Paleontological Views on Diversity Saturation: Historical Context and Future Directions

A significant controversy today about global marine diversity is whether it has grown more or less exponentially over the Phanerozoic, or have there been limits to its growth. Both of these models have implications for taxon dynamics of local and regional ecosystems. If global diversity has increased exponentially, then it implies that diversity of local and regional diversity likewise increased and they are not saturated with taxa. If global diversity has had limits, it implies that taxon diversity has also been saturated at local and regional levels. Historically, marine paleocommunities and regional ecosystems have been viewed as stable entities, changing little in taxonomic composition or abundance relationships for millions of years. Over the last decade this view has begun to change as many studies show paleocommunities and regional ecosystems are characterized more by change than by stability, and the evidence points to a lack of diversity saturation. Future work in understanding the controls on regional ecosystem diversity should investigate how and why saturation, invasion susceptibility, taxonomic turnover, and abundance relationships vary across habitats, among taxa, and through time. Answering these questions will require the quantitative analysis of large data sets collected in a well-constrained temporal and environmental framework. This approach will be illustrated with an example of the effects of a Late Ordovician marine invasion on regional ecosystem structure and stability.