Head turns bias the brain's internal random generator

invasions. Refuges from the invader often remain at some place in a native species' range. Whether native species can stably persist within these refuges is an open question. But in the short term, the number of invasions has far exceeded the number of resulting extinctions. This result has emphasized that most ecological systems are not 'saturated' with species. Two puzzles Nearly all problematic invasive species experience rapid population growth at some point in the invasion process. However, many of these invaders were seemingly present in the exotic range for a considerable period of time prior to exploding in numbers. A number of hypotheses have been proposed for this 'invasion lag', but the true explanation can be hard to pin down and likely varies by system. It may be that the invaders remain sparse until the climatic conditions appropriate for rapid expansion happen to occur. Alternatively, initially small invader populations may require time or multiple introductions to overcome 'Allee effects' or build-up the genetic diversity necessary for population expansion. Lastly, the lag may simply reflect that fact that exponential growth only becomes obvious once populations get relatively large. A second invasions puzzle is why species that are apparently benign in their native range become so dominant in their exotic range. Classic thinking holds that in the introduced range, the invader escapes the specialist predators or pathogens that control its populations in its native range. Indeed, many exotic plants and animals have fewer specialist predators and pathogens in their exotic versus native range. However, the enemies release hypothesis also requires that consumers actually control the exotic species in its native range, and further, that generalist enemies in the exotic range do not compensate for the loss of specialist predators and pathogens. There is much weaker evidence for these more subtle, but equally important requirements of the hypothesis. Moreover, recent work suggests that generalist enemies often prefer exotic over native plants, counteracting benefits of specialist enemy release. In sum, the long-held hypothesis that enemy release is responsible for the runaway success of exotic species is not nearly as well-supported as commonly believed. Developing alternative hypotheses to explain invader dominance in the exotic range may become a key priority. Generality The explosion of research into biological invasions over the last several decades has generated considerable understanding of the patterns and dynamics of individual invasions. The challenge, however, comes when moving from individual case studies …