Data from: Introduced ants reduce interaction diversity in a multi-species, ant-aphid mutualism

Mutualisms contribute in fundamental ways to the origin, maintenance and organization of biological diversity. Introduced species commonly participate in mutualisms, but how this phenomenon affects patterns of interactions among native mutualists remains incompletely understood. Here we examine how networks of interactions among aphid-tending ants, ant-tended aphids, and aphid-attacking parasitoid wasps differ between 12 spatially paired riparian study sites with and without the introduced Argentine ant Linepithema humile in southern California. To resolve challenges in species identification, we used DNA barcoding to identify aphids and screen for parasitoid wasps (developing inside their aphid hosts) from 170 aphid aggregations sampled on arroyo willow Salix lasiolepis. Compared to uninvaded sites, invaded sites supported significantly fewer species of aphid-tending ants and ant-tended aphids. At invaded sites, for example, we found only two species of ant-tended aphids, which were exclusively tended by L. humile, whereas at uninvaded sites we found 20 unique ant–aphid interactions involving eight species of ant-tended aphids and nine species of aphid-tending ants. Ant–aphid linkage density was thus significantly lower at invaded sites compared to uninvaded sites. We detected aphid parasitoids in 14% (28/198) of all aphid aggregations. Although the level of parasitism did not differ between invaded and uninvaded sites, more species of wasps were detected within uninvaded sites compared to invaded sites. These results provide a striking example of how the assimilation of introduced species into multi-species mutualisms can reduce interaction diversity with potential consequences for species persistence.

互利共生作用（mutualism）对生物多样性的起源、维持与组织构建具有根本性贡献。外来入侵物种常参与互利共生作用，但此类现象如何影响本土共生伙伴间的互作模式，目前仍未完全阐明。本研究针对加利福尼亚南部的12对空间匹配的河岸研究样地，对比存在入侵物种阿根廷蚁（Linepithema humile）与无该蚁的样地中，牧蚜蚁（aphid-tending ant）、被牧蚜虫（ant-tended aphid）以及蚜虫寄生蜂（aphid-attacking parasitoid wasp）三者间的互作网络差异。为攻克物种鉴定难题，本研究借助DNA条形码（DNA barcoding）技术，对采自河柳（Salix lasiolepis）上的170处蚜虫聚集群开展物种鉴定，并筛查寄生在蚜虫宿主体内的寄生蜂。与未入侵样地相比，阿根廷蚁入侵的样地中，牧蚜蚁与被牧蚜虫的物种丰富度均显著更低。以入侵样地为例，本研究仅发现2种被牧蚜虫，且它们仅由阿根廷蚁放牧；而未入侵样地中则存在20种独特的蚁-蚜互作关系，涉及8种被牧蚜虫与9种牧蚜蚁。因此，入侵样地的蚁-蚜互作连接密度显著低于未入侵样地。本研究在14%（28/198）的蚜虫聚集群中检测到了蚜虫寄生蜂。尽管入侵与未入侵样地的寄生率并无显著差异，但未入侵样地中检测到的寄生蜂物种数显著多于入侵样地。本研究结果提供了极具代表性的例证，展示了外来物种融入多物种互利共生网络后，如何降低互作多样性，并可能对物种存续产生潜在影响。