No abstract

While the importance of competition in adaptive divergence has recently been demonstrated in a number of systems, its interaction with other ecological factors has been neglected. We attempt a more pluralistic approach in this comparative study of divergence in pumpkinseed sunfish (Lepomis gibbosus) from the Adirondack region of New York. In some lakes, pumpkinseed forms that use shallow‐ and open‐water environments coexist. We tested four predictions relating to an hypothesis of character release: (1) Pumpkinseed divergence is favored in the absence of the closely related bluegill sunfish (L. macrochirus), a known competitor elsewhere, (2) divergence is favored when the combined effects of other planktivorous fish taxa that may impose diffuse competition is reduced, (3) divergence is positively related to lake size because larger lakes provide more distinct and diverse environments, and (4) divergence is favored in lakes dominated by open‐water habitat. In two lakes with introduced bluegill (sympatry), bluegill appear to limit pumpkinseed divergence, as they do elsewhere. Pumpkinseeds were neither found in the open‐water habitat, nor did they eat significant amounts of open‐water prey. In nine allopatric lakes selected for their similarity to the sympatric lakes, pumpkinseeds were ecologically and morphologically divergent, consistent with a character release hypothesis. Morphological divergence followed functional expectations. Open‐water pumpkinseeds generally had longer caudal peduncles, shorter heads, thinner bodies, and smaller paired fins compared to shallow‐water pumpkinseeds (heritable variation in body shape has been demonstrated elsewhere). Among 26 allopatric lakes, other planktivores had a negative effect on intrapopulation divergence as if they collectively imposed diffuse competition. Surprisingly, lake size was unrelated to divergence, and the amount of open‐water habitat had an unexpected negative effect. Mean body size was also negatively related to divergence. Our findings illustrate that a variety of ecological factors, in addition to competition, may influence the divergence of pumpkinseeds, and possibly other fishes in postglacial north temperate lakes.

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We compared the tolerance of three common Ontario crayfish species to low pH under natural and laboratory conditions in water varying in aluminum concentration and hardness. Both transplant and laboratory experiments indicated that exposure to a pH range of 5.4–6.1 in soft water was toxic to attached juvenile stages of Orconectes rusticus and O. propinquus but not to females carrying the broods. In contrast, stage III juveniles of Cambarus robustus molted and survived in soft water at pH 4. Cambarus robustus is clearly far less sensitive to low pH stress than the two Orconectes species. No increased mortality due to the presence of elevated aluminum occurred among stage III juveniles of the three species exposed to pH 4.5–5.0 in soft water. Physiological differences, augmented by differences in life cycle, may account for the continued presence of C. robustus in acid-stressed lakes and streams.