ANALYZING NICHE STABILITY IN LATE ORDOVICIAN ARTICULATED BRACHIOPODS DURING THE RICHMONDIAN INVASION

This study analyzes the relative stability of species’ niches among Late Ordovician articulated brachiopod species preserved in strata of the type Cincinnatian Series across an extrabasinal immigration event known as the Richmondian Invasion. Specifically, the long term impact of invasive species is considered by examining whether incumbent species respond to ecosystem change by (1) conserving the parameters of their ecological niche by tracking their preferred environment laterally or (2) by adapting to the changing conditions by altering their niche dimensions through niche evolution.

Ecological niche modeling (ENM), which combines species occurrence and environmental data to estimate a species’ fundamental niche, was used to model species’ niches at nine time slices through three depositional sequences which encompass the interval before, during, and after the Richmondian Invasion. The Maxent modeling program, which has been demonstrated to achieve high predictive accuracy in analyses of modern taxa, was employed in this study. ENM analyses require robust species occurrence data for all time slices and an estimation of environmental parameters (inferred water depth, limestone bedding style and thickness, etc.) derived from the sedimentary record. The abundant, well preserved fossils and clear temporal framework of the deposits surrounding the Cincinnati, OH region create an excellent setting for ENM analyses. Relative niche stability was assessed by comparing distribution projections from ecological niche models for each time slice with the distribution models produced by projecting the species niche from the current interval onto the environmental layers of the next time slice using ArcGIS. If the original and forward model overlap, the species is interpreted to have conserved its niche parameters; if the original and forward models substantially differ, then niche evolution between time slices is inferred.

Brachiopod species niches in the study area were modeled with a high degree of accuracy by Maxent. Comparisons between temporally adjacent distribution models indicate that incumbent species exhibited substantial niche evolution prior to the Richmondian invasion, but conserved the parameters of their niches to a greater degree during and after the invasion event.