Enemy-Free Space for Parasitoids
Animal trait patterns and variation in the structures of their communities have typically been explained in the context of competition for resources. Progress in research over past decades has shown the importance of mortality from natural enemies as community structuring forces. Selection towards enemy-free space through traits that aid in interactions with these natural enemies can be used to explain community patterns. Insect parasitoids that develop as larvae by continuously feeding within a host herbivore experience many of the same ecological pressures as these hosts, such as predation. Parasitoid communities may be structured by mortality from natural enemies of their hosts, and should avoid hosts with a high risk of mortality. Here, data from multiple field studies conducted in temperate Connecticut forests were combined to assess the impact of enemy-free space on parasitoid fly and wasp community patterns. Using generalized linear models, I examined the relationship between the risk of predation from birds and ants and the probability of parasitism across the community of caterpillar species found on eight deciduous tree hostplant species. I tested the prediction that probability of parasitism should be negatively correlated with risk of predation, while accounting for fixed effects on parasitism related to the caterpillar species and the host-plant species. Predation risk from birds had consistent negative relationships with all measures of parasitism (total, as well as fly and wasp separately) while the risk of ant predation had little to no relationship with probability of parasitism. These relationships were stronger for tachinid fly parasitoids than wasp parasitoids, potentially highlighting differences in their life-histories and the timing of predation. The results support the enemy-free space for parasitoids hypothesis: parasitoids generally avoid caterpillar and host-plant combinations with a high risk of bird predation. Therefore enemy-free space from birds in conjunction with host-plant and caterpillar species-based effects drive parasitoid host choice in this temperate community. These results highlight the importance of considering multiple and varied ecological effects simultaneously when investigating community-level patterns.