GRADIENT ECOLOGY DURING A MARINE BIOTIC INVASION: UPPER ORDOVICIAN OF THE CINCINNATI ARCH

Multivariate ordination is a useful method for revealing underlying gradients that structure the composition of ecological communities. In marine data sets of sufficiently large scale, the primary gradient as indicated by axis 1 of eigenvector-based methods is commonly correlated with depth-related lithofacies or, equivalently, onshore-offshore position. Although ordinations are commonly performed on communities during times of ecological stability, we used ordination (DCA and NMDS) to characterize the nature of change in marine communities during a time of flux, namely, during the Richmondian (Late Ordovician) biotic invasion of the Cincinnati Arch. We conducted over 700 faunal censuses through the six depositional sequences over the entire exposure area of the Cincinnati Arch.

The first three sequences (C1 - C3) all display strongly depth-correlated biofacies along DCA axis 1. Where depositional sequences show similar coverage in terms of lithofacies, values of preferred depth and peak abundance are strongly correlated, indicating considerable ecological stability of individual taxa and ecological gradients as a whole. The early portion of the C4 sequence shares an ecological structure nearly indistinguishable from the C3 sequence, but the late portion of the C4 sequence displays a remarkable breakdown in the previously stable ecological gradients, with a loss of almost all characteristic taxa of the shallow subtidal and a consequent spread of a few ecological generalists across the entire shelf. The C5 sequence records the arrival of a large number of ecological invaders and its ordination structure indicates the presence of multiple strong gradients. Although depth-related biofacies can be recognized, their composition is not stable, as a result of the ongoing influx of new genera into the Cincinnati Arch region.