Why alien invaders succeed: support for the escape-from-enemy hypothesis.

Successful biological invaders often exhibit enhanced performance following introduction to a new region. The traditional explanation for this phenomenon is that natural enemies (e.g., competitors, pathogens, and predators) present in the native range are absent from the introduced range. The purpose of this study was to test the escape-from-enemy hypothesis using the perennial plant Silene latifolia as a model system. This European native was introduced to North America in the 1800s and subsequently spread to a large part of the continent. It is now considered a problematic weed of disturbed habitats and agricultural fields in the United States and Canada. Surveys of 86 populations in the United States and Europe revealed greater levels of attack by generalist enemies (aphids, snails, floral herbivores) in Europe compared with North America. Two specialists (seed predator, anther smut fungus) that had dramatic effects on plant fitness in Europe were either absent or in very low frequency in North America. Overall, plants were 17 times more likely to be damaged in Europe than in North America. Thus, S. latifolia's successful North American invasion can, at least in part, be explained by escape from specialist enemies and lower levels of damage following introduction.