Assessing abundance-diversity relationship in the context of time-averaging: implications for evaluating baseline and novel states in conservation paleobiology

Species diversity increases with time averaging according to the species-timespan relation. This scaling effect can exaggerate geographic or temporal variability in diversity because the time averaging of fossil assemblages can vary by several orders of magnitude owing to variability in sedimentation, mixing and/or disintegration, and thus can affect analyses focusing on the detection of baseline and novel community states in the Holocene stratigraphic record or in live-dead comparisons. However, the relation between abundance and sample size-independent diversity can be used to detect the fingerprints of time averaging. A decrease in sediment accumulation rate should lead to higher abundance and diversity, and the relation is thus expected to be positive. Consistently with this hypothesis, we find that sedimentation rates in the Holocene record of the northern Adriatic correlate negatively with abundance and diversity. Moreover, as sedimentation rates decrease, correlation between abundance and diversity becomes increasingly positive. This scaling-induced relation differs from a negative abundance-diversity relationship observed in living (non-averaged) communities. We suggest that the positive abundance-diversity relation will be a diagnostic of scenarios where variability in fossil abundance and diversity is determined by temporal scaling controlled by variability in sediment accumulation rate rather than by variability in natural or anthropogenic processes.