Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change

Biological invasions into wholly new regions are a consequence of a far reaching but underappreciated component of global environmental change, the human-caused breakdown of biogeographic barriers to species dispersal . Human activity moves species from place to place both accidentally and deliberately-and it does so at rates that are without precedent in the last tens of millions of years. As a result , taxa that evolved in isolation from each other are being forced into contact in an instant of evolutionary time. This human-caused breakdown of barriers to dispersal sets in motion changes that may seem less important than the changing composition of the atmosphere , climate change , or tropical deforestation-but they are significant for several reasons. First , to date , biological invasions have caused more species extinctions than have resulted from human-caused climatic change or the changing composition of the atmosphere . Only land use change probably has caused more extinction, and (as we later discuss) land use change interacts strongly with biological invasions. Second, the effects of human-caused biological invasions are long-term: changes in climate, the atmosphere, and land use may be reversible in hundreds to thousands of years, but the breakdown of biogeographic barriers has resulted in self-maintaining and evolving