A NEW METHOD FOR COMPARING SPECIES ECOLOGY IN TIME AND SPACE: EVALUATING FAUNAL STABILITY DURING THE LATE ORDOVICIAN, CINCINNATI ARCH

Following long-standing approaches in community ecology, a Gaussian distribution defined by three variables can describe the ecologic distribution of taxa within environments. Preferred environment (PE) describes the environment in which a taxon is most likely to be found and is the mean value of the Gaussian distribution. Environmental tolerance (ET) describes the degree to which a taxon can tolerate environments other than its preferred environment and is given by the standard deviation of the Gaussian distribution. Peak abundance (PA) is the probability that the taxon will be found in a sample obtained from that taxon's preferred environment. These parameters can be calculated from any ordination technique, such as detrended correspondence analysis, that produces axes that are linearly correlated with primary environmental variables. Here we demonstrate how such parameters may be compared from separate studies to evaluate faunal stability through time or across space.

Marine benthic faunas from the M5 through C3 depositional sequences of the Late Ordovician of the Cincinnati Arch have been previously regarded to indicate an interval of relative faunal stability, as shown by relatively low rates of genus origination and extinction in brachiopods. Comparison of PE, ET, and PA for the M5-M6 sequences and the immediately superjacent C1 sequence indicates unexpectedly low degrees of ecological stability within all marine benthic invertebrates. Values of PE display an overall correlation of 0.28, but the correlation improves to 0.87 if only the seven most abundant taxa are included. This pattern may indicate that errors in estimates of PE are lowest for abundant taxa or it may indicate a rule of assembly in which abundant taxa are constrained to specific habitats whereas rare taxa can operate more or less as ecological free agents. Values of ET have a correlation of 0.42, which suggests that the habitat breadth of taxa is loosely maintained over time. Values of PA have the strongest correlation (0.84), indicating that the maximum abundance of a taxon is the most strongly conserved aspect of a taxon's ecologic distribution. Future applications of the method should allow ecological stability to be evaluated on a taxon-by-taxon basis and allow different forms of ecological stability to be distinguished.