Host dynamics determine responses to disease: additive vs. compensatory mortality in a grasshopperpathogen system

Disease is often expected to limit host populations, but diseases do not always dramatically reduce host numbers and often have no effect. The impact of a fungal pathogen (Entomophaga grylli pathotype 1) on grasshopper (Camnula pellucida) populations was studied in a field experiment. We tested whether the effects of disease on grasshopper survival were additive, with disease mortality summing with other mortality sources to determine the total population mortality rate; or whether they were compensatory, where disease mortality simply replaces mortality from other sources so that the total population mortality rate remains unchanged. We examined grasshopper survival in relation to differences in disease exposure, host density levels, and host developmental stage. The effects of disease varied with grasshopper developmental stage and density. Disease mortality increased by 60% at high grasshopper density compared to low-density treatments, and decreased when grasshoppers fully matured. Despite increased rates of disease mortality at high densities, the total mortality rate was not notably higher in diseased grasshoppers (87%) compared to disease-free counterparts at high densities (83%), indicating that a large percentage of disease mortality simply replaced mortality from food limitation. Additive responses were supported in early and late instars, with disease exposure resulting in decreased grasshopper survival. In contrast, the effect of disease on adults was inconclusive. Yet, the disease did not affect adult survival, suggesting that adult disease mortality is compensatory. Therefore, disease reduction of grasshopper populations (additive mortality) is more likely to occur during earlier developmental stages, when hosts are most vulnerable to disease, and at low host densities when food is abundant. Combined, our results emphasize the importance of host dynamics and food availability in how this host–pathogen system responds to disease. Accordingly, compensatory vs. additive mortality may need to be considered when examining how disease ultimately affects host population dynamics.