Biotic homogenization and conservation prioritization

Quantitative studies of biotic homogenization can provide useful insights into conservation problems when used appropriately, but can be dangerously misleading when they are not. By separating the concept of biotic homogenization at the global scale from the study of biotic homogenization at spatiallyand temporally-explicit scales, researchers can avoid many of the subtle pitfalls inherent in homogenization studies. 2006 Elsevier Ltd. All rights reserved. As human activities accelerate rates of species invasion and extinction, biological diversity changes in fundamentally different ways at different spatial scales. While species richness is declining globally, species gains are frequently observed at regional and local scales as range-expanding habitat generalists (both native and non-native in origin) invade new species pools typically at the expense of rare, and often endemic, native species that disappear (Hobbs and Mooney, 1998). The process describing this non-random reshuffling of species pools is coined biotic homogenization (McKinney and Lockwood, 1999); referring to an increase in the taxonomic similarity of two or more species pools through time as the result of species invasions and extinctions (Olden and Poff, 2003; Olden and Rooney, 2006). er Ltd. All rights reserved du (T.P. Rooney). Because species invasions and extirpations all too often result in the wake of environmental degradation, biotic homogenization seems an important dimension of the biodiversity crisis. It extends beyond the narrow focus on elevated extinction rates to incorporate the other side of the equation: species introductions are also well above background levels. Biotic homogenization conjures the image of Quammen’s (1998) distopian ‘‘Planet of Weeds’’ scenario and the prospect of Kunstler’s (1993) ‘‘The Geography of Nowhere’’ in which biotic distinctiveness is gradually dissolving over time. While the application of biotic homogenization to conservation problems may appear relatively straightforward, it is anything but. Most importantly, extinction and invasion events at sub-global scales do not automatically increase