145 captures
02 Aug 1997 - 19 Mar 2022
Jul
AUG
Sep
02
1996
1997
1998
success
fail
About this capture
COLLECTED BY
Organization:
Alexa Crawls
Starting in 1996,
Alexa Internet
has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to the
Wayback Machine
after an embargo period.
Collection:
Alexa Crawl GR
Crawl GR from Alexa Internet. This data is currently not publicly accessible.
TIMESTAMPS
The Wayback Machine - https://web.archive.org/web/19970802184301/http://nitro.biosci.arizona.edu:80/courses/EEB105/lectures/extinction/extinction.html
Extinction
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
Ne = 4 - 4/n
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
