Weak morphological constraints on species roles and partner identity in a facultative ant–plant mutualism

Abstract Mutualisms contribute substantially to biodiversity and ecosystem functioning, yet the factors structuring facultative mutualisms remain poorly understood. Trait-based approaches suggest that functional traits may shape species roles and interaction patterns by influencing resource access, exploitation efficiency, and competitive ability, but whether they constrain partner identity remains unclear. We studied a network of interactions between ants and plants bearing extrafloral nectaries (EFNs) in the Caatinga, a seasonally dry forest in northeastern Brazil. We observed ant–plant interactions over 12 months and analysed network structure, species roles, and trait associations. The network exhibited low connectance and moderate specialisation, consistent with a diffuse mutualistic system dominated by generalist species. None of the measured EFN traits were associated with plant species roles. In ants, species specialisation increased with relative clypeus size, whereas no other morphological trait remained significant after correction for multiple testing. We also found no association between the functional traits of interacting ant–plant pairs. Overall, our results provide limited evidence that morphological traits contribute to variation in species roles and no evidence that they constrain partner identity in this facultative mutualism. The association between relative clypeus size and ant specialisation suggests that specific morphological attributes may contribute to interaction selectivity, but the absence of relationships for most traits indicates that morphological constraints on network structure are generally weak. These patterns are consistent with ecological context and behavioural flexibility playing important roles in shaping diffuse ant–plant interaction networks.