| Pumpkinseed sunfish (Lepomis gibbosus) and redear sunfish (L. microlophus), phylogenetic sister-species, are morphologically and behaviorally specialized molluscivores. Except for a small region of natural overlap, their native ranges were allopatric. For purposes of sport fishery enhancement, fisheries managers have introduced redear into lakes of southern Michigan, creating a large zone of artificially sympatric pumpkinseed and redear. Relative to pumpkinseeds, redear display greater specialization than pumpkinseed and were predicted to be better molluscivores and competitively dominant.; I utilize the large scale introduction of redear sunfish into lakes, native to pumpkinseeds, to examine the impacts of this introduced species and also to study the mechanisms of competition between these two size-structured specialists. I performed across lake comparisons, a target-neighbor pond experiment and laboratory feeding trials that highlighted the functional linkages between feeding performance, resource use and competitive abilities.; Redear were found to possess crushing strength that was approximately two times greater than that of pumpkinseed. As a result, redear shifted to a diet of snails significantly earlier in their ontogeny and they were substantially more molluscivorous than sympatric pumpkinseeds.; Several patterns were evident in the field and experimental data that support the inference of pumpkinseed and redear competition (1) Multiple lake surveys showed that following redear introduction, adult pumpkinseed densities declined by approximately 80%; (2) In Michigan lakes without co-occurring redear, adult pumpkinseed diets were dominated by snails ({dollar}>{dollar}80%). Whereas, in lakes with redear, pumpkinseed diets were dominated by soft-bodied invertebrates, and snails contributed only about 30% to their prey biomass; and (3) Snail biomass tended to be lower in lakes with introduced redear and in experimental treatments with redear neighbors. This suggests that redear may reduce resources for pumpkinseeds and drive their reduced molluscivory.; Pumpkinseed growth rates, however, did not differ between lakes with and without redear, yet short-term results from the pond experiment suggest that redear competition limited pumpkinseed growth. A verbal model is proposed that suggests density reductions of pumpkinseed as a compensating mechanism allowing unaffected adult pumpkinseed growth rates in the field. |
