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.

评估种群动态变化对社会性物种种群内攻击行为与群体间竞争结果的实验研究极为匮乏。这一认知空白意义重大，不仅因为社会性物种是众多陆地生态系统的核心组成部分，更因为社会群体间的干扰竞争往往涉及受种群与环境背景影响的类决策过程。在东非，共生蚁金合欢举腹蚁（Crematogaster mimosae）是共优势竞争者，其种群会为独占宿主金合欢树展开高死亡率的种内与种间争斗。我们通过移栽携带蚁栖结构（domatia）的枝条提升金合欢举腹蚁所栖息的金合欢宿主树上的工蚁密度，并通过局部树干熏蒸降低工蚁密度，同时调控蚁群扩张至邻近宿主树的机会，以此探究这些因素对蚁群攻击行为、扩张成功率及长期存活能力的影响。我们在两种不同风险情境下，检验了工蚁密度调控对攻击行为与即时扩张成功率的影响：将目标宿主树分别连接至实验清空的宿主树（低风险处理组），或连接至被黑足举腹蚁（Crematogaster nigriceps）占据的宿主树（高风险处理组）。对于对照组与高工蚁密度蚁群，其扩张至清空邻近宿主树的成功率达100%，但低工蚁密度蚁群的扩张成功率仅为50%——尽管该系统中宿主树属于限制性资源。而在扩张至被异种竞争者占据的宿主树时，低工蚁密度、对照组与高工蚁密度蚁群的扩张成功率分别为36%、40%与79%。研究结果表明，由干扰或种群间争斗引发的工蚁密度变化，有可能破坏竞争等级秩序。工蚁密度调控同样会影响蚁群的长期存续。行为与遗传数据显示，在扩张发生12个月后，100%的低工蚁密度蚁群，以及25%的对照组与高工蚁密度蚁群，均被邻近的机会性同种个体取代。金合欢举腹蚁的种内攻击行为成本或相对较低，因为具有攻击性的蚁群可整合来自被击败竞争者的工蚁或蚁后。同种金合欢举腹蚁之间冲突的意外高发生率，加之相较于种间冲突，种内竞争的成本有所降低，这或许为该蚁植共生系统中的弱势竞争者提供了生存契机。