Eco-Defense against Invasions

O nce thought only science fi ction, alien invasions are one of today's major scientifi c challenges. The "aliens" in question are nonnative, or exotic, species capable of outcompeting natives and, ultimately, taking over the ecosystems to which they are introduced (Figure Invasive alien species are a worldwide problem, now found in every ecosystem on Earth. Staving off the further advance of invasions-identifi ed by the Millennium Ecosystem Assessment (http:www.millenniumassessment.org/en/ index.aspx), a United Nations-backed audit of ecosystem health, as one of the most important drivers of ecosystem change-is a key environmental priority.