An invasive slug exploits an ant-seed dispersal mutualism
Plant-animal mutualisms, such as seed dispersal, are often vulnerable to disruption by invasive species. We examined the effects of native and invasive ants, invasive slugs, and native rodents on seed dispersal, seed predation, and elaiosome robbing of a native spring ephemeral, Asarum canadense. Like other ant-dispersed plants, A. canadense seeds have elaiosomes that attract seed-dispersing ants (myrmecochory), as well as other insects, rodents, and gastropods. These taxa may also effectively disperse seeds, or they may consume seeds (seed predation) or just their elaiosomes (elaiosome robbing).
We examined the potential interactive effects of different groups of organisms to determine whether elaiosome robbing or seed predation have negative effects on seed dispersal. We experimentally manipulated ant, slug, and rodent access to seed depots to determine which taxa dispersed, consumed, or elaiosome-robbed A. canadense seeds. We also video-recorded depots to determine what other taxa interact with seeds besides ants, slugs, and rodents. Finally, we used lab and mesocosm experiments to determine how elaiosome robbing by invasive slugs, as well as slug density, affect seed removal by native (Aphaenogaster rudis) and invasive (Myrmica rubra) ants.
Our field trials and video recordings showed that A. rudis ants were the main dispersers of A. canadense seeds, and that the invasive slug, Arion subfuscus, consumed elaiosomes without dispersing seeds (i.e., elaiosome robbing). Rodent visitation was rare, and rodent exclusion had no significant effect on seed or elaiosome removal. Although elaiosome robbing by slugs did not reduce seed dispersal by ants in the field, probably because ants and slugs did not co-occur at high enough densities, elaiosome robbing by slugs prevented ants from dispersing seeds in lab assays. Similarly, in field mesocosms, seed dispersal by ants was higher when slugs were absent. At high slug densities, the invasive ant, M. rubra, dispersed more seeds than A. rudis, and thus was less affected by elaiosome robbing by slugs, likely due to the quicker discovery rate of seeds by M. rubra. Overall, elaiosome consumption by slugs reduced the attractiveness of seeds to ants, preventing them from being dispersed. Taken together, our results show how elaiosome robbing by the invasive slug, A. subfuscus, reduces seed dispersal by the keystone ant disperser, A. rudis, and suggest that invasive slugs can have profound negative effects on seed dispersal mutualisms.