The relative influence of biotic and/or abiotic factors shapes community assembly, and this balance may change with latitude. Patterns of functional richness, or the amount of functional space occupied by species as defined by their traits, can help differentiate community assembly processes. With increasing taxonomic richness, theory predicts that traits will fill more functional space when biotic filters limit the similarity of traits among species to allow coexistence (i.e., limiting similarity). However, abiotic factors demand common adaptations and cause functionally similar traits to persist resulting in functional redundancy (i.e., environmental filtering). Studies have shown environmental filtering and limiting similarity to be primary assembly mechanisms, with their relative importance changing with latitude or elevational gradients. We built multivariate functional richness indices (FRic) and examined the relationship between functional and taxonomic richness of sessile marine invertebrate communities from tropical and temperate regions to resolve assembly mechanisms. Linear and asymptotic (i.e., saturating) models were used to infer limiting similarity and environmental filtering, respectively. With increasing taxonomic richness, we expect to find a limit in trait space occupation in temperate regions due to environmental filters, while in tropical regions differentiation of traits and trait space occupation should increase due to stronger biotic interactions.
Results/Conclusions
We show that species within communities from temperate and tropical latitudes differ in their patterns of functional richness, suggesting differences in dominant assembly mechanisms. In temperate regions, functional richness is asymptotically related to taxonomic richness suggesting traits are functionally redundant, therefore environmental filtering may have a greater influence on community assembly. In the tropics, functional space occupation linearly increases with taxonomic richness indicating trait differentiation may occur as a mechanism to cope with stronger biotic interactions. We used a dimension of functional diversity, the volume of multivariate trait space occupation, to examine the relative influence of different mechanisms contributing to community assembly across large biogeographic gradients. Our data suggest abiotic and biotic factors influence community assembly in temperate and tropical regions, respectively. Additionally, the relative influence of biotic/abiotic filters across latitudes may help us understand the different mechanisms that contribute to the maintenance of contemporary patterns of biodiversity, a long-held debate among ecologist.