Habitat isolation moderates the strength of top-down control in experimental pond food webs.

Habitat isolation is well known to alter patterns of species' abundance, richness, and the ratios of predators : prey. Less clear, however, is how isolation alters interactions within food webs. Here, we present the results from an experiment performed in artificial ponds (mesocosms) manipulating habitat isolation crossed with a predator reduction treatment to disentangle how isolation mediates the top-down effect of predators. The strength of the trophic cascade, from predators, through herbivores, to producers, was considerably stronger in connected than in isolated habitats. We further found that the overall richness of both predator and herbivore species declined strongly with isolation. Experimental predator reductions suggest that the mechanism underlying the herbivore response was likely mediated by a keystone predator effect; when predators were reduced, herbivore richness was lower, and there was no discernable effect of isolation on herbivore richness. Finally, we found that the composition of predators in more isolated habitats consisted of species that were smaller and likely less effective predators than species that persisted in less isolated habitats. In all, our experiment showed that habitat isolation can alter the structure of communities by a combination of direct effects of the species in question, as well as effects mediated through their interactions in the food web.

Effects of three organic wastewater contaminants on American toad, Bufo americanus, tadpoles.

Recent surveys of aquatic habitats suggest that organic wastewater contaminants (OWCs) may be common in aquatic ecosystems. However, relatively little is known about the impacts of OWCs on amphibians. We studied the lethal and sublethal effects of three OWCs (acetaminophen, caffeine, and triclosan) on American toad (Bufo americanus) tadpoles. High concentrations of triclosan increased activity, whereas acetaminophen had a significant effect on activity but there was no discernable pattern or trend with concentration. Caffeine did not affect activity in B. americanus tadpoles. None of the OWCs we studied had a significant effect on growth. Caffeine had no effect on survivorship. Higher concentrations of acetaminophen increased mortality. Intermediate concentrations of triclosan had a negative effect on survivorship, but the highest concentration apparently had a positive effect on survivorship. Our results suggest that there is variation in the toxicity of the three OWCs we studied.