Environmental Engineering Reference
In-Depth Information
LAURASIA
120
°
80
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40
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80
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120
°
120
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80
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80
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120
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225 million years ago
135 million years ago
EURASIA
AFRICA
120
°
0
°
40
°
120
°
120
°
80
°
120
°
MADA-
GASCAR
T
65 million years ago
Present
Figure 4-8
Geological processes and biological evolution:
over millions of years, the earth's continents
have moved very slowly on several gigantic plates. This process plays a role in the extinction of species as
land areas split apart and in the rise of new species where once isolated areas of land combine. Rock and
fossil evidence indicates that 200-250 million years ago all of the earth's present-day continents were locked
together in a supercontinent called Pangaea (top left). About 180 million years ago, Pangaea began splitting
apart as the earth's huge plates separated and eventually resulted in today's locations of the continents (bot-
tom right).
(20-60 million years apart) during the past 500 million
years (Figure 4-9).
In periods of
mass depletion,
extinction rates are
much higher than normal but not high enough to clas-
sify as a mass extinction. In both types of events, large
numbers of species have become extinct.
A mass extinction or mass depletion crisis for some
species is an opportunity for other species. The exis-
tence of millions of species today means that speciation,
on average, has kept ahead of extinction, especially
during the last 250 million years (Figure 4-10, p. 74).
response to changing environmental conditions. Al-
though extinction is a natural process, humans have
become a major force in the premature extinction of
species.
According to biologists Stuart Primm and Edward
O. Wilson, during the 20th century, extinction rates in-
creased by 100-1,000 times the natural background
extinction rate. As human population and resource con-
sumption increase over the next 50-100 years, we are
expected to take over a larger share of the earth's sur-
face and net primary productivity (NPP) (Figure 3-20,
p. 50). According to Wilson and Primm, this may cause
the premature extinction of at least one-fifth of the
earth's current species by 2030 and half of those species
by the end of this century. If not checked, this trend
could bring about a new
mass depletion
and possibly a
new
mass extinction.
Wilson says that if we make an “all-
out effort to save the biologically richest parts of the
world, the amount of loss can be cut at least by half.”
Effects of Human Activities
on the Earth's Biodiversity
Human activities are decreasing the earth's
biodiversity.
Speciation minus extinction equals
biodiversity,
the
planet's genetic raw material for future evolution in
