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.

Red wolf

Coyote

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, black-footed 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.