Density of hosts and density-dependent pathogens are highest at the core of species ranges

To reconcile the controversy surrounding the center–periphery hypothesis, we first test this hypothesis in hosts using density data from 572 fish and 408 macroinvertebrate genera compiled by two of the most spatially and temporally extensive and standardized freshwater biodiversity monitoring programs in the contiguous United States: the U.S. Geological Survey (USGS) BioData program, and the United States Environmental Protection Agency (USEPA) National Rivers and Streams Assessment (NRSA) program (USEPA 2008). Then, using an independent global database containing 4361 parasite-prevalence records from 1210 host-parasite species pairs across a wide diversity of animal hosts, we test the hypothesis that density-dependent parasites are also more prevalent at the centers of host ranges than at their margins, whereas frequency-dependent parasites show no such pattern. Finally, we examined whether patterns in host and parasite abundance depend on host and parasite taxonomy and traits.