Paper No. 4
Presentation Time: 8:45 AM


HOLLAND, Steven M., Department of Geology, Univ of Georgia, Geology Building, Athens, GA 30602 and SCLAFANI, Judith A., Department of Geosciences, Pennsylvania State University, State College, PA 16801,

Marine invertebrate communities show an increasing mode and variance of Hubbell's theta through the Phanerozoic, with theta ranging from 2-15 in the early Paleozoic and 2-35 in the Mesozoic-Cenozoic. This pattern does not appear to be caused by changes in sampling intensity, depositional environment, lithology, lithification, or preservation of aragonite. Although Hubbell's theta can be treated purely as a shape parameter of a species relative-abundance distribution with no implications for process, Hubbell's theta takes on important implications if communities follow neutral rules of assembly in which individuals of all species are competitively equal. If neutrality holds, Hubbell's theta links diversity and relative abundance across a broad range of scales, from local or alpha levels through provincial scales, and it explains the parallel Phanerozoic increase in evenness.

If neutrality is supported, it would allow for a linkage of diversity across a wide range of scales, providing a mechanism explaining the Phanerozoic correlation of global and alpha diversity. Arguments based on additive diversity partitioning questioned the importance of the linkage between alpha and global diversity, and these arguments suggested that the primary cause of the Phanerozoic rise in diversity must lie in an increasing number of provinces through the Phanerozoic or their increasing distinctiveness. If marine communities behave as neutral systems, the increase in Hubbell's theta would suggest that the rise in Phanerozoic diversity was also driven by diversification at the alpha through provincial levels.

In addition, the Phanerozoic increase in Hubbell's theta would also be reflected in an increase in evenness. Such an increase in evenness was previously seen as a complication because rarefaction requires communities to have the same evenness. If neutrality holds, an increase in evenness is an expected result of an increase in theta: it would be a symptom of increasing diversification, not a complication.