Colonization and extinction mediate environmental effects on the phylogenetic diversity of invertebrate communities

Phylogeny offers a powerful framework for understanding mechanisms driving community assembly. Yet, most empirical studies in community phylogenetics rely on observational approaches. In this study, we explore how two important drivers of community assembly-habitat size and predator presence-shape species richness and phylogenetic relatedness of prey communities by altering colonization and extinction processes. Using bromeliad invertebrate communities as our study system, we combined surveys of natural communities with experiments that manipulated habitat size and predator presence. Colonization and extinction were isolated in separate experiments to test whether effects of habitat size and predator presence differed across stages of community assembly. Following species-area theory, we expected larger habitats to increase species richness and, given the strong consumptive effects of the top predator (a damselfly larvae), we expected species richness to decline in the presence of predators. Under a community phylogenetics framework, if traits mediating responses to these factors are phylogenetically conserved, we expected the phylogenetic structure of the community (i.e., relatedness) to have deterministic patterns along both gradients. Specifically, if habitat size functions as an environmental filter, small bromeliads would host phylogenetically clustered assemblages; alternatively, if it functions as a mediator for coexistence among close relatives, larger habitats would exhibit greater relatedness. Likewise, we expected the generalist top predators to increase relatedness when closely related taxa have shared defensive traits. As traits mediating community assembly may vary in their phylogenetic distribution across lineages, we also anticipated relatedness patterns to vary across taxonomic scales. We found a positive effect of habitat size on species richness, which was driven by colonization mechanisms. Habitat size also affected relatedness, but the direction depended on the taxonomic scale, with positive relationships at broad scales and negative relationships at narrower scales. By contrast, predators reduced species richness through extinction mechanisms, although these effects were masked in natural communities by continuous replacement of individuals through colonization. Predator effects on relatedness were variable across taxonomic scales, suggesting the involvement of multiple traits at different phylogenetic depths. Together, our findings highlight the complex interplay between environmental factors and community assembly in structuring taxonomic and phylogenetic dimensions of diversity.