225 Paleontology III: Diversity, Extinction, and Origination

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 205AB

THE ROLE OF COMPETITION AND LIFE-MODE ON GLOBAL BIODIVERSITY THROUGH TIME

Paper number: 225-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

The influence of fine-scale processes, such as competition and predation, on global biodiversity patterns continues to be a controversial topic. At fine scales, competition tends to maintain populations at or below carrying capacity, the number of individuals which the environment can support. Thus, population growth is density-dependent: as populations grow, the rate of growth must decline. Sepkoski (1996) suggested that global diversity has a saturation point, in essence a scaling up of carrying capacity. Saturation of taxa would manifest as a roughly flat diversity curve through time, interrupted by declines during major extinctions. Recently, Alroy (2010) demonstrated support for some density-dependence of marine invertebrate genera through time. Case-studies indicate that evidence for saturation is more prevalent among clades consisting primarily of sessile organisms than among mobile clades. This is consistent with modern ecological studies, which have established that competition is more influential on sessile organisms, as they cannot relocate to relieve competition.

To test if the differential influence of competition on sessile vs. mobile benthic marine invertebrates scales up to global diversity, we generated Phanerozoic generic richness curves (10 my bins) for each of the two groups using the Paleobiology Database (www.pbdb.org). Regardless of whether the curves were based on raw data, exponential resampling, or quorum subsampling, the sessile curve was statistically flatter than the mobile curve, especially after the Devonian. In addition, the rank-correlation r-values for change in log diversity (a measure of turnover rate) vs. standing diversity of the preceding temporal bin were greater for “sessiles” than for “mobiles”, indicating greater density-dependence for sessiles. This was also true if the independent variable in the correlation was the standing diversity two bins earlier (as opposed to the preceding bin), an approach which eliminates autocorrelation. The results corroborate the hypotheses that (a) sessile organisms have experienced saturation and density-dependence to a greater degree than mobile taxa; (b) competition has influenced the post-Devonian diversity history of sessiles; and (c) global patterns can be affected by fine-scale processes.