Shifting thermal regimes influence competitive feeding and aggression dynamics of brook trout (Salvelinus fontinalis) and creek chub (Semotilus atromaculatus).

The natural distributions of freshwater fish species are limited by their thermal tolerances via physiological constraints and increased interspecific competition as temperatures shift toward the thermal optima of other syntopic species. Species may mediate stress from temperature change physiologically, behaviorally, or both; but these changes may compromise competitive advantages through effects on feeding and social behavior. In the Appalachian Mountains of North America, creek chub (Semotilus atromaculatus) are found in warm-water and cold-water streams and overlap in range with brook trout (Salvelinus fontinalis) across lower thermal maxima, where they compete for food and space. As stream temperatures continue to increase due to climate change, brook trout are under increasing thermal stress which may negatively affect their ability to compete with creek chub. To examine the influences of temperature on competitive interactions between these species, we observed feeding behavior, aggression, and habitat use differences at three temperatures approaching brook trout thermal maxima (18°C, 20°C, and 22°C) among dyad pairs for all combinations of species in experimental flow-through tanks. We also examined feeding and habitat use of both species under solitary conditions. We found as temperature increased, feeding and aggression of brook trout were significantly reduced in the presence of creek chub. Creek chub pairs were more likely to occupy benthic areas and refugia while brook trout pairs used surface water more. Space use patterns significantly changed by pairing treatment. Aggression and space use shifts allowed increased exploitative and interference competition from creek chub when paired with brook trout that was not present in conspecific pairs. The decreased dominance of a top predator may lead to diverse impacts on stream community dynamics with implications for the future range restriction of brook trout and demonstrate possible mechanisms to facilitate competitive advantages of warm water generalist species under thermal stress.

Habitat light and dewlap color diversity in four species of Puerto Rican anoline lizards.

Closely related species often have signals that differ dramatically in design. The evolution of such differences may be important in the process of speciation. Selection for signal detectability under different habitat conditions has been proposed as a mechanism leading to the evolution of signal diversity. We examined dewlap color in four closely related species of Anolis lizards that occupy habitats with different light conditions. Initially, we tested the hypothesis that lizards choose specific light conditions within each habitat in which to signal. We rejected this hypothesis for all four species. We next calculated the detectability of the dewlap color of all four species at display locations in each habitat. If selection for detectability under the different light conditions explained the divergence in signal design, the occupant of a given habitat was predicted to have the highest signal detectability in that habitat. However, the rank order of detectability of the four dewlap colors was nearly the same in all four habitats. We concluded that divergent selection for signal detectability does not, by itself, explain the evolution of dewlap color diversity. We hypothesize that the evolution of dewlap color diversity results from simultaneous selection for multiple functions of dewlap color.

Field evidence of an airborne enemy-avoidance kairomone in wolf spiders.

Hogna helluo, Pardosa milvina, and Trochosa ruricola are co-occurring species of wolf spiders within agricultural fields in the eastern USA. The largest species, H. helluo, is a common predator of the two smaller species, P. milvina and T. ruricola. H. helluo frequently resides within soil fissures where P. milvina and T. ruricola may be attacked when they enter or walk near these fissures. We tested the ability of P. milvina and T. ruricola to avoid H. helluo-containing burrows by detecting airborne enemy-avoidance kairomones associated with H. helluo. To simulate soil fissures and control for visual and vibratory means of predator detection, we baited funneled pitfall traps with one of the following (N = 20 traps/treatment): (1) blank (empty trap); (2) one house cricket (Acheta domesticus); (3) one adult female H. helluo; and (4) one adult male H. helluo. Over two separate 3-d periods, we measured pitfall capture rates of P. milvina and T. ruricola as well as other incidentally captured ground-dwelling arthropods. During the day, male P. milvina showed significant avoidance of pitfall traps baited with H. helluo of either sex but showed no avoidance of empty traps or those containing crickets. At night, male T. ruricola showed a qualitatively similar pattern of avoiding H. helluo-baited traps, but the differences were not statistically significant. We found no evidence that other ground-dwelling arthropods either avoided or were attracted to H. helluo-baited traps. This study suggests that an airborne enemy-avoidance kairomone may mediate behavior among male P. milvina in the field.