Overview

Although Lois was probably talking about society's tendencies away from excellence; many of the "dragons" in our environment have come at our own invitation, as intentionally introduced species. Unlike the chemical stressors we discussed in Chapters 3 through 5, these are living organisms. They are usually introduced into ecosystems where no natural predators can check the newly arrived species' numbers and geographic range.

Introduced species cause us to rethink our concept of pollution. Like so many other issues in this book, the type of pollution caused when an opportunistic, invasive organism colonizes and outlives its welcome is a systematic one. The threat is not usually due to a single species, although this could be very important if a species were already threatened or endangered prior to the invasion. It is usually a problem of the whole ecosystem. Something is out of balance.

The concept of invasion is not one that is consistently applied. For example, the very diligent Invasive Species Specialist Group (ISSG) has reluctantly listed the 100 "worst" invasive species in the world (see Table 6.1). In ISSG's own words, the task is difficult:

Species and their interactions with ecosystems are very complex. Some species may have invaded only a restricted region, but have a huge probability of expanding, and causing further great damage (for example, see Boiga irregularis: the brown tree snake). Other species...