Does infection alter resource consumption by hosts? Trait-mediated indirect effects of disease on resources in a Daphnia-yeast host-parasite system

Background/Question/Methods

Host-parasite-resource models typically assume that infected and uninfected individuals differ only in survivorship and/or fecundity. However, disease might also affect traits such as feeding behavior. Infected individuals might consume more resources to offset immune costs. On the other hand, disease may reduce food consumption through metabolic changes or physical inhibition of feeding mechanisms. When disease affects feeding rate, what are the indirect effects of disease on resources? We used a zooplankton-yeast host-parasite system to quantify effects of infection on resource consumption, and to model indirect effects of disease on resources. We measured algal consumption (feeding rate: FR, mL indiv^-1 hr^-1, size-specific feeding rate: SSFR, mL mm^-2 hr^-1) of infected and uninfected Daphnia from three genotypes and seven age cohorts. All infected Daphnia had been exposed to the parasite at age 6 days, such that age during the FR assay correlated with number of days post-exposure (i.e., older Daphnia had been infected longer). We used these data to parameterize mathematical models of host-parasite-resource dynamics incorporating density (survivorship and fecundity)-mediated and trait (FR)-mediated effects of disease on resources in lake food webs.

Results/Conclusions

Effects of disease on body size (length) and resource consumption (FR, SSFR) depended on host age (days post-exposure, DPE) and genotype. Averaged across genotypes, infected Daphnia 10, 14, and 18 DPE were 6%, 8%, and 14% smaller, respectively, than same-aged uninfected Daphnia. In genotype A, infected Daphnia were significantly smaller 4 DPE, while size diverged 10-18 DPE in genotypes B and C. Averaged across genotypes, infection reduced FR by 58%, 75%, and 97% at 10, 14, and 18 DPE. However, infected hosts did not eat less simply because they were smaller. Averaged across genotypes, infection reduced SSFR by 53%, 71%, and 96% at 10, 14, and 18 DPE. Furthermore, infection decreased FR before body sizes diverged. In genotype A, Daphnia at 2 DPE already had significantly lower FR and SSFR compared to same-aged uninfected Daphnia. In genotypes B and C, infection decreased FR and SSFR by 6-10 DPE. We will present results of host-parasite-resource models in which we explore the potential for disease to affect resource abundance, and extend these models to consider effects of consumption on infectious spores. Our results suggest that trait-mediated effects of disease on resource consumption might affect flows of energy and nutrients through food webs.