USING HUBBELL'S NEUTRAL THEORY TO TEST THE SPECIES-AREA RELATIONSHIP IN THE LATE ORDOVICIAN OF LAURENTIA

Hubbell's theta links diversity and abundance structure at local and regional scales through a positive correlation with metacommunity habitat area. Because theta can be calculated from preserved relative abundance distributions, it is possible to test for this correlation in the fossil record. Late Ordovician strata of Laurentia are divided into four geochemically and biologically distinct regions, which reflect original oceanographic differences in water properties. Because these regions are geochemically distinct, there was likely little mixing between the water masses, which in turn suggests that each region was a single metacommunity. We test whether Hubbell's theta is positively correlated with the area of these four regions, corresponding roughly to the Appalachian Basin, Cincinnati Arch, Upper Mississippi Valley, and western U.S. and Canada.

We use existing bed-level census data of Patzkowsky and Holland from the Cincinnati Arch and Wyoming, with newly obtained censuses from the Appalachian Basin, Upper Mississippi Valley, and Manitoba. To minimize faunal differences caused by differences in water depth, censuses were conducted from only the deep subtidal (between fair weather and storm wave base) depositional environment. Each sample was stratigraphically constrained and contained greater than two dozen individuals. Abundances were tallied by identifying individuals in each sample to genus and using whole-faunal counts with a minimum number of individuals approach. Hubbell's theta was calculated from the relative abundance distribution of each census, and province area was calculated from published maps.

Preliminary results indicate a positive but non-linear relationship between province area and Hubbell's theta. This positive relationship suggests that provincial area influences diversity at the local scale, but the nonlinearity suggests that other factors also affect theta and local diversity structure. Since provincial diversity is an important component of global biodiversity, determining the nature of its relationship with area in the fossil record has implications for understanding how diversity is assembled globally throughout the Phanerozoic.