An invasive ant increases an ecosystem service: The impact of co-introduced partners on seed-dispersal mutualisms

Background/Question/Methods

Myrmecochory, or ant-mediated seed dispersal, is an important ecosystem service, in which ant assemblages play an integral role in structuring plant assemblages. Originally described as a generalized or diffuse mutualism, it has more recently been described as unevenly diffuse with certain species of ants playing disproportionally large roles as seed-dispersers (keystone dispersers). Thus, myrmecochory is likely sensitive to changes in ant partner identity, and some invasive ants have shown to cause collapses in seed dispersal mutualisms. Myrmica rubra is a seed-disperser in its native range and has been co-introduced with its plant partners into North America. In its introduced range, it co-occurs with a native keystone disperser, Aphaenogaster rudis, and native plants. We examined the influence of co-introduced mutualists on seed-dispersal mutualisms and evaluated the potential for ‘invasional meltdown’ (i.e., invasion success facilitated by introduced species). In a field experiment, we set up mesocosms (1m diameter) containing native and invasive ants and plants. Ant assemblages were manipulated to mimic natural distributions and densities: M. rubra and A. rudis at low (single colonies) or high density (two colonies), co-occurring (single colonies of each species), and no ant controls. We also conducted lab and field behavioral trials to further examine these interactions. 

Results/Conclusions

Compared to A. rudis, M. rubra dispersed more seeds of all plant species and at faster rates in both the mesocosm experiment and in field trials. Seed dispersal decreased when these species co-occurred in low resource conditions (i.e., for a plant that produced seeds early in the season). Myrmica rubra also increased the performance (seedling emergence and biomass) of the invasive herb, Chelidonium majus, compared to A. rudis, because dispersed seedlings benefitted from reduced competition from parent plants. Ant species did not differ in their preference for seeds, with both species having high preferences for the invasive plant, C. majus. This study suggests that M. rubra acts as a superior seed disperser in this ecosystem, likely enhancing this ecosystem service. Our results also provide some evidence for ‘invasional meltdown’ as M. rubra facilitated the success of C. majus. Emergence of native plants will be measured in the spring to see how the dynamics of these co-introduced species influence native plant assemblages.