Invasive ants cause size-biased differences in seed survival

Background/Question/Methods

Although studies often focus on the direct effects of invasive species on native taxa, invasive species may also change native communities by altering interactions among native species. The red imported fire ant (Solenopsis invicta) has the potential to alter seed survival directly (i.e. by consuming seeds), but also indirectly, by altering behavior of native arthropods and rodents. We tested the effects of invasive fire ants on rodent and arthropod granivory, at eight replicated sites, by observing seed removal from seed depots that differed in the granivore guild(s) given access (arthropods and rodents or arthropods only), and proximity to an active fire ant mound (0.1m or 4.0m). We used seeds of two different sizes to elucidate the indirect effects of fire ants on different plant species; sand blackberry (Rubus cuneifolius), a small-seeded species, is consumed by both ants and rodents, while black cherry (Prunus serotina), a large-seeded species, is consumed primarily by rodents. Use of these two differently sized seeds allowed us to disentangle direct and indirect effects of fire ants; because fire ants cannot directly consume large-seeded P. serotina, any changes in P. serotinaremoval with proximity to mounds were likely the result of ant-mediated changes in rodent granivory.

Results/Conclusions

We found that overall removal of P. serotina was low, regardless of distance from mound or exclosure type, indicating no impact of S. invicta on rodent removal of this species. In contrast, removal of R. cuneifolius depended on proximity to a mound as well as which granivore guilds were granted access. Removal of R. cuneifolius 4.0m from an active mound was 45-50% higher than next to a mound, but only when rodents were excluded. Our results suggest that seed removal by native arthropods is influenced by competition with both invasive fire ants and native rodents. The removal of R. cuneifolius near fire ant mounds was low, with no difference between depots that allowed or excluded rodents. Away from fire ant mounds, the decrease in seed removal by arthropods observed when rodents are allowed access may have been caused by inadvertent seed burial by foraging rodents, which reduced availability of seeds to arthropods. Our results indicate that exotic and native species can interact to collectively affect seed survival, the consequences of which may be influenced by seed size. By differentially affecting seed survival of various species, these complex interactions have the potential to alter plant community composition.