Surprisingly enough, we know very little about natural extinctions

In the past, known only from fossil records Physical evidence of cause rarely

preserved Cause and Effect hard to establish Post hoc ergo propter hoc danger Even if cause established, what’s the

mechanism?

Habitat Disruption› Volcanic Eruptions› Asteroid Impact

Habitat Modification› Climate Change› Mountain-Building› Sea Level Change

“Exotic” Species› Continental Drift

Epidemics› Rapid co-evolution of disease and host

Evolution of New Competitors in Place› Existing organisms already well-adapted

Excessive Predation (Food, fur, collecting, pest eradication, etc.)

Habitat Destruction Destruction of keystone species Introduction of Exotic Species

› Competitors› Predators› Diseases

Pollution and Contamination

Humans Show Up and Megafaunas Go Extinct

Australia 40,000 years ago Americas 15,000 years ago Madagascar 1000 years ago New Zealand 1000 years ago

Contentious: Threatens Image of Early Humans As Stewards of Environment

Immigrants From Arctic Wouldn’t Have Fine-tuned Cultural Sense of How to Manage Temperate Environment

American Fauna Not Accustomed to Humans

Climate Change?› Rode out 20+ Previous Glacial Cycles

Change in Ecology?› C3 and C4 Grasses

Refers to chemical reactions during photosynthesis (3- versus 4-carbon molecules)

C3 Grasses are cool climate, C4 grasses are warm climate

C4 grasses are richer in silica particles and wear teeth faster

Why Didn’t All Megafauna go Extinct?› Bison, Pronghorn, Deer, Grizzly Bears

Did Humans Really Hunt Megafauna?› Central Asian Mammoth-bone Huts, but

Rabbits Are Main Bones in Food Dumps› What killed off Saber-Tooth Cats?

Did Humans Kill Off Some Keystone Species?

Timing is Sure Suspicious

Steller’s Sea Cow› Cold-Water Relative of Manatee› Extinct 1768

Great Auk› Flightless, Penguin-like North Atlantic

Bird› The Original “Penguin”› Nice Example of Convergent Evolution› Extinct 1844

May once have been the most numerous bird on the planet

Estimated 5 billion Made up 30-40% of all North

American birds Flocks 1 mile wide, 300 miles long Evolved to travel and breed en masse Protection against most predators

Unlike other predators, humans exploited the mass flocks of the passenger pigeon

Netting, mass shooting Railroads shipped pigeons to market,

created demand Declines noted by 1860 Species could probably have survived

even this predation, except….

Pigeons were hunted in nesting sites

Hunters used telegraph to learn of colonies

Conservation laws too little, too late

Last wild pigeons shot Wisconsin, 1899 and Ohio, 1900

Scattered birds could not breed Captive breeding attempts failed Last bird died in Cincinnati Zoo,

September 14, 1914, 1 PM The only extinction we can time

to the minute

Eastern race of the prairie chicken Once ranged from Maine to Virginia Hunting caused visible decline by

1800, steep by 1830 By 1870, restricted to Martha’s

Vineyard, Massachusetts By 1906, only 50 left 1907, Sanctuary established

1907: Sanctuary established for last 50 birds

By 1915, number had grown to 2000

Species had been rescued?

1907-1915: Heath hen had grown from 50 to 2000 birds

1916: Fire destroyed most of refuge Harsh winter and influx of hawks

further damaged species Flock attacked by disease from

domestic turkeys By 1927, only 13 left, mostly male Last bird died, 1932

Only Parrot Native to U.S. Once ranged from Virginia to Texas Adapted readily to agriculture and

became regarded as a pest Widely hunted Rare by 1880’s Last Seen in Florida about 1920

Cheetahs once ranged worldwide Remaining 20,000 are genetically

identical Near extinction 10,000 years ago Generations of close inbreeding Were able to re-occupy large range

because nothing had filled ecological niche

American Chestnut was once a major food crop and lumber source

Accounted for half the value of eastern timber

Devastated by blight 1904-30 Isolated trees and viable roots still

survive Research on blight immunization Even if blight cured, other trees have

filled ecological niche

Scales of organization genetic -- diversity of genetic

information found within species and populations

species -- diversity of species community -- diversity of community

composition ecosystem -- diversity of assemblages

of communities (Fox River watershed) landscape -- diversity of assemblages of

ecosystems (Western Great Lakes)

Single islands (mountain tops) always have fewer species than areas on the “mainland” of similar size

Because islands are isolated, it will be harder for species to immigrate to them, lowering the rate of immigration.

Because of limited resources on islands, carrying capacity will be lower, decreasing population sizes and increasing extinction rates.

Theory of island biogeography has been termed the 'First Law of Conservation Biology.' 

Because of human actions, natural habitats are becoming increasingly isolated and island-like. 

By identifying potential mechanisms underlying the loss of species diversity, Island Biogeography Theory may help suggest ways in which we can design nature reserves to maximize their ability to maintain diversity.

Biodiversity often increases when habitats are fragmented

Many species need large areas› Typically large ranges› Availability of food › Protection from predators and invaders

(Example: cowbirds and songbird decline)

Corridors as solution?

Volunteers – natural chance immigrants (cattle egrets)

Unintentional (rats, English sparrows) Escaped ornamentals (kudzu, purple

loosestrife) Escaped pets (feral cats, house finches) Escaped domestic animals (pigs, goats) Bio-control gone haywire (mongooses) Most exotics not street smart Vigorous exotics have no natural predators Hawaii: 80% overrun by exotic species

A large reserve is better than a small reserve

A single undivided reserve is better than a number of small reserves

A few large reserves are better than a number of small reserves

Reserves should be spaced equally from another, not linearly

Linear reserves should be connected with corridors

If reserve is small and isolated, it should be circular and not linear

Sixth Extinction by Richard Leakey and Roger Lewin 1995

Are we creating a mass extinction to rival the other major events in the geologic past?

The higher the taxonomic level, the lower the extinction level

Easy to wipe out a species, hard to wipe out a family

250 m.y. ago: 90% of species lost, 50% of families, some orders, no phyla

Groups that vanish during mass extinctions and then reappear

Where do they go? Why don’t they change?

About 2100 dinosaur fossils in museums

285 genera, 336 species May have been 1000-1300 genera

total Compare to 1300 living mammal

genera About 30,000 marine invertebrate

genera (more genera living now)

Incomplete Many organisms will never be fossilized

› No hard parts› Rare or very restricted› Environments where fossilization unlikely

Often impossible to distinguish species› Have to rely on skeletons, shells, hard parts› No information on coloration› No information on internal organs

Most sediment is transported by running water

Most fossils are in water-laid rocks Bias toward aquatic organisms

› Shells› Favorable setting

Terrestrial fossils preserved erratically

Dusky seaside sparrow (color variant only)

Ivory-Billed Woodpecker, California Condor, Steller’s Sea Cow (never abundant)

Most rain forest species (too restricted, not likely to be fossilized)

Extinction of Pleistocene megafauna Extinction of Passenger Pigeon Reduction in range of bison, large

carnivores Expansion of human domestic

animals Reduction in rain forest, changes in

land cover Humans and artifacts

The biggest change so far (Pleistocene extinction) was prehistoric

Have been very significant shifts in vegetation and fauna

Not many extinctions would show up in the fossil record

Little change in easily fossilized marine faunas