Understanding Ecology and the environment: Edexcel IGCSE Biology Explained

Ecology is where biology meets the real world. The Edexcel IGCSE Biology specification devotes an entire section to how organisms interact with each other and their physical surroundings, and these concepts appear frequently on both exam papers.

Section 4 of the 4BI1 specification covers four topics: the organism in the environment, feeding relationships, cycles within ecosystems, and human influences on the environment. These edexcel igcse biology ecology and the environment revision notes work through each topic systematically, building from basic ecological definitions to the complex environmental issues that examiners increasingly favour in extended-response questions.

The organism in the environment

Key definitions

The specification requires precise use of ecological terminology. Imprecise definitions are a common source of lost marks.

Term	Definition	Example
Population	All the organisms of one species living in a habitat at a given time	All the rabbits in a field
Community	All the populations of different species living in a habitat at a given time	The rabbits, foxes, grasses and insects in a field
Habitat	The place where an organism lives	A pond, a woodland, a rock pool
Ecosystem	A community of organisms interacting with each other and with their physical environment	A pond ecosystem includes the water, the organisms in it, sunlight, temperature and dissolved minerals
Biodiversity	The variety of different species of organisms in a habitat or on Earth	A tropical rainforest has higher biodiversity than a desert

Sampling with quadrats

The specification requires you to investigate population size and the distribution of organisms using quadrats. A quadrat is a square frame (commonly 0.5 m x 0.5 m) placed on the ground to sample a representative area. To estimate the population of a species:

Place quadrats randomly across the habitat (use random number coordinates to avoid bias).
Count the number of the target organism in each quadrat.
Calculate the mean number per quadrat.
Scale up to the total area of the habitat.

Worked example: A field measures 100 m x 50 m. A student places ten 0.25 m² quadrats randomly and counts the following numbers of daisy plants: 3, 5, 2, 4, 6, 3, 4, 5, 2, 6.

Mean per quadrat = (3+5+2+4+6+3+4+5+2+6) / 10 = 40 / 10 = 4
Total area = 100 x 50 = 5000 m²
Estimated population = (4 / 0.25) x 5000 = 16 x 5000 = 80,000 daisies

The logic: 4 daisies per 0.25 m² = 16 daisies per m². Multiply by total area.

Abiotic and biotic factors

Abiotic factors are non-living: temperature, light intensity, water availability, soil pH, carbon dioxide concentration. Biotic factors are living: predation, competition (for food, water, territory, mates), disease. Both affect population size and distribution. A change in an abiotic factor (e.g. reduced rainfall) can cause a population to decline, which then has knock-on effects on the organisms that feed on it or compete with it.

Feeding relationships

Trophic levels

Organisms in an ecosystem occupy different trophic levels depending on how they obtain their energy:

Producers (trophic level 1): Organisms that make their own food by photosynthesis (e.g. green plants, algae)
Primary consumers (trophic level 2): Herbivores that eat producers (e.g. rabbits, caterpillars)
Secondary consumers (trophic level 3): Carnivores that eat primary consumers (e.g. frogs, small birds)
Tertiary consumers (trophic level 4): Top predators that eat secondary consumers (e.g. hawks, foxes)
Decomposers: Organisms (bacteria and fungi) that break down dead organic matter, returning nutrients to the soil

Food chains, food webs and pyramids

A food chain shows the flow of energy from one organism to the next. A food web is a network of interconnected food chains in an ecosystem, giving a more realistic picture of feeding relationships.

The specification requires understanding of three types of pyramid:

Pyramid type	What it shows	Shape
Pyramid of numbers	The number of organisms at each trophic level	Can be inverted (e.g. one large tree supports many insects)
Pyramid of biomass	The total mass of organisms at each trophic level	Almost always a regular pyramid shape
Pyramid of energy transfer	The amount of energy transferred at each trophic level	Always a regular pyramid shape

Energy transfer between trophic levels

Only about 10% of energy is transferred from one trophic level to the next. The rest is lost through respiration (released as heat), egestion (in faeces) and excretion (in urine, etc.). This is why food chains rarely have more than four or five trophic levels: there is simply not enough energy left to support another level.

Common mistake: Writing that energy is "lost" from an ecosystem. Energy is not destroyed (that would violate the laws of physics). It is transferred to the environment as heat through respiration and is no longer available to the next trophic level. The correct phrasing is that energy is transferred rather than lost.

Cycles within ecosystems

The carbon cycle

Carbon is constantly recycled through living organisms and the environment. The main processes are:

Photosynthesis: Plants absorb CO₂ from the atmosphere and convert it to glucose. Carbon enters the food chain.
Respiration: All living organisms respire, releasing CO₂ back into the atmosphere.
Decomposition: Decomposers (bacteria and fungi) break down dead organisms and waste, respiring and releasing CO₂.
Combustion: Burning fossil fuels (coal, oil, natural gas) releases CO₂ that was locked away millions of years ago.

The nitrogen cycle

Nitrogen makes up about 78% of the atmosphere, but most organisms cannot use nitrogen gas (N₂) directly. The nitrogen cycle converts nitrogen into usable forms and back again, involving four types of bacteria:

Nitrogen-fixing bacteria: Convert atmospheric N₂ into ammonia/ammonium compounds. Found in soil and in root nodules of leguminous plants.
Decomposers: Break down dead organisms and animal waste, releasing ammonium compounds into the soil.
Nitrifying bacteria: Convert ammonium compounds into nitrites, then into nitrates. Nitrates are the form most plants can absorb.
Denitrifying bacteria: Convert nitrates back into atmospheric N₂, completing the cycle. These bacteria thrive in waterlogged, anaerobic conditions.

Plants absorb nitrates through their roots and use them to make amino acids and proteins. Animals obtain nitrogen by eating plants or other animals.

Human influences on the environment

This topic is commonly examined in extended-response questions, often requiring you to evaluate the consequences of human activity. The edexcel igcse biology notes for this topic cover air pollution, the greenhouse effect, water pollution, eutrophication and deforestation.

Air pollution

Sulfur dioxide is released by burning fossil fuels. It dissolves in rainwater to form acid rain, which damages plants (killing leaves), acidifies lakes (killing aquatic organisms) and corrodes buildings.

Carbon monoxide is produced by incomplete combustion of fossil fuels (e.g. in car engines). It is a colourless, odourless, toxic gas. It binds irreversibly to haemoglobin in red blood cells, reducing the blood's oxygen-carrying capacity. High concentrations can be fatal.

The greenhouse effect

Greenhouse gases (water vapour, carbon dioxide, nitrous oxide, methane and CFCs) trap heat in the atmosphere. This natural greenhouse effect keeps Earth warm enough to support life. The problem is the enhanced greenhouse effect: human activities such as burning fossil fuels, deforestation and farming livestock have increased the concentration of greenhouse gases, trapping more heat and potentially leading to global warming.

Consequences of global warming include rising sea levels (due to thermal expansion and melting ice caps), changing weather patterns, species extinction as habitats change, and shifts in the distribution of organisms.

Water pollution

Sewage pollution: Untreated sewage entering waterways provides nutrients for bacteria. Bacteria multiply rapidly and use up dissolved oxygen through aerobic respiration. This reduces the oxygen available for fish and other aquatic organisms, which may die.

Eutrophication: Excess mineral ions (nitrates and phosphates) leach from fertilisers into rivers and lakes. Algae at the surface multiply rapidly (algal bloom), blocking light to aquatic plants below. The plants die. Decomposing bacteria break down the dead plants, using up dissolved oxygen. Fish and other aquatic organisms suffocate.

Deforestation

Removing large areas of forest has several biological consequences:

Leaching: Without tree roots to hold the soil, minerals are washed away by rain.
Soil erosion: Bare soil is carried away by wind and water.
Disturbance of evapotranspiration: Fewer trees means less water vapour released into the atmosphere, potentially reducing local rainfall.
Carbon cycle disruption: Trees that absorbed CO₂ by photosynthesis are removed. If burned, they release stored carbon. Fewer trees means less CO₂ is removed from the atmosphere, contributing to the enhanced greenhouse effect.
Reduced biodiversity: Many species lose their habitats.

Worked example: eutrophication sequence

Question: Explain the process of eutrophication and its effects on aquatic organisms. (6 marks)

Model answer (6 marking points):
1. Excess fertiliser (containing nitrates/phosphates) is washed from fields into a river or lake.
2. The minerals stimulate rapid growth of algae on the water surface (algal bloom).
3. The algal bloom blocks sunlight from reaching aquatic plants below the surface.
4. Aquatic plants cannot photosynthesise and die.
5. Decomposing bacteria feed on the dead plant matter and multiply, using aerobic respiration.
6. Dissolved oxygen in the water is used up, and fish/other aquatic organisms die due to lack of oxygen.

Self-check practice questions

Test your understanding of the ecology and the environment edexcel igcse content with these edexcel igcse biology practice questions. They reflect the kinds of questions that regularly appear on the exam.

Define the terms population, community and ecosystem.
Describe how you would use quadrats to estimate the population of dandelions in a field. Include how you would avoid sampling bias.
Explain why only about 10% of energy is transferred from one trophic level to the next.
Outline the role of denitrifying bacteria in the nitrogen cycle.
Explain how burning fossil fuels contributes to the enhanced greenhouse effect.
Describe the sequence of events in eutrophication, starting from the application of fertiliser to a field.
State three consequences of deforestation.

The igcse 4bi1 ecology and the environment section is commonly tested on both Paper 1 (2 hours, 110 marks) and Paper 2 (1 hour 15 minutes, 70 marks). Paper 2 in particular favours application questions where you must apply ecological principles to unfamiliar scenarios, for example interpreting population data from a field study or evaluating the environmental impact of a proposed development. Solid knowledge of these edexcel igcse biology explained concepts, combined with practice at structuring 6-mark responses, will serve you well. Master the definitions, learn the cycles, and practise the calculations. The ecology section rewards clarity and precision, and the edexcel igcse biology revision notes above give you the detail the specification demands.