COS 243-1 - Thermal fluctuations increase disease transmission of a fungal parasite in a zooplankton host at the individual and population scale

Abstract: Thermally fluctuating environments are ubiquitous, and most organisms exhibit many nonlinear responses to temperature. Under these conditions, theory predicts that biological processes, rates, and interactions will all differ compared to those at constant temperatures. Though previous studies have quantified the impacts of thermal fluctuations on organismal level processes, few have considered how different magnitudes of fluctuations alter species interactions (e.g. host-parasite interactions) or biological processes across scales. We used a Daphnia host - fungal parasite system to 1) quantify how different magnitudes of fluctuating temperature (+/- 0, 1, 3, or 7°C around a mean of 20°C) affected traits of host and parasites at the organismal scale (e.g. host birth rate, transmission rate, and spore yield (number of parasite spores per infected host)), 2) evaluate whether thermal performance curves generated from a gradient of constant temperatures (15, 20, and 25°C) predicted these responses, and 3) test whether effects of temperature fluctuation at the organismal scale predicted effects of temperature fluctuations at the population scale (i.e., size of epidemics in a multi-generational mesocosm experiment).



We found that all traits at the organismal level (host birth rate, transmission rate, and spore yield) were elevated under fluctuating temperatures compared to constant 20°C, regardless of the size of fluctuation. These results were qualitatively consistent with theoretical predictions based on traits evaluated at a gradient of constant temperatures (i.e., the thermal performance curves). However, the traits peaked at different magnitudes of fluctuation. Host fitness and spore yield were greatest under moderate magnitudes of temperature fluctuation (+/- 1 and 3°C) , whereas transmission rate was highest under the largest magnitude of fluctuation (+/- 7°C). At the population level, we observed larger epidemics in terms of mean prevalence of infection under fluctuating temperatures (+/- 3°C) than at a constant mean temperature (20°C), but did not observe significantly different epidemics as measured by the total number of infected hosts over the course of the epidemics. Our results suggest that thermal fluctuations can alter disease dynamics at both organismal and population scales, and that some of these responses are predictable based on existing theory.