Biotic homogenization and the breakdown of biogeographic barriers.
McKinney, Michael 1, Lockwood, Julie*,2, 1 Department of Earth and Planetary Sciences, Knoxville, Tennessee, USA2 Ecology, Evolution and Natural Resources, New Brunswick, New Jersey, USA
ABSTRACT- Exponential growth in the establishment of exotic species is reducing the distinctiveness among biotic regions while reducing global biodiversity. As global transportation increases and human-dominated habitats expand, the rate, degree and extent of this future "Homogeocene" will likely greatly exceed, by several orders of magnitude, any biotic mixing episode that has ever occurred in Earth's history. Biologists should focus much greater attention to biotic exchange, which includes not only exotic species but also the expansion and contraction of native species' ranges. There are many complexities to the homogenization process that have received virtually no attention. The vast majority of studies have used species tabulations. Examples include presence-absence similarity indices and species-area curves. But species tabulations ignore the central issue of species abundance which may often play a huge role in homogenizing communities. We present evidence from plant homogenization studies that, for the first time, include abundance data. Species tabulations also ignore other kinds of biotic blending, such as genetic (hybridization) and ecological function. Another unstudied complexity is how taxa differ in the pattern and process of homogenization. We show evidence that, at many scales, plants have much higher percentages of established exotic species than birds or mammals. A final complexity is the role of scale. At local scales, exotic species not only often increase biodiversity they may also reduce similarity among communities, just the opposite of homogenization. We provide evidence of this and discuss the causes, such as human importation of different exotics into different areas.
Key words: homogenization, plants, invasions, extinction
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