Data from: Elevated atmospheric carbon dioxide concentrations promote ant tending of aphids

1. Animal mutualisms, which involve beneficial interactions between individuals of different species, are common in nature. Insect-insect mutualism, for example, is widely regarded as a keystone ecological interaction. Some mutualisms are anticipated to be modified by climate change, but the focus has largely been on plant-microbe and plant-animal mutualisms rather than those between animals. 2. Ant-aphid mutualisms, whereby ants tend aphids to harvest their honeydew excretions and, in return, provide protection for the aphids are widespread. The mutualism is heavily influenced by the quality and quantity of honeydew produced by aphids, which is directly affected by host plant quality. Since predicted increases in concentrations of atmospheric carbon dioxide (eCO2) are widely reported to affect plant nutritional chemistry, this may also alter honeydew quality and hence the nature of ant-aphid mutualisms. 3. Using glasshouse chambers and field-based open top chambers we determined the effect of eCO2 on the growth and nutritional quality (foliar amino acids) of lucerne (Medicago sativa). We determined how cowpea aphid (Aphis craccivora) populations and honeydew production were impacted when feeding on such plants and how this affected the tending behaviour of ants (Iridomyrmex spp.). 4. eCO2 stimulated plant growth but decreased concentrations of foliar amino acids by 29% and 14% on aphid-infested plants and aphid-free plants, respectively. Despite the deterioration in host plant quality under eCO2, aphids maintained performance and populations were unchanged by eCO2. Aphids induced higher concentrations of amino acids (glutamine, asparagine, glutamic acid and aspartic acid) important for endosymbiont-mediated synthesis of essential amino acids. Aphids feeding under eCO2 also produced over three times more honeydew than aphids feeding under ambient CO2., suggesting they were imbibing more phloem sap at eCO2. The frequency of ant tending of aphids more than doubled in response to eCO2. 5. To our knowledge, this is the first study to demonstrate the effects of atmospheric change on an ant-aphid mutualism. In particular, these results highlight how impending changes to concentrations of atmospheric CO2 may alter mutualistic behaviour between animals. These could include positive impacts, as reported here, shifts from mutualism to antagonism, partner switches and mutualism abandonment.1. 动物间互利共生指不同物种种群个体间的有益互作，在自然界中广泛存在。例如，昆虫间的互利共生被普遍视为关键生态互作。部分互利共生关系预计会因气候变化发生改变，但以往研究大多聚焦于植物-微生物与植物-动物互利共生，而非动物间的互作类型。 2. 蚁蚜互利共生是一类分布广泛的共生关系：蚂蚁通过照料蚜虫以获取其排泄的蜜露，同时为蚜虫提供保护。该共生关系很大程度上受蚜虫所产蜜露的质量与数量影响，而这一指标又直接取决于寄主植物的品质。由于已有大量研究表明大气二氧化碳浓度升高（elevated atmospheric carbon dioxide, eCO2）会改变植物的营养化学成分，因此这一变化可能也会改变蜜露质量，进而影响蚁蚜互利共生的本质。 3. 本研究借助温室培养箱与野外开顶式气室，探究了eCO2对紫花苜蓿（*Medicago sativa*）生长及其营养品质（叶片氨基酸含量）的影响；同时测定了取食此类植株的豆寇蚜（*Aphis craccivora*）的种群动态与蜜露产生情况，并分析了这一变化对虹臭蚁属（*Iridomyrmex* spp.）蚂蚁照料蚜虫行为的影响。 4. eCO2可促进植株生长，但分别使受蚜虫侵染与未受蚜虫侵染的植株叶片氨基酸浓度降低29%与14%。尽管eCO2环境下寄主植物品质有所下降，但蚜虫的生长表现未受影响，其种群数量也未因eCO2发生显著变化。蚜虫会诱导产生更高浓度的氨基酸，包括谷氨酰胺、天冬酰胺、谷氨酸与天冬氨酸，这类氨基酸对于共生菌介导的必需氨基酸合成至关重要。在eCO2环境下取食的蚜虫所产蜜露量是本底二氧化碳（ambient CO2）环境下的3倍以上，表明它们在eCO2环境下摄入了更多韧皮部汁液。响应eCO2，蚂蚁照料蚜虫的频率增加了一倍以上。 5. 据我们所知，本研究首次揭示了大气变化对蚁蚜互利共生关系的影响。尤为关键的是，这些结果凸显了即将到来的大气CO2浓度变化可能如何改变动物间的互利共生行为，其中既包括本研究观测到的正向影响，也可能出现从互利共生向拮抗作用的转变、共生伙伴的更替以及互利共生关系的废弃等多种情况。