BEYOND THE SNAPSHOT: TIME LAG ANALYSIS AS A TOOL TO VERIFY PATTERNS OF LIVE-DEAD FIDELITY FOR CONSERVATION PALEOBIOLOGY

Live-dead fidelity is measured by directly comparing the taxonomic compositions between a living biological community (living assemblage; LA) and the co-occurring accumulations of skeletal remains (death assemblage; DA). Such comparisons have historically been used to understand taphonomic processes and paleoecological patterns within the fossil record, and more recently, to identify ecological changes on the timescales of anthropogenic impacts. However, LAs are typically only sampled once at the same time as the DA without replication or cover only short time series. These “snapshots” of the living community make live-dead fidelity difficult to reliably assess because they may fail to account for the natural range of variation or dynamics of community change at a given location. Understanding the consequences of these limitations requires the analysis of long-term LA time series, often only available in regions where dedicated biomonitoring programs have been established.

Do multidecadal temporal dynamics among LAs correspond to particular outcomes of live-dead fidelity? To explore this question, I used a 30-year (1989-2019) time series of bivalve communities at nine long-term monitoring stations across Puget Sound to quantify each station’s range of temporal variation (beta dispersion) and rate of community change (time lag analysis). DAs were recovered from the 2019 survey and used to measure live-dead fidelity as the (1) Jaccard-Chao index of taxonomic similarity, (2) Spearman rho coefficient of rank-order abundance, and (3) distance from the station’s DA to the multivariate centroid of LAs. Eight of the nine stations demonstrated significant community change, and this was strongly correlated to overall beta dispersion. The slopes of time lag regressions were also strongly correlated to mean Jaccard-Chao index values and centroid distances, but there was no apparent relationship with Spearman rho values. The rate of community change observed over recent decades was thus a powerful predictor of several common metrics for live-dead fidelity within the purview of conservation paleobiology. Living community time series – whenever available – should be assessed in tandem with DAs to verify interpretations of ecosystem condition.