Data from: Impacts of worker density in colony-level aggression, expansion, and survival of the acacia-ant Crematogaster mimosae

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.

评估社会物种种群统计变化对社群攻击行为与群体间竞争结局影响的实验研究目前仍较为罕见。这一认知空白意义重大：一方面，社会物种是众多陆地生态系统的基础组成部分；另一方面，社会群体间的干扰竞争往往涉及受种群统计与环境背景调控的类决策过程。在东非地区，共生性的含羞草举腹蚁（Crematogaster mimosae）是共优势竞争者，为独占寄主金合欢树，会引发高死亡率的种内与种间战斗。我们针对含羞草举腹蚁的寄主金合欢树，操控其工蚁密度，并调整蚁群扩张至邻近树木的机会，以探明这些因素如何影响蚁群层面的攻击行为、扩张成功率与长期存活情况。我们通过移植携有蚁栖结构的枝条提升目标树上的工蚁密度，并通过局部树干熏蒸处理降低其工蚁密度。我们在两种不同风险情境下，探究了工蚁密度操控对攻击行为与即时扩张成功率的影响：将目标树分别与经实验清除蚁群的邻树（低风险处理组），或与被黑头顶举腹蚁（Crematogaster nigriceps）占据的邻树（高风险处理组）进行关联。尽管寄主金合欢树在该系统中属于限制性资源，但对照组与工蚁密度提升组的蚁群向清空邻树的扩张成功率均为100%，而工蚁密度降低组的扩张成功率仅为50%。向异种竞争者占据的树木扩张的成功率，在工蚁密度降低组、对照组与密度提升组中分别为36%、40%与79%。我们的研究结果表明，因干扰或群体间争斗引发的工蚁密度变化，有可能破坏竞争等级体系。工蚁密度操控同样会影响蚁群的长期存续能力。行为学与遗传学数据显示，扩张发生12个月后，工蚁密度降低组的所有蚁群（100%）以及对照组与密度提升组中25%的蚁群，均被邻近的机会主义同种蚁群取代。含羞草举腹蚁的种内攻击行为或许成本更低，因为具有攻击性的蚁群可以吸纳战败竞争者的工蚁或蚁后。含羞草举腹蚁同种个体间冲突的频率远超预期，加之相较于种间冲突，该物种存在降低种内竞争成本的行为策略，这或许为该蚁植共生系统中的劣势竞争者提供了生存契机。