Ecological Aspects of a Late Ordovician (Cincinnatian) Epibiofacies Gradient: Understanding the Roles of Colonial Organisms

Detailed study of epibiont taxonomic composition and areal coverage of brachiopod-encrusting epibionts (mainly bryozoans) in the Ordovician of the Tri-state (OH, IN, KY) has permitted a rather refined epibiofacies zonation related, in part, to a light-sensitive/relative depth gradient. Several ecological trends can be seen along the gradient when epibionts are quantified by areal coverage as opposed to occurrence. Species richness, richness ratio (ratio of number of species occurring in a given sample/sum of all species in all samples), community equitability, and taxonomic composition along the gradient can be compared using only coverage, only occurrence, or both, with differing results. Richness, richness ratio, and percent coverage decrease significantly with depth/loss of light in the Ordovician of the Tri-state, and these trends are mirrored in extant epibiont communities seen on experimentally deployed shells in a modern environment. Understanding the ecological relationships of epibionts within a community may yield a framework for interpreting the trends seen through time.

Metrics of community equitability vary greatly when examined by methods of areal cover vs. simple counts of occurrences, and that occurrence data do not fully reflect the structure and organization of an epibiont community gradient, or epibiofacies. Patterns based on occurrence and areal coverage data are most similar in shallow depths (shallow euphotic zone). Differing reproductive strategies may be hypothesized based on comparison of areal coverage dominance with occurrence dominance. In particular, colonial epibionts (primarily bryozoans) with high occurrences may focus reproductive efforts on recruitment by spatting (many small, simple colonies), primarily sexual reproduction. Conversely, those with low numerical abundance (few colonies), but high areal cover on shells, when present, may exhibit higher colonial complexity, primarily asexual reproductive strategies, and greater overgrowth competitiveness. The distribution of such opportunistic vs. equilibrium reproductive strategies may vary along ecological stress gradients in light availability, oxygenation, and water turbulence.