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

Abstract

Invasive insects represent major threats to ecosystems worldwide. Yet their effects on the functional dimension of biodiversity, measured as the diversity and distribution of traits, are overlooked. Such measures often determine the resilience of ecological communities and the ecosystem processes they modulate. The fire ant Solenopsis invicta is a highly problematic invasive species occurring on five continents. Its impacts on the taxonomic diversity of native ant communities have been studied but its impacts on their functional diversity are unknown. 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. We calculated the functional diversity of individual species, including the trait variation from intraspecific polymorphisms, and 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. Our results suggest that studies based only on taxonomic measures of diversity or indices describing trait variety risk underestimating the full ramifications of invasions. Investigating the diversity and distributions of traits at species, community and landscape levels can reveal the cryptic impacts of alien species which, despite causing little taxonomic change, may substantially modify the structure and functioning of ecological communities.