COMPARING QUANTITATIVE TECHNIQUES FOR EVALUATING LIVE-DEAD AGREEMENT IN AN INTERTIDAL HABITAT IN NORTH CAROLINA

Fidelity between the taxonomic composition of a living community and its death assemblage has been used to indicate the degree of anthropogenic environmental change in a system. Low fidelity of taxonomic composition, indicated by a mismatch between the living community and its death assemblage, indicates anthropogenic stress on a system, whereas high fidelity, or very similar taxonomic compositions, may indicate little environmental change. There are currently a number of analytical methods used to identify and quantify agreement or mismatch. One commonly-used metric is from Kidwell, which plots the Jaccard-Chao coefficient of similarity of taxonomic composition against Spearman’s rank-order correlation coefficient (ρ) to specifically assess shifts in taxonomic composition due to anthropogenic eutrophication. This measure of fidelity is considered to be conservative, and it is recommended that additional methods be used in conjunction with this method to identify differences between live communities and death assemblages that might be hidden by this metric.

Here we sampled the live molluscan community and death assemblage from a shelly, intertidal muddy-sand flat near Masonboro Sound, Wilmington, NC, in summers from 2008-2012. Mollusks were identified to species level and abundances tallied in concurrently sampled live and dead assemblages. This area has been dredged and harvested, and it is possible that these activities changed the mollsucan community over time.

Analyses reported here are for bivalves; live gastropods were rare in our samples. All analyses showed concordance between live and dead assemblages sampled each year. Spearman’s ρ ranged from 0.41 – 0.83 and was statistically significant (p<0.05) in all cases. Plots of Jaccard-Chao values vs. ρ place all yearly results within the upper right quadrant, the “pristine” quadrant of the graph, as expected from the conservative nature of the metric. We will explore various other methods of quantifying the difference between the live and dead assemblages including cluster analyses, and comparing other methods of ecological community assessment including species proportional abundance, richness, and evenness. These other metrics may show discordance between the live community and death assemblage not captured by the more conservative metric.