BackgroundSkeletal and eye abnormalities in amphibians are not well understood, and they appear to be increasing while global populations decline. Here, we present the first study of amphibian abnormalities in Alaska.ObjectiveIn this study we investigated the relationship between anthropogenic influences and the probability of skeletal and eye abnormalities in Alaskan wood frogs (Rana sylvatica).MethodsFrom 2000 to 2006, we examined 9,269 metamorphic wood frogs from 86 breeding sites on five National Wildlife Refuges: Arctic, Innoko, Kenai, Tetlin, and Yukon Delta. Using road proximity as a proxy for human development, we tested relationships between skeletal and eye abnormalities and anthropogenic effects. We also examined a subsample of 458 frogs for the trematode parasite Ribeiroia ondatrae, a known cause of amphibian limb abnormalities.ResultsPrevalence of skeletal and eye abnormalities at Alaskan refuges ranged from 1.5% to 7.9% and were as high as 20% at individual breeding sites. Proximity to roads increased the risk of skeletal abnormalities (p = 0.004) but not eye abnormalities. The only significant predictor of eye abnormalities was year sampled (p = 0.006). R. ondatrae was not detected in any Alaskan wood frogs.ConclusionsAbnormality prevalence at road-accessible sites in the Kenai and Tetlin refuges is among the highest reported in the published literature. Proximity to roads is positively correlated with risk of skeletal abnormalities in Alaskan wood frogs.

Abstract. Toxicants may harm predators or prey differentially, hindering clear determination of multiple stressor effects on predation dynamics in polluted aquatic systems. We built on a prior field study in which we demonstrated that a chemical contaminant, copper (Cu) and odonate predators were correlated with more frequent observations of skeletal abnormalities in Alaskan wood frog (Rana sylvatica) tadpoles. Our prior study established a multiple stressor effect linked to an important environmental phenomenon (malformed amphibians) but did not provide clear mechanisms that might guide management. We here investigated behavioral mechanisms because of their potential to produce large changes in predation dynamics, and because in published studies low concentrations of Cu produced behavioral changes in predator-detection in fish. Surprisingly, low but environmentally relevant concentrations of Cu (5 lg/L) combined with chemical cues from a predator (Aeshna sitchensis) to produce large changes in the behavior of larval amphibians. Experiments demonstrated that a low concentration of Cu did not inhibit the ability of wood frog tadpoles to detect chemical cues of predators by olfactory means, but produced strong behavioral effects, causing tadpoles to reduce activity and alter microhabitat use. These results occurred with Cu at an exposure level lower than any we could find reported as toxic to amphibians in the literature. When Cu and predator cues were administered together, the activity reduction was additive and stronger at earlier life stages. We suggest that Cu intoxication would be disadvantageous to larval amphibian prey with prolonged exposure to predators during development, and we present field data from 2010 that support this assertion. Our study demonstrates the need to use sensitive behavioral assays and to investigate multiple stressor mechanisms to understand how multiple threats combine to affect organisms in nature.Key words: Alaska; amphibian; copper; frog; Lithobates sylvaticus; multiple stressor; predator; toxicity; trait-mediated interaction; Rana sylvatica.

The repeated occurrence of abnormal amphibians in nature points to ecological imbalance, yet identifying causes of these abnormalities has proved complex. Multiple studies have linked amphibian abnormalities to chemically contaminated areas, but inference about causal mechanisms is lacking. Here we use a high incidence of abnormalities in Alaskan wood frogs to strengthen inference about the mechanism for these abnormalities. We suggest that limb abnormalities are caused by a combination of multiple stressors. Specifically, toxicants lead to increased predation, resulting in more injuries to developing limbs and subsequent developmental malformations. We evaluated a variety of putative causes of frog abnormalities at 21 wetlands on the Kenai National Wildlife Refuge, south‐central Alaska, USA, between 2004 and 2006. Variables investigated were organic and inorganic contaminants, parasite infection, abundance of predatory invertebrates, UVB, and temperature. Logistic regression and model comparison using the Akaike information criterion (AIC) identified dragonflies and both organic and inorganic contaminants as predictors of the frequency of skeletal abnormalities. We suggest that both predators and contaminants alter ecosystem dynamics to increase the frequency of amphibian abnormalities in contaminated habitat. Future experiments should test the causal mechanisms by which toxicants and predators may interact to cause amphibian limb abnormalities.