A clever experiment conducted along the Eel River in northern California sheds new light on a seeming contradiction in the field of ecology about whether a diverse ecosystem is inherently more resistant to invasion by alien species. The question has become more important in recent years as scientists campaign to preserve the planet's biodiversity, focusing on areas with the largest concentration of species, or attempt to restore native habitats and native species in the face of dramatic increases in the number of non-native invaders. Paradoxically, while many studies have shown that more diverse communities are better at resisting invaders, surveys of diverse ecosystems show they contain the highest number of exotic, non-native species. In a series of experiments reported in this week's issue of Science, Jonathan Levine, a graduate student in integrative biology at the University of California, Berkeley, shows clearly that both are true. Small communities with high diversity are better at resisting invading species, but at larger scales, invaders are more likely to get established in diverse ecosystems.
Places are diverse for a reason.
They are rich in resources, they lack predators, they have a moderate amount of natural disturbance - in general, they are hospitable to a broad range of species. So, you can't rely on biodiversity alone to prevent invasion. You have to keep exotic species from getting a foothold in the first place. Levine and one of his faculty advisors, Carla D'Antonio, associate professor of integrative biology at UC Berkeley, wrote a review article for the ecological journal Oikos last year that questioned the theoretical foundation and scientific support for the claim that diversity reduces susceptibility to invasion. People want to believe that, if we have more diversity in the ecosystem, it will be more resistant to invasion.