Print PageAmphibian larvae develop in a variety of wetlands where they may be concurrently exposed to multiple biotic and abiotic stressors with the potential to influence their development and growth rates. For example, in agricultural regions wetlands have variable hydroperiods, food resources, predator abundances, and could be exposed to herbicides and nutrient enrichment. Some amphibian species have evolved phenotypically plastic development rates to optimize survival or growth in variable environments. The most well studied amphibian example is the acceleration of larval development in wetlands where desiccation risk (maximising survival) or prolonging development in wetlands where desiccation risk is low (maximising growth). However, many other factors including food resources, temperature, and competitor and predator abundances have also been shown to influence development rate. Anthropogenic factors such as exposure to herbicides and nutrient enrichment can also affect development and growth rates, either through direct effects on the hypothalamus-pituitary axis, or indirectly by impacting natural cues such as increasing or reducing algal food availability, increasing UVB exposure through reduction of macrophytes, or altering predator densities. In 2008 at the Long Term Experimental Wetlands Area (LEWA) in New Brusnwick Canada, we split 24 natural wetlands in half with an impermeable plastic barrier; one side was a treated with herbicide or herbicide and nutrients and the other side was left as an untreated control.
Results/Conclusions
Using a split-wetland design, we first investigated the direct effects of exposure to a widely used glyphosate-based herbicide (RoundUp WeatherMax™) and nutrient enrichment on wood frog (Lithobates sylvaticus) survival, growth and development over two years. We did not detect direct effects of herbicide or nutrient enrichment on any amphibian endpoints. However, concurrent studies in the same wetlands indicated that both herbicide and nutrient exposure had effects on other biotic and abiotic factors; ex. herbicide application resulted in a dramatic reduction in macrophyte cover. These changes undoubtedly altered several environmental cues that have been hypothesized to influence amphibian development rates. If amphibian development is a plastic trait which has evolved to maximise survival or growth in variable environments then observed changes in development and growth rates should be predictable based on the observed changes to ecosystem structure. Here we provide evidence for indirect effects of herbicide and nutrient exposure and estimate the relative importance temperature, hydroperiod, competitors, predators and food resources, on amphibian growth and development in natural wetlands.
