The ecology of fear framework was developed to describe the impacts of predators on prey beyond consumption. Negative physiological effects of stress, altered feeding behaviour, reduced reproductive opportunities, etc, that occur during predator exposure can be as impactful on prey populations as predation itself. Recent work extends these non-consumptive effects (NCEs) to describe relationships beyond predator prey interactions, in particular host-parasite systems. Currently, the fitness and population level impacts of parasites on hosts outside of infection is only beginning to be understood. However, given their ubiquity, parasites may be exerting wide-spread but underestimated NCEs. We investigate the NCEs of ectoparasitic Macrocheles subbadius on fly hosts, (Drosophila nigrospiracula). We hypothesised the NCEs of parasites could suppress host population growth in the absence of infection. We used physiological and fitness assays to measure the NCEs of parasite exposure on flies held in proximity to mites with mesh barriers preventing infection. Using experimentally determined parameters for individual flies, we simulated fly populations using matrix models. Simulated flies experienced no effects of parasites, only NCEs, or non-consumptive effects and consumptive effects (i.e. infection). 1000 populations were simulated for 100 days (~5 fly generations), and the final population size and growth rates were calculated.
Results/Conclusions
Respirometry showed proximity to mites increased fly metabolic rate by 70% suggesting an energetic cost of exposure. In fitness assays individual female flies exposed to mites, without infection risk, lived 23% less long and produced 13% fewer offspring. Individual flies experienced deleterious effects in the short and long term due to mite exposure sans infection. Counterintuitively, NCEs alone did not reduce the population growth of simulated populations relative to populations without parasite effects (𝞴=1.51 versus 𝞴=1.50 respectively). In fact, the average final population size was ~200 flies larger in the simulations including NCEs. By contrast simulated populations experiencing both infection and NCEs had substantially reduced growth rates (𝞴=1.03) and had 4-10 times lower final population sizes. The impact of parasites in the NCEs and infection condition was sensitive to the prevalence of infection among flies. A final prevalence of ~25% reduced the final population size by ~50%. Despite individual-level negative impacts of parasite exposure on fly fitness, simulations suggest NCEs may not manifest on a population level. This counterintuitive result may be driven by substantial variation in egg production among flies experiencing NCEs, suggesting potential compensation for early mortality.
