BOULDING, E.G.; Univ. of Guelph, Ontario: A Field Experiment Simulating an Invasion of Predatory Crabs.

Wild populations have only a limited ability to adapt to sudden biotic changes in their environment – many populations have gone extinct after the invasion of an exotic predator or competitor. I have investigated the theoretical conditions under which rapid genetic adaptation to environmental change may prevent such extinctions. Among the factors influencing the outcome is the amount of gene flow from surrounding predator-free populations relative to the intensity of selection caused by the invading predators. I created an experimental predator invasion by building crack refuges for predatory shore crabs (Hemigrapsus nudus) near Bamfield, B.C. (48�49’N; 124�41’W) in 1993. Crabs inhabiting the refuges behaved as central place foragers on local Littorina spp. which resulted in strong selection for increased shell thickness among gastropod prey living near the refuges but little selection for shell thickness among prey more than three meters away. I estimated migration rates between populations of a direct-developing gastropod species (Littorina subrotundata) near the refuges and those a few metres farther away. Migration rates were high (&sigma = 2.4 – 5.8 m per year from tagging and Fst = 0.001 from microsatellite markers). The extremely localized scale of selection and the high migration rates may explain why rapid evolutionary change is not observed more often in wild populations of Littorina. I conclude that such localized biotic changes may be more likely to cause extinction than more widespread changes that affect neighboring populations as well.