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.