Global declines in amphibians likely have multiple causes, including widespread pesticide use. Our knowledge of pesticide effects on amphibians is largely limited to short-term (4-d) toxicity tests conducted under highly artificial conditions to determine lethal concentrations (LC50). We found that if we used slightly longer exposure times (10 -16 d), low concentrations of the pesticide carbaryl (3-4% of LC50 4-d) killed 10 -60% of gray treefrog (Hyla versicolor) tadpoles. If predatory cues also were present, the pesticide became 2-4 times more lethal, killing 60 -98% of tadpoles. Thus, under more realistic conditions of increased exposure times and predatory stress, current application rates for carbaryl can potentially devastate gray treefrog populations. Further, because predator-induced stress is ubiquitous in animals and carbaryl's mode of action is common to many pesticides, these negative impacts may be widespread in nature.

Pesticides are widely used by humans to eliminate or reduce populations of unwanted species. These pesticides often cause collateral damage by killing nontarget species and altering biological communities. Our study examined the relative importance of direct and indirect effects of the insecticide carbaryl on southern leopard frog tadpoles, Rana sphenocephala, in a simple aquatic community. We did not detect any significant direct effects of carbaryl on the anuran life‐history traits examined in this study. Rather, environmentally relevant concentrations of carbaryl indirectly affected R. sphenocephala life‐history traits by causing changes in the intensity of competition and predation within the community. Carbaryl generally increased the survival of R. sphenocephala tadpoles. However, the increase in survival was greatest in mesocosms containing predators, largely due to pesticide‐induced mortality of the predators. Carbaryl also exacerbated the effects of competition by decreasing periphyton abundance, which resulted in smaller metamorphs. Thus, we conclude that investigations of the effects of pesticides on a species of interest should go beyond standard toxicological protocols and examine the effects of the pesticide within the context of the community, placing special emphasis on understanding how competition and predation mediate indirect effects.

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Oxygen available to amphibian embryos fluctuates widely and is often very low. We investigated the effects of oxygen partial pressure (1. 3-16.9 kPa) on embryonic development and hatching of two salamander (Ambystoma) and two frog (Rana) species. In Ambystoma, chronic hypoxia resulted in slowed development, delayed hatching, and embryos that were less developed at the time of hatching. Although hypoxia was not lethal to embryos, temporary developmental abnormalities were observed in Ambystoma at oxygen partial pressures of 3.8 kPa and below. Posthatching survival decreased below 3.3 kPa. In Rana, hypoxia did not affect developmental rate, presumably because hatching occurs at a very early stage of development relative to Ambystoma. However, Rana embryos hatched sooner in hypoxia than in normoxia, resulting in less developed embryos at the time of hatching. The results suggest that embryonic hypoxia may negatively affect survival and fitness in these species.