Population-level consequences of the interaction between host stage-structure and disease

Multiple life stages are a universal feature of organismal life cycles. Disease impacts life stages differently, and, in some cases, affects only a single stage in a population. Because disease outbreaks are expected to increase worldwide, understanding the population-level consequences of these stage-specific disruptions is vital for predicting future health impacts on natural, agricultural and human systems. Here we use a continuous time, stage-structured population model consisting of two stages (juvenile and adult) to elucidate the effects of stage-specific disease. We explore dynamics for populations that, in the absence of disease, are inherently unstable (i.e., exhibit persistent oscillations due to the existence of a stable limit cycle) due to negative density-dependent feedbacks operating on juvenile maturation. We find that unstable populations are more susceptible to disease invasion than their stable counterparts (i.e., systems exhibiting a non-trivial stable point equilibrium) due to higher average abundance in oscillating populations. Further, disease can stabilize these unstable systems. However, stabilization is not guaranteed- whether it occurs depends on the stage attributes affected and, under some assumptions, can break down for high disease transmission. We report emergence of alternative stable states and hysteresis in many of the scenarios explored. Thus, even simple stage-structured populations may exhibit a wide-range of dynamical behavior in response to disease, with important ecological implications.