Mutualistic interactions between species underpin biodiversity and ecosystem function, but may be lost when partners respond differently to abiotic conditions. Except for a few prominent examples, effects of global anthropogenic change on mutualisms are poorly understood. Here we assess the effects of urbanization on a symbiosis in which the plant Cordia nodosa house ants in hollow structures (domatia) in exchange for defense against herbivores. We expected to find that mutualist ants would be replaced in the city by heat‐tolerant opportunists, leaving urban plants vulnerable to herbivory. In five protected forest sites and five urban forest fragments in southeast Perú, we recorded the identity and heat tolerance (CTmax) of ant residents of C. nodosa. We also assayed their plant‐defensive behaviors and their effects on herbivory. We characterized the urban heat‐island effect in ambient temperatures and within domatia. Forest plants housed a consistent ant community dominated by three specialized plant ants, whereas urban plants housed a suite of 10 opportunistic taxa that were, collectively, about 13 times less likely than forest ants to respond defensively to plant disturbance. In the forest, ant exclusion had the expected effect of increasing herbivory, but in urban sites, exclusion reduced herbivory. Despite poor ant defense in urban sites, we detected no difference in total standing herbivory, perhaps because herbivores themselves also declined in the city. Urban sites were warmer than forest sites (daily maxima in urban domatia averaged 1.6°C hotter), and the urban ant community as a whole was slightly more heat tolerant. These results illustrate a case of mutualism loss associated with anthropogenic disturbance. If urbanization is representative of increasing anthropogenic stressors more broadly, we might expect to see destabilization of myrmecophytic mutualisms in forest ecosystems in the future.

中文翻译：

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城市化导致蚂蚁-植物共生中伴侣的转换和共生关系的丧失


物种之间的互惠互动是生物多样性和生态系统功能的基础，但当伙伴对非生物条件的反应不同时，这种相互作用可能会消失。除了少数突出的例子外，人们对全球人为变化对共生主义的影响知之甚少。在这里，我们评估了城市化对共生关系的影响，其中植物 Cordia nodosa 将蚂蚁安置在空心结构 （domatia） 中，以换取对食草动物的防御。我们预计会发现，城市中的共生蚂蚁将被耐热的机会主义者所取代，使城市植物容易受到食草动物的攻击。在秘鲁东南部的 5 个保护林地点和 5 个城市森林碎片中，我们记录了 C. nodosa 蚂蚁居民的身份和耐热性 （CTmax）。我们还分析了它们的植物防御行为及其对食草动物的影响。我们描述了环境温度和 domatia 内部的城市热岛效应。森林植物栖息着一个由三种特化植物蚂蚁主导的一致蚂蚁群落，而城市植物栖息着一套 10 个机会性分类群，这些分类群对植物干扰做出防御性反应的可能性总体上比森林蚂蚁低约 13 倍。在森林中，排斥蚂蚁具有增加食草的预期效果，但在城市地区，排斥减少了食草。尽管城市地区的蚂蚁防御能力很差，但我们检测到总站立食草动物没有差异，这可能是因为城市中的食草动物本身也有所下降。城市蚂蚁群落比森林遗址更暖和（城市蚂蚁的日最大值平均高出 1.6°C），城市蚂蚁群落整体的耐热性略高。这些结果说明了与人为干扰相关的共生主义丧失的案例。 如果城市化代表了更广泛的人为压力源的增加，那么我们可以预期未来森林生态系统中肉豆蔻共生关系的不稳定。