The way I have just described the situation of the native flora in Hawaii is the way we usually describe the impact of invasive exotics. We implicitly presume that the species composition of communities is static and that all species invasions are ``bad.'' In fact,
Just as species have come and gone through the course of evolutionary history, so have their ranges expanded and contracted. Why should we care that species formerly restricted to Europe and the Mediterranean are now found in California, South Africa, Chile, Argentina, and Australia?
First of all, species naturally come and go from communities. Thus, a focus on invasions is too narrow. We should also consider the impact of species deletions. Both invasions and deletions are an integral part of natural communities. Human activities alter the frequency of both and, by so doing, greatly affect community structure.
Although these impacts are less well documented than impacts on species extinctions, the case for concern about human impacts on community structure is analogous to that for concern about extinction. The rate at which human activities impose changes in community structure are likely to be far greater than those they have endured in the past (Figure 2. Moreover, the pattern of change, especially in human-dominated, disturbed ecosystems, is for the biota to become more homogeneous. The ease with which we and our associates now travel the globe mean that homogenization is possible to a far greater extent now than it was even a few decades ago.
McKinney and Lockwood (Table 1) point out that biotic invasions are in many respects the flipside of extinctions. There are, however, many more species declining than increasing in number. For example, We are replacing declining species with a small number of widespread species whose abundance is increasing. The result will be a much more biologically homogeneous world.4
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