The ailing invader.

T he nature of biological invasions prompts researchers to focus on questions of great importance: How far will a nonnative species go? How fast? And what are the consequences to native species? To crops? To us? This perspective had crystallized long before Charles Elton wrote his classic treatise on the subject in 1958 (1). In it, Elton noted that scientific accounts of invasion have ‘‘a sense of urgency and scale’’ reflecting the uncertainties and high stakes surrounding the arrival of invaders such as brown tree snakes, Russian wheat aphids, and the West Nile virus. The resulting body of research has greatly increased our ability to estimate eventual distribution (2, 3) and the most effective modes of intervention (4). Nevertheless, the prevailing focus has left other questions virtually unasked. In this issue of PNAS, Brown et al. (5) take on a good one: What is the impact of invasion on the invader? Perhaps invasion biologists may be forgiven for overlooking such a question. After all, invasive species that gain our attention are highly successful in ecological terms. Despite being displaced from their native turf, they have managed not just to gain a toehold in a new environment but to thrive. Like wealthy neighbors or bright classmates, their problems may tend to escape our notice.