TAPHONOMIC INERTIA TO SHIFTING ECOLOGICAL BASELINES

To what extent do death assemblages diverge in composition from living communities not because of genuine post-mortem bias, but because human activities have recently shifted the local living community (LLC) outside the range of natural variability that produced the time-averaged death assemblage (TADA)? The magnitude of such taphonomic inertia – the tendency for death assemblage composition to lag behind changes in the living community -- would depend upon a number of factors. For example, if population sizes or mortality rates of new species are low relative to the quantity and loss rate of dead material already present, and/or the preservation potential of new input is low, then response of the TADA might be very slow. This possibility was first raised by Staff & Powell (1999), and strong evidence for it has now emerged. Meta-analysis of 37 datasets from open shelves, scored using independent evidence for anthropogenic eutrophication (AE = 17 of 30 large datasets) and bottom-trawling (25/30; no power for test), finds no effect from AE on live/dead agreement in richness and evenness, but significantly lower live/dead taxonomic similarity and lower agreement in species rank-order abundance. With increasing AE, live and dead first diverge in composition (more live-only and dead-only species) and then also in species rank-order abundance (dominants in LLC are rare in TADA) (N.B. On natural shelves, median live/dead taxonomic similarity is ~1 and median Spearman rank-order agreement is +0.4, equivalent or better than “live/live” agreement among repeated censuses of the LLC). The taphonomic inertia implicit in these differences is probably multi-factorial, including a relatively low preservation potential for AE LLC, which can include many small, nacreous, thin-shelled bivalves. Live/dead analyses in “non-natural” study areas thus do tend to underestimate the actual reliability of death assemblages for paleoecologic interpretation; the positive finding for environmental managers is that modern-day TADA can preserve useful portraits of pre-impact conditions. More precise information is needed on the timing and magnitude of anthropogenic impacts in study areas, along with information on shell ages in local TADA, but the results here suggest that TADA can resist resetting by AE LLC for several decades at least.