Environment / Ecosystems

"With invasions in a place like San Francisco Bay, you've got more species than you had before," says Carlton. "If you've added 250 new species since 1850 with no species known to have gone extinct, why don't we consider that good? I think that's a very good question."

One answer, ecologists say, is that there are at least two kinds of diversity to keep in mind: alpha diversity, the number of species in any given location, and beta diversity, the relative diversity between any two locations. If a New York snail invades San Francisco Bay and a San Francisco snail invades New York Harbor, the alpha diversity in both locations has increased, but the beta diversity has decreased, because the two environments now share two species. Each place is that much less unique. Moving species around the world may increase local diversity, but it doesn't increase the overall number of species on the planet; that number only goes down.

The larger cost of invasions is hard to discern. Most of us live small, local lives and are grateful for whatever manages to grow on our windowsills; we live in alpha-diversity worlds. In contrast, beta diversity is visible only on a grand scale and requires effort to see and take in. Its appreciation is a luxury, although perhaps no less valuable for being one.

A head count of species—native or introduced—in one place or another is one way to measure the impact of biological invasion, but it may not be the most telling. "We should focus on ecosystem management, not just species management," says Grosholz. "Extinction is a warning sign, but equally important are fundamental changes in ecosystem structure. Where do we draw the line? Maybe we have to say, 'We care above this line, and we don't care below that one.' "

Since Elton's time, ecologists have struggled to account for the distribution and spread of alien species. They now realize that the key factor is opportunity. The more frequently and persistently a foreign plant or animal is exposed to a new environment, the better its odds of invading. Ecologists call this propagule pressure. It amounts to a kind of Noodle Theory: Throw enough different kinds of noodles at a wall long enough and eventually one will stick, no matter whether the noodle is buckwheat or soba or the wall is cement or wood paneled.