THE TAPHONOMY OF DIVERSITY: LIVE-DEAD COMPARISONS OF EVENNESS METRICS

Measures of species diversity are one of the primary tools paleoecologists use to analyze the structure of fossil communities, so determining their robustness in light of taphonomy and time-averaging is critical. In this study, we evaluate the fidelity of four metrics of community evenness (uniformity in species abundances in an assemblage) using a database of 85 molluscan life and death assemblages: Pielou's J (based on Shannon’s H), Hurlbert's PIE (complement of Simpson’s l), Peters' E_ss, and the complement of PATT (Proportional Abundance of Top Taxon). Initial results indicate that death assemblages tend to show equal or greater (rarely lower) evenness than samples of the local live community. Subsampling analysis and correlation of evenness values indicate that death assemblage evenness is NOT an artifact of the greater sample size (number of individuals, N) or richness (number of species, S) of death assemblages compared to counterpart life assemblages. Although evenness increases with S, this is a statistical artifact due to the rarity of assemblages with high N and low S or vice versa. In study areas where repeated samples of in situ live fauna can be pooled to simulate time-averaging, evenness of the pooled life assemblages falls within the range of single-sample live values rather than approaching that of the corresponding death assemblage. This suggests that in terms of evenness, death assemblages are typically not passively time-averaged accumulations of static local communities. This result requires additional taphonomic processes that diminish the relative importance of top live taxa. These may include (1) geographic admixing of abundant shells from exotic species, (2) temporal mixing of ecologically volatile local communities, or (3) preferential removal/destruction of specimens from the most abundant (small-bodied, short-lived, opportunistic) species. Regardless of specific scenarios, absolute values of evenness metrics should at this point be used with caution in assessing ancient community structure: death assemblages are not passive accumulations of live species abundance-frequencies, and although the general pattern is for greater evenness in death, no universal correction factor is immediately apparent.