PRESERVATION OF ENVIRONMENTAL GRADIENTS BY BIVALVE DEATH ASSEMBLAGES IN SOFT-BOTTOM HABITATS

Paleoecological analyses implicitly assume that death assemblages capture variation in species and communities along environmental gradients accurately. We assess nine datasets from modern, relatively undisturbed soft-bottom habitats to evaluate the live-dead agreement in the extent to which the species composition of living molluscan assemblages and their local death assemblages vary as a function of environmental conditions and geographic separation. First, optima of bivalve species along bathymetric and sedimentary gradients in death assemblages show significantly positive rank correlations to species optima in living assemblages in seven of nine datasets. Post-mortem processes thus do not strongly bias the record of species optima. This result also indirectly supports the consistency of differences in species niches over time, since species optima in the time-averaged death assemblage are being compared to a single snapshot characterization of optima in the living assemblage. The average live-dead agreement of species environmental tolerances (measured by the weighted standard deviation) and of species carrying capacities (maximum abundance predicted by generalized additive models) are lower than the agreement of species optima, but still significantly positive. Second, the amount of variance explained by environmental gradients (niche separation) in death assemblages is exceptionally similar to that of counterpart living assemblages, and compositional dissimilarities are significantly related to environmental differences to a similar degree as in living assemblages. Death assemblages thus capture environmental gradients similarly strongly as do living assemblages. This analysis of the environmental resolution of death assemblages using multiple independent datasets thus supports fine-scale niche and paleoenvironmental analyses in the fossil record.