Photo by John Hayes

Seminar summary: Climate change drives infectious disease across spatial and temporal scales

By: Daniel Evans, Vratika Chaudhary, and Marta Prat Guitart

Dr. Jeremy Cohen is a post-doctoral researcher at the University of South Florida whose research focuses on climate change, spatial and temporal scale, and temperature related to infectious disease. Climate change impacts wildlife through geographic range shifts, phenological shifts (such as the timing of migration), loss of coastal habitat and associated species, changes in body size, and disease (increased susceptibility). Dr. Cohen expressed a need to use cross scale global predictions to asses these changes, and emphasized that it is important to consider the impacts at different scales. Scale can influence perceived species interactions and scale can change the factors identified as influencing community assemblies. Most studies either ignore scale or only focus on a single scale. One part of Dr. Cohen's dissertation research was focused on how both spatial and temporal scales are important.

Distribution of a species is affected by three primary factors; biotic, abiotic, and humans. In ecology, large spatial or temporal scales tend to focus on environmental affects (abiotic), while small spatial or temporal scales focus on species interaction affects (biotic). Dispersal and population of humans tends to matter across intermediate and large scales. It is possible that the scale of a study is at an intermediate level and catches the influence of all three factors. Dr. Cohen noted that there are few comprehensive datasets of multiscale analysis. To address the question of scale, three diseases found in the U.S. were considered. Chytrid disease in amphibians was used to assess the biotic factor, West Nile Virus in birds was used for the abiotic factor, and Lyme disease in mammals was used for the human dispersal factor. Seven different spatial scales were considered. The results suggested as you move from small to large scale the relative importance of factors moves from biotic to abiotic, with an intermediate effect not being seen. The take away message was that "common single-scale analyses can misrepresent the true impact of anthropogenic modifications on biodiversity and the environment" (Coehn et al. 2016).

Dr. Cohen then discussed his work on climate change, temperature, and diseases. Species that are faster at acclimation /adaptation to changes in temperature were able to perform better under a wider temperature range than species that were slow to acclimate. Overall, smaller body species tend to be able to acclimate better than larger species. Parasites have an advantage, being able to quickly acclimate, allowing them to preform when conditions are variable and under a greater breadth of temperature and conditions. Dr. Cohen then introduced the Thermal mismatch hypothesis that if parasites are adapting to new conditions faster than their host species, hosts should be more susceptible to disease under conditions far from conditions they normally experience. The conditions that influence the susceptibility of amphibians to Chytrid disease varied greatly in the literature, from increased disease in cold conditions to increased disease in warm conditions, as well as wet vs dry seasonal difference, as well as difference with what is happening in lab isolation.

Dr. Cohen and collogues set up a series of experiments to study susceptibility to the fungal pathogen Batrachochytrium dendrobatidis (Bd) in three species of amphibians; Atelopus zeteki (golden frog), southern toad, and Cuban tree frog, under different temperatures. The three amphibian species were exposed to a temperature gradient to determine each species' optimal conditions. Cuban tree frogs and southern toads were warm adapted species, while Atelopus preferred a cooler temperature. Animals were acclimated to their specific temperatures, and then exposed to Bd either under opposite temperature conditions or the same temperature conditions. Growth of Bd was related to host species and temperature, with highest growth of Bd on species adapted to warm temperatures occurring at cold temperatures, and highest growth on the cold adapted species occurring at warmer temperatures. These results were compared to the literature on Bd research in wild populations between 1955 and 2005 that included swabbed samples. Both mean temperature and rainfall were compiled for each study site. The results from the literature matched the lab results; populations adapted to cool regions were likely to have disease outbreaks at warmer temperatures than warm adapted species. Additionally, populations of Atelopus that went extinct in the wild experienced a change in temperature just prior to a Bd outbreak. Extant populations had not experienced changes in temperature.

Through his research, Dr. Cohen was able to demonstrate how climate change can facilitate Bd outbreaks under certain conditions, the contribution of temperature variability on Atelopus declines, and that the thermal mismatch hypotheses can explain why some species are more susceptible to Bd caused extinction.

Cohen, J., Civitello, D., Brace, A., Feichtinger, E., Ortega, N., Richardson, J., Sauer, E., Rohr, J. 2016. Spatial scale modulates the strength of ecological processes driving disease distributions. Proceedings of the National Academy of Sciences, 113, E3359-E3364