ARE NICHE STABILITY LEVELS CONGRUENT ACROSS CLADES? AN ECOLOGICAL NICHE MODELLING TEST OF NICHE CONSTANCY IN THE TYPE CINCINNATIAN

The relative stability of ecological niches through time and across clades is assessed by analyzing the niche dynamics of a diverse set of invertebrate taxa from the type Cincinnatian (Late Ordovician) strata, which outcrop in Ohio, Kentucky, and Indiana. The focal taxa experienced both gradual and rapid environmental changes due to sea level fluctuations and species introductions. In this study, ecological niche modeling (ENM) is employed to examine the niche dynamics of individual genera from several phyla at a fine temporal and spatial resolution.

Ecological niche models for each genus were generated for each of three sequences (C3, C4, and C5) using the maximum entropy (Maxent) algorithm. Environmental data from sedimentological proxies were compiled from prior publications, and fossil occurrence data were acquired from the Paleobiology Database. Niche models were compared temporally by projecting the niche model for one time slice onto the environmental layers of the next time slice. Then the predicted geographic distribution of the forward model was compared with that of the original model. Statistical similarity of consecutive models was also assessed. High degrees of similarity and range overlap indicate niche conservatism, whereas low degrees indicate niche evolution. Models were also compared between genera to determine if taxa have similar responses.

Prior studies of Cincinnatian brachiopod species document niche stability except during intervals of rapid change (specifically the biotic invasion). Results of this study indicate that different taxa with unique niches and varying adaptive abilities (ecological generalists vs. specialists) responded with varying levels of niche stability, but with the same general trend: niche stability is more prominent during intervals of gradual change, and niche evolution is evident during the interval of biotic invasion. These results augment the current paleontological data in support of niche evolution in deep time.