Plant–animal mutualisms, such as seed dispersal, are often vulnerable to disruption by invasive species. Here, we show for the first time how a non-ant invasive species negatively affects seed dispersal by ants. We examined the effects of several animal species that co-occur in a temperate deciduous forest—including native and invasive seed-dispersing ants (Aphaenogaster rudis and Myrmica rubra, respectively), an invasive slug (Arion subfuscus), and native rodents—on a native myrmecochorous plant, Asarum canadense. We experimentally manipulated ant, slug, and rodent access to seed depots and measured seed removal. We also video-recorded depots to determine which other taxa interact with seeds. We found that A. rudis was the main disperser of seeds and that A. subfuscus consumed elaiosomes without dispersing seeds. Rodent visitation was rare, and rodent exclusion had no significant effect on seed or elaiosome removal. We then used data obtained from laboratory and field mesocosm experiments to determine how elaiosome robbing by A. subfuscus affects seed dispersal by A. rudis and M. rubra. We found that elaiosome robbing by slugs reduced seed dispersal by ants, especially in mesocosms with A. rudis, which picks up seeds more slowly than M. rubra. Taken together, our results show that elaiosome robbing by an invasive slug reduces seed dispersal by ants, suggesting that invasive slugs can have profound negative effects on seed dispersal mutualisms.
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We are grateful for the help in the field provided by G. Cho, N. Macalasdair, T. H. Q. Powell, and S. Khadar. We thank A. Weis, J. Stinchcombe, S. Schneider, and other staff for providing support at Koffler Scientific Reserve at Joker’s Hill. Comments from two anonymous reviewers improved this manuscript. We acknowledge funding from NSERC’s Discovery Grant and Undergraduate Student Research Award programs, the Ontario Ministry of Economic Development and Innovation, and the University of Toronto.
Author contribution statement
SAMD, KMP, and MEF conceived and designed the experiments. SAMD and KMP implemented the experiments. SAMD, KMP, and MEF analyzed the data and wrote the manuscript.
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