Summary

Description

Amphibian abnormalities and diseases are not well understood, and appear to be increasing while global populations decline. OBJECTIVES: The goals of this study were to identify stressors associated with amphibian abnormalities on the Kenai Refuge and assess whether anthropogenic factors contributed to these abnormalities. METHODS: Between 2004 and 2006, we assessed 38 breeding sites for prevalence of abnormal wood frogs. We chose 21 ponds for more intensive study, measuring the following variables known to cause abnormalities in amphibians: UVB, temperature, basic water quality, contaminants, and abundance of predatory invertebrates. On a subset of frogs, we assessed gonadal structure, DNA integrity, and biomarkers of genetic damage, and identified and enumerated parasites. We analyzed field data with logistic regression, using AIC to compare competing models. RESULTS: Of 5,716 metamorphic wood frogs examined, 450 7.9 had skeletal or eye abnormalities. We documented 558 abnormalities in these 450 abnormal frogs because frogs often had more than one abnormality. Over 25 abnormality types were seen. The four most common were micromelia small limb, ectromelia truncated limb, amelia no limb, and unpigmented iris. We found evidence for two diseases of conservation concern, Batrachochytrium dendrobatidis, a fungal pathogen responsible for global amphibian population declines, and an undescribed protozoan, quite virulent in Kenai study populations. We also observed intersex frogs, 41 of 163 frogs 25 examined had abnormal gonadal morphology. None of the 448 frogs assessed for parasites were infected with the abnormalityinducing trematode, Ribeiroia ondatrae. We quantified predatory invertebrates in study sites, including dragonfly larvae, damselfly larvae, water beetles, leeches, and spiders. Organic and inorganic contaminants exceeded toxic thresholds in study site sediments. PCBs were found in every pond, and DDT was higher than toxic thresholds in four sites. Arsenic, iron, selenium, cadmium, copper, and nickel were all higher than toxic thresholds in sediments, and barium, iron, cadmium, and copper surpassed thresholds in water. In statistical analyses, we identified dragonflies, toxic metals, and temperature as predictors for skeletal abnormalities and malformations. Metals that correlated with skeletal abnormalities included arsenic, cadmium, copper, and selenium in sediment and barium, iron, potassium, and lead in water. Environmental factors predictive of disease were temperature, acidity, metals, and total dissolved solids. Controlled experiments showed toxicity but not teratogenicity from abiotic site media. DISCUSSION: We propose the ultimate cause of skeletal abnormalities in Kenai wood frogs is amputation injury, probably by dragonfly larvae. The significant effects of metals and temperature in our statistical analyses suggest one or both of these factors may be disrupting the normal predatorprey relationship between dragonflies and wood frogs. Contaminants in sediment may slow development or interfere with normal predator detection and avoidance strategies. Warmer temperatures may increase abundance of dragonfly larvae or change the timing of dragonfly presence relative to tadpole growth. Higher temperatures and poor water quality were positively associated with disease. Two initial hypotheses for the intersex condition are high temperatures and PCBs, both previously shown to cause endocrine disruption in amphibians. Anthropogenic disruption of climate and consequent high temperatures appear linked to three of the four abnormality types we documented. These temperature effects may be particularly significant in the face of further predicted global change.