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
Top Related