Invasive species

Three different words get used as one. Non-native means it came from elsewhere. Established means it persists. Invasive means it spreads and causes harm — and most non-native species never get that far.

Start with the attrition, because it is the part most often left out. Of the species carried to a new place, most fail to survive at all. Of those that survive, most fail to establish a self-sustaining population. Of those that establish, most spread slowly or not at all and produce no measurable effect on anything. A minority spread widely, and a smaller minority of those cause serious documented harm — and that last group includes cases as severe as the extinction of island bird faunas, so the fact that it is a minority does not make it a small matter. The disagreement in the literature is about what follows from that. One position holds that since origin predicts impact poorly, species should be assessed on measured effects rather than on where they came from — and points to introduced species providing functions that lost natives once did, and to native species causing comparable damage when conditions change. The other holds that lag times between establishment and detectable impact can run to decades, so apparent harmlessness is weak evidence, and that the documented harm from the damaging minority is severe enough to justify caution about the whole category. Both agree on the attrition and on the severity of the worst cases. They disagree about how to act under uncertainty. This is also a subject where scientific and political disagreements are easily confused. NatureHQ reports the scientific one — about lag times, predictive traits and how impact should be measured — and takes no position on the political questions about borders and belonging that sometimes attach themselves to the vocabulary. The earthworm case is a useful anchor here, because it involves an animal nobody has strong feelings about, doing exactly what earthworms do, and transforming forests that had none.