Ecological theory suggests that predators keep herds healthy by decreasing parasitism in their prey. Much of this theory, and the tests of its predictions, rely on assumptions of homogeneity the predator populations. However, in nature, predators vary in their predation strategy and the abundances and activity levels of different types of predators vary seasonally. Avian and terrestrial predators exhibit different prey preferences with respect to prey size, which often correlates with parasite intensity. And these different predation strategies likely determine the degree to which the predator selectively preys on infected prey. In our study area many avian predators are only seasonally resident while most terrestrial mesopredators are year-round residents. We conducted two parallel predator exclusion experiments: (i) terrestrial meso-predators were experimentally excluded from 40ha plots, decreasing terrestrial predation and increasing avian predation as compared to control plots (ii) avian predators were experimentally excluded from 5000 m2 plots. We trapped small mammals in both experiments and measured gastrointestinal parasite load in the dominant prey species, cotton rats (Sigmodon hispidus) and cotton mice (Peromyscus gossypinus). We asked (i) does the effect of predator exclusion on parasites in small mammals vary seasonally? And (ii) does the effect differ for avian and terrestrial predators?
Results/Conclusions
We identified three common gastro-intestinal parasites in our cotton rat populations: 2 strongyle nematodes, and Physaloptera hispida. Both terrestrial and avian predator exclusion had no effect on either strongyle nematode but both significantly interacted with seasonality to influence intensity of P. hispida. The two predator exclusion treatments drove parasite loads in opposite directions during periods where the significantly differed. We identified two common gastro-intestinal parasites in our cotton mouse population, which was only present in our terrestrial predator exclusion experiment, both of which were strongyle nematodes. The interaction between terrestrial predator exclusion treatment and season was significant in predicting intensity of both parasites. In both cotton rats and cotton mice, the significant interaction between season and predator treatment was largely driven by significant differences occurring in the fall and winter. These findings suggest that seasonal changes in predator behavior and abundance change the effect of predators on parasite dynamics in prey populations and that predator groups differ both in their effect on parasites in prey and in the seasonality of these effects.