ADDITIVE DIVERSITY PARTITIONING OF A MARINE BIOTIC INVASION: UPPER ORDOVICIAN OF THE CINCINNATI ARCH

Understanding the factors that control diversity in large geographic regions requires a hierarchical approach, where diversity is examined at progressively larger geographic scales to determine the relative influence of each scale to the total diversity in the system. Additive Diversity Partitioning (ADP) is a relatively new approach developed by ecologists that is based on a hierarchical sampling scheme and permits partitioning of regional diversity into alpha and multiple beta components. This approach holds great promise for evolutionary paleoecology because it is possible to determine how the components of diversity in a region, or globally, change over time in response to environmental or biotic perturbations.

Here, we apply ADP to understand how the components of regional diversity changed as a result of a marine biotic invasion in the Upper Ordovician of the eastern United States. We partition diversity into three components (Alpha - collections, Beta1 - within facies, Beta2 -- between facies) that sum to give regional diversity. We investigated two metrics of diversity, taxonomic richness and dominance (Simpson's D). We found that the largest component of regional richness in the upper Ordovician occurs within facies (Beta1), reflecting both habitat heterogeneity and a depth-related gradient component within this coarse subdivision of the seafloor. However, the increase in regional diversity that results from the marine biotic invasion is proportionately larger in the between-facies Beta2 component and can be explained by larger increase in richness in deep subtidal environments. Simpson diversity also increases more in deep subtidal environments as a result of the invasion, suggesting that the invaders were also the dominant taxa within communities.