COMPETITION–COLONIZATION TRADE-OFFS IN A GUILD OF AFRICAN ACACIA-ANTS

Abstract

In systems where disturbance creates colonization opportunities, competition–colonization trade-offs can lead to coexistence between competitors sharing a single, limiting resource. In upland East Africa, four species of obligate acacia-ants compete for possession of Acacia drepanolobium host trees but still coexist at fine spatial scales. Previous experiments revealed a hierarchy of competitive dominance among mature ant colonies in battles over occupied trees (Crematogaster sjostedti > C. mimosae > C. nigriceps > Tetraponera penzigi), and the pattern of ant species turnover on single host trees over time is consistent with this hierarchy. Here we present evidence that competition–colonization trade-offs are likely to facilitate coexistence within this guild. The population of host trees is highly dynamic. Although most suitable host trees are occupied by ants, mature A. drepanolobium trees may be vacated after fire, extensive damage, or drought. Smaller host trees with limited nesting space become available when they recruit into the population as saplings or coppices. These unoccupied trees may be colonized by expansion of nearby, mature colonies as well as by foundress queens attempting to initiate new colonies. We identified competition–colonization trade-offs between two stages of colony development: competitive colony expansion onto nearby trees, and competition among foundress queens for nest-initiation sites. All three Crematogaster species readily occupy nearby, empty trees by colony expansion. In contrast, T. penzigi colonizes empty trees almost exclusively by dispersive foundress queens. Colonies of C. sjostedti are competitively dominant over those of all other species, but foundress queens of this species rarely attempt to start independent colonies. Foundress queens of the secondary competitive dominant (C. mimosae) are restricted mostly to low-quality, small saplings. In contrast, competitively subordinate colonies of T. penzigi and C. nigriceps produce foundress queens that disproportionately colonize empty mature trees, the highest quality host plants that are available. Competition between foundress queens is intense. Queens are significantly hyper-dispersed among potential nest-initiation sites, indicating that the presence of resident foundresses deters subsequent colonization attempts, and competitive dominance among foundresses who fight during colony founding is the reverse of that seen among mature colonies. Coexistence within this guild may therefore be promoted by ontogenetic reversals in relative competitive ability between early and late stages of colony development, a pattern that is consistent with the niche hypothesis for competition–colonization trade-offs.