Differences in foraging phenology dampen the impact of an invasive slug on ant-mediated seed dispersal of understory forest plants

Abstract

Seed-dispersal mutualisms are important ecosystem functions that, if disrupted, influence plant fitness and community structure. Anthropogenic stressors such as invasive species and climate change may independently or synergistically influence animal-mediated seed dispersal. Aphaenogaster sp. ants are the primary dispersers of myrmecochorous plants that produce diaspores or seeds with lipid-rich appendages (elaiosomes) that make them attractive to ants in eastern North American forest understories. An invasive slug, Arion sp., damages diaspores by consuming elaiosomes, rendering seeds unattractive to ants. Here, in one forest site, we examine if slugs negatively affect seed dispersal by assessing interactions between seed-dispersing ants and invasive slugs on four species of myrmecochores that release diaspores at different times. We performed an exclusion experiment, excluding slugs and ants to diaspores on depots, when diaspores of four myrmecochore species were released from fruits. We also conducted a similar experiment presenting diaspores of all species at one time in the field and slug preference trials in the lab to determine if particular myrmecochore species are vulnerable to the invasive slug. We found that slugs did not reduce dispersal by ants despite slugs damaging diaspores. This was because peak ant and slug activity differed, with slugs damaging most diaspores of early-dehiscing Sanguinaria canadensis, and ants dispersing most diaspores of later-dehiscing Trillium grandiflorum. When diaspores were presented at the same time, ants and slugs preferred the same species, T. grandiflorum. While we did not find overlap in the phenology of ant and slug interactions in this study, phenological interactions are likely context-dependent. Thus, high-preference species may be vulnerable to invasive slugs if interacting species overlap in different contexts, including under climate change.