Host energetics explain variation in parasite productivity across hosts and ecosystems

Parasites are thought to play a role in ecosystem energetics, in part because some ecosystems harbor a substantial amount of parasite biomass. Nevertheless, the extent to which parasite biomass accurately reflects the flow of energy from hosts to parasites, and the linkages between their energetics, remain unclear. Here, we estimate parasite community energetics at the host and ecosystem-level and test predictions for parasite energetics using the metabolic theory of ecology. Across 27 host species, parasite community abundance declines with average individual parasite energy use Rp as Rp-0.50 and increases with host metabolic rate Rh as Rh 0.63, which is inconsistent with metabolic theory. We next test whether the fraction of host energy that is allocated to parasitism is invariant across hosts. Our empirical analysis demonstrates that 85% of the variation in parasite community energy use can be explained by differences in host metabolic rate. However, parasite community energy use increases allometrically with host metabolic rate   as   suggesting that the fraction of host energy used by parasites declines with host metabolic rate. At the ecosystem-level, we show that the energy flowing through parasite communities scales allometrically with the total rate of energy use by their fish hosts across three ecosystems. Importantly, directly examining energy flux revealed variation in parasite energy use among ecosystems that was not apparent when examining differences in biomass. Taken together, these results establish strong empirical links between host and parasite energetics, but our findings often did not align with predictions based on metabolic theory.