ENVIRONMENTAL THRESHOLDS IN THE PALEOECOLOGICAL NICHES OF ATLANTIC BIVALVE SPECIES

In both modern and fossil species, ecological niche modeling (ENM) has been used to characterize the range of environmental conditions that determine the species’ geographic range and distribution (Peterson et al., 2011). The distribution of occurrences of a species within that ecological niche depends on the relationship between habitat suitability and likelihood of occurrence, which is generally assumed to be correlated with fitness (Aldridge and Boyce, 2007; Hirzel and Le Lay, 2008). In n-dimensional niche-space, a negative relationship between species abundance and distance to the niche center would suggest the niche had an “abundant center”, such that there is a point where environmental conditions are optimal that would result in the highest number of occurrences (Brown, 1984; Martínez-Meyer et al., 2013; Osorio-Olvera et al., 2020). However, a thresholded relationship between abundance and niche centrality may suggest that the deviation from “ideal” environmental conditions is not the only driving factor in the species geographic distribution (Briske et al., 2006; Francesco Ficetola and Denoël, 2009). For example, geographic factors such as the bathymetric change at a continental shelf break may well have a stronger influence on the distribution of shallow marine bivalves than the temperature or salinity of the water at that shelf break locality. This study used Maxent to measure the ecological niche of six bivalve species found in the Atlantic and Gulf coastal planes, which have been present throughout the Pleistocene and are still extant today. Models were performed using integrated climate models across four time bins – the Early, Middle and Late Pleistocene, and the Holocene, as well as a cumulative model integrating all four time bins. Mahalanobis distances calculated from the niche centroids were plotted against associated habitat suitability generated by Maxent to determine whether a distributional pattern could be found. Preliminary data shows no evidence for an abundant niche center but may support an environmental threshold as a driver of bivalve distributions, particularly in cumulative models incorporating all the climatic variability present across the Pleistocene and Holocene.