THE HiSToRy oF LiFE 465
geographic distributions resulted from normal processes of dispersal and establish-
ment in new favorable areas, coupled with extinction of populations in areas that
became climatically unfavorable to a species. Many species were extirpated over
broad areas; for instance, beetle species that occurred in England during the Pleis-
tocene are now restricted to such far-flung areas as northern Africa and eastern
Siberia [21]. Many species that had been broadly and rather uniformly distributed
became isolated in separated areas (refuges, or refugia) where favorable conditions
persisted during glacial episodes. Some such isolated populations diverged geneti-
cally and phenotypically, in some instances becoming different species. In some
cases, populations have remained in their glacial refugia to this day, isolated from
the major range of their species (see Fig ure 18.8). However, many species have rap-
idly spread over broad areas from one or a few local refugia and have achieved their
present distributions only in the last 8000 years or fewer. Studies of fossil pollen
show that since the glaciations, the geographic distributions of plant species have
changed incessantly, and that the species composition of ecological communities
has changed kaleidoscopically, mostly because of fluctuations in climate [40].
Aside from changes in species’ geographic distributions, the most conspicu-
ous effects of the changes in climate were extinctions. At the end of the Pliocene,
many shallow-water marine invertebrate species became extinct, especially tropical spe-
cies, which may have been poorly equipped to withstand even modest cooling [39,
88]. No major taxa of marine animals became entirely extinct, but on land the story
was different. Except in Africa, a very high proportion of large-bodied mammals and
birds became extinct in the late Pleistocene and Holocene. These animals included
mammoths, saber-toothed cats, giant bison, giant beavers, giant wolves, ground
sloths, and all the endemic South American ungulates. Archaeological evidence,
mathematical population models, and the timing of extinctions relative to human
population movements and climate change indicate that both human hunting and
climate change were major causes of this megafaunal extinction [4, 61, 72].
The most recent glaciers had hardly retreated when major new disruptions
began. The advent of human agriculture about 11,000 years ago began yet another
reshaping of the terrestrial environment. For the last several thousand years,
deserts have expanded under the impact of overgrazing, forests have succumbed
to fire and cutting, and climates have changed as vegetation has been modified
or destroyed. At present, under the impact of an exponentially growing human
population and its modern technology, species-rich tropical forests face almost
complete annihilation, temperate zone forests and prairies have been eliminated
in much of the world, marine communities suffer pollution and appalling overex-
ploitation, and global warming caused by combustion of fossil fuels is changing
climates and habitats so rapidly that many species are unlikely to adapt [44, 74].
An analysis of the numbers of threatened species of terrestrial vertebrates sug-
gests that biological diversity could be reduced by 75 percent (a proportion compa-
rable to the K/Pg mass extinction) within the next 900, and perhaps as few as 240,
years (see Box 22.B) [8]. Even if these estimates are twice as pessimistic as they
should be, one of the greatest ecological disasters, and one of the greatest extinc-
tions of all time, appears to be under way, and can be mitigated only if humans act
decisively and quickly.
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