ALPHA, BETA, AND GAMMA: DIVERSITY PARTITIONING DURING A MARINE BIOTIC INVASION, UPPER ORDOVICIAN, CINCINNATI ARCH

How local and regional ecosystems adjust to invasions of exotic species can provide important clues into the processes that govern local, regional, and ultimately global diversity. We determined how alpha (local), beta (between-habitat), and gamma (regional) diversity adjusted to a major marine biotic invasion during the Late Ordovician on the Cincinnati Arch. Our analysis is based on a data set of over 700 whole-fauna collections and over 40,000 individual specimen counts from six depositional sequences (C1-C6) and three depositional environments, ranging from shallow subtidal to offshore. Alpha diversity is calculated as the mean collection diversity for a given combination of depositional sequence and environment. Gamma diversity is given by the sample-standardized diversity for a sequence-environment combination. Beta diversity is calculated as the ratio of gamma to alpha diversity (Whittaker's beta) and as the quantified Jaccard coefficient between depositional environments within a sequence.

The Richmondian Biotic Invasion took place largely during the C5 sequence and is characterized by wholesale biofacies replacement with turnover including many of the dominant biofacies players. We find that diversity increased in the C5 sequence by 26% as a result of the invasion, but that the increase was not spread evenly across environments. Gamma diversity did not change in the shallow subtidal, but it increased by 40% in the deep subtidal environment. Likewise, alpha diversity did not change in the shallow subtidal, but it increased by13% in the deep subtidal. Overall, 68% of the gamma diversity increase in the C5 sequence was taken up by an increase in beta diversity in the deep subtidal environment. The increase in beta diversity may reflect an increase in species packing along a depth gradient or an increase in habitat heterogeneity caused by associated oceanographic changes.