| The major goal of comparative paleoecology is to understand the origin and maintenance of species richness within communities. Traditional paleoecological methods have focused on using taxonomical or morphometrical methods, but these approaches are limited because they require significant similarity in the units that are compared. Other, more ecological approaches have made progress, but their success is limited by the synoptic scale at which such analyses are typically undertaken. What is needed is a method for comparing ecological entities that is less dependent on taxonomy, that is derived from theory, and that is applicable to any ecological unit, whether an individual, a community, or a lineage. In this dissertation, I present such a method, called the ecospace framework, that can operationalize the functional traits, or adaptive strategies, of ecological entities. Its development necessitates the creation of a new ecological unit that can be used to measure the ecological disparity of these entities. This new unit, the hutch, is defined as a discreet combination of functional traits describing the basic autecological characteristics of ecological entities. In combination with a measure of the dispersion of these hutches within communities, it is possible to distinguish the large-scale ecological trends in the history of natural communities: redundancy, partitioning, and expansion.; As an example of the application of these methods, I evaluate ecological diversification of marine communities within a single habitat, the deep-subtidal, soft-substrate marine habitat. In Chapter One, the ecospace framework is applied to a comparison of thirteen Ordovician and fifty modern marine assemblages spanning 450 million years. Although these biotas have many differences in taxonomy, they are rather similar functionally, especially when the effects of fossilization are considered. Although modern assemblages have greater numbers of free-living, mobile infauna, assemblages from both periods are equally rich in terms of the numbers and dispersion of hutches. In Chapter Two, I briefly present an efficient and accurate method for estimating the skeletal body volume for a variety of marine invertebrates, spanning many sizes, shapes, taxonomic affinities, and ecological habits. Although this analysis has little direct bearing on our understanding of the ecological diversification of marine life, it has important applications for incorporating body size into paleoecological studies. Finally in Chapter Three, I conclude with an analysis of the ecological diversification of nearly 450 marine assemblages during the Cambrian through Devonian, the first 200 million years of apparent animal life. During this interval, average species richness within assemblages increased fourfold from approximately eight during the Cambrian to thirty during the Silurian, possibly in concert with dramatically increasing population density and decreasing evenness. While changes in the functional traits of species within these assemblages can be linked to taxonomical changes, and in particular to the transition from primarily raptorial, trilobite-dominated biotas in the Cambrian to primarily filter-feeding, brachiopod-dominated biotas in the later Paleozoic, ecological disparity and taxonomical diversity describe largely different patterns. Cambrian assemblages also contain fewer numbers of hutches than later Paleozoic assemblages, but all are equally disparate when standardized to a common number of species. Although these assemblages have changed appreciably in terms of their general functional strategies, the overall organization of these biotas has remained remarkably constant. |
