Density manipulations influence colony aggression, expansion, and survival success of the mutualistic ant-acacia species Crematogaster mimosae (Santschi)

Background/Question/Methods

Social species engage in aggressive confrontations to gain or defend territory against competitors. In social insects, group size, group personality, and collective organization all determine group success. The whistling-thorn tree Acacia drepanolobium is host to four symbiotic ant species, whose colonies may span multiple trees. Colonies compete for resources fighting intraspecific and interspecific neighboring trees. Despite its ecological relevance, we know little about the factors modulating aggression, and a colony’s tendency to expand or contract. Ant density and resource quality are likely to be major determinants of aggression and group size. On this system, previous studies demonstrated that larger colonies win most interspecific confrontations, but have not examined intraspecific interactions. Further, the role of ant density as a potential cyclical regulator of colony expansion and the role of intraspecific competition in maintaining long-term coexistence of these acacia-ants remains unknown. We manipulated ant densities and assessed its effects in aggression, and expansion success into new territories available free of mortality (empty neighbor tree), or with high risk (occupied heterospecific neighbour tree). We examined the relative importance of inter and intraspecific competition as determinants of post-expansion colony survival success using molecular markers to disentangle intraspecific transitions. 

Results/Conclusions

Experimental density manipulations significantly altered the number of Crematogaster mimosae ants patrolling trees, and strongly affected colony aggression levels. On average increased density colonies had 3.8 times more ants patrolling branches, and were 45% faster recognizing and attacking C. nigriceps intruders. The interaction between density and neighbor treatment strongly determined initial expansion success. The odds of successfully expanding for increased density colonies joined to experimentally emptied trees were 3.5 those of the controls. Compared to the other two treatments, decreased density colonies were less successful expanding into both neighbor treatments. Although colonies that expanded onto occupied trees were not significantly less likely to survive subsequent attacks than those that expanded onto empty trees, decreased density focal trees were 6 times more likely to subsequently being killed by opportunistic neighbors compared to the controls. Further, examination of non-focal tree takeover rates confirmed that low worker density is a core determinant of takeover risk by neighboring colonies. Across all trees taken-over by opportunistic neighbors, 97% of focal and 70% of non-focal takeovers involved intraspecifics. Our study shows that density is an important force influencing colony aggression and expansion success, and highlights the importance of intraspecific competition in shaping the ecological and evolutionary dynamics maintaining this system.