Climate change and shrinking salamanders: Alternative mechanisms for changes in plethodontid salamander body size

Abstract: An increasing number of studies have demonstrated correlations between climate trends and body size change of organisms. In many cases, climate might be expected to influence body size by altering thermoregulation, energetics or food availability. However, observed body size changes can result from ecological processes (e.g., growth, selection, population dynamics), yet may also be due to imperfect observation. We used two extensive datasets to evaluate alternative hypotheses for recently reported changes in the observed body size of plethodontid salamanders. We found that mean adult body size of salamanders can be highly sensitive to survey conditions, particularly rainfall, such that smaller individuals are more likely to be sampled under dry conditions. This systematic bias in the detection of individuals across a range in body size would result in a signature of body size reduction in relation to reported climate trends when it is simply observation error. We also identify considerable variability in body size distributions among years that shows correspondence with rainfall. This suggests that annual variation in growth or shifting population age structure could also result in a correlation between climate and mean body size. Finally, our study demonstrates that measures of mean adult body size can be highly variable among surveys and that large sample sizes may be required to make reliable inferences. Identifying the effects of climate change is critical for future research in ecology and conservation. Researchers should be aware that observed body size changes in some organisms may be a result of true ecological processes or systematic bias due to non-random sampling of populations. Ultimately, the credibility of ecological research related to climate change depends on researchers demonstrating a thorough consideration of alternative hypotheses for observed changes in species.