Download Extinction Processes

Extinction
Extinction is a normal process. Can
result from demographic failure or
genetic swamping. Speciation is
another. Anagenesis is another.
Goal of conservation biology is NOT to
abolish extinction, but to keep it from
overpowering the rate of speciation.
Normal extinction and speciation rates have
probably balanced each other over much of
geologic time.
Rates differ for different groups.
Marine inverts – new species every 2-20 years
Birds – estimate of extinction rate of one species
every 83.3 years.
Extinction rates have not remained constant
over time. Some controversy about the
relative magnitude of these “mass
extinctions”.
Mass Extinctions of the Phanerozoic
North America supported a rich
“megafauna” until about 11,000 years
ago. Rivaled modern-day Africa
Fauna included 31 general of large
mammals. Only 14 genera remain. Some
22 genera of birds also disappeared.
Bonus: Can you
name the 14
remaining genera
of large, North
American
mammals?
What the heck happened?
Could it have been early humans?
Man colonized the
Americas from north to
south, and the movement
seems to have been
coupled with local
extinctions.
Martin has hypothesized that, in all areas except Africa
and Asia, man arrived on the scene as a skilled hunter
and encountered a “naïve” local fauna, with overkill
resulting.
Whatever happened, the extinction of many large
mammals had a influence on the local vegetation, and
probably led to the extinction of many plant species.
In the Pacific, the arrival of humans may have led to rapid extinction of
many animals, particularly birds. Polynesia and Micronesia were
populated by humans between 1500 BC and 400 AD. Reached New
Zealand about 800 AD. It looks like they wiped out many bird species as
they went.
Rate has gone up even more recently.
Why? Early on, it was primarily because of
hunting and other exploitation. Now, it has
more to do with exotic species and habitat
destruction.
Stresses that increase mortality and decrease fecundity
push the population toward extinction. Killing, habitat
destruction, disease, etc. are such processes. A more
detailed study in birds showed that killing by humans
and the effects of introduced predators have been the
major causes of extinctions since 1650.
Man has been responsible for a
number of extinctions through
simply killing the species off.
Black rhino
Another major cause of extinctions has
been introduced species, whether
predators (often rats or cats) or
competitors.
The brown tree snake reached Guam in
late ‘40s or early ‘50s. Probably in fruit
shipments. It has eliminated most of
the native bird species.
Other harmful introductions:
kudzu
zebra mussels
carp
Another threat - genetic swamping
Has led to the extinction of 15 or more fish
species in North America.
In New Zealand, the native gray duck has been
driven nearly to extinction by hybridization with
mallards.
Coyote
Red wolf
Perhaps the biggest threat now is habitat destruction.
ALMOST ALL RECENT EXTINCTIONS
ARE ATTRIBUTABLE TO HUMAN
ACTIVITES.
Stochastic processes may play a big role in
extinction in small populations.
Demographic stochasticity – chance variation in
mortality and reproduction. Small populations are
also subject to genetic stochasticity. Genetic
diversity may be lost by purely random processes.
Environmental stochasticity may affect small
populations more heavily than large populations.
What makes a species vulnerable?
Biggest thing: small population size.
Because of this – large species. Migratory
species.
Especially vulnerable
Large species with low reproductive potential – whales, rhinos,
great apes, condors, whooping cranes.
Species with high economic value – whales, sea turtles,
elephants, spotted cats, rhinos.
Species at the end of long food chains – birds of prey, cats,
reptiles.
Species restricted to local, insular habitas – snail darter, Key
deer.
Highly specialized species, as for feeding – giant panda, blackfooted ferret.
Migratory species – monarch butterflies, marine mammals,
many birds.
GLOBAL EXTINCTION RATES
How do we estimate? Tough.
Usually use species-area
curves. Curve usually steeper
for islands than for continental
areas. If we get the equation
for a curve, we can estimate
the loss of species if we lose a
certain amount of area.
Discuss.
Has been used
to estimate that
a 90% loss of
habitat would
lead to a 50%
loss of species.