CIE A Level Biology (9700) 2019-2021

Revision Notes

18.3.1 Threats to Biodiversity

Threats to Biodiversity

  • Many aquatic and terrestrial ecosystems around the globe are facing major declines in their biodiversity
  • The biodiversity of ecosystems is made up of three components: the variety of habitats, species and genetic diversity within species
  • These three components can be negatively affected by biotic and abiotic factors
  • Many of the major threats to vulnerable ecosystems are due to human activity:
    • Habitat Loss and Degradation
    • Climate change
    • Pollution
    • Overexploitation

Habitat loss & degradation

  • Natural events such as hurricanes, landslides and tsunamis can cause major damage to habitats however it is often only temporary and habitats can recover
  • Over the past 1000 years humans have made major advances in agriculture, construction and industry but it has had major permanent consequences for habitats
  • Causes of aquatic habitat loss include: destructive fishing techniques, dredging of wetlands, damage from ships, tourism and pollution
  • Causes of terrestrial habitat loss include: inland dams, deforestation, desertification, agriculture and pollution
  • When a species’ habitat is destroyed or degraded then they no longer have the support systems they need to survive

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 aquatic and terrestrial ecosystems around the world
  • These include: increasing temperatures, rising sea levels, acidification of the oceans, more intense and frequent wildfires and faster melting of the ice caps
  • These changes have massively impacted some species as the habitat becomes unsuitable for them to live in
  • Coral colonies are the foundation of several complex marine ecosystems and many have been irreversibly destroyed by coral bleaching
    • Coral bleaching is the whitening of coral due to the loss of the algae that usually lives inside them
    • It is caused by a change in ocean temperature and increased ocean acidification
  • Large parts of Australia were decimated by wildfires in 2020
    • Thousands of plants and animals were killed
    • Many animals were also displaced and left without a habitat


  • Pollution is the addition of contaminants into a natural environment
    • It negatively impacts the environment
    • Fertilizers, fossil fuels and non-biodegradable plastics are all examples of pollution that can seriously harm ecosystems
  • Many species within aquatic ecosystems have been negatively affected by large crude oil spills. The toxic oil doesn’t break down for years and it can kill thousands of birds and fish
  • Sewage and runoff from factories can contaminate the soil and upset the nutrient balance in terrestrial ecosystems. It can cause the death and displacement of many plant and animal species


  • Humans harvest many plant and animal species
  • When humans remove individuals from a natural population at a rate greater than the population is able to match with its growth rate then overexploitation is occurring
  • Continued overexploitation of a species can drive it to become extinct
  • The popular commercial fish tuna has seen a dramatic decline in numbers in Europe due to overfishing
    • To combat this the Council of Fisheries in the European Commission has introduced total allowable catches (TACs) or catch limits (measured by weight or no. of fish)
  • Many tropical rainforests are under threat
    • They have major ecological and economic value
    • The trees are being cut down and harvested at a rate much faster than reforestation takes place
    • There are many initiatives and organisations around the world with the aim of protecting tropical rainforests

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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