Page 300 - Environment: The Science Behind the Stories

Extinction and Biodiversity Loss

Biodiversity at all levels is being lost to human impact, most
irretrievably in the extinction of species. Once vanished, a
species can never return. Extinction (pp. 76–77) occurs when
the last member of a species dies and the species ceases to
exist. The disappearance of a particular population from a
given area, but not the entire species globally, is referred to
as extirpation. The black rhinoceros has been extirpated from
most of its historic range across Africa (Figure 11.6), but as a
species it is not yet extinct. However, no individuals of the
subspecies of black rhino known as the western black rhino
have been seen since 2006, and this subspecies is now thought
to be extinct. Extirpation is an erosive process that can, over
time, lead to extinction.

Extinction occurs naturally

Human impact is responsible for most extirpation and extinc-
tion today, but these processes also occur naturally, albeit at a
much slower rate. If species did not naturally go extinct, we
would be up to our ears in dinosaurs, trilobites, ammonites, African Black Rhino
and the millions of other creatures that vanished from Earth Current distribution
during the immense span of time before humans appeared. Former distribution
Paleontologists estimate that roughly 99% of all species that
ever lived are now extinct. Thus, the wealth of species gracing
our planet today represents just 1% of the species that have Figure 11.6 The black rhinoceros has disappeared from
ever existed. most of its range across Africa.
Most extinctions have occurred one by one for independ-
ent reasons, at a pace referred to as the background rate of our planet experienced five major mass extinction events
extinction. By studying traces of organisms preserved in the (p. 77). Each event eliminated more than one-fifth of life’s
fossil record (pp. 73, 76), scientists infer that for mammals families and at least half its species (Table 11.1). The most
and marine animals, each year, on average, 1 species out of severe episode occurred at the end of the Permian period
every 1–10 million has vanished.
(see aPPeNDiX e). At this time, 248 million years ago, close
to 90% of all species went extinct. The best-known episode
Earth has experienced five mass occurred 65 million years ago at the end of the Cretaceous
extinction events period, when an asteroid impact (and possibly volcanism)
brought an end to the dinosaurs and many other groups.
Extinction rates rose far above this background rate at sev- Evidence exists for earlier mass extinctions during and before
eral points in Earth’s history. In the past 440 million years, the Cambrian period, more than half a billion years ago.

Table 11.1 Mass extinctions
DATE (MILLIONS OF TYPES OF LIFE PERCENTAGE OF LIFE
EVENT YEARS AGO [MYA]) CAUSE MOST AFFECTED DEPLETED CHAPTER 11 • Bi odiv ER si T y A nd Cons ER vAT i on Bi ology
Ordovician 440 mya Unknown Marine organisms; terrestrial record >20% of families
is unknown
Devonian 370 mya Unknown Marine organisms; terrestrial record >20% of families
is unknown
Permo-Triassic 250 mya Possibly Marine organisms; terrestrial record >50% of families; 80–95%
volcanism is less known of species
End-Triassic 202 mya Unknown Marine organisms; terrestrial record 20% of families; 50% of
is less known genera
Cretaceous- 65 mya Likely asteroid Marine and terrestrial organisms, 5% of families; >50% of
Tertiary impact including dinosaurs species
Current Beginning Human impacts Large animals, specialized organisms, Ongoing
0.01 mya island organisms, organisms
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