Functional traits drive the competitive assembly of mangrove ant communities and influence colony performance in competition mesocosms

1. Studies along broad spatial and habitat gradients evidence that organisms’ traits can influence community assembly through their impact on dispersal and environmental filtering. However, the role of traits in structuring local faunal communities within habitats remains poorly understood. In particular, the often-assumed role of traits in affecting the outcome of competition among animal species is rarely tested. 2. We coupled a field study with a mesocosm experiment to explore how ant species’ traits shaped competition and community structure, leveraging a small mangrove ecosystem which conceivably imposed minimal dispersal- and environmental filtering effects on ant community assembly. 3. We first surveyed the ant communities inhabiting 115 mangrove trees using over 2000 carbohydrate and protein baits, and directly measured multiple morphological, physiological (critical thermal maximum) and dietary (stable isotope trophic position) traits of all ant species. We then coupled co-occurrence network analyses with meta-analytical models to uncover the trait-based mechanisms structuring species co-occurrences in the field. Finally, in a mesocosm experiment, we reared 100 colonies of eight ant species from the mangrove over 30 days under different competition treatments to investigate trait-mediated effects of competition on ant colony performance. 4. Patterns of ant species co-occurrences and bait recruitment indicated strong competition for limited protein-rich resources. Accordingly, dissimilarities in three traits – eye size, pronotum width and antennal scape length – consistently explained species co-occurrences, suggesting that the communities were competitively assembled by a partitioning of resource acquisition strategies among species. Species co-occurrences were also to a lesser extent explained by similarities in critical thermal limits, suggesting mild environmental filtering. 5. In the mesocosm experiment, increasing hierarchical differences in eye size and pronotum width between neighbouring ant colonies exacerbated interspecific competitive effects on colony survival and growth. 6. Our results empirically demonstrate that traits linked to resource acquisition influence competition outcomes and community structure in ants. Importantly, they also suggest that the effects of species trait differences on competition are context-dependent: whereas dissimilarities in species’ traits facilitated resource partitioning in the mangrove, hierarchical differences in trait values distinguished species’ competitive abilities for shared resources in the mesocosms.