ABSTRACT
The movement of species has been a constant occurrence in natural history—like the occurrence of fire. Before the people commit a lot of money to controlling exotic species, it might be helpful to understand why the people should treat alien species any differently than the people treat native ones. To justify a generalization one has to show that the bad apples are characteristic or representative of the group—for example, that exotic species are much more likely than native ones to cause ecological damage or economic harm. Three ecologists have recently written, "Symptoms of degrading ecosystem conditions include the prevalence of exotic species". One could hypothesize that the ecosystem with more species is the one that has been colonized—but this would suggest that colonization, by increasing diversity, improves ecosystems. Biological and ecological science, to some extent, can describe what may happen if nonnative oysters, swans, and so on are allowed to prosper in the Chesapeake Bay.
