The macroecological pattern known as Taylor’s Power Law (TPL) represents the pervasive tendency of the variance in population density to increase as a power function of the mean, which has important implications for understanding species aggregation and system stability. Despite empirical illustrations in systems ranging from viruses to vertebrates, the mechanisms underpinning this relationship and its biological significance continue to be debated. Here we combined collection of a unique dataset involving amphibian hosts and their trematode parasites with experimental measurements of core epidemiological outcomes to explicitly test the contributions of hypothesized biological processes in driving aggregation among 1,189 parasite populations from 14,319 hosts.
After using feasible set theory to properly account for mechanisms acting indirectly on aggregation, we detected strongly consistent influences of host and parasite species identity over seven years of sampling. Incorporation of field-based measurements of host body size, its variance, and spatial heterogeneity accounted for host identity effects, while experimental quantification of infection competence and especially virulence from the 20 most common host-parasite combinations highlighted the role of species-by-environment interactions. The ubiquity of TPL across biological and non-biological systems has challenged the assumption that system-specific mechanisms are present in TPL. By uniting constraint-based theory, controlled laboratory experiments, and syntopic field surveys, we illustrate how biological processes predictably affect parasite aggregation, with implications for population regulation, ecological stability, and disease management.