CIE A Level Biology复习笔记18.3.1 Reasons for Extinction

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


  • 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 ate 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 & loss of habitats

  • This is the main cause of species extinction
  • Over recent years humans have aggressively destroyed animals habitats by cutting down forests, draining wetlands and polluting the water, soil and air
  • This is highly problematic as species are adapted to survive within their specific habitat that has particular environmental conditions
  • Without their habitat organisms will not get the resources they need to survive
  • As their habitat area gets reduced a species will:
    • Search for other suitable habitats
    • Compete with others for the remaining habitat
  • Eventually the range of habitat can become so small or non-existent that a species is not able to survive and goes extinct

Exam Tip

All of the factors above explain how the population of a species can dramatically decrease and become very small. It is worth thinking about why small populations are so much more vulnerable to extinction.Several things are happening at the genetic level. A smaller population has increased levels of inbreeding, which reduces the genetic variation in the population. Genetic drift has a larger impact on a small population leading to an even further decrease in genetic variation. As we know from natural selection genetic variation is important as it allows a species to adapt and survive environmental change; it improves its fitness. So a small population has a lower fitness meaning increased mortality and decreased reproduction.