Time-Averaging Transfers Diversity from Beta to Alpha Levels

The fidelity with which time-averaged death assemblages capture variation in species abundances among sites or time intervals remains unexplored. In this study, we address the preservation of variation in species composition and in species richness using 11 regional datasets based on samples of living molluscan communities and their co-occurring time-averaged death assemblages. We find that compositional differences among living assemblages (LA) within datasets are significantly positively rank-correlated to differences among counterpart pairs of death assemblages (DA), demonstrating that pairwise variation in living species composition within a study area has a good preservation potential in the fossil record. However, the average variation in species composition – beta diversity - is consistently smaller in DAs than in LAs (9 of 11 datasets). This damping of beta diversity might arise from DAs generally having a larger sample size, but the reduction (by ~10-20%) persists in DAs in sample size-standardized analyses (4 to 7 of 11 datasets). The reduction in beta diversity can be attributed to the loss of temporal resolution that occurs with time-averaging, without invoking taphonomic bias (from differential preservation or post-mortem transportation) or sample-size effects (DAs are usually larger than LAs). Therefore, time-averaged DAs do capture variation among samples but tend to damp the magnitude, making them a conservative means of inferring change over time or variation among regions in species composition and diversity. In contrast to evenness that is overrepresented at alpha level and underrepresented at beta level in death assemblages, richness is partitioned similarly in both LAs and DAs, with only ~15% of total gamma richness captured at the alpha level.