Dynamic mean and variance of microparasite load give key insights into population dynamics and underlying mechanisms

Heterogeneity among individuals, in number of parasites, body size, etc., can have critical interactions with population dynamics. We tease these out with a relatively simple model of such interactions in a case study of microparasite load with empirically supported assumptions to ask a key question. How does variance in microparasite load interact with population-level dynamics? We show how the mean and variance of infection load vary throughout an epidemic. Further, we show how mean and variance have mutual negative feedbacks on each other mediated by high disease-induced death rates at high loads. Helpfully, we find that mean and variance provide information into underlying processes as well. Population-level trends in the mean and variance reveal underlying trends in within-host processes, e.g., differentiating host evolution of defence that manifests as tolerance, constitutive resistance, inducible resistance, or acquired resistance. Such inference may be critical for managing endangered Mountain Yellow Legged Frog populations recovering from Batrachochytrium dendrobatidis epidemics. Lastly, we demonstrate the impact of pathogen load variance on host fitness, pathogen fitness, and host population depression by pathogens. Our results demonstrate the importance of trait heterogeneity and the insights available from relatively simple models, both for microparasite infection load and possibly other traits.