The complexity of a parasite’s life-cycle, characterized by the number of obligate intermediate hosts and/or free-living developmental stages, has been hypothesized to be a key determinant of a parasite’s sensitivity to climate change, but a clear picture has yet to emerge. Probabilistic arguments suggest that parasite extinction probability increases with life-cycle complexity due to an increasing chance of losing at least one obligate host with environmental change. Behavioral considerations, by contrast, suggest that the behavioral thermoregulation of ectotherm intermediate hosts may confer an advantage to complex life-cycles by sheltering parasites from thermal extremes. Moreover, it even remains unclear how life-cycle complexity affects the temperature-sensitivity of parasite fitness: Whether fitness increases or decreases with temperature depends on the relative strengths of positive and negative temperature effects on different life-cycle transitions (e.g. faster development vs decreased survival). This net effect is readily calculated for simpler life-cycles, but additional life stages and transitions, along with potentially differing temperature-sensitivities between life stages, quickly increase the number of interacting temperature-dependencies and possible outcomes for complex life-cycles. Given these complexities, should we expect systematic relationships between life-cycle complexity, the net balance of positive and negative temperature impacts on different life-cycle transitions, and consequent climate change impacts?
Results/Conclusions
We developed a series of life-cycle models for host-parasite dynamics of increasing complexity, and parameterized the temperature-sensitivities of survival, development, reproduction, infection, and parasite impacts on hosts using the Metabolic Theory of Ecology. We show that, regardless of complexity, the temperature of peak parasite performance can be predicted by a “balance equation” that amalgamates the temperature-sensitivities of all host and parasite life history traits into a single, temperature-dependent measure of parasite fitness. The balance equation also shows (i) that the thermal sensitivity of parasite performance depends on the relative strengths of positive and negative temperature influences on different life-cycle components (ii) elucidates which life stages have the largest influence on a parasite’s thermal sensitivity, and (iii) whether overall parasite fitness is likely to decrease or increase in a warming climate. Predicted patterns depend on life-cycle structure, and specifically the number of intermediate hosts, the presence/absence of free-living parasite stages, transmission mode, the body masses of hosts and parasites, whether parasites live in or on hosts, and on whether hosts are endotherms or ectotherms. Our balance equation suggests systematic relationships between these variables and parasite sensitivity to climate change, and thus allows climate change impact predictions even for understudied species.