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42057-GB8
Is Rarity Tied to Extinction During Background Intervals? A Case Study Using Mollusk Species From the Mid-Atlantic Coastal Plain
Introduction.- Ecological studies suggest that rare taxa are more likely to go extinct than abundant ones, but the relationship between rarity and extinction in the fossil record has received surprisingly little attention (McKinney and Frederick 1992; Lockwood 2003). The purpose of this research is to examine the link between rarity (defined here to include mean abundance, spatial and temporal variability in abundance, geographic range, and habitat specificity) and extinction during background intervals in the molluscan fossil record of the mid-Atlantic Coastal Plain. I am focusing on three questions:
(1) Are rare taxa more likely to go extinct?
(2) Does the relationship between rarity and extinction differ in mass versus background extinction intervals? And
(3) Does selectivity differ according to metric of rarity?
Geologic setting.- This project focuses on the molluscan fossil record of the mid-Atlantic Coastal Plain. Sampling is limited to Miocene and Pliocene units exposed in Maryland, Virginia, and North Carolina, focusing almost exclusively on the Eastover, and Yorktown Formations. These deposits accumulated in the predominantly shallow shelf, open marine setting of the Salisbury and Albemarle Embayments and contain a wealth of well-preserved molluscan material (Ward and Blackwelder 1980, 1987; among many others).
Fieldwork.- The fieldwork component of this project is now complete and has involved collecting and processing 45 bulk samples from 10 localities in Virginia, Maryland, and North Carolina. At each locality, the exposed units were measured, sedimentology and sedimentological features described, and replicate bulk samples collected in an attempt to assess spatial and temporal variability in molluscan abundance. Sediment samples were also collected for supplemental grain size and geochemical analyses.
Data compilation.- Bulk sample data from field-collected bulk samples was supplemented with two large datasets donated by collaborators. Susan Barbour Wood and Gwen Daley generously provided access to their dissertation datasets, which included molluscan abundance data for 228 bulk samples collected from the Eastover and Yorktown formations in Virginia and North Carolina.
Laboratory work.- Samples were standardized (according to volume), sieved, and molluscan material greater than 2mm (following Kidwell 2001) was sorted and identified to species level whenever possible. Both right and left valves of bivalves were tallied and the larger value of the two was used to represent the number of individuals for a particular species in a particular sample. Fragments of bivalve and gastropod specimens were not included in abundance analyses unless the umbo or apex/aperture is present, respectively (following Aller 1995). Two different metrics were used to measure abundance: rank and percent abundance. Geological durations (i.e., stratigraphic ranges) of species were compiled from the literature and the Paleobiology Database (paleodb.org) based on faunal occurrences and used as a proxy for survivorship. Spatial and temporal variability in abundance was quantified using the coefficient of variation for each abundance metric. Occupancy was quantified as the percentage of samples in which a taxon occurred. The link between rarity and extinction has been preliminarily assessed through the correlation of fossil abundance data (including mean abundance and variability in abundance) with species duration, calculated at a variety of spatial and temporal scales.
Preliminary results.- Despite the great increase in sample size (from 30 to 273 samples in the past year), our results still show little or no evidence of a link between abundance and survivorship in mollusks in the mid-Atlantic Coastal Plain. The relationship is extremely weak and differs according to both rarity metric (abundance, variability, or occupancy) and extinction proxy (survivorship or duration) used. In the next year, I will continue to expand the chronological and geographic scope of this database (mainly using literature data) to include background intervals from the Eocene of the Gulf Coastal Plain.
Significance.- This research is significant in two major respects: (1) it is one of the first attempts to explore the link between rarity and extinction during background intervals in the fossil record and (2) it will contribute to our understanding of how ecological, environmental, and taphonomic factors affect fossil abundance data. Thus far, the project has supported seven undergraduate students at the College of William and Mary and one Ph.D student at Virginia Polytechnic Institute (Barbour Wood). This project has formed the foundation for a long-term research project focusing on the macroinvertebrate paleoecology of the mid-Atlantic Coastal Plain (in collaboration with Dr. Michal Kowalewski from Virginia Polytechnic Institute, Dr. Susan Barbour Wood from Western Carolina University, Dr. Gwen Daley from Winthrop University, and Dr. Lauck Ward from the Virginia Museum of Natural History).
References
Aller, J. Y. 1995. Palaeogeography, Palaeoclimatology, Palaeoecology 118:181-212.
Kidwell, S. M. 2001. Science 294:1091-1094.
Lockwood, R. 2003. Proceedings of the National Academy of Sciences 100: 2478-2482.
McKinney, M. L. and D. Frederick. 1992. Geology 20: 343-346.
Ward, L. W. and B. W. Blackwelder. 1980. USGS Bull. 1482-D, p. 1-61.
Ward, L. W., and B. W. Blackwelder. 1987. Smithsonian Contributions to Paleobiology 61: 113-283.
