A case for studying biotic interactions in epiphyte ecology and evolution

Biotic interactions are widely accepted as an important driver of ecological and evolutionary patterns, contributing to the structure of systems as diverse as tropical tree seedlings, intertidal barnacles, and wildflower-pollinator networks. Species interactions within a trophic level, such as competition and facilitation, can drive patterns of community change over time, yielding both fundamental ecological theories of succession as well as insight vital to predicting biodiversity conservation priorities. One system in which biotic interactions are poorly explored is epiphytes, or structurally dependent, non-parasitic organisms. This is a topic of broad interest because epiphytes—including vascular plants, bryophytes, and lichens—exist in practically all terrestrial ecosystems throughout the world. From lichens acting as pollution-sensitive indicator species in urbanized landscapes, to the multimillion-dollar commercial market for horticultural bromeliads, to tropical orchids representing striking examples of rapid speciation, epiphytes make substantial contributions to theory, biodiversity, ecosystem services, and the global economy. This review is the first to broadly synthesize the underlying biotic interactions important to epiphyte ecology and evolution. We first draw from theory to discuss where and when biotic or abiotic processes are likely stronger drivers of epiphyte dynamics. We then systematically review the literature across the major interaction modes, highlighting areas where different groups of epiphytes (e.g., vascular versus nonvascular) and ecosystems have contrasting patterns or expectations. Throughout, we illustrate where research efforts have focused and where large gaps in knowledge exist. Our review is organized around the major biotic interactions, rather than the specific organisms interacting with the epiphytes, to highlight general processes and set epiphytism within the framework of ecological and evolutionary theory. Our review encompasses pollination and dispersal, intratrophic facilitation and competition, mycorrhizal mutualisms, epiphyte-host interactions, parasitism and pathogens, and herbivory, focusing on the impact of these interactions on the epiphyte. Finally, we provide a simple conceptual framework distilling open questions in the field, expand our findings to the community and ecosystem level, and summarize the biodiversity conservation implications of ignoring biotic interactions in epiphytes. Our synthesis brings together currently disparate literature from tropical and temperate systems on vascular and nonvascular plants and lichens. We hope our review stimulates further research and inspires cross-disciplinary collaboration.