EvoluTion And SoCiETy 587
pest. Likewise, herbivorous insects used to control weeds or invasive plants must
be screened to be sure they will not also attack crops or native plants. A good
approach is to see whether they have the potential to feed on or adapt to plants that
are related to the target plant species [45].Conservation
There is little doubt that a major extinction event has been initiated by the huge
and accelerating impact of human activities on every aspect of the environment
(BOX 22.B). So far, the main human threat to other species has been elimina-
tion of their habitats by land use and climate change. In this context, the most
important means of conservation are obvious and require mostly ecological, polit-
ical, legal, and economic expertise: save natural habitats in preserves, establish
and enforce limits on the exploitation of fish populations and other biological
resources, reduce pollution and global warming. But evolutionary biologists alsoFor the first time in the history of life, a single species has
precipitated a major extinction. Within the next few cen-
turies, the diversity of life will almost certainly plummet at
a pace that may well equal any mass extinction in Earth’s
history.
The human threat to Earth’s biodiversity has accelerated
steadily with the advent of ever more powerful technology
and the exponential growth of the world’s human popula-
tion, which has surpassed 7 billion. The per capita rate of
population growth is greatest in the developing countries,
which are chiefly tropical and subtropical, but the per capita
impact on the world’s environment is greatest in the most
highly industrialized countries. An average American, for
example, has perhaps 140 times the environmental impact
of an average Kenyan, because the United States is so
profligate a consumer of resources (harvested throughout
the world) and of energy (with impacts ranging from strip
mines, fracking, and oil spills to greenhouse gases that
cause global warming).
Some species are threatened by hunting or overfish-
ing and others by species that humans have introduced
into new regions. But by far the greatest cause of extinc-
tion, now and probably over the course of the twenty-first
century, is the destruction of habitat [101]. It is largely for
this reason that 29 percent of North American freshwater
fishes are endangered or already extinct, and that about 10
percent of the world’s bird species are considered endan-
gered by the International Council for Bird Preservation.
The numbers of species likely to be lost are highest in
tropical forests, which are being destroyed at a phenome-nal and accelerating rate in Asia, Africa, and tropical Amer-
ica. As E. O. Wilson said, “in 1989 the surviving rainforests
occupied an area about that of the contiguous forty-eight
states of the United States, and they were being reduced by
an amount equivalent to the size of Florida each year” [120].
Several authors have estimated that 10–25 percent of tropi-
cal rainforest species—accounting for as much as 5–10 per-
cent of Earth’s species diversity—will become extinct in the
next 30 years. To this toll must be added extinctions caused
by the destruction of species-rich coral reefs, pollution of
other marine habitats, and losses of habitat in areas such as
Madagascar and the Cape Province of South Africa, which
harbor unusually high numbers of endemic species. Even if
extinction rates are much lower than Wilson estimated, they
will still equal or exceed those described by paleontolo-
gists, such as the huge end-Permian mass extinction [104].
In the long run, an even greater threat to biodiversity may
be global warming caused by high and increasing con-
sumption of fossil fuels and production of carbon dioxide
and other greenhouse gases. Earth’s climate has warmed
by a global average of 0.6°C during the last century, and
the rate of warming is much faster than most of the cli-
mate changes that have occurred in the past. The effects
of climate change vary geographically; for example, some
regions are becoming much drier.
Some species may adapt by genetic change, but the rate
of climate change is so high that the rate of evolution of
species’ “climate niches”—the range of climate conditions
they actually occupy—would have to be more than 10,000(continued)BOX 22B
The Current Extinction Crisis
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