A Grasshopper and Its Fungal Pathogen: A Model System for Disease Dynamics

Understanding how pathogens limit host populations is a central question in population ecology. However, disease ecology is still an emerging field with strong theoretical and laboratory work, but little experimentation in natural settings. This dissertation examined how disease affects host population dynamics using a fungal pathogen-grasshopper model system. Three main questions were addressed in a series of field experiments: 1) How do host density level and life stage affect host-pathogen interactions? 2) Do ants act as mechanical vectors and affect disease dynamics of this host-pathogen system? 3) How do temperature and moisture impact host-environment and pathogen-environment outcomes?

Manipulative field experiments were conducted in a bunchgrass prairie at the National Bison Range in western MT from 2010-2013. Grasshopper density, grasshopper life stage, ant presence, temperature, and moisture were manipulated in field cages. Grasshopper survival time and disease mortality rates were measured as well as temperature, relative humidity, and grass biomass.

The experiments show that both biotic and abiotic factors are important in this grasshopper-pathogen system. These factors interact to produce pathogen limitation under some conditions and no pathogen limitation under other conditions. Host dynamics was a key biotic factor with pathogen limitation occurring during earlier developmental stages when hosts are most vulnerable to disease and at low host densities when food is abundant. Despite increased rates of disease mortality at high densities, a large proportion of disease mortality simply replaced mortality from starvation as intraspecific competition also increased with host density level. While ant scavengers did increase disease mortality rates, the effect on grasshopper survival was minimal due to the ephemeral nature of conidia. Moisture supplementation resulted in a three-fold increase in disease mortality rates. Warming had both negative and positive effects on the grasshopper host with increased temperatures resulting in decreased disease mortality rates and decreased grass biomass (i.e. food availability). This research indicates that the effects of pathogens on host population dynamics can be very complex depending on both abiotic and biotic factors.