BIOTIC IMMIGRATION EVENTS, SPECIATION, AND THE ACCUMULATION OF BIODIVERSITY IN THE FOSSIL RECORD (Invited Presentation)

Biodiversity accumulates separately at the local and global levels via processes that are mediated by geographic connectivity, isolation, and regional differentiation. Biotic immigration events (BIMEs), the large-scale dispersal of taxa from one biogeographic area to another, significantly impact geographic connectivity and regional differentiation. In this presentation, we present a conceptual model for biodiversity accumulation that links BIMEs and geographic isolation with regional (α) diversity, global (γ) diversity, and regional (β) diversity metrics. This conceptual model is then tested using seven case studies from the paleontological record: the Great Ordovician Biodiversification Event (GOBE), the regional Richmondian Invasion, the Hirnantian Migration Event, the Great Devonian Interchange, Neogene mollusk migrations, the Great American Biotic Interchange, and Beringian dispersals.

Although each case study presents certain idiosyncrasies, major patterns recur. First, intervals of active immigration are characterized by minimal to no successful speciation events, increased species richness within regions, reduced differentiation among areas, and limited impact on global species richness. Once dispersal pathways disconnect and areas become isolated, speciation within basins increases, which results in greater α-, β-, and γ-diversity. Case studies in which dispersal and vicariance regimes alternate, such as the Great Ordovician Biodiversification Event, exhibit profound global diversity increases.

The diversity partitions (α, β, γ) are semi-independent. Each increases and decreases within different contexts and under distinct ecological pressures, but they are linked through biogeographic processes. Migration events alone are insufficient to produce biodiversity accumulation. However, BIMEs are important facilitators of geographic range expansion, which produce widespread species and establish the foundation on which speciation processes can operate during the next interval of regional isolation. Oscillation of area connectivity and isolation can generate a robust and effective mechanism for substantial accumulation of new biodiversity and persistence of existing species.