Richness and diversity of amphibian parasite communities in North America: A macroecological approach
The search for the determinants of biodiversity and its spatial variation remains a central aim in ecology. Although a small set of factors account for much of the geographical variation in free-living diversity, the search continues for general drivers of parasite species richness (PSR) among host species. Because parasites constitute a substantial proportion of biodiversity, elucidating the factors shaping variation in their diversity is crucial to identify disease risks and conservation targets. We perform a large-scale, quantitative analysis of the environmental drivers of metazoan parasite richness within amphibians, focusing on widely-used presumed predictors of PSR patterns including host diversity (anuran richness), latitude and longitude. Our dataset consists of 431 sites sampled across 43 US states from 2000-2014, encompassing 30 amphibian species. Only sites that had at least ten hosts surveyed were included in our analyses, and of those, only data from the first year a site was visited was used. PSR was recorded as the total number of helminths, arthropods, and protists groups observed across all host species sampled at a site. Statistical analyses were run using generalized additive models (GAMs). Anuran richness was obtained by layering IUCN species range maps and extracting site-level cumulative richness values.
Results revealed strong albeit non-linear associations among PSR, host richness and spatial position. At the continental scale of the analysis, anuran (but not caudate) richness positively predicted PSR. The positive correlation we found between species richness of parasites and their anuran hosts matches patterns between resource diversity and consumer diversity seen among free-living communities, reinforcing the fact that host-parasite interactions provide an ideal system to explore congruence of biodiversity patterns across trophic levels. This relationship persisted after accounting for associations with latitude and longitude, which were best captured using a GAM. PSR tended to increase with latitude before subsequently declining at more temperate latitudes, whereas longitude correlated with several peaks in PSR across the US. Our finding that PSR tended to be higher further from the equator runs counter to the trend observed in free-living organisms. We expect the peaks in richness associated with longitude stem from historical glaciation and overlap with major bird migratory routes. Our findings support the tight ecological and evolutionary association between host richness and PSR and provide evidence that patterns of parasite diversity exhibit latitudinal gradients that differ from free-living taxa. Further comparative analyses will be necessary to pinpoint the main drivers of variability in parasite biodiversity patterns.