Island biogeography for an urbanizing world: how extinction and colonization may determine biological diversity in human-dominated landscapes

Urbanization is increasing worldwide with potentially important implications to biological diversity. I show that bird diversity is responsive to the reduction of forest cover associated with urbanization in the Seattle, WA, USA metropolitan area. Bird diversity peaks at intermediate levels of human settlement primarily because of the colonization of intermediately disturbed forests by early successional, native species. Extinction of native forest birds and colonization of settlements by synanthropic birds have lesser effects on the overall pattern of avian diversity with respect to the level of urbanization. However, extinction increases linearly with loss of forest and colonization by synanthropic species decreases curvilinearly with reduction of urbanization. These findings have biological, theoretical, and practical implications. Biologically, intermediate disturbance appears to drive diversity by increasing the heterogeneity of the local land cover. Theoretically, I present a graphical model and use it to derive testable hypotheses about how extinction and colonization are affected by urbanization to determine local diversity. Practically, maintaining high local diversity without reducing regional or global diversity will require planning so that the same landscapes are not promulgated everywhere. This will require cooperation among a diverse group of planners, ecologists, policy makers, home owners, educators, and activists.