Were Local Ecological Interactions Linked to Secular Trends In Alpha Diversity In Level-Bottom Marine Communities?

Fossil assemblages with high taxonomic richness also have high ecological richness (i.e., many modes of life). Bambach (1983) suggested that alpha diversity increased from the Paleozoic to the Cenozoic because new ecological modes of life were added to local ecosystems, and that diversification had depended on animals invading new, distinct niches. In one interpretation, this hypothesis implies "assembly rules" for local assemblages: once a particular ecological mode of life is occupied, competitors are excluded, and additional diversity requires using space or resources differently. However, analyses of level-bottom assemblages from the mid-Paleozoic and late Cenozoic reject structured assembly rules as the drivers of increased alpha diversity over time, implicating instead the general ecological effects of increased predation and disturbance. When organisms must balance a greater number of ecological needs, a greater number of ecological/morphological strategies become equally effective overall, as argued by Niklas and Marshall. In the Paleozoic, lower levels of predation and disturbance meant that feeding was the primary need, and only a few ecological strategies were optimal (two modes of life, both epifaunal and suspension feeding, accounted for over 80% of average local abundance in the data set). In the Cenozoic, animals must feed while avoiding intense predation pressure and frequent disturbance, and there are many ecological lifestyles that balance these demands. Therefore, no modes of life were superdominant; rather, a greater number were common or moderately common. Increased disturbance and predation may have facilitated the rise of initially less important modes of life by requiring animals to cope with several, rather than a few, equivalent needs, making a diversity of energetically more demanding ecological niches more viable. Local interactions such as predation and biological disturbance may be more important than competition in long-term alpha diversity trends, at least for the suspension feeders that dominate many marine fossil assemblages.