Species introductions and their ecological consequences: An example with congereric sunfish

Pumpkinseed (Lepomis gibbosus) and redear sunfish (L. microlophus) are sister species with largely allopatric native ranges. For purposes of sport fishery enhancement, redear have been introduced into lakes of southern Michigan, and as a result, a large zone of artificial sympatry of pumpkinseed and redear has been created. Redear and, to a lesser extent, pumpkinseed are morphologically and behaviorally specialized molluscivores, and we hypothesized that introduced redear would have strong negative effects on pumpkinseed due to competition for snails, which are each species\u27 main adult resource. Specifically, we predicted that redear would reduce snail availability, alter pumpkinseed diet, and reduce pumpkinseed growth and density. To examine these predictions, we surveyed pumpkinseed diets, growth, and densities, as well as snail availability in lakes with and without introduced redear. We also conducted a controlled field experiment (target–neighbor design) in which relative neighbor densities of each species were manipulated and the effects on target individuals of each species were measured. This experiment was designed to explore mechanisms underlying the competitive interactions suggested by the longer‐term field study. Together, the field patterns and short‐term experimental results demonstrate that the introduction of redear negatively affected the native pumpkinseed: (1) In lakes where redear had been introduced, the abundance of pumpkinseed declined an average of 56%, while average pumpkinseed abundance increased 60% in lakes without introduced redear. (2) In the presence of redear, pumpkinseed had fewer snails in their diets and showed only a weak ontogenetic niche shift to feeding on snails. (3) Snail biomass was ∼69% lower in lakes where redear had been introduced, and snail biomass was ∼50% lower in experimental treatments containing redear instead of pumpkinseed. In the experiment, pumpkinseed growth was reduced in the presence of redear. However, contrary to our predictions, pumpkinseed growth rates did not differ between lakes with redear vs. those without redear. We suggest that a reduction in pumpkinseed growth rate is an expected short‐term response to redear introduction, whereas the long‐term response is a reduction in pumpkinseed density