Summary of a Talk Presented to the Sonoita Crossroads Community Forum
March 25, 2006
Carl E. Bock, Jane H. Bock, Zach F. Jones
University of Colorado at Boulder
Throughout much of the American West, former cattle ranches are being converted to low-density 'exurban' housing developments. The Sonoita Valley is one such place. Exurbanization of western landscapes is likely to have major impacts on natural ecosystems, including their plants and wildlife, but these effects have been surprisingly little-studied. Between 2003 and 2005 we counted plants, grasshoppers, butterflies, lizards, rodents, rabbits, large mammals, and birds at 48 sampling points in the Sonoita Valley that were grazed by livestock, or that were parts of exurban developments, or both, or neither. Twelve of the points were on the Appleton-Whittell Research Ranch, and these served as ungrazed and undeveloped control plots. Other plots were located on local cattle ranches or in housing developments in the valley. Some but not all of the developments permit homeowners to keep livestock (usually horses) on their properties.
We found that the responses of plants and wildlife to subdivision versus ranching depended very much on the individual species' habitat needs and sensitivities. Mule deer, pronghorn, and ground-dwelling lizards were scarce in the subdivisions compared to undeveloped lands. By contrast, rabbits, many birds, and some butterflies were attracted to exurban landscapes, probably because of increased resources such as water, shade, and nectar that are associated with home sites. Grasshoppers were particularly common on exurban properties where homeowners kept horses, probably because of the increased abundance of weedy plants associated with patchy but locally intense grazing. Rodents as a group were more abundant on ungrazed than on grazed lands, largely independent of the presence or absence of housing developments.
A key aspect of our study, in terms of planning, was to determine if there were differences in effects of development depending on housing density. A recurring pattern among certain species of birds, butterflies, lizards, and rodents was an apparent threshold effect at about one home per 20 acres. Below this level, these species' abundances in housing developments were as high or higher than in undeveloped areas, but above it the same group of species rarely achieved high densities. Likely causes were losses of native habitat, more roads, and increased abundances of domestic pets. This apparent threshold effect was apparent only for select species and not for biological diversity as a whole. Because only eight of our 48 sampling plots were in areas with housing densities above one home per 20 acres, our statistical support for evidence of a development threshold effect is weak, and this requires further study.
Overall, the results of our study suggest that the Sonoita Valley as configured in 2003-2005 supported an extraordinary abundance of plants and wildlife. It appears that the present mixture of low-density housing developments, well-managed cattle ranches, and other open spaces, provides the resources sufficient to sustain much native biological diversity. No small amount of credit must go to the individual property owners who manage their ranches, and their "ranchettes," in ways that keep them in a natural condition. However, our results also indicate that there may be a development threshold out there (about one home per 20 acres?), above which certain components of the biological diversity of the valley will be lost. Future work should include increased sampling in areas with higher housing densities, to more fully document possible threshold effects. Future studies also should consider the ecological costs and benefits of clustered versus dispersed housing Ð that is, not only the overall housing density but also its spatial arrangement. The challenge facing members of the Sonoita Crossroad Community Forum is to determine what sorts of landscapes will sustain for future generations the very things that attracted most of us to the valley in the first place, and then to make a plan and see it implemented. We hope that the results of our work will prove useful in that endeavor.
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