THE RELATIONSHIP BETWEEN PROPORTIONAL RARITY AND ABUNDANCE DISTRIBUTIONS AT MULTIPLE GEOGRAPHIC SCALES

Most of the taxonomic richness within samples, communities, or regions is comprised from uncommon or rare taxa. At the same time, most of the specimens or occurrences within a typical bulk sample are derived from a few common or abundant taxa. Connolly et al. (2005) illustrated, in modern ecological settings, that the abundance distributions of taxa in a community will exhibit log-normal curve shapes, with many taxa occurring infrequently while fewer taxa are represented by a large number of individuals, more precisely illustrating the known relationship between taxonomic richness and rarity. Further, when increasingly larger geographic areas were sampled, the distributions changed shape to reveal a less truncated log-normal abundance curve. This change indicated that with increasingly large geographic sampling regions, more uncommon taxa were recovered from behind the so called “veil line,” of previously unsampled rare taxa.

In this research, I investigated how the abundance distributions of fossil taxa changed with increasing geographic area to see if the relationships observed by Connolly et al. could be observed with a dataset from the Paleozoic. The data used here were comprised of field and literature-derived samples from the Kope Formation of the type Cincinnatian of Kentucky, Indiana and Ohio (Upper Ordovician). To establish the relationship between geographic area and abundance distributions, data from this region were divided into equal-area binning schemes (geographic halves, quarters, ninths and sixteenths), partitioning the area using latitude and longitude boundaries. Log-normal distributions of these geographically binned data indicated that there was an unveiling effect, with larger geographic areas revealing proportionally more rare taxa. This finding was consistent with Connolly et al., although in this study, assuming a log-normal abundance distribution, there were still many apparently unsampled taxa hiding behind the veil line. It remains to be seen whether additional exhaustive sampling in the region could reveal more taxa and further exemplify the apparent relationship between geographic area and the shape of the abundance distributions.