How Humans Pave the Way for Plant and Animal Invasions

Humans, it seems, are not the only species to seek refuge from crowded, competitive environments. Given the opportunity, plants and animals move in droves to relatively impoverished habitats that have been virtually cleared of natural rivals, according to a paper published in the Sept. 6 issue of the weekly scientific journal Science. The article by geology professor Geerat J. Vermeij, a paleontologist at the University of California, Davis, may help scientists predict what will happen in a region if resident species become extinct. It also may help answer basic questions in ecology and evolutionary biology. "This has interesting implications for biotas, because we are bringing about habitat destruction on a large scale," Vermeij said. "We might be setting the stage for geographical expansion of a great many species." Biota are the flora and fauna of a region. During the last 25 million years, known as the Neogene period, physical barriers separating historically distinct biota have been breached on an unprecedented scale, according to Vermeij. One of the most spectacular terrestrial examples is the formation of the isthmus of Panama, joining huge continents in the northern and southern hemispheres. Slightly more than half the living South American savannah-adapted mammals can trace their origins to North America. About the same time, 3 million years ago, the Beiring Strait opened. The Arctic exchange involved hundreds of species and an extremely disproportionate ratio. For every 10 creatures that moved from the North Pacific into the Arctic Atlantic basin, only one ventured in the opposite direction. According to Vermeij, people have accelerated the rate of biotic interchange. Species are transported through human commerce around the world. For example, larvae riding in huge volumes of ballast water in ships from Japan have settled down into estuarine homes on the west coast of North America. After the Suez Canal opened in 1869, hundreds of marine species from the Red Sea entered the Mediterranean. As barriers break down, species often invade from one side more than they do from the other. The asymmetric pattern of interchange has puzzled paleontologists, who have advanced several theories. One conjecture assumes the invaders' biological superiority in competition, defense or reproduction. The most important reason for the lopsided pattern, Vermeij proposes, is that large-scale extinctions of species can turn a biota into a sort of biological vacuum that is especially prone to invasions. "The presence of incumbent species in the recipient biota evidently inhibits the establishment of invaders, even if potential invaders occasionally disperse into that biota," he says. As evidence for his hypothesis, Vermeij has synthesized 20 years of biotic interchange research. His documentation rests upon historical evidence -- particularly mammals and molluscs, which tend to leave a good fossil record -- as well as advances in several fields of historical ecology. Large-scale marine invasions do not seem to threaten native species, Vermeij said. In some biotas, in fact, interchange has pushed diversity to levels higher than the pre-extinction number of species. However, a plausible case could be made that land and fresh-water interchanges may directly or indirectly cause extinctions of some native species of mammals and fish. The study of historic biotic interchange may help ecologists determine if given communities can support more species than are actually found there. Vermeij points out that biotas with little prior extinction tend to remain virtually unaffected by invasions, which may mean these biotas are "intact" and close to saturation. "On the other hand," he says, "it may imply only that none of the species in the available pool of potential invaders happens to fit into the recipient biota." Applying his theory to the possibility of a sea-level Panama Canal, Vermeij argues that more species would invade from the eastern Pacific to the more biologically impoverished and less biologically sophisticated western Atlantic. Biotic history needs to be integrated into the traditional studies of living communities and into predictions about the ecology of the future, Vermeij said. "As environments around the world are being disturbed and as species are being exploited and eliminated on an ever-increasing scale, this phenomenon of geographical release is likely to become more common," he says.

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