Scaling Temperature Dependence of Disease Dynamics from Individuals to Populations

Abstract

There is growing interest among ecologists to develop mechanistic models that can generate generalizable predictions across multiple levels of biological organization. A key question is whether and how host and parasite contributions to infection can be disentangled, and to what degree these separate contributions affect population level transmission dynamics. The Metabolic Theory of Ecology (MT) provides a framework to investigate physiological processes such as metabolism, reproductive rates, host defenses, and parasite infectivity. The amphibian fungal pathogen <Batrachochytrium dendrobatidis (Bd) is a popular host-parasite system to experimentally investigate temperature dependent infection dynamics. In Chapter 2, I validated methods for decontaminating nitrile and cotton gloves after handling Bd infected Xenopus laevis. In Chapter 3, I conducted two Bd infection experiments to quantify temperature-dependent infection on individual X. laevis and found persistent acclimation effects and increased infection loads on frogs following a drop in temperature, consistent with the Temperature Variability Hypothesis (TVH). In Chapter 4, I quantified MT-based thermal performance curves (TPC) to estimate a key metabolic parameter (activation energy, EA) for 60 amphibian species, allowing me to investigate taxonomic and environmental predictors of species specific EA. Temperate species had wider thermal breadths than tropical species and EA was positively correlated with mean environmental temperature within order Caudata, consistent with the Climate Variability Hypothesis (CVH). To test whether MT-based thermal mismatch models can successfully describe individual infections, I built and parameterized models using independent proxies for host and parasite metabolic performance. These models successfully captured the individual infection dynamics observed in Chapter 3 and provided new insights into the thermal biology of this infection. To investigate population-level transmission dynamics, I conducted a controlled temperature outdoor mesocosm Bd transmission experiment with small populations of X. laevis and tracked infections via weekly skin swabs (Chapter 6). Frog populations at 10 C had dramatically higher infection loads compared to individually housed frogs at the same temperature. In Chapter 7, I developed and fully parameterized an Individual Based Model (IBM) that nested individual level MT-based mismatch models within population level transmission dynamics, successfully predicting the higher infection loads and mortality rates observed in the population level experiment.