Relaxation Time and the Problem of the Pleistocene

Steven Holland

doi:10.3390/d5020276

Abstract

Although changes in habitat area, driven by changes in sea level, have long been considered as a possible cause of marine diversity change in the Phanerozoic, the lack of Pleistocene extinction in the Californian Province has raised doubts, given the large and rapid sea-level changes during the Pleistocene. Neutral models of metacommunities presented here suggest that diversity responds rapidly to changes in habitat area, with relaxation times of a few hundred to a few thousand years. Relaxation time is controlled partly by metacommunity size, implying that different provinces or trophic levels might have measurably different responses to changes in habitable area. Geologically short relaxation times imply that metacommunities should be able to stay nearly in equilibrium with all but the most rapid changes in area. A simulation of the Californian Province during the Pleistocene confirms this, with the longest lags in diversity approaching 20 kyr. The apparent lack of Pleistocene extinction in the Californian Province likely results from the difficulty of sampling rare species, coupled with repopulation from adjacent deep-water or warm-water regions.

Highlights

Because sea level strongly affects the area of shallow-marine habitats [1,2,3,4,5,6], paleobiologists have long argued that sea level should be an important control on the diversity of shallow-marine communities [7,8,9,10,11,12,13,14,15]
Because θ is linearly proportional to speciation rate and to the number of individuals, which is in turn the product of habitat area and the density of organisms [29], the model predicts that larger habitat areas, increased density of organisms within that habitat, and increased per-individual speciation rates should all result in larger equilibrium diversity
All of these simulations indicate that neutral metacommunities should have relaxation times of no more than a few thousand years, even when the changes in habitat area are large and unrealistically abrupt as simulated here

Summary

Introduction

Because sea level strongly affects the area of shallow-marine habitats [1,2,3,4,5,6], paleobiologists have long argued that sea level should be an important control on the diversity of shallow-marine communities [7,8,9,10,11,12,13,14,15]. A related concept is extinction debt, which refers to the extinction that has yet to take place before a new, lower equilibrium diversity is reached following habitat loss [27]. All of these simulations suggest short relaxation times, and they imply that the diversity of shallow marine faunas should have been able to closely track sea-level changes, even during times such as the Pleistocene, when sea level underwent rapid and wide changes [23,24,25]. Simulating the diversity of the Californian Province during the Pleistocene follows the same procedure as models described above, except that habitat area is allowed to change during every time step

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Discussion

Conclusion