Just as species have come and gone through the course of evolutionary history, so have their ranges expanded and contracted. 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. Moreover, the pattern of change, especially in human-dominated, disturbed ecosystems, is for the biota to become more homogeneous.