TOWARDS AN ASSESSMENT OF PHANEROZOIC TRENDS IN BETA DIVERSITY: THE IMPORTANCE OF ENVIRONMENTAL AND GEOGRAPHIC SCALE

Recent assessments of Phanerozoic diversity appear to confirm that alpha (“within-community”) diversity increased significantly through the Phanerozoic. It has also been suggested that, in aggregate, global diversity during the same interval did not increase as substantially as once envisaged. This raises an important question: was there a trend in beta (“between-community”) diversity through the Phanerozoic? The answer to this question is essential for determining the aggregate, global trend, because, if the increase in alpha diversity outpaced the aggregate increase, the only way to accommodate this difference is with a DECREASE in beta diversity. On the other hand, if beta diversity remained unchanged or even increased, and assuming that alpha diversity also increased, it would follow that recent suggestions of a muted Phanerozoic global increase may be in error.

In our presentation, we will discuss our initial efforts to assess the Phanerozoic history of beta diversity, starting with a comparison of two exemplars: the Caradocian stage of the Ordovician and the Eocene epoch of the Paleogene. In this context, it is important to consider the geographic or environmental scale at which the compositions of collections are compared to one another (the heart of any assessment of beta diversity). While it might be argued that beta diversity during any interval should be analyzed with samples collected from paleocommunities arrayed along local environmental gradients, there are clearly much broader spatial scales at which biotic turnover is also meaningful, including the degree of differentiation among major, paleocontinent-scale regions of the world. With this in mind, we are investigating the nature of biotic differentiation at multiple geographic and environmental scales, to determine: a) whether, during a given interval, the degree of biotic differentiation did, indeed, vary as a predictable function of geographic or environmental scale; and b) whether, for a given scale, the degree of biotic differentiation varies significantly between the intervals under investigation. This will permit a more sophisticated assessment of beta diversity trends than would be possible by focusing on just a single scale, or by attempting to summarize beta diversity for a given interval with a single approximation.