'Invasive' Species Increase Biodiversity

New Zealand is home to 2,065 native plants found nowhere else on Earth. They range from magnificent towering kauri trees to tiny flowers that form tightly packed mounds called vegetable sheep.

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, 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.

“I hate the ‘exotics are evil’ bit, because it’s so unscientific,” Dr. Sax said.

I agree with Dr. Sax. I find the term 'invasive' species to be pejorative and prefer immigrant species. While it is possible that immigrant species can cause extinctions and lower biodiversity, that is not certain. As Dr. Sax's research shows, in fact, it is more likely that immigrant species will add to local biodiversity.

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.

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. 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.

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.

Take a look at these two charts to see how immigrant species have increased local biodiversity on tropical islands.

Beyond just adding to local diversity, immigrant species can also help create new species.

These scientists also point out that exotics can actually spur the evolution of new diversity. A North American plant called saltmarsh cordgrass was introduced into England in the 19th century, where it interbred with the native small cordgrass. Their hybrid offspring could not reproduce with either original species, producing a new species called common cordgrass.

In a recent paper in the journal Science, Peter Roopnarine of the California Academy of Sciences and Geerat Vermeij of the University of California, Davis, looked at the history of invasions among species of mollusks, a group that includes mussels, clams and whelks. About 3.5 million years ago, the mollusks of the North Pacific staged a major invasion of the North Atlantic. Before then, the Arctic Ocean had created a barrier, because the mussels could not survive in the dark, nutrient-poor water under the ice.

A period of global warming made the Arctic less forbidding. Yet the migration did not lead to a significant drop in the diversity of the Atlantic native mussels. Instead, the Atlantic’s diversity rose. Along with the extra exotic species, new species may have arisen through hybridization.

Aside: I am having a hard time picturing an "invasion of mollusks". Wait, I think it must look something like this.

This research makes me think that greater immigration of species would lead to more biodiversity around the world and in general should be promoted rather than restricted.

via NY Times