Although ecologists have extensively documented how communities change along environmental gradients, understanding the ecological processes that create those patterns has been more elusive. Recent initiatives to move beyond taxonomic based assessments and incorporate functional trait and phylogenetic information into community ecology holds great promise for deepening our understanding of community assembly. One approach is to examine whether coexisting species are functionally or phylogenetically more similar (underdispersed) or less similar (overdispersed) than expected from a random sampling of the regional species pool. This approach can be used to identify whether the dominant process is one that limits functional or phylogenetic similarity (such as competition) or one that favors species with similar traits (such as environmental filtering). Nonetheless, few studies have simultaneously examined patterns of functional and phylogenetic diversity to identify assembly processes along environmental gradients (e.g. altitudinal or climatic). Here, we present a novel conceptual framework for using patterns of functional and phylogenetic over- and underdispersion along environmental gradients to infer the strength of processes structuring communities along those gradients. We contend that this framework will give deeper insight into the mechanisms underlying community structure.
We consider all pair wise combinations of functional and phylogenetic under- and overdispersion and synthesize possible community assembly processes that could result in these combinations. We then describe the functional and phylogenetic patterns we would expect to occur along a hypothetical environmental gradient and connect these patterns to the processes that may drive them. For simplicity, we consider a gradient from benign to stressful conditions for organisms, so that our framework can be applied to any environmental gradient that possesses these conditions. We show how considering both functional and phylogenetic diversity can provide additional information or a different interpretation than would have been obtained by considering either set of information alone. The generality of this framework will facilitate the identification of processes that mold community structure along the environmental gradients.