Extinction

• It’s a natural process that’s been taking place since the beginning of life.

• Earth’s earlier organisms have become extinct because of climatic, geological and biotic changes. However, today’s organisms are mostly threatened by human activity.

• The current rate of extinction causes the biodiversity crisis.

• The fossil record is the history of life shown by fossils, remains and imprints of organisms from previous geological periods.

• The fossil record shows that the normal ‘background’ rate of extinction is 10⁻⁶ y⁻¹.

• Each year, one species in a million becomes extinct.

• Well documented examples of extinction following exposure to humans include the passenger pigeon, in North America, and the thylacine, which is a type of megafauna, from Tasmania.

• Evidence shows that humans have been responsible for for the extinction of many megafauna (animals with an average weight over 100 pounds).

• Examples of this are the moa, a giant flightless bird from New Zealand, and Megatherium, the giant ground sloth from South America, although climatic stressors may have contributed (changes due to climate change eg. temperature change, sea level rise).

• It is estimated that human activity in tropical areas alone has increased extinction rates between 1,000 and 10,000 times.

• Around one third of Earth’s marine fish species rely on coral reefs. Data suggests that all warm water coral reefs could die out by 2050, at the current rate of destruction.

• Geological evidence shows that the majority of species have been made extinct by a catastrophic change five times since life’s beginning. This has ranged from low global temperature to ocean acidification. These large extinctions are known as mass extinctions.

• The sixth mass extinction is on its way, caused by human activity.

Why Species Are at Risk

• Human activities are changing ecosystems which species depend on.

• Habitats are destroyed around the world by agriculture (farming), urban development (cities), forestry (gathering wood), mining and environmental pollution.

• There is a biodiversity crisis because species are becoming deprived of the biotic (living) and abiotic (non-living) factors they need.

• This results in less successful reproduction so the population decreases.

• Sometimes this leads to extinction

Endangered Species

• The existence of many animals are threatened by human activity.

• The IUCN (the International Union for the Conservation of Nature) makes assessments of plants and animals and grades them based on their vulnerability to extinction. Species are rates depending on their numbers, rate of decline and distribution.

• The grades in order of least to most worried are:

  • Least concern (LC)

  • Near threatened (NT)

  • Vulnerable (VU)

  • Endangered (EN)

  • Critically endangered (CR)

  • Extinct in the wild (EW)

  • Extinct (EX)

• Many species are given the rating DD (data deficient) or NE (not evaluated).

• Results are published in Red Data Lists

The Reasons Species Are Threatened

• Natural selection. This occurs when individuals less suited to prevailing conditions reproduce less successfully. Their numbers decrease, which may lead to extinction. Human activities are causing habitats to change faster than the time it takes for new mutations to allow species to adapt, so are driven to extinction at a faster rate.

• Non-contiguous populations. The total number of individuals in a species may suggest that numbers are sufficient to continue their species, however, some species may exist in isolated groups. If groups of individuals are isolated from each other, they cannot interbreed and each group functions as a separate population. There may not be enough genetic diversity to ensure a healthy population which leads to extinction. An example of this is the black rhinos in Africa

• Loss of habitat. For example, by:

  • Deforestation.

  • Drainage of wetlands.

  • Hedgerow loss. Hedgerows have provided habitats, nesting sites, food and varying light intensity and water availability for diverse plants for centuries. They enable reptiles, birds and mammals to move from one area to another, maintaining biodiversity. Their removal has destroyed large areas of specialised habitat. Herbivores and other consumers reduce in numbers, therefore, organisms at higher trophic levels also reduce in numbers.

  • Farmers often sow crops in autumn rather than spring, so plants are an unsuitable height for birds to build nests with. This has led to a decrease of many well-known birds, such as the skylark or lapwing.

• Overhunting by humans, including:

  • Trophy hunting. Countries that allow this claim that only old or sick animals are targeted.

  • Some traditional medicinal practices, such as the use of tiger body parts and rhino horns.

  • The bush meat industry where primates, and others, are killed for food.

  • Overfishing.

  • Agricultural exploitation, which is the need to increase efficiency and intensity of food production to meet increasing demands by a rapidly increasing human population.

• Competition from introduced species, including domestic animals and the accidental introduction of organisms.

  • The dodo was driven to extinction because, as well as deforestation and hunting, rats were brought on European ships. The rats ate the dodo eggs. Rats were also brought to the Galapagos Islands where rats have diminished native species.

  • The North American signal crayfish has invaded UK streams and rivers, so the native crayfish is being outcompeted.

  • Red squirrels in the UK have declined due to habitat loss, as well as being outcompeted by the North American grey squirrel.

• Pollution.

  • Oil is shipped worldwide in supertankers. Accidental discharge of oil into the sea occurs, such as when the water is no longer deep enough for ships to float. An example of this is the largest ever petroleum spill in freshwater from a Royal Dutch Shell tank ship on the coast of Argentina in 1999. Oil floats and prevents oxygenation of surface water. Animals that break through the surface are covered in a film of oil. If this happens to birds, their feathers clump together so they can’t insulate themselves and freeze to death. Oil washed up on beaches is ingested by shore-dwelling animals, which are poisoned by it.

  • PCBs are polychlorinated biphenyls. They were formerly used in industrial and consumer products but were banned because they are neurotoxins, carcinogens and hormone disruptors. They have been dumped into the wildlife, so are ingested by animals.