Fig. 4.

Individual effects of lethal (survival) and sublethal (resource intake, fecundity) components of parasite virulence on producers (green), host herbivores (black), and adult parasites (orange). (A) Parasite-free communities result in relatively high host biomass and low producer biomass compared to communities with parasites included, demonstrating top down-regulation of primary production by herbivorous hosts (shown for clarity of comparison with other panels). When considering parasite effects on individual host traits, (B) reduced host survivorship (C) reduced host resource intake, or (D) reduced host fecundity all trigger trophic cascades; these outcomes result in reduced host herbivore biomass and elevated producer biomass. In addition, while negative effects of infection on (B) host survivorship or (C) host resource intake stabilize resource–host oscillations, infections that reduce host fecundity (D) are destabilizing. Lines and shaded envelopes in A and D indicate the mean ± SE for each density, given the cycling dynamics (herbivore data obscures producer data in A) during the last 10,000 time steps of the model after initial transient dynamics. The model was parameterized with values corresponding to the producer–caribou–Ostertagia system (Table 1) with herbivores and adult parasites expressed as number per square kilometer and producer biomass expressed in kilograms per square kilometer.