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.