Since spring break is just finishing at some schools and just about to start at others, I have been thinking about islands. Lots of people will go on cruises, or to spend the week on an island, scuba diving or just surfing or lounging. But all those visitors have had some serious effects on biodiversity on those islands. Since the 1600s, when humans first caused the extinction of the island bird called the dodo by bringing dogs, pigs, rats, and other animals that found eggs lying on the ground an easy breakfast, island birds have gone extinct in great numbers. Guam’s native birds were wiped out by the introduction of the brown tree snake, and avian malaria has done a number on unique Hawaiian birds.
Plants, on the other hand, have not gone extinct in large numbers on islands due to introduced species. That is according to a 2008 study by Dov Sax and Steven Gaines. There are certainly examples where domestic goats have run wild and eaten off virtually all of the remaining slow-growing and unique silverswords in Hawaii. Rob Robichaux, a faculty member at University of Arizona, along with a number of other scientists and dedicated volunteers and professionals, are trying to save those silverswords.
So why do introduced plants tend to naturalize without causing extinction of native plants? Maybe they are using resources in a different way and will continue to coexist. They certainly can change the ecosystem’s look, structure, and functioning (how much biomass grows, how humid the climate is, etc.). So maybe they are causing extinctions, just more slowly than a disease epidemic. The paper by Sax and Gaines asks some of those questions, but scientists do not know the answers yet.