CRB: Subdivision versus Ranching: Effects of Livestock Grazing and Exurban Development on the Biodiversity of a Southwestern Grassland/Savanna

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

PROJECT SUMMARY Across the interior American West,cattle ranches are being converted to subdivisions,in order to accommodate exurban population expansion.What are the biodiversity consequences of this profound land use change?That is the core question addressed by this proposed research. Conversion of rangelands to subdivisions may negatively impact native biodiversity through direct habitat loss.It has been argued that finding ways to keep ranchers from selling their lands to developers is vital to conservation.On the other hand,most western ranches include some public lands that cannot be sold,and even on private lands the developments usually are low density.Therefore,western exurban development often results in relatively little direct habitat loss.If a landscape is converted by scattered subdivisions,but simultaneously released from the controlling influence of livestock grazing,will this result in net gains or losses of regional biodiversity?So far,a volatile mixture of prediction and passion has fueled this debate,but rarely has it been informed by data. The purpose of this proposed study is to quantify components of native biodiversity in a grassland/savanna in the Sonoita Valley of southeastern Arizona,where some cattle ranches are being converted into subdivisions.This study will compare flowering plant,grasshopper, butterfly,bird,and rodent populations in landscapes that are being grazed,or subdivided,or both,or neither.It also will quantify the demographics of three bird species known to be particularly responsive to the effects of livestock grazing and/or landscape conversion through subdivision.The data will be used to test a series of hypotheses and predictions about the independent and interactive effects of exurban development and livestock grazing on 1)species richness and evenness of native biotic communities,2)invasions of exotic species into those communities,3)population and community stability,and 4)avian demographics. The centerpiece of this work will be the Appleton-Whittell Research Sanctuary,a 3200 ha preserve that has been ungrazed and otherwise undisturbed since 1968,and that has been the focus of extensive fieldwork since the early 1970 's.Unlike many land use studies in the West, this one will have a control,to the degree that 33 years protection have permitted patterns of native biodiversity to express themselves in the absence of either agriculture or exurban development. Results of this study will address basic and unanswered questions about the relative impacts of local forces (livestock grazing)versus landscape conversion (subdivision)on biodiversity in arid, non-forested western ecosystems,where 1)natural landscape heterogeneity is high,2) development is distant from the controlling influences of large urban centers,and 3)subdivision usually consists of scattered landscape intrusions rather than fragmentation of the grassland into isolated patches.The study will make societal contributions that are both timely and urgent, given present rates of development in the rural West,and the many uncertainties about its ecological consequences.