Why do parasites exhibit reverse latitudinal diversity gradients? Testing the roles of host diversity, habitat and climate.

The latitudinal diversity gradient (LDG) - in which species richness decreases from the equator toward the poles - is among the most fundamental distributional patterns in ecology. Despite the expectation that the diversity of parasites tracks that of their hosts, available evidence suggests that many parasites exhibit reverse latitudinal gradients or no pattern, yet the rarity of large-scale datasets on host-parasite interactions calls into question the robustness of such trends. Here, we collected parasitological data from a host group of conservation importance - lentic-breeding amphibians - to characterize the form and direction of relationships among latitude, parasite richness, and parasite load. The contiguous USA. 2000 to 2014. Lentic-breeding frogs and toads and their helminth parasites. We collected information on parasite richness and infection load for 846 amphibian populations representing 31 species. We combined these data with environmental and biological data to test for LDGs and potential mechanisms. Both parasite richness and abundance increased across 20 degrees of latitude - a reverse LDG. For parasite richness, this pattern was partially explained by latitudinal increases in wetland area, landcover diversity, and the richness of waterbirds - which function as definitive hosts for many amphibian parasites. Host body size also correlated positively with latitude and helminth richness, potentially reflecting increased habitat availability, greater host longevity, or a persistent phylogenetic signal. Parasite abundance associated positively with wetland area and landcover diversity, but negatively with amphibian taxonomic richness. Longitude exhibited non-linear relationships with parasite abundance and richness, which we suggest stem from large-scale variation in host availability (e.g., migratory bird flyways). With growing interest in the distribution of parasites and pathogens, these results highlight the importance of inverse latitudinal gradients while emphasizing the explanatory influence of host body size, habitat availability, and host diversity.