Conservation biologists typically view invasive species as a scourge that drives native plants and animals to extinction. But Brown University’s Dov Sax isn’t sure exotic plants deserve their bad reputation.
Sax looked for patterns in hundreds of years’ worth of data on plant and vertebrate extinctions on the world’s islands. He confirmed that invasive animals sometimes devastate native species. But he found that exotic plants usually live alongside natives quite peacefully. Consider this contrast: In New Zealand, more than one-third of native land birds have gone extinct. During the same period, only three of more than 2,000 plant species have been eliminated, even as the island has become home to thousands of exotics.
“It’s not to say that plant extinction isn’t happening, but it doesn’t appear to be because of invasives,” Sax says.
Sax’s research, reported in Proceedings of the National Academy of Sciences, found that oceanic islands absorb a massive number of exotic plants with little effect on native botanicals. In fact, these islands seem able to take on almost as many new species as they had starting out, thereby doubling plant richness.
The paper is controversial, and Sax agrees that scientists need to better understand the mechanisms of extinction. He says it’s particularly important for scientists to learn whether there is a significant lag between extinctions and the events that trigger them. In the meantime, he says, this investigation demonstrates that “sometimes the stereotypes are wrong.” ❧
Sax, D. and S.D. Gaines. 2008. Species invasions and extinction: The future of native biodiversity on islands. Proceedings of the National Academy of Sciences 105:11490-11497.