ARE ECOLOGICAL NICHES STABLE IN GEOLOGIC TIME? ANALYZING NICHE STABILITY LEVELS AMONG A DIVERSE SET OF LATE ORDOVICIAN TAXA

Changes in niche stability levels in deep time are evaluated by assessing the niche dynamics of a diverse suite of invertebrate taxa from the Late Ordovician Cincinnatian series of North America. During this interval, periodic sea-level fluctuations, storm events, and an extrabasinal species invasion disrupted stable shallow-marine communities as taxa responded to both rapid and gradual environmental changes. Variations in relative niche stability through time and across clades are determined using ecological niche modeling (ENM) to produce and compare models for 20 individual genera at high spatial and temporal resolution.

The maximum entropy (Maxent) algorithm was used to generate ecological niche models for each genus for 9 time-slices across three 3^rd order sequences. Previous studies provided sedimentologically derived environmental layers, and geo-referenced occurrence data were compiled from primary field research augmented by published data. To evaluate the relative frequency of niche stability, models were compared both geospatially and in environmental space across each time-slice. Environmental parameters modeled to define individual ecological niches were tested for statistically significant dissimilarity between time-slices. In geographic space, the percent overlap between the predicted range and their actual geographic distribution was determined by projecting the previous time-slice niche model forward onto the environmental layers of the next sequential time-slice. High degrees of similarity and range-overlap indicate niche stability, whereas statistical dissimilarity and low percentage range overlap indicate niche evolution.

Complimentary ENM studies of brachiopod species across this interval have documented higher levels of niche evolution during intervals of rapid sea-level rise and during the Richmondian invasion. The results of this study indicate that different clades responded similarly, with niche evolution being the dominant response across the invasion interval, whereas niche stability prevailed during intervals of gradual abiotic change. Although species level trends are dampened at the genus level, these results augment current paleontological data in support of niche stability varying with the scale and style of different environmental drivers.