Two-Stage Life Histories and Species Interactions: Models and Tests with Sunfishes

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In this study a simplified two.life.stage model to examine the interaction between two competing sunfish species and their prey resources. The model can be used to predict patterns of species abundances, size.specific growth rates, and resource densities under the following general conditions: 1) species found in sympatry and allopatry, and 2) species coexisting across gradients in resource productivities. Preliminary field data suggest that the model, and that the observed responses run counter to expectations based on traditional, not.structured models. Comparative surveys among six northern Wisconsin lakes (where the pumpkinseed sunfish is found in sympatry and allopatry with the bluegill) will be used to test a species' predicted response to the presence/absence of its competitor. Comparative surveys among eight southern Michigan lakes will test the predicted responses of two competing species and their resources to variation in resource productivity . A series of experimental studies in Michigan lakes will elucidate the mechanisms driving the observed patterns of species abundance and individual responses. The responses observed at the individual level (e.g. diet shifts and growth patterns) will be integrated with the responses of populations (changes in densities), to provide rigorous tests of the model.