It is plausible that introduced species in the beginning of an invasion would be subjected to selection pressure to disperse rapidly so as to access unexploited habitats, but be subjected to selection pressure for more sedentary behavior once the available habitat was fully colonized. Using a mix of previously-published and new data from the introduced Brown Treesnake (Boiga irregularis) population on Guam we evaluated this scenario and found that: 1) Brown Treesnakes accessed a food bonanza when colonizing Guam, and then depleted it; 2) during the irruption, high-dispersing Brown Treesnakes most-likely benefitted by accessing undepleted or less-depleted food areas; and 3) after the irruption, high-dispersing Brown Treesnakes probably wasted effort through superfluous movement, and Brown Treesnake activity areas on Guam decreased in size in the 1990s, possibly in response to selection pressure for reducing superfluous movement. Nonetheless, 4) confined Brown Treesnakes had better fitness measures in 2004–2006 than snakes free to disperse, suggesting that selection pressure to reduce movements was ongoing. We conclude that selection in the post-irruption period promoted reduced dispersal and we observed appropriate change over generations, but we do not know the extent to which the observed changes reflect genotypic or phenotypic responses.
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Vol. 3 • No. 2