Extinction

Robert KinlockeDepartment of Geography and GeologyUWI, Mona

GEOG2202 – The Biosphere

Introduction • Extinction is the failure of a taxonomic group to produce

descendants resulting in its disappearance world wide. • Over the last 600 million years, about 4 billion species have

existed but no more than 10% exist today. • Extinction is a natural, inevitable process and the loss of one

species by extinction may leave an ecological niche that can be filled by others

• Extinctions are measured on the basis of the number of families that made their last appearance in the fossil records.

• Records of extinction are much more abundant in marine environments than they are in terrestrial environments because the fossil record is much better for marine environments

Estimated number of extinctions between 1600 and 1980

Biota Taxa # Extinct

Mammals SpeciesSub species

6352

Birds SpeciesSub species

8883

At the end of this session you should be able to:

• Discuss the different types of extinction • Explain why some organisms are more prone

to extinction than others • Describe the pattern of extinction

Types of Extinction

• Extirpation– Refers to a simple reduction in the range of an organism. – It means a loss of genetic diversity in the local

population. – The loss can however be reversed by reintroduction. – Very often extirpation is the beginning of global

extinction.– e.g: In late Pleistocene, elephants were found in Africa,

Europe, Asia and the Americas but today they are restricted to Africa and India

• Terminal or true extinction -Is the global disappearance of a species or higher taxon such as the disappearance of mammals like the Cape lion and birds such as the Dodo.

Dodo and Cape lion

• Quasi extinction– Refers to a situation in which species are on the

verge of extinction. – e.g: tigers, Cheetahs, chimpanzees, blue and white

marlin, leatherback turtles etc.

• Pseudo extinction or Phyletic extinction– There is no real extinction. As a result of

evolutionary change, a taxon evolves into another of either equal or higher rank and this gives the impression that the ancestor has become extinct. However, the evolutionary lineage continues.

e.g.: Homo habilis disappeared with the evolution of Homo sapiens.

– 20% of all extinctions are pseudo extinction.

Background extinction• Background extinction is a continuous , low level

rate of extinction, representing the normal rate of replacement of one organism by another.

• It is estimated that background rate for all species is about one per year.

• Occurs due to diseases, species interaction, pseudo extinction and small scale environmental changes.

• Events are abnormal, but not high rates of extinction in a region in short space of time.

Mass extinction

• Mass extinction is an excessively high loss of species within a brief geological time period.

• The extinction rates may be twice the background rate and they affect a much broader range of organisms.

• Mass extinction is either gradualistic that is occurring over a period of 100-1 million years or catastrophic occurring within a generation or up to 1000 years.

Extinction susceptibility

• Animals are more susceptible than plants.• Plants are more resistant because of their

mechanisms of propagation:– Ability to regenerate through roots, rhizomes,

seeds etc.– Ability to survive the loss of leaves and severe

biomass mortality

• Certain characteristics make taxa extinction prone:– Large body size– Reproductive type and rate– Carnivorous nature– Taxa with specialized diets– Taxa with low dispersal abilities and those that

occupy a restricted geographical ranges. – Species showing tropical distribution

Body size • Large animals require greater quantities of

resources and therefore exist is relatively smaller populations

• Environmental change could easily affect the entire group

• e.g.: some species of dinosaurs weighed over 50,000 kg. This made temperature regulation difficult.

• Small-bodied animals or plant parts (e.g., leaves) heat up and cool down faster; bigger and/or thicker bodies heat up and cool down more slowly

• smaller/thinner bodies have a larger surface area to volume ratio. Bodies gain and lose heat out of the surface of their body; more surface area means greater gains and losses.

• But this association is not precisely linear• Burbidge and McKenzie (1989) observed that

extinction rates were higher among species of intermediate size range 3500 -5500 g. This was termed the “critical weight range” and was based of observations of Australian terrestrial fauna

•

Figure 1: Body-size frequency distribution of Australian terrestrial mammals, with distributions for extinct and endangered species superimposed

Reproduction type and rate• Species which reproduce asexually tend to be at

higher risk of extinction• Species with lower birth rates tend to be more prone

to extinction. e.g.: California Condor • Correlates of birth rate may include size and trophic

level. Carnivores tend to have lower birth rates • Longer gestation periods. e.g.: elephants have a

gestation period of 22 months, pandas = 90 – 160 days

Carnivorous Nature• As meat-eating animals become larger they are forced to prey

on animals larger than themselves. Catching little animals like mice and rabbits requires more energy than it's worth.

• Once they cross that line and start to prey on larger animals, the predators develop larger jaws and teeth and generally become more specialized. This makes them more vulnerable to environmental change

• Additionally, because they are at higher trophic levels they require larger quantities of food

SpecializationSpecies with specialized diets or habitats are

particularly vulnerable.e.g.: Giant Pandas have a diet which is

comprised of 99% bamboo.

Dispersal ability and geographical ranges• Dispersal rate for a species depends on either the long-

distance dispersal of colonists (e.g., seeds or migrating animals), or rapid iterative colonization of nearby habitat. Most trees, for instance, can change the boundaries of their range by 1 to 45 kilometres per century.

• Species with intrinsically low colonization ability may face extinction as the present habitat becomes unsuitable. Predicted shifts of up to 100's of kilometres per century are obviously beyond the dispersal ability of most trees. Such species would face extinction as climatic zones shift.

• Endemic species with narrow ranges are particularly prone to extinction

• e.g. Jamaican Iguana and Black Billed Parrot (at risk of extinction)

Species showing tropical distributions• Abundance tends to be lower in the tropics

The End