Associate Professor University of Puget Sound Tacoma, Washington, United States
Background/Question/Methods
Biotic interactions are widely accepted as important drivers of ecological and evolutionary patterns, contributing to the structure of systems as diverse as tropical tree seedlings, intertidal barnacles, and wildflower-pollinator networks. One system in which biotic interactions are not well-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 and make substantial contributions to theory, biodiversity, ecosystem services, and the global economy. Here, we report quantitative results from a systematic review of existing literature on epiphyte biotic interactions. From our synthesis of 304 articles, we collected data on biotic interaction direction (positive, negative, neutral), interaction mode (e.g., pollination, epiphyte-epiphyte competition), broad taxonomic group of epiphytes (vascular plant, bryophyte, lichen), and growth form of vascular epiphytes (hemi- or holo-epiphyte). We highlight areas where different groups of epiphytes and ecosystems have contrasting patterns, and fit those into predicted expectations from theory. We provide a 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.
Results/Conclusions
Overall, our understanding of epiphyte biotic interactions has grown in recent years, with total number of publications increasing over time. However, biotic interactions, in their various forms, are still not a major point of attention in the field. Our collective research effort is uneven both taxonomically and across biotic interaction modes. For example, while over 100 of our 304 reviewed articles focused on vascular plant-pollinator interactions, fewer than 10 publications exist for any type of biotic interaction within the nonvascular epiphyte system. In lichens, herbivory was the most-studied interaction (16 publications), but herbivory was one of the least-studied interactions in both vascular epiphytes and bryophytes. We only found one publication that focused on the effect of parasites or pathogens on any type of epiphyte. Epiphyte biotic interactions have several implications for ecosystem-level characteristics, including bird diversity, nutrient cycling, and insect-mediated habitat cascades. Excitingly, epiphyte biotic interactions have been used in conservation and agroforestry, signaling opportunities for progress in applied ecological fields. All in all, our synthesis brings together currently disparate literature from tropical and temperate systems on vascular and nonvascular plants and lichens, and argues for more consideration of biotic interactions as contributors to epiphyte ecology and evolution.
