Extinction

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The most well understood mass extinction
occurred about 65 million years ago it marked
the end of the Cretaceous, and in it the
dinosaurs disappeared. The Alvarez hypothesis to
explain it is now widely accepted. It suggests a
17km diameter asteroid struck earth in the Gulf
of Mexico, just off the Yucatan peninsula.
Extinction The fossil record indicates five times
in the history of life on earth there have been
massive extinctions. Those mass extinction events
mark the ends of major geological periods.
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The nuclear winter that resulted deposited
exotic elements (Iridium) in a thin layer over
the surface of the globe. The impact cloud
darkened the earth for months years. Reduced
photosynthesis affected the entire food chain.
Reduced temperature affected both aquatic and
terrestrial animals. According to very recent
evidence, the Permian mass extinction may also
have been caused by impact of an asteroid around
250 MYBP. This mass extinction killed 88-96 of
marine species, and 30-40 of terrestrial
species. The direct causes of other mass
extinctions are not yet well understood. Each
caused the extinction of a significant fraction
of all species alive before the event. There
must have been large scale changes in
environmental conditions to drive the pattern of
biological change.
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Heres a figure for family level extinctions and
diversity
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At the time of transition from the Permian to the
Triassic something else of importance
happened At that time continental drift brought
all the major land masses of earth together into
a giant supercontinent called Pangaea. Assembly
would have changed continental shelves and the
climate in many areas then removed from
coastlines. Then separation would have caused
massive climate changes.
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And here are indications of when pairs of
continents fragmented or became joined (at
least with respect to dispersal between them).
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Probably the most important element in explaining
the climate change accompanying continental drift
is whether polar regions are covered by
landmasses (or landlocked seas) or permit free
oceanic circulation. The distribution of thermal
energy over the surface of the earth is far more
even when there is free circulation, and
temperate climates extend to near the poles.
Without that circulation, polar regions become
very cold, and temperate climates reach only far
more limited latitudes. As recently as 30 MYBP
much of North America had tropical plant
communities. Less than 100 MYBP Antarctica had
rich temperate communities, and still had
connection to Australia with an island-hopping
connection to South America. So, climate change
has caused many historical extinctions
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Extinctions are occurring today at a rate
probably unprecedented in the history of life on
earth (at a rate comparable to the periods of
mass extinction). Some occur naturally, but
humans are a major cause through their actions,
both direct and indirect. For extinctions
occurring in the U.S. (and, by implication, over
the rest of the globe), the primary causes
are 1) habitat reduction or modification 2)
small population size of endangered species 3)
introduction of exotic species, and 4)
overexploitation 5) pollution
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There is a direct parallel (a correlation)
between the rate of human population growth since
1600 and the rates of extinction of birds and
mammals
We can also consider not just the number of
extinctions, but whether they occurred on islands
or mainland, and what fraction of the taxonomic
group they represent
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First, how do the different factors that can
cause extinction compare in importance? No
assigned cause 56 of recorded
extinctions Introduced (exotic)
species 17 Habitat destruction 16 Hunting
(overexploitation) 10 Other causes 1
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How do the factors I listed compare when we look
at numbers of species that are endangered (i.e.
dangerously close to extinction)?
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Here are some examples of species weve driven
extinct
Stellars sea cow passenger pigeon
dodo Carolina parakeet
a and b were hunted to extinction for c and d
habitat loss was also important
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Habitat reduction and overexploitation are
particularly important in developing countries.
Why is that important? Because the regions of
highest diversity and highest endemism are almost
all in the developing world. Norman Myers used
endemism, diversity, and population pressure to
determine the 25 regions where conservation
efforts should be concentrated. These areas have
high human population growth rates. They
represent only a few percent of land area, yet
hold almost half of all plant species and more
than one third of terrestrial vertebrate
species. Obviously, tropical forest areas are on
the list, as are isolated islands with high
endemism, but there are others
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Massive forest destruction on Madagascar has
already driven 1/2 of lemur and tarsier species
extinct. Montaine Central America and the Andes
of northern South America have very high habitat
diversity, and many parts are severely endangered
by development. There is a famous ridge in
Ecuador that was cleared for agriculture, causing
the extinction of more than 100 endemic
species. Edges of major biomes, like the areas
around the Mediterranean have much higher
diversity than latitude might suggest. The
Pacific rainforest of Oregon, Washington, and
British Columbia is very diverse, and subject to
development pressure (from the forest industry).
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This last is an excellent example of how knowing
too little can be dangerous. The Pacific yew was
considered a junk tree, until the discovery
that Taxis brevifolia produced a secondary
chemical called taxol. It is a powerful
anti-cancer agent important in the treatment of
breast cancer. The yew is now grown in
plantations to harvest/extract taxol. How many
other, similarly useful chemicals are being lost
with the loss of 2 or so of the total remaining
tropical forest per year. I cant tell you how
many extinctions occur with that loss, because we
dont know, havent named, or researched the
adaptations of most of the species in the areas
cut.
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So, what are we to do? The first, and most
important problem is us! There are well over 6
billion of us now, and population projections
suggest an increase to 10 billion within about 25
years. The earth cannot sustain that population
in the lifestyles we currently enjoy. Reduced
(controlled) fecundity must be established very
soon to stabilize the human population at a
supportable size. Lifestyles must be modified to
increase energy efficiency. The use of energy for
heating and cooling needs to be reduced. Our
diets need to obtain more of their calories from
lower in the food chain (more veggies, less meat).
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Are either of those minimal necessary changes
likely? Population control In the developed
world planned parenthood is an established
reality, child bearing is occurring later in
life, ZPG has largely been achieved. But In the
developing world population growth is still
occurring at an alarming rate. The developing
countries have larger family size (fertility) and
high rates of increase.
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Here are fertility rates and recent rates of
increase for some areas Region Fertility Rate
rate of increase North America 1.9
0.71 Europe 2.0 0.22 Former USSR 2.3
0.68 Latin America 4.0 1.94 Asia 4.0
1.82 Africa 6.0 3.01 How can we reduce
the rate of global population growth? By
contraception. In 1992, 70 of couples in
developed countries used contraception, while it
was only 45 in developing countries. In Africa,
it was only 14!
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Governments in developing countries need to
provide family planning methods, information, and
support. Yet there are still 31 developing
countries where citizens have virtually no access
to family planning. Women in most of those
countries want to have fewer children than they
actually bear. Only in sub-Saharan Africa do
women want family sizes of 5 or more. Elsewhere
the desired number is around 2 or even
less. Developed countries need to fund and
support family planning in the developing world.
But that doesnt provide income to very many
corporations in those developed countries,
so Lets get back to ecology what can we do
to conserve species?
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Try to conserve species by protecting their
habitat. This is particularly important in those
hot zones with high endemism and high
diversity. Which species do we protect? We cant
protect everything. One approach is to choose a
species of apparent interest (a warm-furry or
feathered) whose habitat needs encompass the
needs of many others. Such a species is termed an
umbrella species. How large a population do we
need to protect? There is a broad stroke
generalization known as the 50-500 rule. Protect
a population of at least 500 individuals
including at least 50 genetically diverse mating
pairs.
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As a first approximation, use the rules for
reserve design drawn from island
biogeography. Use any means necessary to keep key
species around. For the last wild California
condors, that meant bringing them into captivity,
incubating and raising their eggs, and eventually
releasing young birds back into the wild in areas
where their success was more likely. The condor
program has been a great success. Instead of
extinction, the California condor population now
numbers almost 200 individuals.
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The case of the spotted owl brings up the
problems conservation policy faces. The spotted
owl lives and breeds successfully only in old
growth forests of the northwest U.S. and British
Columbia. That is prime timber land. The key
question is How do you balance the rights of the
individual (or corporation) with the public good?
Strix occidentalis
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The opposing points of view
Save a logger, Eat an owl. The Timber Industry
Save an owl, Ban logging. Environmentalist
In the end, conservation begins with an
understanding and control over human ecology.
We cannot continue to rape the earth and yet
expect it to sustain future generations. Global
population growth must soon slow, then stop if we
are to live in balance with the earths
ecological potential. Energy use in the developed
world must be brought under control, and a much
larger fraction of the energy must come from
renewable sources.