The effects of houses and road networks on ecological processes in a southwestern grassland

Background/Question/Methods

Large areas of grassland in semiarid/arid parts of the southwestern United States and northern Mexico are yielding to low-density, dispersed, non-urban housing. In the United States, exurban land use occupies 5 to7 times more area than land with urban and suburban densities, and has grown at a rate of about 10 to 15% per year. This widespread land-use change, exurbanization, has profound implications for regional biological diversity and ecosystem function. A series of “development hazard maps” were produced for the Sonoita Plain, southeastern Arizona (USA) as part of an initial investigation into the ecosystem, ecosystem services, and natural resources of a semiarid grassland area threatened by exurban development. Houses and roads have effects on ecological processes beyond their physical boundaries and the distances that road and house effects extend outward were considered in defining areas impacted. Values reported in the scientific literature and derived from high-resolution aerial photographs and field observations were synthesized to generalize impact zone extent.

Results/Conclusions

Although the effects zones selected for this study were conservative (30-100m), the impacts of houses and roads are affecting ecological processes on nearly 20% of this sparsely populated landscape. The fire hazard maps for the Sonoita Plain also show that areas with higher housing densities also have high FRID (Fire Return Interval Departure) values. In all but one case, where there was a recent burn, higher density housing clusters had FRID values greater than 1. This suggests that fire suppression measures around homes have altered the natural fire regime; fire return intervals (years since fire) have surpasses historic values and fire risk is now high. Overall, 81% of this area is at high to extreme risk of fire.  Water hazard maps show a relationship between increasing population density and greater depth to groundwater, lower saturation zone, and greater well depth. Risk of spread of invasive species is “Very High” around two towns (Sonoita and Patagonia) and “High” around the third (Elgin). “High” risk for invasive species spread was found around four other higher density housing clusters. These clusters represent housing developments away from towns and are characteristic of exurban sprawl. These dispersed settlement patterns create practical complications for natural resource management and planning. By identifying areas most impacted by exurban development, the spatial statistics and maps generated may help to guide management and planning efforts.