IMPACTS OF THE RICHMONDIAN INVASION IN THE NASHVILLE DOME: ANALYSIS OF NICHE STABILITY USING ECOLOGICAL NICHE MODELING

The Richmondian Invasion was the immigration of a diverse suite of marine taxa into southeastern Laurentia during the Late Ordovician (Katian), preserved in the strata of the Cincinnati Basin and Nashville Dome of North America. The evolutionary and ecological impacts of the invasion have been well studied in the Cincinnati region; however, faunal change in the Nashville Dome is poorly constrained. In this study, Paleo-Ecological Niche Modeling was applied to test the hypotheses that niche lability patterns in the Nashville Dome were similar to the Cincinnati region, specifically that niche stability was high among taxa before the invasion, but niche stability declined during and after the invasion. Late Ordovician strata of the Nashville Dome comprise highly fossiliferous limestone and shale units. In-situ species-level identifications for different fossil taxa and sedimentological data were collected to develop species occurrence and environmental proxy data for niche modeling. Niche models were developed using MAXENT, an R-based modeling package. Models were produced for taxa with at least seven geographic occurrence points among twenty locations spanning the western edge of the Nashville Dome. Articulated brachiopods, bryozoans, gastropods, and few other benthic clades were included in the modeled taxa. Sedimentary proxies, like carbonate bedding style and thickness, and limestone/shale percentage were used to construct environmental parameters. Niches were characterized across the invasion in the Nashville Dome by examining changes in environmental parameters coupled with taxa distribution data. Results demonstrate that taxa maintained a high level of niche stability throughout this interval but demonstrated elevated niche conservatism in times of biotic disturbance due to the introduction of invasive taxa. Results were compared with similar analyses previously conducted for taxa of Cincinnati region. Comparisons demonstrate that niche response differs between Nashville and Cincinnati taxa across the invasion. Nashville taxa exhibited overwhelming niche stability whereas Cincinnatian taxa underwent substantive niche contraction. This discrepancy invokes differences in tectonic position and sedimentary regimes of the two regions, differences in timing, differences in pre-invasion taxa and community structures, and differences in the identity of the invaders. This analysis improves the current understanding of how Biotic Immigration Events alter ecology in geologic time.