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Extinctions: past and present
Bio 415/615
Questions
1. How do species go extinct?
2. How is the ‘background’ extinction rate
calculated?
3. What caused mass extinctions in the
geologic record?
How many documented extinctions
have there been in the last 500
years?
~75 plant extinctions
~600 animal extinctions
Some species are more vulnerable to
extinction
Species on islands (or isolated habitats)
e.g., ground nesting birds
e.g., ‘naïve’ species: megafauna
Extinction due to predation, not (usually)
competition
Mauritius
Raphus cucullatus: extinct mid/late 1600s
Tambalacoque
Historic extinctions
Some species are more vulnerable to
extinction
Species with small geographic ranges
narrowly distributed endemic species
Species with 1 or a few populations
(e.g. Devil’s Hole Pup Fish, Nevada)
Five ways to go extinct
Extinction via reticulation
Grey Ducks & Mallards
Extinction via competition/predation
e.g., extinction on islands, the
lighthouse keeper’s cat
Stephens Island Wren (NZ)
Extinction via over-exploitation
Pleistocene megafauna?
NZ moas
Extinction via habitat destruction
Ivory-billed woodpecker (?)
Dusky seaside sparrow: extinct in the wild since 1979,
last died in captivity in 1987 (DisneyWorld)
Extinction via changing
environmental conditions
Golden toad (Cranopsis periglenes), formerly of Costa Rica
Mass Extinction –
A major episode of extinction for many
taxa, occurring fairly suddenly in the
fossil record
5 historic mass extinction
events
Mass Extinction Events & Change in diversity
What causes mass extinctions?
Flood basalts and vulcanism
Central Atlantic group (180 mya)
Columbia River group (17-14 mya)
Siberian traps (250 mya)
Deccan traps (65 mya)
Returning to the K-T Boundary:
? Why did Dinosaurs go extinct?
? Why did Mammals and Birds become the
dominant terrestrial vertebrates?
Because of,
A. a giant asteroid impact like the movie
Armageddon
B. global patterns of cooling
C. the extinction of food the Dinosaurs
depended on
Alvarez et al. 1980. Extraterrestrial
cause for the Cretaceous-Tertiary
extinction. Science 208: 1095-1108.
Believed that a giant asteroid had hit
the earth, that this had caused a dense
cover of particulate matter in the
atmosphere that blocked the sun,
causing global cooling, and the death of
many plants
They predicted that an iridium layer
should separate cretaceous from
tertiary strata
Swisher et al. 1992, also published in Science,
found the asteroid crater on the ocean
covered continental shelf of the Yucatan
Mammals and Birds came to dominance
after the Dinosaurs went extinct 65 million
years ago.
This happened at the boundary between
the Cretaceous and Tertiary Periods (or
the K-T boundary), which is also the
boundary between the Mesozoic and
Cenozoic Eras
Mass Extinction – a major episode of
extinction for many taxa, occurring fairly
suddenly in the fossil record
We don’t know how quickly the
extinctions occurred following the
asteroid impact:
Bad weekend versus bad century
(versus bad few million years?)
Either way: a lot of species (millions)
went extinct in a relatively short period
of time
Are human mediated changes in the
environment causing another mass
extinction?
Three questions:
1. Have there been more extinctions
recently then we expect by chance?
2. What are ‘normal’ background levels
of extinctions?
3. Do we expect more rapid extinction
events in the near future?
Background levels of extinction:
Estimates of paleo extinctions derived from
wide-ranging marine animals.
Average species may survive for 1-10 million
years
If 10 million species on the planet then estimate
the loss of a total of 1-10 species per year
So ‘background’ extinction rate is 0.00010.00001% of species per year
Current rate of bird and mammal loss is 0.01 %
per year
100 – 1000 times greater then background levels
Background levels of extinction:
But…
1. Regan (2001) examined estimate and
readjusted calculations to 36-78 times
background levels
2. Estimates based on wide-ranging taxa may
lead to erroneous conclusions
3. Average values calculated on short time
intervals (e.g. the last 500 yrs) may lead to
erroneous conclusions
Pimm et al. 2006: birds
• About 10,000 bird species known
• 129 recorded extinctions since 1500
= 26 extinctions per million species per year
(compare to ‘background’ of ~1 E/MSY)
• BUT:
– We keep finding new remains of extinct species
– Many species are ‘on the verge’ or already gone but
remain on lists
– Conservation efforts have prevented many extinctions
• Added to total, these estimates of more cryptic
extinctions yields ~150 E/MSY
• If present habitat loss continues, rate could get
to 1500 E/MSY by 2100
The big question:
Do we expect rates of extinction to
accelerate in the near future?
The big question:
Do we expect rates of extinction to
accelerate in the near future?
We might because of:
1. Habitat destruction
2. Global climate change
The big question:
Do we expect rates of extinction to
accelerate in the near future?
We might because of:
1. Habitat destruction
2. Global climate change
But what if ‘sensitive’ species have already gone extinct?