Using ecological niche modeling to assess biogeographic and niche response of brachiopod species to the Richmondian Invasion (Late Ordovician) in the Cincinnati Arch

Abstract Within the Cincinnati, Ohio region, the beginning stage of a large-scale biotic invasion is recorded in the C4 depositional sequence (Late Ordovician, Richmondian Stage), which includes the Arnheim Formation and its correlates. The fauna and strata of this region are well studied and the ecological changes present provide a unique opportunity to study biogeographic changes associated with the onset of a biotic invasion. In this study, GIS-based ecological niche modeling (ENM) is used to examine the biogeographic and paleoecological impacts of the biotic invasion during the C4 sequence. In particular, this study assesses whether species respond to invasion pressure through niche conservatism (exhibiting stasis and maintaining the parameters of their ecological niche through time) or niche evolution (altering the ecological parameters of their niche through adaptation). Ranges of ten rhynchonelliform brachiopod species were modeled for three temporal intervals within the C4 sequence. Modeled ranges provide the basis for quantitatively assessing the shifting roles of native and invasive species throughout the C4 sequence. GARP (Genetic Algorithm for Rule-set Prediction), a computer-learning based ENM program, was used to model species' ecological niches from environmental data as estimated from sedimentological variables associated with known species occurrence data. Once environmental parameters of the niche were modeled, the resulting rule set was used to project the geographic boundaries of potential habitat and estimate the spatial extent occupied by each species. Three prominent trends are apparent within the fauna. (1) Generalist species tended to track their preferred ecological niches closely from the early into the middle time slice and maintained or expanded the size of their geographic range during this transition. (2) Ecologically specialized species exhibited decreasing geographic range size through the sequence and were more likely to shift geographic areas inhabited within the study area. (3) The amount of niche evolution increased as the invasion progressed; species that adapted to the new biotic regime though niche evolution persisted through the invasion interval while species with high levels of niche conservatism did not. During early C4 sequence, species respond individualistically to changes in abiotic conditions (shallowing upward), but do so primarily by tracking their previously preferred environmental conditions laterally and maintain high fidelity of niche dimensions through time. This matches the null hypothesis that species will exhibit niche conservatism during intervals of slow or gradual environmental change. Following the geologically sudden introduction of the intrabasinal invaders in the second half of the C4 sequence, native species responded to the combined abiotic (shallowing upward) and biotic (invader introduction) changes in the Cincinnatian environmental by altering the parameters of their ecological niches and through fundamental changes in geographic distribution. Extrapolating these results to modern ecosystems, suggests that most species should exhibit niche conservatism and habitat tracking in response to gradual changes in environmental conditions, while species should exhibit niche evolution and adaptive response following rapid environmental changes. Furthermore, modern species with reduced potential for adaptive response may also be the most susceptible to extinction.