fore should have many separate biotic provinces, at least on land. The opposite occurs during high-oxygen times: there will be relatively few biotic provinces and a worldwide fauna.

The drop in oxygen did more than make mountain ranges barriers to migration. It made most areas higher than 3,000 feet uninhabitable during the late Permian-Triassic time interval. Huey and I recognized that there might be a further effect on life’s history: it may have contributed to the Permian and Triassic mass extinctions. We called this “altitudinal compression.” The removal of habitat because of altitudinal compression would have caused species from highlands to migrate toward sea level or die out. Doing so would have increased competition for space and resources and perhaps would have introduced new predators, parasites, or diseases in the previously populated lowlands, causing some number of species to go extinct. We calculated that by the end of the Permian more than 50 percent of the planet’s land surface would no longer have been habitable because of altitudinal compression. There may even have been extinction caused by the effects modeled long ago by Robert MacArthur and E. O. Wilson in their Theory of Island Biogeography. These two scientists noted that diversity is related to habitat area and that species died out when islands or reserves of some sort became smaller. Altitudinal compression would accomplish the same by making the continental landmasses functionally lower in terms of usable area.

THE FATE OF PLANTS

While plants probably were not overtly affected by the gradual drop in oxygen during the latter half of the Permian period, the concomitant rise in temperature coupled with poisonous hydrogen sulfide in the air surely got their attention. Plant species today are highly sensitive to temperature, both high and low, and will migrate to follow their required temperature ranges during climate change intervals. (Unlike mobile animals, which can move into shelter from freezing winds or find shade in blazing heat, plants must just sit there rooted and take it.) The Permian shows two trends—a change in the kind of flora during the period and a substantial extinction of plants at the end.



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