Habitat complexity modifies ant-parasitoid interactions: implications for community dynamics and the role of disturbance

Species must balance effective competition with avoidance of mortality imposed by predators or parasites to coexist within a local ecological community. Attributes of the habitat in which species interact, such as structural complexity, have the potential to affect how species balance competition and mortality by providing refuge from predators or parasites. Disturbance events such as fire can drastically alter habitat complexity and may be important modifiers of species interactions in communities. This study investigates whether the presence of habitat complexity in the form of leaf litter can alter interactions between the behaviorally dominant host ants Pheidole diversipilosa and Pheidole bicarinata, their respective specialist dipteran parasitoids (Phoridae: Apocephalus sp. 8 and Apocephalus sp. 25) and a single species of ant competitor (Dorymyrmex insanus). We used a factorial design to manipulate competition (presence/absence of competitors), mortality risk (presence/absence of parasitoids) and habitat complexity (presence/absence of leaf litter). Parasitoid presence reduced soldier caste foraging, but refuge from habitat complexity allowed increased soldier foraging in comparison to treatments in which no refuge was available. Variation in soldier foraging behavior correlated strongly with foraging success, a proxy for colony fitness. Habitat complexity allowed both host species to balance competitive success with mortality avoidance. The effect of fire on habitat complexity was also studied, and demonstrated that the immediate negative impact of fire on habitat complexity can persist for multiple years. Our findings indicate that habitat complexity can increase dominant host competitive success even in the presence of parasitoids, which may have consequences for coexistence of subordinate competitors and community diversity in general.