Wars of attrition: colony size determines competitive outcomes in a guild of African acacia ants

Abstract

Territorial aggression is a primary mode of population regulation among ants, and frequently leads to the formation of linear dominance hierarchies. However, few experimental studies have investigated the proximate mechanisms that underlie these hierarchies. In this study I examine the mechanisms underlying dominance relationships in interference competition for nest sites in a guild of acacia ants residing on Acacia drepanolobium. I show that (1) interspecific conflicts are fought as simple wars of attrition, with mortality ratios among combatant species approximating 1:1, (2) interspecific competitive outcomes, both within and between habitats, closely parallel differences in estimated average colony size among the four acacia ant species, and (3) experimental manipulation of colony size effectively reverses the dominance hierarchy. This is the first study to demonstrate experimentally that size asymmetries underlie dominance relationships in an entire guild of competitors. These results also highlight the contingent nature of ecological dominance in this community. Because dominance is dictated by characteristics of a trait shared by all ant species (colony size), interspecific competitive outcomes may vary if species differ in growth and/or mortality responses to factors such as disturbance or environmental conditions. Contingencies in competitive outcomes among these acacia ants may play an important role in promoting coexistence in this intensely nest-site-limited community.