Extinction
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These are lecture notes from an experimental non-majors biology class taught in 1995 by
Bruce Walsh, University of Arizona.
Other lectures of potential interest:
Origins of life
Cosmic impacts and life on Earth
Genetics class lecture notes
Extinction
- 99.9% of all species have become extinct
- Without extinction, we would not be here
- Mass extinctions vs. background extinction rate
Mass extinctions
- many mass extinctions
- Cretaceous-Tertiary (KT) extinction
- dinosaurs
- many marine species
The Cambrian Extinction (500 MYA)
- 50% of all animal families went extinct
The Devonian Extinction (345 MYA)
- 30% of animal families went extinct
The Permian Extinction (230 MYA)
- 50 % of all animal families
- 95% of all marine species
- all trilobites
- many trees
Triassic extinction (80 MYA)
- 35% of all animal families
Quaternary (Pleistocene epoch) extinction 20,000 YA
- massive extinction of large mammals and birds
- Possible causes:
- The ice-age
- hunting by prehistoric man
- idea due to Paul Matrin (here at Arizona)
Causes of extinction: Overexploitation
- Hawaii : Arrival of Polynesians causes extinction of at least 39 species of endemic land birds
- New Zealand: Colonization by Polynesians (1000 YA) followed by extinction of Moas and other large flightless birds
- Introduction of humans:
- 20,000 YA into North America
- 30,000 YA into Australia
- Recent examples: The great whales
Causes of extinction: Introduction of Pests, Predators and competitors
- Rats, house cats:
- decimated endemic pacific island birds which are mostly ground-nesting
- Lost of mutualists
- In Hawaii, many species of the Lobelia tree are endangered, due to the declining populations (extinction) of native nectar feeding birds
Causes of extinction: Habitat destruction
Risks faced by small populations
- Demographic stochasticity
- random changes in the number of individuals due to random births and deaths
- Genetic stochasticity
- Genetic drift, the loss of genetic information by chance.
- inbreeding depression
Demographic stochasticity
- probability of extinction depends on both the population size and fine details of the population demography
- sex ratio, age structure, age-specific birth and death rates
- General formulae for time to extinction
- T = a * Nb
Genetic stochasticity
- Genetic drift
- fixation of random genes do to small population size
- Deleterious genes can be fixed
- inbreeding depression
Effective population size, Ne
- Rate of drift proportional to 1/Ne
- want to maximize Ne in captive populations
- Equal sex ratio
- equal contribution from all members
- More members of a breeding group, the better
Ne with unequal sex ratios
- Ne = 4 N*r*(1-r)
- N = total population size, r = % female
- extreme case Suppose only a single male used
Short-term Genetic risks faced by small populations
- A population of genetically identical individuals is at high risk of disease
- Wheat rusts
- Cheetahs and FLV
- inbreeding depression
Long-term Genetic risks
- Populations require a certain amount of genetic variation in order to respond to changes in the environment
Genetic Vs. demographic stochasicity
- Once alleles are lost, they cannot be recovered
- mutation to reintroduce genetic variation is extremely slow (at least tens of thousands of generation)
Why save species?
- genetical resources
- ecosystem stability
- ethical issues