Biodiversity: extinction

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Transcript Biodiversity: 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.
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.
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.
Here’s a figure for family level extinctions and diversity…
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.
And here are indications of when pairs of continents
fragmented or became ‘joined’ (at least with respect to
dispersal between them).
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
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…
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
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…
% of taxon
extinct since
First, how do the different factors that can cause extinction
compare in importance?
No assigned cause
56% of recorded extinctions
Introduced (exotic) species
Habitat destruction
Hunting (overexploitation)
Other causes
How do the factors I listed compare when we look at
numbers of species that are endangered (i.e. dangerously
close to extinction)?
Here are some examples of species we’ve driven extinct:
sea cow
a and b were hunted to extinction; for c and d habitat loss was
also important
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…
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).
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 can’t tell you how many extinctions occur
with that loss, because we don’t know, haven’t named, or
researched the adaptations of most of the species in the areas
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).
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.
Here are fertility rates and recent rates of increase for some
Fertility Rate
% rate of increase
North America
Former USSR
Latin America
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%!
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 doesn’t provide
income to very many corporations in those developed
countries, so…
Let’s get back to ecology – what can we do to conserve
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 can’t 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.
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.
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
The opposing points of view:
Save a logger,
Eat an owl.
Save an owl,
Ban logging.
The Timber Industry
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 earth’s 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.