PHANEROZOIC INCREASES IN ALPHA DIVERSITY AND EVENNESS: LINKED CONSEQUENCES OF INCREASED ECOSPACE USE

A true increase in alpha (within-assemblage) diversity between Paleozoic and late Cenozoic marine assemblages would suggest global biodiversity also increased, but concerns about taxonomic evenness bring the nature of the observed increase in alpha diversity into question. For example, two communities might contain the same total number of taxa, but the one in which the taxa were more evenly distributed in relative abundance could appear more diverse because of the difficulty of sampling rare species in a finite sample from a highly dominated (less even) community. In our database of fossil assemblages, rarefied genus richness and evenness both increase between the middle Paleozoic and the late Cenozoic. We assigned each genus in the dataset to an ecologic category (ecotype) from a theoretical ecospace based on tiering, motility, and feeding type. Genera classed in the same ecotype presumably interacted and competed more directly than genera in different ecotypes. In both the Paleozoic and the Cenozoic, the individual ecotypes within fossil samples have similar diversity-abundance structure (i.e., their rarefaction curves are similar). Similarly, the evenness within ecotypes does not change much between the eras (i.e., the number of abundant vs. rare taxa within ecotypes stays fairly constant). However, after sampling-standardization, the average Cenozoic sample contains more ecotypes than the average Paleozoic sample. Adding ecotypes to a paleocommunity tends to increase evenness because each ecotype tends to have some common taxa, and adding similarly common taxa to a sample increases evenness. Most of the increase in evenness is attributable to the greater number of ecotypes in Cenozoic samples. Adding ecotypes also increases alpha diversity because the taxa in the new ecotypes are new to the community. The increases in taxon richness and evenness between Paleozoic and Cenozoic assemblages resulted primarily from the addition of new ecotypes and their taxa, not from the addition of new taxa to already utilized ecotypes or from changes in the relative abundances of taxa within ecotypes. The increase in alpha diversity from the Paleozoic to the Cenozoic appears to be real and supports the traditional view that global diversity was greater in the late Cenozoic than in the Paleozoic.