Trait-similarity and trait-hierarchy jointly determine fine-scale spatial associations of resident and invasive ant species

Interspecific competition, a dominant process structuring ecological
communities, is influenced by species' phenotypic differences.
Limiting similarity theory holds that species with similar traits should
compete intensely ("trait-similarity"). In contrast, competing
theories including modern coexistence theory emphasize that species with
traits conferring competitive advantages should outcompete others
("trait-hierarchy"). Either or both of these mechanisms may
drive competitive exclusion, but their relative importance and interacting
effects are rarely studied. Here, we explore empirically whether
trait-similarity and trait-hierarchy can explain fine-scale spatial
associations observed between invasive and native ant species in a
tropical assemblage. We find that pairwise co-occurrences between the
invasive red imported fire ant Solenopsis invicta and 28 other species
across relatively homogenous grasslands can be explained largely by an
interaction of trait-similarity and trait-hierarchy in a single
morphological trait, relative pronotum width. Specifically, higher
trait-hierarchy values are associated with negative co-occurrences;
however, these effects are counteracted when species are increasingly
dissimilar in their trait ranges. These findings are consistent with the
notion that limiting similarity and competitive hierarchies are
interactive rather than discrete mechanisms driving competitive exclusion.