Page 193 - Introduction to Paleobiology and The Fossil Record

180 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
first 20–25 myr of the Triassic. It was only in
the Late Triassic that forests reappeared. Tet-
rapods on land had been similarly affected,
and ecosystems remained incomplete and
unbalanced through the Early and Middle
Triassic until they rebuilt themselves in the
Late Triassic with dinosaurs and other new
groups (see p. 454).
Life recovers slowly after mass extinctions.
A fl urry of evolution happens initially among
disaster taxa, species that can cope with harsh
conditions and that can speciate fast. These
disaster taxa are then replaced by other species
that last longer and begin to rebuild the
complex ecosystems that existed before the
mass extinction. The mass extinction crisis
may have affected life in two ways: conditions
after the event may have been so harsh that
nothing could live, and the crisis probably
knocked out all normal ecological and evolu-
tionary processes.
Extinction today
We started this chapter with the dodo, a rep-
resentative of how humans cause extinction.
There is no question that the extinction of the
dodo was regrettable, as is the extinction of
any species. But where should we stand on
this? Some commentators declare that we are
in the middle of an irreversible decline in
species numbers, that humans are killing 70
species a day, and that most of life will be
gone in a few hundred years. Others declare
that extinction is a normal part of evolution,
and that there is nothing out of the ordinary
happening.
The present rate of extinction can be cal-
culated for some groups from historic records.
For birds and mammals, groups that have
always been heavily studied, the exact date of
extinction of many species is known from
historic records. The last dodo was seen on
Mauritius in 1681. By 1693, it was gone, prey
to passing sailors who valued its fl esh, despite
the fact that it was “hard and greasie”. The
last Great auks were collected in the North
Atlantic in 1844 – ironically, the last two
Great auks were beaten to death on Eldey
Island off Iceland by natural history collec-
tors. Some sightings were reported in 1852,
but these were not confi rmed.
Human activity has not simply caused the
extinction of rare or isolated birds. The last
150
100
Number oif recorded extinctions
50
0
1600 1700 1800 1900 2000
Year
Figure 7.13 The rate of historic extinctions of
species for which information exists, counted in
50-year bins. Note the rapid rise in numbers of
extinctions in the period 1900–1950; the
apparent drop in the period 1950–2000 is
artificial because complete counts have not been
made for that 50-year period yet.
Passenger pigeon, named Martha, died at
Cincinnati Zoo in 1914. Only 100 years
earlier, the great ornithologist John James
Audubon, had reported a fl ock of Passenger
pigeons in Kentucky that took 3 days to go
by. He estimated that the birds passed him at
the rate of 1000 million in 3 h. The sky was
black with them in all directions. They were
wiped out by a program of systematic shoot-
ing, which, at its height, blackened the land-
scape with Passenger pigeon carcasses as far
as the eye could see.
These datable extinctions can be plotted
(Fig. 7.13) to show the rates of extinction of
birds, mammals and some other groups in
historic time. The current rate of extinction
of bird species is 1.75 per year (about 1% of
extant birds lost since 1600). If this rate of
loss is extrapolated to all 20–100 million
living species, then the current rate of extinc-
tion is 5000–25,000 per year, or 13.7–68.5
per day. With 20–100 million species on
Earth, this means that all of life, including
presumably Homo sapiens, will be extinct in
800–20,000 years. These fi gures are startling
and they are often quoted to compare the
present rate of species loss to the mass extinc-
tions of the past.
A reasonable response to this calculation
would be to query the annual loss fi gure and
the validity of extrapolating. The birds that
have been killed so far are mainly vulnerable
species that lived in small populations on