The Interactions of Temperature Change and Contaminant Toxicity in Ambystoma barbouri

Amphibians are especially vulnerable to the effects of climate change because they are ectothermic and because of their permeable skin allows for toxins to enter their body easily. Despite these characteristics, their mobility on land lets them escape from toxins in water for periods of time, but while in their embryonic and larval stages, amphibians have limited mobility and cannot escape from the contaminants in their water environments. To help prevent prolonged exposure to contaminants in these stages, development of amphibians can be accelerated by increasing temperature. Rohr et al. (2011) used embryonic and larval forms of the salamander Ambystoma barbouri to test the combined effects of the contaminant atrazine and temperature on hatching, metamorphosis, growth, and survival. They found that increases in concentration of atrazine impeded hatching and metamorphosis and decreased growth and survival. Increases in temperature resulted in more growth and faster development, but also lower survival rates in embryos. Instead of contributing to the negative effects of atrazine, temperature helped the salamanders combat the effects of atrazine by decreasing the time spent exposed to the contaminant. —Isabelle Heilman
Rohr, J., Sesterhenn, T., Stieha, C., 2011. Will climate change reduce the effects of a pesticide exposure of amphibians?: partitioning the effects on exposure and susceptibility to contaminants. Global Change Biology 17, 657–666.

           
            Usually only the negative effects of rising temperature are considered when talking about climate change. Rising temperatures can accelerate the growth rates of organisms, reducing the amount of time they spend in their vulnerable embryonic and larval stages. More developed organisms have a greater chance of survival because of their improved biological processes. For amphibians, this means that they have the ability to leave toxic environments and reduce their exposure to damaging chemicals. Using the embryonic and larval forms of the salamander Ambystoma barbouri, Rohr et al. observed the effects of temperature and the toxin atrazine. The researchers hypothesized that rises in temperature would raise both the toxicity of atrazine and the developmental speed of the salamanders, which would lower the total amount of time the salamanders were exposed to atrazine.
            The study measured the effects of temperature and atrazine on hatching, metamorphosis, growth, and survival separately for embryonic and larval stages. For each experiment, 12 randomly chosen A. barbouri were put into a glass bowl, for a total of 64 bowls. These bowls were controlled for light and water conditions. Four solutions of atrazine were prepared in 0, 4, 40 and 400 μg L-1 concentrations, to simulate the range of atrazine concentrations found in nature. Each bowl was treated with one solution; however after the first metamorphosis in the larval experiment, 8 bowls in each temperature were treated with an acetone control solution. For the embryonic experiment, the bowls were divided into two groups, one at 13ºC and another at 19ºC.  The larval experiment covered a greater range of temperatures, separating the bowls into four groups, at 16ºC, 19ºC, 22ºC, and 25ºC. The A. barbouri were observed until all of the embryos had hatched or died (49 days) and all of the larvae had metamorphosed or died (78 days). Those salamanders still alive at the end of the experiment were euthanized, preserved, and weighed.
            The data from the experiment were statistically analyzed after a log transformation, which allowed for a better statistical fit. Generalized linear model tests were used to analyze the effects of atrazine and temperature in the embryonic experiment, while regression-based tests were used for the larval experiment because of the lack of a duplication of each temperature and concentration treatment. Results from the embryonic statistical tests showed that although the embryos hatched sooner and were heavier at the 19ºC treatments, they were also more likely to die at this temperature than at 13ºC. Atrazine negatively affected hatching time and embryonic survival. All four concentrations increased embryonic mortality rates, but only the highest concentration of atrazine significantly affected hatching time.  In the larval experiment, atrazine concentration was found to negatively predict larval survival and also decrease larval mass. Temperature was positively correlated with developmental rate. Interestingly, the effect of atrazine on survival rate did not depend on temperature or amount of time exposed to the contaminant. Rising temperature was not found to increase the toxicity of atrazine.
            Although in this experiment temperature had a positive effect on development and lowered the amount of exposure to atrazine, the results of this experiment should not be interpreted to mean that rising temperatures provide an overall benefit for salamanders. The authors point out that global climate change not only affects temperature, but also precipitation, which is another factor to consider for the overall effects of atrazine and other contaminants. To most benefit the A. barbouriand other organisms, the focus should be placed on reducing the contaminants that end up in these habitats in the first place. 

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