Biodiversity Gradients in Obligate Symbiotic Organisms: A Case Study in Lichens in a Global Diversity Hotspot (NSF Dimensions)

James Lendemer and collaborators

Obligate symbioses are relationships between two or more species that depend entirely on each other for growth and survival. Such symbioses characterize some of the most common and ecologically important relationships on Earth, ranging from human gut bacteria and diseases to corals and specialized plant-pollinator relationships. Yet, many of these obligate symbioses are imperiled by unprecedented rates of environmental change and permanent biodiversity losses. Compared to single branches on the Tree of Life such as birds, flowering plants, or mammals, much less is known about the factors that facilitate or limit the distributions of the obligate symbioses that abound in nature. Moreover, research on biodiversity distributions has focused largely on abiotic factors (e.g., temperature, precipitation, elevation) rather than on biotic factors (i.e., interactions with other organisms) that influence geographical distributions. This project will uncover the factors that impact diversity and distributions of obligate symbiotic lichens as a model system. The geographic focus is the southern Appalachian Mountains of the southeastern U.S. – a global lichen diversity hotspot and unique natural laboratory. In addition, this project will develop human capital and infrastructure in U.S. lichenology through student training, collaborations, and outreach to resource managers and the public. It will establish a publicly available, high-quality dataset and improve physical and digital resources that advance lichen conservation for a global biodiversity hotspot.

Despite generations of research on, and proposed theories for, factors that generate and maintain biodiversity, consensus is lacking. Biotic factors have long been proposed as drivers, but they are rarely studied due to the difficulty of assessing the multitude of possible interactions. Because of the inherent biotic interaction between obligate symbionts, lichens offer a unique opportunity to explore both biotic and abiotic drivers of biodiversity across multiple dimensions. This project will utilize both field and genomic inventories of lichens in a global biodiversity hotspot. The research will generate and synthesize data from multiple symbiotic biodiversity dimensions initiated from a single information source: a unique museum voucher. Across local, regional, and landscape scales, the project will record site-specific metrics for phylogenetic (including taxonomic) and functional diversity together with site-specific metrics for a mostly unexplored genetic dimension, potential of diversity, that quantifies the availability of compatible symbiont propagules in the environment. Analysis of these metrics in light of biotic and abiotic variables enables assessment of factors that impact the dimensions of biodiversity. These analyses will also permit understanding of interactions among dimensions, for example, whether all are positively correlated and predicted by the same sets of variables, or in what contexts other types of correlations exist. This project is designed to yield two major conceptual advances in ecology and evolutionary biology. First, it will reveal new, emergent properties of biodiversity gradients in symbiotic organisms. Second, deconstructing constraints on individual partners of the symbiosis and quantifying feedbacks between and among them will make possible a full analysis, including biotic constraints, of the factors that impact diversity and distribution of the symbiotic organism as a whole.