COS 266-5 - Increased disease risk among populations at range edges after extreme temperature events

Abstract: Populations of hosts and parasites at the core and periphery of their geographic ranges can often experience very different weather and historical climates that can drive present-day adaptations to environmental conditions. A common ecological assumption is that densities of species decline towards their range limits (‘center–periphery’ hypothesis [CPH]), and as a consequence, we hypothesize that density-dependent parasite transmission might also decline. To test this hypothesis, we compiled: i) a spatiotemporal dataset of infectious disease prevalences for thousands of wild animal populations and ii) the geographic range of each host species in the disease dataset. Consistent with our hypothesis, prevalence decreased with latitudinal distance from the geographic core across all categories of hosts and for protozoal, viral, bacterial, fungal, and arthropod parasites, but not for helminth parasites. To develop predictions for temperature-dependent changes in disease risk from the core to the periphery of hosts’ ranges, we annotated each surveyed population in our disease dataset with historical climate and contemporaneous weather data. In addition, we borrowed concepts from the well-supported thermal mismatch hypothesis (TMH), which proposes that ectothermic hosts adapted to cooler climates should be at the greatest risk of infection under abnormally warm conditions, and vice versa for warm-adapted hosts. Given that species tend to be better adapted to conditions at their range core and are often living on the margins of their abiotic tolerance at range edges, extreme events at edges are more likely to push them outside their tolerance limits; thus, we hypothesized that the risk of infection should increase from the core to the periphery in cold-adapted hosts exposed to abnormally warm temperatures and for warm-adapted hosts exposed to abnormally cold temperatures. This is precisely the pattern we found for ectothermic cold-adapted hosts but not for ectothermic warm-adapted hosts. In the case of endothermic hosts, the risk of infection increased from the core to the periphery in warm-adapted exposed to cool temperatures and decreased from the core to the periphery in cold-adapted exposed to abnormally warm temperatures. As climate change intensifies and extreme weather events become more common, understanding the mechanisms driving host-parasite interactions at the core and periphery of host ranges becomes increasingly essential to predict and mitigate disease outbreaks, emergence, and range expansions.