Ecological fidelity and temporal acuity of marine molluscan death assemblages as time capsules of biodiversity

Sedimentary cores from estuarine and marine environments contain shelly records of past communities. However, using these to develop time-lines of ecological change for the last several millennia to centuries -- or even using death assemblages from grab samples to characterize "modern" diversity -- requires quantifying the reliability of species abundance data (e.g., differential destruction and post-mortem transportation of species) and the magnitude of time-averaging (memory of past generations). Observational and manipulative experiments, direct dating of death assemblages, meta-analysis, and modeling now permit quantification of many aspects of data quality and reveal underlying controls and tradeoffs. For example, "live-dead" comparisons of shelled mollusks, where counts of living individuals sieved from quantitative grabs are compared with counts of dead individuals in the same samples, indicate that average "dead richness" is ~25% greater than a single live census, reflecting the accrual of rare species during time-averaging (114 subtidal datasets; some shells are several thousand years old, but the vast majority are <a few hundred yr). Species relative abundance is quite robust: rank-order agreement of live and dead species lists is 0.31 (richness-weighted meta-analytic average Spearman rho of 173 comparisons), comparable to "live-live" agreement when two successive censuses of the same habitat are compared (rho = 0.34; N = 1732 comparisons). If only "pristine" habitats are considered, average live-dead rho increases to 0.38. Live-dead agreement declines significantly with increased anthropogenic eutrophication, bottom-trawling, or other human impacts, owing to dead composition lagging behind the shifting community baseline  (rho = 0.24; taphonomic inertia). Poor live-dead agreement thus carries a strong signal of recent community change. Death assemblages represent time capsules that, because they time-average across short-term volatility, capture the pre-impact community composition as well as single live censuses.  These are generally encouraging results for a variety of ecological analyses from molluscan death assemblages.