Biodiversity and the impact of human activities on ecosystems

What you'll learn

This revision guide covers the AQA GCSE Biology specification content on biodiversity and how human activities impact ecosystems. You'll learn about waste management, pollution types, land use changes, deforestation, global warming and peat destruction. Understanding these concepts is essential for Paper 2 and will help you answer both short-answer and extended-response questions on environmental topics.

Key terms and definitions

Biodiversity — the variety of all the different species of organisms on Earth, or within an ecosystem

Pollution — the introduction of harmful substances or products into the environment

Deforestation — the permanent removal of trees from forests to clear land for other uses

Global warming — the gradual increase in Earth's average surface temperature caused by increased concentrations of greenhouse gases in the atmosphere

Eutrophication — the process by which nutrients accumulate in water bodies, leading to excessive plant and algal growth

Peat — partially decomposed plant material that has accumulated in waterlogged acidic conditions over thousands of years

Biogas — fuel produced by anaerobic decay of biological material containing a mixture of methane and carbon dioxide

Invasive species — organisms introduced to ecosystems where they do not naturally occur, often causing harm to native species

Core concepts

The importance of biodiversity

Biodiversity refers to the variety of living organisms in an area. High biodiversity indicates a healthy, stable ecosystem with many different species occupying various ecological niches.

The importance of maintaining biodiversity includes:

• Future food security — different species may contain genes useful for developing new crop varieties resistant to disease or climate change
• New medicines — many pharmaceutical drugs originate from compounds found in plants, animals and microorganisms
• Industrial materials — organisms provide raw materials like wood, fibres and oils
• Ecosystem services — different species contribute to pollination, nutrient cycling, water purification and climate regulation
• Cultural value — many people appreciate the aesthetic and recreational value of diverse natural environments

Biodiversity is being reduced globally due to human activities. The current rate of extinction is estimated to be 100–1000 times higher than the natural background extinction rate.

Human population growth and waste production

The human population has grown exponentially, from approximately 1 billion in 1800 to over 8 billion today. This rapid increase places significant pressure on ecosystems.

As populations grow and living standards improve, humans use more resources and produce more waste. This leads to:

• Increased demand for food, water and energy
• More land required for housing, agriculture and industry
• Greater volumes of waste requiring disposal
• Increased extraction of natural resources

Pollution occurs when harmful substances are released into the environment in quantities that damage ecosystems. The main types relevant to GCSE are:

Water pollution:

• Sewage and fertiliser runoff cause eutrophication in rivers and lakes
• Chemical pollutants from industrial processes contaminate water supplies
• Plastics accumulate in aquatic environments, harming wildlife

Air pollution:

• Combustion of fossil fuels releases carbon dioxide (contributing to global warming), sulfur dioxide (causing acid rain), and particulates (damaging respiratory health)
• Smoke and gases from factories and vehicles reduce air quality

Land pollution:

• Toxic chemicals from agriculture (pesticides, herbicides) contaminate soil
• Landfill sites take up space and may leak harmful substances
• Nuclear waste remains radioactive for thousands of years

Eutrophication in detail

Eutrophication is a particularly important process you must understand in detail for GCSE examinations.

The process occurs in stages:

1. Excessive nutrients (nitrates and phosphates) enter water bodies from agricultural fertiliser runoff or sewage discharge
2. These nutrients stimulate rapid growth of algae and aquatic plants, forming an "algal bloom" on the water surface
3. The algal bloom blocks light from reaching plants below the surface
4. Submerged plants cannot photosynthesise and die
5. Aerobic bacteria decompose the dead organic matter, using oxygen from the water during aerobic respiration
6. Oxygen levels in the water decrease dramatically (oxygen depletion)
7. Aquatic animals such as fish and invertebrates die from lack of oxygen

This creates a "dead zone" where few organisms can survive. Eutrophication demonstrates how human activities can cause cascading effects through ecosystems.

Deforestation and its consequences

Deforestation involves large-scale removal of trees for various purposes:

• Providing timber for construction and fuel
• Clearing land for agriculture (crops and livestock grazing)
• Extracting minerals through mining operations
• Building infrastructure (roads, settlements)

The consequences of deforestation are significant and far-reaching:

Impact on biodiversity:

• Habitat destruction causes species extinction
• Fragmented forests isolate populations, reducing genetic diversity
• Loss of food sources and breeding sites for wildlife

Impact on climate:

• Trees remove carbon dioxide from the atmosphere through photosynthesis
• When trees are cut down and burned or left to decay, carbon dioxide is released back into the atmosphere
• Reduced photosynthesis means less carbon dioxide is absorbed from the atmosphere
• This increases the greenhouse effect and contributes to global warming

Impact on soil:

• Tree roots bind soil together and prevent erosion
• Without tree cover, heavy rainfall washes away nutrient-rich topsoil
• This causes flooding downstream and reduces soil fertility

Impact on the water cycle:

• Trees absorb water from soil and release it through transpiration
• Deforestation disrupts local rainfall patterns
• Increased surface water runoff causes flooding

Global warming and climate change

Global warming refers specifically to the rise in Earth's average temperature. Climate change encompasses broader changes in weather patterns, sea levels and ecosystems resulting from global warming.

The greenhouse effect is a natural process essential for maintaining Earth's temperature:

1. Short-wavelength radiation from the Sun passes through the atmosphere and warms Earth's surface
2. Earth radiates heat as longer-wavelength infrared radiation
3. Greenhouse gases in the atmosphere (carbon dioxide, methane, water vapour) absorb this infrared radiation
4. The absorbed energy is re-radiated in all directions, warming the atmosphere and Earth's surface

Human activities have enhanced the greenhouse effect by increasing greenhouse gas concentrations:

Carbon dioxide sources:

• Burning fossil fuels for electricity, heating and transport
• Deforestation and combustion of wood
• Cement production

Methane sources:

• Livestock farming (enteric fermentation in cattle and sheep)
• Rice paddy fields (anaerobic decay)
• Decomposition in landfill sites
• Natural gas extraction

Consequences of global warming include:

• Rising sea levels due to thermal expansion of water and melting ice caps, threatening low-lying areas
• Changes in weather patterns causing more extreme events (droughts, floods, hurricanes)
• Changes in species distribution as organisms migrate to suitable climates
• Increased extinction risk for species unable to adapt quickly enough
• Reduced biodiversity in vulnerable ecosystems
• Changes to food production as agricultural conditions shift

Peat bogs and their destruction

Peat forms in waterlogged, acidic, anaerobic conditions where decomposition is very slow. Dead plant material accumulates over thousands of years, forming thick deposits.

Peat bogs are important ecosystems because they:

• Store vast quantities of carbon that would otherwise contribute to atmospheric carbon dioxide
• Provide unique habitats for specialised species
• Act as natural water filters and flood defences

Peat is extracted for several uses:

• Compost for gardens and agriculture
• Fuel for heating (particularly in Ireland and parts of Scotland)

Problems with peat extraction:

• Releases stored carbon dioxide when peat is burned or when aerobic decay occurs after drainage
• Destroys rare peat bog habitats and threatens specialist species
• Peat forms extremely slowly (approximately 1 mm per year), making it a non-renewable resource
• Contributes significantly to greenhouse gas emissions

Alternatives to peat-based compost include composted bark, coir (coconut fibre) and recycled green waste.

Maintaining biodiversity: conservation and management

Several strategies help maintain biodiversity and reduce human impact on ecosystems:

Breeding programmes:

• Captive breeding of endangered species in zoos and wildlife parks
• Genetic diversity maintained through careful selection of breeding pairs
• Reintroduction programmes return species to their natural habitats (e.g., red kites and beavers in the UK)

Protection and regeneration of habitats:

• Establishing nature reserves and national parks
• Protecting rare habitats through legal designation (Sites of Special Scientific Interest in the UK)
• Reforestation and rewilding projects
• Preventing building on ecologically important areas (greenfield sites)
• Creating wildlife corridors to connect fragmented habitats

Reduction of deforestation:

• Sustainable forestry with selective cutting and replanting
• Using alternative materials to timber
• Improving agricultural efficiency to reduce land clearance
• Legal protections for vulnerable forests

Recycling and waste reduction:

• Reducing consumption of resources
• Recycling metals, plastics, paper and glass to conserve raw materials
• Composting organic waste
• Reducing single-use plastics to prevent marine pollution

Biogas production:

• Anaerobic digestion of organic waste produces methane-rich biogas
• Can be used as a renewable fuel source
• Reduces methane emissions from landfill
• Produces nutrient-rich digestate suitable as fertiliser

Worked examples

Example 1: Eutrophication sequence (4 marks)

Question: A farmer applies large amounts of fertiliser to fields next to a river. Heavy rain washes some fertiliser into the river. Explain how this could lead to the death of fish in the river.

Answer:

• Fertiliser contains nitrates/nutrients [1]
• This causes increased growth of algae/algal bloom [1]
• Algae block light, so plants below the surface die / cannot photosynthesise [1]
• Bacteria decompose dead plants using oxygen / aerobic respiration [1]
• Oxygen levels decrease, so fish die / fish cannot respire [1]

(Any 4 points for 4 marks)

Example 2: Deforestation impacts (6 marks)

Question: Large areas of rainforest in South America are being cut down. Explain the impact of this deforestation on biodiversity and climate change.

Answer: Impact on biodiversity:

• Loss of habitat for species [1]
• Many species become extinct / loss of species variety [1]
• Loss of food sources for animals [1]

Impact on climate change:

• Trees remove carbon dioxide from atmosphere through photosynthesis [1]
• When trees are burned or decay, carbon dioxide is released [1]
• Less photosynthesis occurs, so less carbon dioxide is removed from atmosphere [1]
• This increases greenhouse effect / global warming [1]

(Any 6 points for 6 marks — must cover both parts of the question)

Example 3: Peat extraction (3 marks)

Question: Explain why gardeners should avoid using peat-based compost.

Answer:

• Peat extraction destroys peat bog habitats [1]
• Releases carbon dioxide stored in peat / contributes to global warming [1]
• Peat is non-renewable / forms very slowly [1]

(Any 3 points for 3 marks)

Common mistakes and how to avoid them

• Confusing global warming with the greenhouse effect — global warming is the increase in temperature; the greenhouse effect is the natural process that has been enhanced by human activity. Use precise terminology.

• Incomplete eutrophication explanations — always include the full sequence: nutrients → algal bloom → light blocked → plants die → bacteria decompose → oxygen depleted → fish die. Missing steps loses marks.

• Saying trees "produce" or "make" oxygen rather than describing photosynthesis — be specific that trees remove carbon dioxide during photosynthesis and that cutting them down reduces this carbon dioxide removal.

• Not stating both the source and the consequence — when describing pollution, identify what causes it AND what effect it has. For example, "Fertiliser runoff causes eutrophication which leads to death of aquatic organisms."

• Vague statements about biodiversity — instead of "biodiversity is good," explain specific reasons: provides future medicines, ensures food security, maintains ecosystem stability.

• Confusing methane and carbon dioxide sources — methane comes from livestock, rice fields, and decomposition; carbon dioxide primarily comes from combustion of fossil fuels and deforestation.

Exam technique for "Biodiversity and the impact of human activities on ecosystems"

• Command word awareness: "Describe" requires you to state features or characteristics; "Explain" requires reasons or mechanisms with linking words like "because," "so," "this causes." Extended response questions worth 6 marks typically require explanations with logical sequences.

• Link human activity to consequence clearly: Structure answers as [human activity] → [immediate effect] → [consequence]. For example: "Burning fossil fuels releases carbon dioxide, which is a greenhouse gas, causing increased global temperatures."

• Use named processes and correct terminology: Answers mentioning "eutrophication," "anaerobic decay," or "greenhouse effect" by name demonstrate understanding and gain credit more easily than vague descriptions.

• In calculations or data questions about population or pollution: show your working, include units, and relate numbers back to the biological concept being tested. For 6-mark questions, plan your answer briefly before writing to ensure you cover all required points in a logical order.

Quick revision summary

Human population growth increases resource use and waste production, causing pollution of water, air and land. Eutrophication occurs when excess nutrients cause algal blooms, oxygen depletion and death of aquatic life. Deforestation destroys habitats and contributes to climate change by reducing photosynthesis and releasing stored carbon. Global warming results from enhanced greenhouse effect due to increased carbon dioxide and methane. Peat extraction releases stored carbon and destroys habitats. Biodiversity can be maintained through breeding programmes, habitat protection, sustainable practices and waste reduction. Understanding the detailed mechanisms and consequences of human impacts is essential for exam success.