BETWEEN-YEAR COMPARISONS OF BEACH ACCUMULATIONS OF SHELLS AND THE UTILITY OF BEACH DEATH ASSEMBLAGES FOR STUDIES OF DRILLING PREDATION

Because shell accumulations on modern beaches often are used as analogs for paleoecological samples, paleontologists have begun to test the validity of beach assemblages for such work. To address whether beach shell accumulations are useful analogs in studying drilling predation, I compared drilling predation, diversity metrics and similarity indices for assemblages collected at a single locality over multiple years. For such assemblages to be useful analogs for paleoecological samples, metrics should be consistent from year to year. Beach assemblages, like fossil assemblages, are likely to be averaged to some degree over time and space; I thus expected beach assemblages to be similar from year to year, in contrast to live assemblages, which live-dead studies have shown to vary among successive years.

Beach death assemblages were bulk sampled at the same locality on Oak Island, southernmost North Carolina, in June 2003, 2010, and 2014. Assemblages were sieved with a 5mm mesh and bivalve beaks and gastropod apices were picked and identified to species level. Specimens > 85% complete were examined for drilling traces and drilling frequency (DF) and the incidence of failed drilling determined for various subsets of the fauna. I used Spearman’s rho to compare rank-order abundance and the Jaccard-Chao index to compare taxonomic similarity between years; I also compared Oak Island samples with a death assemblage from an intertidal sand flat near Wilmington, NC, using rho and the Jaccard-Chao index.

Oak Island samples averaged 1712 (5137 total) specimens. When rarefied to the size of the smallest sample (1440 specimens), richness did not differ significantly among years (43.9 - 46.4 species). Taxonomic similarity was very high (0.97 -0.98) and species rank-order abundance was significantly correlated between years as well as in comparison to the intertidal death assemblage. No significant differences between years occurred in DF for the total fauna, gastropods, bivalves, infaunal bivalves, or arcoid bivalves. DF for the 6 most common infaunal bivalve species did not differ significantly between years (18 comparisons) except for Anadara ovalis (2003 vs 2010 and 2014) and A. transversa (2003 vs 2010). Failed drilling was very rare (<2%) for all years. The results support the utility of beach samples in predation studies.