Reserve size, local human density, and mammalian extinctions in US protected areas

We examined quantitatively the interaction of reserve size and surrounding local human density in the United States and their relative effect on extinction of large mammals in 13 national parks of the western United States. Data on reserve size and human density were obtained from publicly available sources. Local human density was calculated as the mean density in the 50- or 100-km zone surrounding the reserves' borders. Reliable extinction data are extraordinarily hard to find. Using a variety of definitions of extinct , we collated information on extinctions of large mammals (>5 kg) that spanned the size of U.S. national parks as a whole. Human density surrounding reserves varies considerably. Overall, small reserves were in areas of higher human density than were large reserves (p < 0.0001, r(2) = -0.24, n = 864; excluding Hawaii), and many of the small reserves were at higher local density than the mean for the contiguous United States. Extinction rates of large mammals correlate significantly with local human density, but not with park area. These findings together emphasize that (1) processes occurring outside of a reserve's boundary may unexpectedly strongly affect species within the reserve; (2) small reserves might suffer the double jeopardy of not only their size but also their situation in especially adverse surrounds; and thus (3) small reserves might suffer more intense edge effects and be more isolated than large reserves. If so, conservation workers need to incorporate the relationship into their models and management decisions.