“I hate the ‘exotics are evil’ bit, because it’s so unscientific,” Dr. Sax said.
Dr. Sax and his colleagues are at odds with many other experts on invasive species. Their critics argue that the speed with which species are being moved around the planet, combined with other kinds of stress on the environment, is having a major impact.
There is little doubt that some invasive species have driven native species extinct. But Dr. Sax argues that they are far more likely to be predators than competitors.
In their new paper, Dr. Sax and Dr. Gaines analyze all of the documented extinctions of vertebrates that have been linked to invasive species. Four-fifths of those extinctions were because of introduced predators like foxes, cats and rats. The Nile perch was introduced into Lake Victoria in 1954 for food. It then began wiping out native fish by eating them.
“If you can eat something, you can eat it everywhere it lives,” Dr. Sax said.
But Dr. Sax and Dr. Gaines argue that competition from exotic species shows little sign of causing extinctions. This finding is at odds with traditional concepts of ecology, Dr. Sax said. Ecosystems have often been seen as having a certain number of niches that species can occupy. Once an ecosystem’s niches are full, new species can take them over only if old species become extinct.
But as real ecosystems take on exotic species, they do not show any sign of being saturated, Dr. Sax said. In their paper, Dr. Sax and Dr. Gaines analyze the rise of exotic species on six islands and island chains. Invasive plants have become naturalized at a steady pace over the last two centuries, with no sign of slowing down. In fact, the total diversity of these islands has doubled.
Fish also show this pattern, said James Brown of the University of New Mexico. He said that whenever he visits a river where exotic fish have been introduced, “I ask, ‘Have you seen any extinctions of the natives?’ ” “The first response you get is, ‘Not yet,’ as if the extinction of the natives is an inevitable consequence. There’s this article of faith that the net effect is negative.”
Dr. Brown does not think that faith is warranted. In Hawaii, for example, 40 new species of freshwater fish have become established, and the 5 native species are still present. Dr. Brown and his colleagues acknowledge that invasive species can push native species out of much of their original habitat. But they argue that native species are not becoming extinct, because they compete better than the invasive species in certain refuges.
Granted the oscars and the peacock are not threatening the ecosystem they are in, but had non-natives species been banned for sale after that happened, the likelihood of someone having a lionfish in their aquarium would be minimal!
