Applicability of research findings to conservation policy.

Baldwin et al. (2007 [this issue]) raise a number of concerns about our efforts to discern the independent effects of human activity levels and distributions on biodiversity (Brown & Laband 2006). Their central point of contention seems to be that we failed to acknowledge properly in our work that “land-use change due to sprawl is a leading threat to biological diversity in the United States.” We agree with much of their presentation, including (1) that populations of most states are growing; (2) that cities within most of those states are “sprawling”; and (3) that numerous species around those growing cities are affected negatively—even imperiled—by this dual-action process of urban population growth and subsequent urban expansion. We agree further that public policies should be implemented that address the cause(s) of this emerging and undisputed rise in species imperilment. Baldwin et al. appear to favor “exurban planning” and other urban-sprawl mitigation policies. We support policies that address specific (i.e., independent) causes of imperilment and derive from conclusions based on relevant empirical research. Insofar as we are aware, our research is the first and only effort to disentangle the independent and separate effects of changing levels of human presence and changing distributions of that presence. We sought to identify the individual effect of human population diffusion on species imperilment, controlling separately for the size or level of that population. Our concern was that the dual-action process of sprawl may affect biodiversity differently depending on whether (1) increased populations live within fixed areas; (2) fixed populations live within expanded areas; or (3) increased populations live within expanded areas. The definition of sprawl by Baldwin et al. does not differentiate between these possibilities even though they each may generate unique sets of policy prescriptions. The empirical chal-