MODERN NONMARINE ECOLOGICAL GRADIENTS AND THEIR IMPLICATIONS FOR THE FOSSIL RECORD

Elevation in nonmarine settings changes predictably with transgressions and regressions, owing to the lengthening and shortening of the fluvial profile and the vertical movement of the sea-level datum. Because of this, the relative abundance of species and the composition of species assemblages may change stratigraphically if there are elevation-correlated ecological gradients. Ecologists have documented such gradients for over 200 years, but many of these studies span elevations of thousands of meters, far more than the few hundred meters typically preserved in nonmarine basins. Although several dozen modern studies document these smaller-scale gradients in several taxa, particularly trees, herbs, and fishes, they are commonly based on relatively few sites from a limited area. Here, we explore elevation-correlated gradients in a wide variety of taxa from the passive margin sedimentary basin of Georgia, South Carolina, and North Carolina, USA, by harnessing occurrence records in the GBIF database. Occurrences were grouped into collections defined by 0.2° bins of latitude and longitude; these collections were ordinated with non-metric multidimensional scaling. We find that most higher taxa have ecological gradients correlated with elevation and distance to the coast; these include Plantae, magnoliopsids, bivalves, gastropods, arthropods, fishes, birds, and several of their orders (Charadriiformes, Passeriformes, Pelecaniformes). These ecological gradients tend to be most pronounced within a few tens of kilometers of the coast and at elevations lower than 25 m. Inland gradients in species composition are much gentler to indetectable in some taxa. Other taxa, notably terrestrial mammals and reptiles, do not display gradients correlated with elevation or distance to the coast. The small sample sizes for these groups and their spatially heterogeneous sampling raises concerns about whether sufficient data exists. These results and previous ecological studies suggest that similar elevation-correlated ecological gradients are likely common in the fossil record. Their detection is important because stratigraphic changes in community composition may have a simpler origin in local elevation changes rather than ecosystem perturbations.