Carl Zimmer writes in a recent New York Times piece "Friendly Invaders", that invasive species might not actually be that bad for ecosystems. He bases this interesting argument on new research from Dov Sax, at Brown. Dr. Sax has published a paper in TRENDS in Ecology and Evolution that argues, based on statistics, that invasive species, particularly plants, contribute to diversity rather than denigrate it. "Effects of exotic species on evolutionary diversification."
"Exotic species invasions create almost ideal conditions for promoting evolutionary diversification: establishment of allopatric populations in new environmental conditions; altered ecological opportunities for native species; and new opportunities for hybridization between previously allopatric taxa...Our review indicates that, although the well-documented reductions to biodiversity caused by exotic species might outweigh the increases resulting from diversification, a complete understanding of the net effects of exotic species on biodiversity in the long term will require consideration of both."
Aldo Leopold said, "To keep every cog and wheel is the first precaution of intelligent tinkering". But unfortunately for this machine metaphor, unexpected results can also arise from adding new, self-propagating parts. Dr. Sax urges science to remain unbiased in evaluating the cumulative impacts of invasive species, and statistically it may well be true that many invasives, particularly plants, do no harm and should be given their "green card" (e.g. Dandylions Taraxacum offiniale), if not honorary citizenship (e.g. Cleome serrulata). However, based on my personal experience, other introductions have perhaps the greatest potential to harm ecological systems. Some examples:
Buffel Grass in Tucson : this is a no-holds-barred fight for the very existence of the Sonoran Desert. This invasive threatens the extinction of hundreds of species and even our "way of life".
Tamarix in New Mexico: while not threatening extinction of the cottonwood bosques that line mid and low-elevation streams, is responsible (combined with anthropogenic disturbance) for degraded ecosystems throughout the American Southwest. The complex web of riparian corridors that sustains delicate desert and upland ecosystems cannot be repaired without bringing Tamarix under control.
Rio Grande Cutthroat Trout : threatened by interbreeding and habitat loss from non-native invasives. See, e.g. Comanche Creek Restoration Project.