COS 187-2 - Experimental evaluation of extreme heat events on Pacific chorus frog (Pseudacris regilla) behavior, microbiome, growth, and development.

Abstract: Northwestern North America experienced an extreme heatwave event in the summer of 2021 which exposed the region to scorching temperatures for a period of five days. Analysis of these heat dome impacts have centered on human mortality, agricultural, and economic tolls, with little attention paid to freshwater ecosystems and species of conservation concern. Freshwater populations likely experienced increased mortality rates and sub-lethal effects, such as altered behavior, growth, and development. Our study explored amphibian responses to a heat dome simulation under chronic temperature conditions representing current and predicted future climate scenarios. We posited that tadpoles experiencing chronic temperature stress (predicted future climate regime) would be primed to tolerate extreme temperature spikes relative to tadpoles reared under current climate conditions. We experimentally assessed Pacific chorus frog (Pseudacris regilla) responses over ontogeny, quantifying survivorship, tadpole behavior and microbiome diversity, metamorphic timing, and metamorphic body size. Survivorship did not vary across treatments, but tadpole activity rates were reduced under heat dome conditions. Microbiome analysis detected a decrease in skin and gut microbial diversity with increasing temperature stress, with the highest diversity detected in tadpoles held under current climate conditions without heat dome temperature exposure. Time till metamorphosis, body size, and weight decreased in individuals exposed to future mean climate regimes. Interestingly, exposure to heat dome and future climate temperatures resulted in metamorphic individuals taking a longer time to develop relative to individuals only exposed to future conditions. The combination of the two stressors likely exceeded a tolerance threshold, slowing development and extending the larval life stage without the associated benefit of larger body sizes. Our results indicate that Pacific chorus frogs were not primed for extreme temperature variability by chronic exposure to future temperature conditions. Rather, temperature extremes interacted with climate, producing larvae that were unable to metamorphose early when stressed. This study emphasizes the need to assess multiple endpoints, as tadpole development coupled with reduced microbiome diversity may result in shifts in disease resistance and population dynamics. Further, the ecological role of Pacific chorus frogs in freshwater ecosystems will depend on interactions between mean and extreme temperature conditions. This work will become increasingly important as climate change shifts seasonal temperatures and extreme heating events become more common across the Northwestern US region.