Does ant-plant mutualism have spillover effects on the non-partner ant community?

Mutualism benefits partner species and theory predicts these partnerships can affect the abundance, diversity, and composition of partner and non-partner species. We used 16 years of monitoring data to determine the ant partner species of tree cholla cacti (Cylindriopuntia imbricata), which reward ants with extrafloral nectar in exchange for anti-herbivore defense. This long-term data revealed one dominant ant partner (Liometopum apiculatum) and two less common partners (Crematogaster opuntiae and Forelius pruinosus. We then used short-term characterization of the terrestrial ant community via pitfall trapping to sample partner and non-partner ant species across ten plots of varying cactus density. We found that the dominant ant partner tended a higher proportion cacti in plots of higher cactus density, and was also found at higher occurrence within the pitfall traps in higher density plots, suggesting strong positive feedbacks that promote ant partner occurrence where plant partners are available. Despite the strong association and increased partner occurrence, ant community-wide effects from this mutualism appear limited. Of the common ant species, the occurrence of a single non-partner ant species was negatively associated with cactus density and with the increased presence of L. apiculatum. Additionally, the composition and diversity of the ant community in our plots were insensitive to cactus density variation, indicating that positive effects of the mutualism on the dominant ant partner did not have cascading impacts on the ant community. This study provides novel evidence that exclusive mutualisms, even those with strong positive feedbacks, may be limited in the scope of their community-level effects.

互利共生可使共生伙伴物种获益，相关理论亦预测此类共生关系会对伙伴物种与非伙伴物种的丰度、多样性及群落组成产生影响。本研究依托16年的监测数据，明确了树团扇仙人掌（Cylindriopuntia imbricata）的共生蚂蚁类群：该仙人掌通过分泌花外蜜（extrafloral nectar）作为报酬，换取蚂蚁提供的抗草食动物防御服务。该长期监测数据显示，树团扇仙人掌的共生蚂蚁包含1个优势类群（Liometopum apiculatum）与2个稀见类群（Crematogaster opuntiae及Forelius pruinosus）。随后，本研究采用陷阱诱捕法（pitfall trapping）对地表蚂蚁群落开展短期表征采样，在10个仙人掌密度各异的样地中采集共生与非共生蚂蚁类群。研究发现，在仙人掌密度更高的样地中，优势共生蚂蚁占据的仙人掌比例更高，且该蚂蚁在高密样地的陷阱诱捕检出率也更高，这表明存在强烈的正反馈机制：当存在植物共生伙伴时，会促进共生蚂蚁的定殖。尽管二者存在强关联且共生蚂蚁的检出率有所提升，但该互利共生对整个蚂蚁群落的影响似乎较为有限。在常见蚂蚁类群中，仅1种非共生蚂蚁的检出率与仙人掌密度及Liometopum apiculatum的丰度提升呈负相关关系。此外，本研究样地内蚂蚁群落的组成与多样性对仙人掌密度变化并不敏感，这表明该互利共生对优势共生蚂蚁的正向作用并未对整个蚂蚁群落产生级联效应。本研究提供了新的证据：即便是存在强正反馈的专性互利共生关系，其群落级效应的范围也可能受到限制。