Pond hydroperiod alters the effect of density-dependent processes on larval anurans

Many studies examine how changes (disturbances) to the abiotic environment alter the intensity of species interactions by directly changing the numbers of individuals involved in the interaction but few studies examine how the abiotic environment affects how strongly individuals interact with each other. We conducted an experiment to evaluate how pond hydroperiod affected the ability of two larval anuran species ( Hyla squirella and Bufo terrestris ) to interact with intraspecific and interspecific competitors. Pond hydroperiod altered how an increase in the abundance of intraspecific competitors affected fitness components of B. terrestris but not for H. squirella. Specifically, an increase in intraspecific abundance caused a reduction in B. terrestris survivorship in short hydroperiod ponds but had no effect on fitness components of B. terrestris in long hydroperiod ponds. Interspecific competition did not occur between these species in either short or long hydroperiod ponds. Our study highlights the point that the strength of density-dependent processes operating in the aquatic environment can depend on pond hydroperiod. Consequently, the extent to which processes operating in the aquatic environment contribute to the regulation of population and metapopulation size for amphibians will also likely depend on pond hydroperiod.

Strong response to anuran chemical cues by an extreme dietary specialist, the eastern hog-nosed snake (Heterodon platirhinos)

Squamate reptiles use the lingual–vomeronasal system to identify food using only chemical cues. In lizards, most of which are dietary generalists that consume a wide variety of arthropods and other small animals, correlated evolution has occurred between addition of plants to the diet and responsiveness to plant chemical cues. In snakes, many of which are dietary specialists, several studies have detected differences in response strength among populations that correspond to the importance of prey types in different geographic locations. In one species of Thamnophis Fitzinger, 1843, such variation in responsiveness has been demonstrated to have a genetic basis. We studied tongue-flicking and biting responses to chemical cues from a range of potential prey types by nine ingestively naive hatchlings of the eastern hog-nosed snake ( Heterodon platirhinos Latreille in Sonnini and Latreille, 1801), which is an extreme toad specialist that less frequently eats other anurans. The snakes responded most strongly to chemical cues from the southern toad ( Bufo terrestris (Bonnaterre, 1789)), as indicated by significantly greater tongue-flick rate. Only two individuals bit in response to chemicals cues, both to the toad cues. Elevated tongue-flick rates were also elicited by chemical cues from the green frog ( Rana clamitans Latreille in Sonnini de Manoncourt and Latreille, 1801), but the mean rate for frog cues was less than for toad cues. Responses to other potential prey types did not differ from those of the control stimuli. Our findings are consistent with those of several other investigators in showing close correspondence between the inclusion and importance of dietary items and the intensity of chemosensory investigation in snakes. Studies of diverse dietary specialists are needed to establish the generality of this relationship in snakes and to demonstrate that diet and chemosensory responses to food cues coevolve.