COLLABORATIVE RESEARCH: Landscape-scale consequences of mutualism disruption: invasive ants threaten a widespread ant-plant mutualism in East Africa

在整个自然界中，人们可以发现成对的物种生活在一起，为彼此提供服务，以实现彼此的利益。这些物种的相互作用被称为共生关系，它们被认为维持了世界上大部分的生物多样性。共生关系也是种子传播、授粉和养分循环等重要生态系统服务的基础。但是，当一个入侵物种取代了共生关系中的一个物种伙伴时会发生什么呢？这就是这个项目将在肯尼亚大草原上解决的问题，在那里，一种入侵的蚂蚁物种正在取代生活在金合欢树上的本地蚂蚁。本地蚂蚁通过攻击大象和其他吃树的食草动物来积极保护树木；作为交换，树木为蚂蚁提供食物（含糖的花蜜）和庇护所（肿胀的荆棘）。相比之下，一种入侵物种，大头蚁，杀死本地蚂蚁，但不保护树木。没有了当地的保镖，树木被大象吃掉和杀死的频率越来越高。该项目将评估与大头蚁入侵有关的树木损失的速度和程度。因为许多野生食草动物，包括濒临灭绝的黑犀牛，都以这样或那样的方式依赖树木，所以这个项目还旨在了解树木的减少如何影响野生动物的栖息地。这项研究提供了一个独特的机会来评估在一个拥有大量大象、长颈鹿和其他标志性大型哺乳动物物种的景观中，关键物种共生崩溃的多米诺骨牌效应。本项目将验证入侵蚁种巨头蚁（P. megacephala）对金合欢（Acacia drepanolobium）与本地蚁之间基础互惠关系的级联效应是依赖于环境的假设。在巨头象入侵的稀树草原上，大象预计会减少树木覆盖，从而降低土壤和植物氮(N)，并对野生动物的饲料和栖息地质量产生负面影响（因为A. drepanolobium是一种固氮剂）。相比之下，在没有大象的地方，P. megacephala的入侵应该会加速树木的生长，因为P. megacephala不诱导花蜜的产生，因此可能会降低宿主植物的代谢成本。从长期来看，这种共生关系的破坏预计会导致入侵地区更高的树木覆盖率和土壤氮。该项目将(1)量化在野生食草动物（特别是大象）存在和不存在的情况下，本地蚂蚁和入侵蚂蚁对树木生长、固氮、繁殖和生物量的影响；(2)利用人口统计学模型预测树木种群增长率；(3)利用遥感技术对野生动物存放区和野生动物灭绝区巨头棘入侵的传播速率进行评估；(4)评价巨头棘入侵对土壤和植物养分有效性及野生食草动物利用生境的影响。

Throughout the natural world, one can find pairs of species living together and providing services for one another to their mutual benefit. These species interactions are called mutualisms, and they are thought to maintain much of the world's biodiversity. Mutualisms are also the basis for important ecosystem services like seed dispersal, pollination, and nutrient cycling. But what happens when an invasive species displaces one of the species partners in a mutualism? That is the question this project will address in the savannas of Kenya, where an invasive ant species is displacing the native ants that live on acacia trees. Native ants aggressively defend trees by attacking elephants and other herbivores that eat trees; in exchange, trees provide food (sugary nectar) and shelter (swollen thorns) for the ants. In contrast, an invasive species, big-headed ants, kill native ants but do not defend trees. Without their native bodyguards, trees are being eaten and killed by elephants more frequently. This project will assess the rate and extent of tree loss associated with the big-headed ant species invasion. Because many species of wild herbivores, including endangered black rhinos, rely on trees in one way or another, this project also aims to understand how tree loss affects habitat for wildlife. This research provides a unique opportunity to assess the domino effects of a critical species mutualism breakdown in a landscape with an abundance of elephants, giraffes, and other iconic species of large mammals. This project will test the hypothesis that the cascading effects of the invasive ant species P. megacephala on the foundational mutualism between Acacia drepanolobium and native ants are context-dependent. In savannas invaded by P. megacephala, elephants are expected to reduce tree cover, thereby lowering soil and plant nitrogen (N) and negatively affecting forage and habitat quality for wildlife (since A. drepanolobium is an N-fixer). In contrast, where elephants are absent, P. megacephala invasion should accelerate tree growth, as P. megacephala does not induce nectar production and thus may reduce metabolic costs to host plants. Over the long-term, this mutualism breakdown is predicted to lead to higher tree cover and soil N in invaded areas. This project will (1) quantify the effects of native versus invasive ants on growth, N-fixation, reproduction and biomass of trees in the presence and absence of wild herbivores, especially elephants; (2) use demographic models to project tree population growth rates; (3) use remote sensing to assess the rate of spread of the P. megacephala invasion in both wildlife-present and wildlife-extirpated areas; and (4) assess the effects of P. megacephala invasion on availability of soil and plant nutrients and resulting habitat use by wild herbivores.