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Permo-Triassic Extinctions: Effects of Area on Biotic Equilibrium

Daniel S. Simberloff
The Journal of Geology
Vol. 82, No. 2 (Mar., 1974), pp. 267-274
Stable URL: http://www.jstor.org/stable/30061979
Page Count: 8
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Permo-Triassic Extinctions: Effects of Area on Biotic Equilibrium
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Abstract

Schopf (1974) suggests that the decrease in number of families of marine invertebrates during the Permian and subsequent increase in the Lower Triassic are due to changes in the area of the shallow marine seas. Shallow marine sea coverage fell by two-thirds, then more than doubled, and the number of families appears roughly to have tracked areal change. Since a change in area cannot be followed immediately by extinction or origination of families, the Permo-Triassic situation may have been a short-term nonequilibrium one, with number of families constantly changing but lagging behind the instantaneous equilibrium number. In the Recent, the widely studied and experimentally observed increase in species number with area, even in the absence of increased habitat diversity, implies a monotonic relationship between area and equilibrium number of families. Increase in number of species with area appears usually to be linear in a log-log plot, and this leads inductively to a nearly linear relationship between log area and log number of families as well. From such an area-family number relationship, I show that the rough correspondence between shallow marine sea coverage and number of marine invertebrate families can indeed be construed as the result of a causal connection between area and biotic diversity, and that Schopf's hypothesis that the Permo -Triassic extinctions were a consequence of changing sea level is biologically plausible.

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