Impacts of worker density in colony‐level aggression, expansion, and survival of the acacia‐ant Crematogaster mimosae

Abstract

Experimental studies assessing the impact of demographic changes on aggression and inter‐group competitive outcomes in communities of social species are rare. This gap in our knowledge is important, not only because social species are foundational elements of many terrestrial ecosystems, but because interference competition among social groups often involves decision‐like processes influenced by demographic and environmental contexts. In East Africa, the symbiotic ant Crematogaster mimosae is a co‐dominant competitor that engages in high‐mortality, intra‐ and interspecific battles for sole possession of host trees. We manipulated worker density on C. mimosae Acacia host trees, and the colony's opportunity to expand onto neighboring trees to identify how these factors influenced colony‐level aggression, expansion success, and longer‐term survivorship. Worker density on focal trees was increased through translocation of domatia‐bearing branches, and was decreased using partial tree fumigations. We examined impacts of density manipulations on aggression and immediate expansion success under two different risk scenarios. We tied focal trees to either an experimentally emptied‐tree (low‐risk treatment), or to a C. nigriceps‐occupied tree (high‐risk treatment). Expansion success onto emptied neighbor trees was 100% for controls and increased‐density colonies, but only 50% for decreased‐density colonies, despite the fact that host trees are a limiting resource in this system. Success expanding onto trees occupied by a heterospecific competitor reached 36%, 40%, and 79% in decreased, control, and increased‐density trees, respectively. Our results show that changes in worker density due to disturbances or inter‐group battles have the potential to disrupt competitive hierarchies. Worker density manipulations also affected longer‐term colony persistence. Behavioral and genetic data revealed that 12 months after expansions 100% of the decreased‐density colonies, and 25% of control and increased‐density colonies, had been supplanted by neighboring opportunistic conspecifics. Intraspecific aggression may have lower costs in C. mimosae because aggressive colonies can incorporate workers or queens from defeated competitors. The unexpectedly high frequency of conflicts between conspecific C. mimosae, in combination with behaviors decreasing the cost of intraspecific competition relative to interspecific conflict, may create opportunities for the survival of subordinate competitors in this ant–plant system.

Fire disturbance disrupts an acacia ant–plant mutualism in favor of a subordinate ant species

“…The effect of large herbivores on ant density is consistent with other work demonstrating that browsing by megaherbivores increases host investment in swollen thorns and nectaries, which leads to higher worker production and aggression to defend higher quality resources (Palmer et al 2008b, Stanton and Palmer 2011, Ruiz Guajardo et al 2017). However, we found a significant interaction between herbivory and fire with burning reducing ant densities per meter of branch dramatically in herbivore-accessible plots but less so in herbivore exclusion plots ( Fig.…”

“…, Ruiz Guajardo et al. ). These four ants defend their host trees against herbivory in exchange for housing and food (Madden and Young ), although the different fitness benefits of each ant species for the host trees is complex (Stanton et al.…”

“…The area is underlain by "black cotton" vertisol soils, where woody vegetation is dominated by Acacia drepanolobium (>98%; Young et al 1997). In this system, four mutually exclusive symbiotic ant species (Crematogaster sjostedti, C. mimosae, C. nigriceps, Tetraponera penzigi) compete intensively for host trees, with the outcomes of aggressive confrontations determined by colony size, ant density, and aggression (Palmer et al 2000, Ruiz Guajardo et al 2017). These four ants defend their host trees against herbivory in exchange for housing and food (Madden and Young 1992), although the different fitness benefits of each ant species for the host trees is complex (Stanton et al 1999, Palmer et al 2008b, Stanton and Palmer 2011.…”

Polygyny does not explain the superior competitive ability of dominant ant associates in the African ant‐plant, Acacia (Vachellia) drepanolobium

“…While differences in competitive and colonizing ability among A. drepanolobium 's ant associates have been well described, our understanding of the mechanistic basis for these differences is still lagging. The higher competitive ability of dominant species, in particular, seems to be driven by larger worker populations (Palmer, ; Ruiz‐Guajardo, Grossenbacher, Grosberg, Palmer, & Stanton, ), but the proximate causes of these larger worker populations are unknown. A primary hypothesis proposed is that more competitive species are polygynous, that is, they have multiple queens per colony (Palmer, ; Rubin et al., ).…”

The Chief Mau Mau Propagandist: Experiences that Prompted Gakaara wa Wanjau into Anti-Colonial Literary Activism

“…The area is underlain by "black cotton" vertisol soils, where woody vegetation is dominated by Acacia drepanolobium (>98%; Young et al 1997). In this system, four mutually exclusive symbiotic ant species (Crematogaster sjostedti, C. mimosae, C. nigriceps, Tetraponera penzigi) compete intensively for host trees, with the outcomes of aggressive confrontations determined by colony size, ant density, and aggression (Palmer et al 2000, Ruiz Guajardo et al 2017. These four ants defend their host trees against herbivory in exchange for housing and food (Madden and Young 1992), although the different fitness benefits of each ant species for the host trees is complex (Stanton et al 1999, Palmer et al 2008b, Stanton and Palmer 2011.…”