Data from: Mechanisms of aggregation in an ant-tended treehopper: attraction to mutualists is balanced by conspecific competition

Understanding the spatial structure of populations and communities has been a dominant focus of ecological research, and spatial structure is increasingly seen as critical for understanding population dynamics. Habitat (or host) preference is a proximate mechanism that can generate aggregation or overdispersion, lending insight into the ultimate consequences of observed spatial distributions. Publilia concava is a univoltine phloem-feeding insect that forms mutualistic associations with ants, which consume honeydew and protect treehoppers from predation. Treehopper adults and nymphs are aggregated at the scale of goldenrod plant stems, and previous studies have suggested that this aggregation is an adaptive response that increases feeding performance or maximizes benefits of ant-tending. Previous studies have also shown experimentally that individual treehoppers preferentially oviposit on plants with ants present, but a complimentary hypothesis that treehoppers prefer to oviposit near conspecifics (e.g., to take advantage of density-dependent ant attraction) remains untested. We show that, as expected, the probability of treehopper oviposition increases with ant-presence and relative ant abundance. However, we also find that treehopper oviposition decreases with increasing treehopper density. Thus our results are inconsistent with the hypothesis that treehopper aggregation is a socially cooperative strategy to attract ants; we suggest that aggregation is a form of conflict and an unavoidable by-product of individual responses to ant-tending levels.

解析种群与群落的空间结构始终是生态学研究的核心议题，而空间结构在种群动态（population dynamics）解析中的关键地位亦日益凸显。生境（habitat）或寄主（host）偏好是驱动种群聚集或过度分散的近因机制（proximate mechanism），可为理解观测到的空间分布的终极生态效应提供重要视角。凹形角蝉（Publilia concava）是一类一化性（univoltine）韧皮部取食昆虫，与蚂蚁形成互利共生（mutualistic association）关系：蚂蚁取食其分泌的蜜露（honeydew），并保护角蝉（treehopper）免受天敌捕食。该物种的成虫与若虫会聚集于一枝黄花（goldenrod）植株的茎秆尺度范围内，既往研究认为这种聚集属于适应性响应，可提升取食效能或最大化蚂蚁护佑带来的收益。既往实验研究还证实，单个角蝉会优先选择有蚂蚁栖息的植株产卵（oviposit），但另一项补充假说——角蝉偏好于同种个体（conspecifics）附近产卵（例如借助密度依赖（density-dependent）的蚂蚁吸引效应）——迄今仍未得到验证。本研究表明，正如预期，角蝉的产卵概率随蚂蚁存在及其相对丰度的升高而提升；但同时也发现，角蝉产卵概率会随自身种群密度的增加而降低。因此本研究结果与‘角蝉聚集是吸引蚂蚁的社会合作策略’这一假说相悖；我们提出，聚集实则是一种种内冲突形式，亦是个体针对蚂蚁护佑水平做出响应时不可避免的副产物。