiverse in the southwest region, however, the raw numbers alone present a misleading picture of biodiversity due to the small land area containing the habitat types that support the highest diversity. Habitats with the highest species richness are riparian, which currently occupy little more than one half of 1% of the total watershed area. The loss of these rare habitat types would severely reduce the overall biodiversity of the watershed and have major implications for watershed health through the loss of landscape corridors, pollinator populations, water provisioning services and more.

The scenarios presented in this research illuminated the potential unintended consequences and trade-offs that may result from the future implementation of regional conservation plans, and in doing so provide information to guide more holistic and sustainable growth planning in the area.

Read the original research here: Biodiversity losses and conservation trade-offs: assessing future urban growth scenarios for a North American trade corridor