Nationwide distributional determinants for a wildlife and human parasite

Background/Question/Methods

Understanding the factors that constrain species distributions is a long-standing goal of ecology, although many such studies involve only free-living species. Studies of disease occurrence and spread often require broader knowledge of distributional overlap for free-living and parasitic species, emphasizing the importance of determining the distributional constraints on parasites. The flatworm trematode, Alaria spp., is a poorly understood but broadly distributed parasite with wildlife and human health implications found in many aquatic systems. Using a 10-year survey of 624 ponds across the contiguous United States, we evaluated the relative roles of climate, geology, and land cover for Alaria spp. occurrence using species distribution modeling (Maxent). We also conducted a step-wise parameterization of Maxent and a sampling bias control method, which may be useful for improving the functionality of the widely used but rapidly evolving Maxent application.

Results/Conclusions

We identified the best fitting Alaria spp. model (Mean Test AUC: 0.829 ± 0.070 SD; average of 10 ensemble models) from 28 total models as one comprised of nine variables including climate, geology, and land cover. Bootstrapping with 20 replicates was found to be the best Maxent method because it maximized mean Test AUC score and decreased mean variance. Geology (i.e. rock type) was the most important variable explaining nearly 25% of the model variance followed by precipitation in the wettest month (15%) and mean temperature of the driest quarter (14%). The role of geology is likely mediated through its effect on water body pH, which can affect the free-living stage of Alaria spp., while climatic variables may affect the behavior and composition of hosts required to complete the life cycle. Land cover, however, which has been proposed as an indicator of biological interactions in species distribution models, was not a substantial contributor to Alaria spp. occurrence (7%). The primary Alaria spp. occurrence areas included Western US, Ohio River Valley, and Midwestern US with a low probability of predicted occurrence in the Rocky Mountains and Southern US. Our results may help inform predictions of infection risk in wildlife and humans.