THE LIVE, THE DEAD, AND THE EXPECTED DEAD: MORTALITY AND TAPHONOMIC BIAS IN RECENT DEATH ASSEMBLAGES

Many factors can lead to bias in the composition of death or fossil assemblages relative to their source living communities. Of these, mortality bias, which is the expected over-representation of short-lived species relative to longer lived species in time-averaged death/fossil assemblages, has long been suspected but has received little attention.

Here we use Recent bivalve death assemblages to examine the relative importance of mortality bias (differential input), taphonomic bias (differential destruction), and time-averaging (summing multiple generations of individuals). In the absence of data on mortality rates, we combine information on lifespans from the ecological and shell-fisheries literature with data on the local living community (LLC) to calculate the expected composition of the death assemblage (EDA), which is then compared with the source LCC as a measure of mortality bias, and with the observed death assemblage (ODA) as a measure of taphonomic bias. Where the LLC is characterized on the basis of repeated censuses, rather than a one-time census (the norm in live-dead studies), we can also assess the role of time-averaging in both mortality and taphonomic bias: time-averaged EDAs can be generated and compared with the LLCs (singly and pooled) and ODA.

Preliminary results using published multi-census live-dead datasets indicate that while each of these biases can alter the abundance structure of death assemblages, bias from all three sources is lower than generally feared, and in most cases falls within the range of variation found from one census of the LLC to the next. Time-averaged EDAs are a better match than single-census EDAs to ODAs, also due to the range of variation found within a LLC over time. To better assess the implications of short-term instability in LLCs for ODAs, we are analyzing bivalve death assemblages from offshore San Diego, CA, where municipal wastewater agencies have generated a ~30-year time series of LLC data. We are again finding that variability in the LLC is greater than or equal to the live-dead discrepancy generated by the 3 bias sources. Death assemblages may be a useful tool for assessing community ecology in Recent systems; with proper taphonomic control (staying with a single taphonomic grade) useful paleoecological comparisons can be made with fossil assemblages.