Top-down control by predaceous insects has rarely been demonstrated in terrestrial communities, but invasive species may be particularly suited to exert such control. The red imported fire ant, Solenopsis invicta, has strong interactions with spiders, other ants and phytophageous insects, and we predicted that it would exert top-down control, cascading to plants in early successional plant communities. In forest gaps in South Carolina, we measured levels of folivory, growth and fecundity for five common herbaceous plant species in plots where ants were present vs. excluded (ant exclusion study) and in plots with varying abundance of fire ants and sympatric ants (ant substitution study). For all plant species in the ant exclusion study, folivory was greater when fire ant abundance was low (P < 0.01). Light levels, spider abundance and herbivore abundance were also related to folivory for some plant species in both studies (P < 0.01). Mean folivory over the growing season was low for all plant species (mean amount of leaf tissue removed for the most-chewed species was 11%). Variation in plant growth and fecundity were not well explained or consistent for any plant species in either study (partial R2 ≤ 0.10 for eight significant variables related to aboveground vegetative biomass and three significant variables related to reproductive biomass). Our results indicated that fire ants are important in determining the level of folivory on early successional plants; however, their top-down influence was not enough to cascade and impact plant growth or fecundity.
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Vol. 146 • No. 1