It is well known that larger areas contain greater numbers of species than smaller ones. Species-Area Relationships (SARs), which have a log-linear relationship, have been well studied in present day terrestrial environments, and considerable research has been devoted to diagnosing differences among these environments in the rate at which diversity accumulates. These measurements, in turn, have been used to estimate diversity at various spatial scales, and have been used recently by conservation biologists to forecast the amount of diversity loss likely to be incurred by a given reduction in habitat area. However, there is currently little knowledge of SARs in oceanic settings, despite growing worries about the loss of marine species. Since large-scale diversity studies of the modern oceans are both challenging and expensive, the fossil record may be the most cost effective place to begin to examine these relationships.
In this study I compare and contrast marine invertebrate Genus-Area Relationships (GARs) across multiple spatial scales and environments within the fossil record. Recently there has been renewed interest in comparing the environmental properties of two broad scale marine settings: epicontinental seas and open-ocean-facing settings. Differences in water circulation, the steepness of onshore-offshore environmental gradients, and migration potential suggest that GARs may differ between the two systems.
This study presents two sets of genus-area analyses, conducted at different spatial scales. Chapter 1 focuses on regional assessments of GARs for epicontinental seas versus open-ocean-facing settings, based on Late Cretaceous marine invertebrate data from the Paleobiology Database; Chapter 2 is a finer-scale, field-based assessment of sample-to-sample and locality-to-locality GARs exhibited in Late Cretaceous strata of the Holz Shale, Santa Ana Mountains, California.
The regional approach in chapter 1, documents clear relationships between fossil genus richness and area for Late Cretaceous regions worldwide, but indicates that as area increases, genus richness increases more per unit area in epicontinental seas than in open-ocean-facing settings. This difference implies that there is a greater degree of compositional heterogeneity as a function of geographic area in epicontinental sea settings.
In the field-based study of the second chapter, the slope of the genus-area relationship on an outcrop scale was in the same range as the slope of regional open-ocean settings. This preliminary observation, based on just a single value, may simply be a coincidence, but it is also possible that the slopes of genus-area relationships are scale transcendent, consistent at both local and regional levels. Furthermore, an analysis of compositional variability within and among stratigraphic intervals at Silverado Canyon demonstrates that, on individual bedding plans, genus-richness does not vary as a function of distance. At the same time, when the relative abundances of genera are considered, samples that are more closely spaced on a given bedding plane exhibit significantly greater similarity than those that are further apart. This difference amplifies the findings of other studies that have demonstrated the importance of considering not only the presence and absence of species, but also their relative abundances, in considering the ecological significance of biodiversity trends at confined spatial and temporal scales.