Data from: Economy of scale: third partner strengthens a keystone ant-plant mutualism

While foundation species can stabilize ecosystems at landscape scales, their ability to persist is often underlain by keystone interactions occurring at smaller scales. Acacia drepanolobium is a foundation tree, comprising >95% of woody cover in East African black-cotton savanna ecosystems. Its dominance is underlain by a keystone mutualistic interaction with several symbiotic ant species in which it provides housing (swollen thorns) and carbohydrate-rich nectar from extra-floral nectaries (EFN). In return, it gains protection from catastrophic damage from mega-herbivores. Crematogaster mimosae is the ecologically dominant symbiotic ant in this system, also providing the highest protection services. In addition to tending EFN, C. mimosae tend scale insects for carbohydrate-rich honeydew. We investigated the role of scale insects in this specialized ant-plant interaction. Specifically, does this putatively redundant third partner strengthen the ant-plant mutualism by making the ant a better protector of the tree? Or does it weaken the mutualism by being costly to the tree while providing no additional benefit to the ant-plant mutualism? We coupled observational surveys with two scale-manipulation experiments and found evidence that this third partner strengthens the ant-plant mutualism. Trees with scale insects experimentally removed experienced a 2.5X increase in elephant damage compared to trees with scale insects present over 10 months. Reduced protection was driven by scale removal causing a decrease in ant colony size and per capita baseline activity and defensive behavior. We also found that ants increased scale-tending and the density of scale insects on trees when EFN were experimentally reduced. Thus, in this system, scale insects and EFN are likely complementary, rather than redundant, resources with scale insects benefitting ants when EFN production is low (such as during annual dry periods in this semi-arid ecosystem). This study reveals that a third-partner strengthens an ant-plant mutualism that serves to stabilize a whole ecosystem.尽管建群种（foundation species）可在景观尺度上稳定生态系统，但其存续能力往往依托于更小尺度下发生的关键种间相互作用（keystone interactions）。镰叶金合欢（Acacia drepanolobium）是一类建群乔木，在东非黑棉土稀树草原生态系统中，其木本植被覆盖占比超过95%。该物种的优势地位，源于其与多种共生蚁类之间的关键互利互作：金合欢为蚁类提供栖息结构（膨大棘刺）与花外蜜腺（extra-floral nectaries, EFN）分泌的富含碳水化合物的花蜜；作为回报，金合欢可获得保护，免受巨型草食动物带来的毁灭性破坏。含羞草举腹蚁（Crematogaster mimosae）是该系统中生态占优的共生蚁类，同时其提供的保护服务也最为高效。除了照料花外蜜腺外，含羞草举腹蚁还会管护介壳虫以获取其分泌的富含碳水化合物的蜜露。我们针对介壳虫在这一特殊蚁植互利互作中的角色展开了研究，具体而言：这一被认为是冗余的第三参与者，究竟是通过提升蚁类对树木的保护能力来强化蚁植互利关系？还是会因给树木带来额外成本，且未为蚁植互利关系提供额外益处，反而削弱该共生体系？我们结合野外观测调查与两组介壳虫操控实验，发现证据表明该第三参与者能够强化蚁植互利关系。在为期10个月的观测周期内，人工移除介壳虫的树木，其遭受大象破坏的程度较保留介壳虫的树木高出2.5倍。保护能力下降的原因在于，移除介壳虫会导致蚁群规模缩小，且工蚁的基础活动水平与个体防御行为均有所降低。此外我们还发现，当人工抑制花外蜜腺分泌时，蚁类会提升对介壳虫的管护强度，同时树木上的介壳虫密度也会上升。由此可见，在该系统中，介壳虫与花外蜜腺更可能是互补而非冗余的资源：在半干旱生态系统的年度旱季等花外蜜腺产量较低的情境下，介壳虫可为蚁类提供额外益处。本研究揭示，第三参与者能够强化蚁植互利关系，进而助力整个生态系统的稳定。