Department of Biology, University of Florida, Gainesville, Florida, 32611, USA.
Mpala Research Centre, P.O. Box 555, Nanyuki, Kenya.
Ecology. 2018 Feb;99(2):335-346. doi: 10.1002/ecy.2104. Epub 2018 Jan 12.
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.
虽然基础物种可以在景观尺度上稳定生态系统，但它们的生存能力往往是由较小尺度上的关键相互作用所支撑的。金合欢树是一种基础树种，占东非黑棉草原生态系统木质覆盖物的>95%。它的优势是由与几种共生蚁种的关键共生相互作用支撑的，在这种相互作用中，它为蚁种提供住房（肿胀的刺）和来自额外花蜜腺（EFN）的富含碳水化合物的花蜜。作为回报，它从巨型食草动物的灾难性破坏中获得保护。Crematogaster mimosae 是该系统中生态上占优势的共生蚁种，也提供最高的保护服务。除了照顾 EFN 外，C. mimosae 还为含糖丰富的蜜露照顾介壳虫。我们研究了介壳虫在这种专门的蚁-植物相互作用中的作用。具体来说，这个假定的冗余的第三个伙伴是否通过使蚂蚁成为树更好的保护者来加强蚁-植物共生关系？或者，它是否通过对树木造成成本而削弱了共生关系，同时对蚁-植物共生关系没有额外的好处？我们将观察调查与两项规模操纵实验相结合，发现有证据表明第三个伙伴加强了蚁-植物共生关系。与有介壳虫存在的树木相比，实验去除介壳虫的树木受到大象破坏的可能性增加了 2.5 倍，这种情况持续了 10 个月。保护的减少是由介壳虫去除导致蚂蚁群体规模和人均基线活动和防御行为减少引起的。我们还发现，当 EFN 被实验性地减少时，蚂蚁会增加对介壳虫的护理以及介壳虫在树上的密度。因此，在这个系统中，介壳虫和 EFN 可能是互补的，而不是冗余的资源，当 EFN 产量低（例如在这个半干旱生态系统的年度干旱期）时，介壳虫会使蚂蚁受益。这项研究表明，第三个伙伴加强了一种蚁-植物共生关系，这种关系有助于稳定整个生态系统。
