The cryptic impacts of invasion: functional homogenization of tropical ant communities by invasive fire ants

Invasive insects represent major threats to ecosystems, with known impacts on the taxonomic diversity of invaded communities. However, their effects on the functional dimension of biodiversity, measured as the diversity and distribution of traits, are poorly known. Such measures can determine the resilience of a community and of the ecosystem processes it modulates. The fire ant Solenopsis invicta is a widespread and highly problematic invasive species. Its impacts on the taxonomic diversity of native ant communities have been widely studied, but not its impacts on their functional diversity. Comparing invaded and uninvaded plots in tropical grasslands of Hong Kong, we investigated how the presence of S. invicta affects the diversity and distribution of ant species and traits within and across communities, the functional identities of communities, and functionally unique species. Using a trait-probability density framework, we estimated the functional diversity of individual species, including the trait variation from intraspecific polymorphisms. We then scaled up these values to calculate functional diversity at the community level. Invasion had only limited effects on species richness and functional richness, which were 13% and 8.5% lower in invaded communities respectively. In contrast, invasion had pronounced effects on taxonomic and functional composition due to turnover in species and trait values. Furthermore, invaded communities were functionally more homogeneous, displaying 23% less turnover and 56% more redundancy than uninvaded communities, as well as greater clustering and lower divergence in trait values. Invaded communities had fewer functionally-unique individuals and were characterized by ant species with narrower heads and bodies and shorter mandibles. Synthesis and applications. Our results suggest that the full ramifications of invasions are likely to be underestimated if based solely on taxonomic measures of diversity or indices describing trait variety. Invasions by S. invicta led to significantly altered functional identity and increased functional homogenization of native ant communities, but only marginally lower taxonomic and functional richness. Investigating the diversity and distributions of traits at species, community and landscape levels can reveal the 9cryptic impacts9 of invasions which, despite causing little taxonomic change, may substantially modify the structure and functioning of ecological communities.