Ecology and the Environment

What you'll learn

This revision guide covers the Ecology and the Environment content from the Pearson Edexcel International IGCSE Biology specification. You'll explore how organisms interact with each other and their environment, understand energy and nutrient flow through ecosystems, and examine both positive and negative human impacts on the natural world. These concepts form the foundation for understanding environmental challenges and conservation strategies.

Key terms and definitions

Ecosystem — a unit containing the community of organisms and their environment, interacting together

Producer — an organism that makes its own organic nutrients, usually using energy from sunlight through photosynthesis

Consumer — an organism that gets its energy by feeding on other organisms

Decomposer — an organism that gets its energy from dead or waste organic material

Trophic level — the position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass

Food web — a network of interconnected food chains showing the feeding relationships within a community

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

Eutrophication — the process by which excess nutrients in water cause excessive growth of algae, leading to oxygen depletion

Core concepts

Energy flow through ecosystems

Energy enters most ecosystems through producers (mainly green plants and algae) via photosynthesis. This energy then flows through the ecosystem as organisms feed on one another.

Food chains show the transfer of energy from one organism to the next, beginning with a producer. Each stage is called a trophic level:

• Producers (first trophic level)
• Primary consumers (herbivores, second trophic level)
• Secondary consumers (carnivores, third trophic level)
• Tertiary consumers (carnivores, fourth trophic level)

Food webs provide a more realistic representation than simple food chains because most animals eat more than one type of food. They show how food chains interconnect within a community.

Pyramids of numbers and biomass

Pyramids of numbers show the population of each organism at different trophic levels. However, they can be inverted (upside down) — for example, one oak tree (producer) supports many caterpillars (primary consumers).

Pyramids of biomass are more reliable as they show the total mass of organisms at each trophic level. They account for the size of organisms, not just their number. Pyramids of biomass are usually pyramid-shaped because:

• Energy is lost at each trophic level through respiration, movement, heat, and waste
• Only approximately 10% of energy is transferred to the next trophic level
• Less energy available means less biomass can be supported

This explains why food chains rarely have more than four or five trophic levels — insufficient energy remains to support higher levels.

Nutrient cycles

Unlike energy (which flows through ecosystems), nutrients are recycled within ecosystems. Two key cycles you must understand are the carbon cycle and the nitrogen cycle.

The carbon cycle

Carbon continuously cycles between the atmosphere, living organisms, and the environment:

Processes that remove carbon dioxide from the atmosphere:

• Photosynthesis by plants and algae converts CO₂ into organic compounds (glucose)

Processes that return carbon dioxide to the atmosphere:

• Respiration by all living organisms releases CO₂
• Combustion (burning) of fossil fuels and wood releases CO₂
• Decomposition by microorganisms breaks down dead material, releasing CO₂ through respiration

Carbon is stored in:

• The atmosphere as carbon dioxide
• Living organisms as organic compounds (carbohydrates, proteins, lipids)
• Fossil fuels (coal, oil, natural gas) formed from dead organisms over millions of years
• Carbonate rocks and shells

The nitrogen cycle

Nitrogen gas (N₂) makes up 78% of the atmosphere, but most organisms cannot use it directly. The nitrogen cycle converts nitrogen into usable forms:

Nitrogen fixation — the conversion of nitrogen gas into nitrogen compounds:

• Lightning provides energy for nitrogen and oxygen to combine
• Nitrogen-fixing bacteria in root nodules of legumes (peas, beans, clover) convert N₂ into nitrates
• Some free-living soil bacteria also fix nitrogen

Nitrification — the conversion of ammonium compounds into nitrates by nitrifying bacteria in the soil

Uptake — plants absorb nitrates from soil through their roots and use them to make amino acids and proteins

Feeding — animals obtain nitrogen compounds by eating plants or other animals

Decomposition — when organisms die, decomposers break down proteins and release ammonium compounds into the soil

Denitrification — denitrifying bacteria in waterlogged soil convert nitrates back into nitrogen gas, returning it to the atmosphere

Human impact on ecosystems

Human activities significantly affect ecosystems, both negatively and positively.

Negative impacts

Deforestation — large-scale removal of forests for timber, agriculture, or urbanisation causes:

• Loss of habitat and reduced biodiversity
• Soil erosion (tree roots no longer hold soil)
• Increased carbon dioxide in atmosphere (fewer trees for photosynthesis, plus CO₂ released from burning)
• Disrupted water cycles

Pollution:

Air pollution from burning fossil fuels releases:

• Carbon dioxide (greenhouse gas contributing to climate change)
• Sulfur dioxide (causes acid rain, damaging plants, aquatic life, and buildings)
• Particulates (cause respiratory problems)

Water pollution from:

• Sewage and fertiliser runoff causes eutrophication: excess nutrients → algal bloom → blocks light → plants die → decomposers increase → oxygen depletion → fish and other organisms die
• Toxic chemicals and heavy metals from industry poison aquatic organisms
• Plastic waste harms wildlife through ingestion and entanglement

Land pollution from:

• Pesticides and herbicides harm non-target species and accumulate in food chains
• Landfill waste takes decades to decompose and may leak toxins

Overharvesting — removing organisms faster than they can reproduce:

• Overfishing depletes fish stocks
• Hunting endangered species reduces biodiversity

Positive impacts and conservation

Conservation strategies protect ecosystems and biodiversity:

Protected areas:

• National parks and nature reserves preserve habitats
• Marine protected areas allow fish populations to recover

Sustainable resource management:

• Sustainable fishing: catch quotas, net size restrictions, fishing seasons
• Sustainable forestry: replanting, selective logging, rotation systems

Breeding programmes:

• Captive breeding of endangered species
• Reintroduction programmes to restore wild populations

International agreements:

• CITES (Convention on International Trade in Endangered Species) regulates trade
• Carbon emission targets reduce greenhouse gases

Recycling and waste reduction:

• Reduces landfill and resource extraction
• Decreases pollution from manufacturing

Alternative energy sources:

• Wind, solar, hydroelectric power reduce fossil fuel dependence
• Biofuels provide renewable alternatives

Population dynamics and competition

Within ecosystems, organisms compete for limited resources. Competition occurs when organisms require the same resources.

Intraspecific competition — competition between members of the same species for:

• Food and water
• Breeding partners
• Territory

Interspecific competition — competition between different species for:

• Food
• Light (plants)
• Space

Competition can limit population size. Predator-prey relationships also affect population dynamics:

• Predator numbers increase when prey is abundant
• Increased predation reduces prey population
• Reduced prey causes predator numbers to decline
• Lower predation allows prey population to recover
• The cycle continues

Sampling techniques and data analysis

Ecologists use various sampling methods to study populations:

Quadrats — square frames used to sample plants and slow-moving animals:

• Place randomly to avoid bias
• Count organisms or estimate percentage cover
• Use multiple samples to calculate mean
• Multiply by total area to estimate whole population

Transects — lines across a habitat to show how distribution changes:

• Belt transect: place quadrats along a line at regular intervals
• Used to study environmental gradients (e.g., distance from seashore)

Capture-recapture — estimate mobile animal populations:

• Capture, mark, and release a sample
• Later, capture another sample and count marked individuals
• Population estimate = (number in 1st sample × number in 2nd sample) ÷ number recaptured marked

Assumptions: no births, deaths, immigration, or emigration between samples; marking doesn't affect survival; marked individuals mix randomly.

Worked examples

Example 1: Energy transfer in food chains

Question: A farmer wants to increase food production. Explain why growing crops to feed humans directly is more energy-efficient than feeding crops to cattle and then eating beef. (3 marks)

Answer:

• Energy is lost between trophic levels through respiration, movement, and heat (1 mark)
• Only approximately 10% of energy is transferred to the next level (1 mark)
• Feeding crops directly to humans involves fewer trophic levels, so more energy from crops reaches humans (1 mark)

Mark scheme notes: Award marks for understanding energy loss between levels and recognising that shorter food chains are more efficient.

Example 2: The nitrogen cycle

Question: Farmers sometimes grow clover on fields before planting wheat. Clover is a legume with nitrogen-fixing bacteria in its root nodules. Explain how this improves wheat growth. (4 marks)

Answer:

• Nitrogen-fixing bacteria convert nitrogen gas from air into nitrates (1 mark)
• When clover is ploughed into soil, it decomposes (1 mark)
• Decomposition releases nitrates into the soil (1 mark)
• Wheat plants absorb nitrates and use them to make amino acids/proteins for growth (1 mark)

Mark scheme notes: Look for understanding of nitrogen fixation, the role of decomposers, and why plants need nitrogen compounds.

Example 3: Eutrophication

Question: A river receives runoff from farmland containing high levels of fertiliser. Describe and explain what happens to the river ecosystem. (5 marks)

Answer:

• Excess nitrates cause rapid algal growth/algal bloom (1 mark)
• Algae block light from reaching plants below surface (1 mark)
• Plants cannot photosynthesise and die (1 mark)
• Decomposers/bacteria feed on dead plants and increase in number (1 mark)
• Decomposers use oxygen in respiration, depleting oxygen in water, causing fish and other organisms to die (1 mark)

Mark scheme notes: Sequence is important. Show the chain of events clearly.

Common mistakes and how to avoid them

• Confusing food chains with energy flow: Remember energy flows in one direction (from sun through producers to consumers), while nutrients cycle. Don't say "energy is recycled."

• Mixing up trophic levels: Producers are always first. Herbivores are primary consumers (not secondary). Count carefully from the start of the chain.

• Incomplete nitrogen cycle explanations: State what bacteria do (e.g., "nitrogen-fixing bacteria convert nitrogen gas into nitrates"), not just "bacteria are involved." Name the specific process.

• Vague pollution effects: Be specific. Don't just say "pollution is bad for organisms." Explain the mechanism: sulfur dioxide dissolves in rain making it acidic, which damages plant leaves and aquatic life.

• Forgetting assumptions in capture-recapture: Examiners often ask about limitations. Remember: no migration, births, or deaths between samples; marks don't affect survival; random mixing.

• Confusing conservation methods: Match the strategy to the problem. Captive breeding helps endangered species; sustainable fishing helps overfished stocks; emissions reduction helps climate change.

Exam technique for "Ecology and the Environment"

• "Explain" vs "Describe": "Describe" requires you to state what happens. "Explain" requires reasons/mechanisms. For example, describe = "plants die"; explain = "plants die because algae block light, preventing photosynthesis."

• Use data from graphs and tables: In questions with data, quote figures to support your answer. Compare values explicitly: "Population increased from 200 to 450 between..."

• Draw complete arrows in cycles: In carbon and nitrogen cycle diagrams, arrows show direction of movement. Label processes on arrows (respiration, photosynthesis), not just compounds in boxes.

• Allocation of marks guides detail: A 1-mark question needs one clear point. A 4-mark question needs four distinct points or developed explanations. Don't write an essay for 1 mark.

Quick revision summary

Ecosystems contain interacting organisms and their environment. Energy flows from producers through consumers via food chains and webs, with approximately 10% transferred between trophic levels. Carbon and nitrogen cycle through ecosystems via photosynthesis, respiration, decomposition, and bacterial processes. Human activities including deforestation, pollution, and overharvesting damage ecosystems by reducing biodiversity, disrupting nutrient cycles, and causing eutrophication. Conservation strategies like protected areas, sustainable management, and breeding programmes help preserve ecosystems. Ecologists use quadrats, transects, and capture-recapture methods to study populations.