Pesticides alter how frogs choose habitats

Chemicals change habitat selection behavior; frogs avoid contaminated sites

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Grey treefrogs with eggs. Image courtesy of Julia Buck/WASU.
Grey treefrogs with eggs. Image courtesy of Julia Buck/WASU.

Pesticide contamination of freshwater ponds may alter the patterns that treefrogs use to choose their breeding sites, according to a new study led by a Virginia Commonwealth University biologist.

The findings show that treefrogs avoid ponds with a history of contamination, suggesting that the effects of pesticides on non-target organisms may be caused by perceived habitat loss or degradation, as well as by any toxic effects.

Amphibians are considered to be important indicators of ecosystem health and their populations are facing global decline. Amphibians are very sensitive to various forms of environmental degradation because they spend time in both aquatic and terrestrial environments during different stages of their life cycles.

“Pesticides are one of a number of factors that may be contributing to the decline of some amphibian populations. As a result, conservation biologists are examining the effects of human chemicals in the environment on amphibians, in particular, agricultural chemicals – herbicides, insecticides and fertilizers,” said James R. Vonesh, Ph.D., an assistant professor of biology at VCU and lead author on the study.

According to Vonesh, most research to date focuses on the direct toxic effects of pesticides, their role as endocrine disrupters, or the interactions between pesticides and natural stressors, like disease or predators. Little attention has been given to the effects of pesticides on habitat selection behavior.

“Our work highlights that the chemicals can have effects that are not necessarily toxic in nature, that contaminants themselves might alter who colonizes in contaminated sites,” he said.

In this study, Vonesh, and his collaborator, Julia Buck, with the Tyson Research Center at Washington University in St. Louis, examined the effects of carbaryl, the active chemical in the widely used commercial insecticide Sevin, on the habitat selection behavior of the gray treefrog through a series of field experiments.

They observed that artificial pools that were uncontaminated with carbaryl received two to three times more eggs than pools treated with commercial Sevin, or technical grade carbaryl. They found this result consistently at different spatial and temporal scales. The study findings were published online in the August issue of the international ecological research journal, Oecologia.

Additionally, the findings suggest that the chemicals could have effects at the landscape-scale by changing the way the animals move among patches, independent of any toxic effects.

“Consider two focal ponds, both uncontaminated, one adjacent to another pristine pond and a second adjacent to a contaminated site,” Vonesh said. “Although our two focal ponds are themselves identical, the number of eggs laid in them – and potentially the number of froglets they produce – may differ if contaminants alter the selection of breeding sites.”

Further, Vonesh said that the focal pool adjacent to the contaminated site may receive proportionally more eggs than the focal site next to the pristine pool if frogs that would have bred in the contaminated site find it unsuitable and redirect their eggs into the remaining uncontaminated site.

“Predicting the summed new froglets produced by each pair of pools depends also on resources limitation, costs of moving between ponds and other such factors. But pesticide effects on habitat selection plays an important, previously overlooked role,” said Vonesh.

This work was supported by grants from the Howard Hughes Medical Institute’s SURF program, Crescent Hills Research Fund and Tyson Research Center postdoctoral fellowship.