A NEW QUANTITATIVE METHOD FOR DETERMINING SUCCESSIONAL ORDER AMONG FOSSIL TAXA

Determination of successional order within modern communities is almost routine. Although successional data from paleocommunities would open new realms of study, acquisition of such data is difficult. Most attempts to determine succession in the fossil record have relied on paleoenvironmental reconstructions or autecological inferences. This study presents a new, quantitative method for determining successional order among fossil taxa that does not rely on potentially subjective environmental or autecological interpretations.

Recent communities typically follow a pattern through succession: (1) after a disturbance, early-stage taxa colonize the empty habitat; diversity is low; (2) over time, late-stage taxa arrive in the ecosystem; these taxa are strong competitors but poor colonizers. The mix of early- and late-stage taxa causes a significant increase in diversity; (3) eventually, late-stage taxa competitively exclude early-stage taxa; diversity decreases. If the ecosystem is disturbed at any time, succession is interrupted and returns to stage 1.

Paleocommunities may be preserved in any of the three stages, depending on local disturbance rates. Frequently disturbed ecosystems will be in stage 1, and so have low diversity (late-stage taxa never have a chance to arrive); ecosystems experiencing rare disturbance will be in stages 2 or 3. However, if disturbance is rare, time-averaging is more likely ? consequently, regardless of whether the ecosystem is in stage 2 or 3, both early- and late-stage taxa are preserved; diversity is high.

The new technique analyzes multiple (> 100) communities from a given time/space. Late-stage taxa only appear in stages 2 or 3, and so are _always_ in high-diversity communities. Early-stage taxa may be preserved in any stage, and appear in either low- or high-diversity systems but the mean diversity of all communities in which an early-stage taxon is present will be lower than that of a late-stage taxon. Thus, calculation of mean diversity for communities in which each given taxon is present provides a ranking of successional order.

We demonstrate and test this new approach using Devonian encruster communities living on host brachiopods. As the order of succession for encrusters can be determined independently, the use of encrusters provides an appropriate test of the method.