CONTRASTING RESPONSES OF FUNCTIONAL DIVERSITY TO DROPS IN TAXONOMIC DIVERSITY: MODERN CLIMATE GRADIENTS VS PAST MASS EXTINCTIONS

Taxonomic diversity of benthic marine invertebrate shelf species today declines by nearly an order of magnitude from the tropics to the poles, tracking changes in climate and other environmental factors. In marine bivalves, a useful model system for integrating past and present biodiversity data, this taxonomic trend is accompanied by a commensurate decline in functional richness (FR, the number of functional groups as defined using categories along attachment mode, substratum relationship, feeding, and mobility axes; cf. Bambach et al. 2007), and an increase in the evenness of taxa distributed among those groups, with maximum functional evenness (FE) in polar waters of both hemispheres. However, although the extinction at or near the end of the Permian also represents a diversity decline of nearly an order of magnitude, FR is nearly or entirely unchanged (depending on views on potentially photosymbiotic taxa), and the FE of the surviving fauna increases. The end-Cretaceous (KPg) mass extinction is not as severe taxonomically, but the fauna contains more functional groups (26 KPg vs 16 end-Permian) and so tests the generality of functional-group survival in the face of severe taxonomic loss; again, few functional groups are lost (the controversy on photosymbiotic rudists adding uncertainty), and FE rises. We hypothesize that the latitudinal loss of taxonomic diversity and FR, and increase in FE, primarily reflects the actions of diversity-dependent factors and may be characteristic of large-scale variation in climate and its correlates, while retention of nearly all functional groups through the two Era-defining mass extinctions more closely resembles the action of diversity-independent factors. In the short term and on local scales, perturbations are expected to deplete FR and decrease FE as a few stress-tolerant or opportunistic species predominate, but over long time-scales the opposite appears to be true. Comparative analyses of different aspects of biodiversity will be a significant step towards understanding strongly contrasting biological consequences of diversity declines of similar magnitudes, potentially decoupling taxonomic and functional patterns in time and space, and can help predict the consequences for functional diversity of different stresses on modern biotas.