Reasons for Extinction (Cambridge (CIE) A Level Biology): Revision Note

Reasons for Extinction

• Extinction is when a species comes to an end or dies out

• Extinction is a natural biological process that happens on planet Earth and studies of fossils and ancient DNA have shown that million of species have gone extinct in the past

• Mass extinction events have also occurred in the past where a very large number of species went extinct at one time

  • The rate of extinction during these periods are very high

• Past mass extinctions were likely caused by major and sudden shifts in the environment such as an Ice Age or an asteroid hitting the Earth

• Scientists have been studying the current rates of extinction in recent years and many believe that the Earth is undergoing a current mass extinction with humans being the main cause

• Possible reasons for extinction include:

  • Climate change

  • Competition

  • Introduction of species

  • Hunting by humans

  • Degradation and loss of habitats

Climate Change

• The large scale burning of fossil fuels by humans in recent years has led to a large increase in the levels of carbon dioxide in the atmosphere, creating the greenhouse effect

• The increased carbon dioxide concentration in the atmosphere has had several knock-on effects on ecosystems around the world

  • There has been an increase in the mean global temperature

  • Sea levels are rising

  • Ocean temperatures and acidity are rising

  • Ice caps are melting

• These knock-on effects have massively changed the habitats of some species, so much so that some are no longer able to survive in the new environmental conditions

  • For example, polar bears are struggling to survive as more of their habitat melts away earlier each year. The earlier melting of the ice caps means they have to swim further to reach seal populations that they hunt for food

Climate Change Causing Extinction - Example

• Human activity has caused elevated CO₂ levels in the air

• CO₂ can dissolve in large bodies of water, such as oceans

• Dissolved carbon dioxide forms a weak acid carbonic acid (H₂CO₃)

• Carbonic acid can dissociate and release H⁺ ions into the water

• This lowers the pH of the ocean water

An effect of climate change on an aquatic ecosystem

• Lower pH affects aquatic life

  • For example, shells of sea organisms are made of calcium carbonate that can  react and be weakened by acid conditions

  • This reduces the calcification of corals and weakens their structure

• Warm water causes coral bleaching, where corals expel the organisms that photosynthesise within them

  • This removes the producer from many food chains and so the food chain collapses, putting selective pressure on many organisms that have lost their habitat

  • The producers (usually forms of algae) are coloured, so when they are removed the coral reef turns white (bleaches)

Staghorn coral (Acropora cervicornis) in its healthy state (left) and bleached state (right)

Adona9 at the English Wikipedia, via Wikimedia Commons Matt Kieffer, CC BY-SA 2.0, via Wikimedia Commons

Competition

• When there is a limited supply of resources within an area competition between individuals for the same resource can occur

• The resources could be food, water, habitat and reproductive mates

• Competition can exist within species and between species

• Competition reduces the population size of a species

  • For example, millions of years ago there were many different species of wild dog that lived in North America

  • When several cat species spread from Asia into North America there were high levels of competition between these carnivores

  • As a result, there are only nine species of wild dog that exist in North America today

• Competition with humans has become a major problem for some species in the last 100 years as humans have taken their food, water and habitat

Introduction of Species

• When humans colonised new land they would often exchange animal and plant species between their home country and the new land

  • These introduced species are non-native

  • Non-native species can be highly problematic as they often have no natural competitors, predators or pathogens that help limit population growth

  • Without these natural population checks, non-native species can massively increase in number

  • The large numbers of non-native species can negatively affect the native species through factors such as competition and disease

• Grey squirrels have led to the decimation of the red squirrel population in the UK

  • Grey squirrels were introduced to the UK in 1876

  • They quickly grew in numbers

  • The larger grey squirrels compete with red squirrels for food

  • They also carry and transmit a disease known as squirrelpox which is fatal to red squirrels

Hunting by humans

• In the past humans would have hunted, killed and eaten wild animal species in order to survive

• Nowadays most of the meat that humans consume comes from domesticated livestock like cattle, sheep and chicken

• The evolution of livestock has negated the need for many humans to hunt wild animals

  • Some humans in underdeveloped countries still have to hunt animals for survival

• The hunting of wild animals is still common and has become a sport for some individuals

  • The rarer and more vulnerable species are often more desirable for a sport hunter

• If too many individuals within a species are killed then the population can become so small that it is no longer able to survive

Degradation and Loss of Habitats

• This is the main cause of species extinction

• Over recent years humans have aggressively destroyed animal habitats by cutting down forests, draining wetlands and polluting the water, soil and air

• This is highly problematic because species are adapted to survive within their specific habitat, under particular environmental conditions

• Without their habitat, organisms will not get the resources they need to survive

  • Search for other suitable habitats

  • Compete with others for the remaining habitatAs their habitat area becomes reduced a species will:

• Eventually the range of habitat can become so small or non-existent that a species is not able to survive and goes extinct