Traditional views of ecology see alien plants and animals as harmful to native species. But that thinking may be wrong.

By CARL ZIMMER

New Zealand is home to 2,065 native plants found nowhere else on Earth.
When Europeans began arriving in New Zealand, they brought with them alien plants - crops, garden plants and stowaway weeds. Today, 22,000 non-native plants grow in New Zealand. Most of them can survive only with the loving care of gardeners and farmers. But 2,069 have become naturalized: they have spread out across the islands on their own. There are more naturalized invasive plant species in New Zealand than native species.
It sounds like the makings of an ecological disaster: an epidemic of invasive species that wipes out the delicate native species in its path. But in a paper published in August in The Proceedings of the National Academy of Sciences, Dov Sax, an ecologist at Brown University in Rhode Island, and Steven D. Gaines, a marine biologist at the University of California, Santa Barbara, point out that the invasion has not led to a mass extinction of native plants. The number of documented extinctions of native New Zealand plant species is a grand total of three.
Exotic species receive lots of attention and create lots of worry.
Some scientists consider biological invasions among the top two or three forces driving species into extinction. But Dr. Sax, Dr. Gaines and several other researchers argue that attitudes about exotic species are too simplistic. While some invasions are indeed devastating, they often do not set off extinctions. They can even spur the evolution of new diversity.
“I hate the ‘exotics are evil’ bit, because it’s so unscientific,” Dr. Sax said.
Dr. Sax and his colleagues do not agree 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. “The overall pattern almost always is that there’s some net increase in diversity,” he said.
But critics, including Anthony Ricciardi of McGill University in Montreal, argue that today’s biological invasions are fundamentally different from those of the past.
It is estimated that humans are now moving 7,000 species a day. In the process, species are being thrown together in combinations that have never been seen before.
“If you pour on more species, you don’t just increase the probability that one is going to arrive that’s going to have a high impact,” Dr. Ricciardi said. “You also get the possibility of some species that triggers a change in the rules of existence.”