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

Raphus cucullatus: extinct mid/late 1600s

TambalacoquTambalacoquee

MauritiusMauritius

Historic extinctions

Species with small geographic ranges

narrowly distributed endemic species

Species with 1 or a few populations

(e.g. Devil’s Hole Pup Fish, Nevada)

Some species are more vulnerable to extinction

Five ways to go extinct

Grey Ducks & Mallards

Extinction via reticulation

Extinction via competition/predation

e.g., extinction on islands, the lighthouse keeper’s cat

Stephens Island Wren (NZ)

Extinction via over-exploitation

NZ moas

Pleistocene megafauna?

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

Deccan traps (65 mya)

Siberian traps (250 mya)

Columbia River group (17-14 mya)

Central Atlantic group (180 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

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?

Are human mediated changes in the environment causing another mass extinction?

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 0.0001-0.00001% of species per year

Current rate of bird and mammal loss is 0.01 % per year

100 – 1000 times greater then background levels

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

Background levels of extinction:

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

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?

The big question:

We might because of:

1.Habitat destruction

2.Global climate change

Do we expect rates of extinction to accelerate in the near future?

The big question:

We might because of:

1.Habitat destruction

2.Global climate changeBut what if ‘sensitive’ species have already gone extinct?