Understanding Biology: Ecology and the environment: Edexcel IGCSE Science Single Award Explained

Ecology sits at the intersection of biology, chemistry and physics, which makes it a natural fit for the Science Single Award

If you are studying Pearson Edexcel IGCSE Science Single Award (4SS0), you already know that the course draws from three disciplines in a single qualification. Ecology and the environment is one of the biology sections that asks you to step back from cells and organs and look at how whole organisms interact with each other and with their surroundings. It is a section where biology meets chemistry (through the carbon cycle) and where quantitative fieldwork skills become just as important as factual recall.

This article covers every specification point in the ecology and environment section of the edexcel igcse science single award biology: ecology and the environment unit. You will find the core concepts broken down, worked examples of the kinds of calculations you might face, common mistakes to avoid, and self-check questions to test yourself. Use these as edexcel igcse science single award revision notes alongside your own class notes and past papers.

The organism in the environment

Before you can study how organisms interact, you need a shared vocabulary. The specification requires you to understand four terms, and examiners expect precise definitions rather than vague approximations.

Term	Definition	Example
Population	All the organisms of one species living in the same area at the same time	All the rabbits in a meadow
Community	All the populations of different species living in the same area at the same time	The rabbits, foxes, grasses, insects and fungi in that meadow
Habitat	The place where an organism lives	The meadow itself, including its soil, moisture and shelter
Ecosystem	A community of organisms together with the non-living (abiotic) components of their environment, interacting as a system	The meadow ecosystem, encompassing its community plus the soil, rainfall, temperature and light

Notice how each term builds on the one before. A population is part of a community, which lives in a habitat, and a habitat combined with its community forms an ecosystem. Examiners regularly test whether you can distinguish between these terms, so practise writing them out from memory until each definition feels automatic.

Estimating population size with quadrats

The specification asks you to investigate the population size of an organism in two different areas using quadrats. A quadrat is a square frame (often 0.25 m² or 1 m²) placed randomly in a habitat. You count the number of individuals of your target organism inside the frame, repeat the process at multiple random positions, and then scale up.

Suppose you are comparing the number of daisy plants in a sunny field and a shaded woodland. You throw ten quadrats (each 0.5 m x 0.5 m = 0.25 m²) in each area and count the daisies inside each one.

Quadrat number	Sunny field (daisies)	Shaded woodland (daisies)
1	12	2
2	9	0
3	15	1
4	11	3
5	10	0
6	14	2
7	8	1
8	13	0
9	11	1
10	7	0

Mean number per quadrat in the sunny field = (12 + 9 + 15 + 11 + 10 + 14 + 8 + 13 + 11 + 7) / 10 = 110 / 10 = 11.

If the total area of the sunny field is 2000 m², you can estimate the total population:

Estimated population = mean per quadrat x (total area / area of one quadrat) = 11 x (2000 / 0.25) = 11 x 8000 = 88,000 daisies.

Common mistake: Forgetting to divide the total area by the area of one quadrat, not by the number of quadrats. The number of quadrats tells you how many samples you took. The area ratio tells you how many quadrats would fit in the whole field. These are different numbers, and mixing them up produces an answer that is orders of magnitude wrong.

Abiotic and biotic factors

The specification requires you to understand how both abiotic and biotic factors affect the population size and distribution of organisms.

Abiotic factors are the non-living components of the environment: light intensity, temperature, water availability, soil pH, mineral content, wind speed, and carbon dioxide concentration. A field with more light will support more photosynthesising plants, which in turn supports more herbivores. A pond with low oxygen levels will support fewer fish.

Biotic factors are the living components: competition (for food, water, territory, mates), predation, disease, and the availability of food. If a new predator is introduced to an ecosystem, the population of its prey will fall, at least initially. If two species compete for the same food source, the one that is better adapted will tend to out-compete the other.

In an exam, you might be given data showing a population change and asked to suggest which factor caused it. The key is to look at the data carefully. A sudden crash might suggest disease or a new predator. A gradual decline might suggest increasing competition or a slow environmental change such as rising temperature.

Feeding relationships

This is the largest part of the ecology section, and it carries several concepts that examiners test in different ways across the edexcel igcse papers.

Trophic levels

Every organism in a food chain occupies a trophic level. The specification requires you to know the names given to different trophic levels:

Producers (trophic level 1): organisms that make their own food by photosynthesis, such as green plants and algae.
Primary consumers (trophic level 2): herbivores that eat producers. Examples include rabbits, caterpillars and zooplankton.
Secondary consumers (trophic level 3): carnivores or omnivores that eat primary consumers. Examples include frogs eating caterpillars, or small fish eating zooplankton.
Tertiary consumers (trophic level 4): predators that eat secondary consumers. Examples include hawks eating frogs, or large fish eating smaller fish.
Decomposers: organisms (mainly bacteria and fungi) that break down dead organic matter and waste products, returning nutrients to the soil. They operate at every trophic level because dead organisms and waste exist at all levels.

Food chains, food webs and pyramids

A food chain shows a single pathway of energy transfer from producer to top consumer. For example: grass (producer) to rabbit (primary consumer) to fox (secondary consumer). A food web shows the interconnected food chains in an ecosystem and gives a more realistic picture of feeding relationships, because most organisms eat more than one type of food and are eaten by more than one predator.

The specification also requires you to understand three types of pyramid:

Pyramid type	What each bar represents	Shape
Pyramid of numbers	The number of individual organisms at each trophic level	Usually widest at the base, but can be inverted if the producer is very large (e.g. one oak tree supporting thousands of insects)
Pyramid of biomass	The total mass of living material at each trophic level at a given time	Almost always widest at the base and narrowing upwards
Pyramid of energy transfer	The total amount of energy flowing through each trophic level in a given time	Always widest at the base, never inverted

Exam tip: Pyramids of energy transfer are always the correct shape (widest at the bottom), because energy is lost at every step. Pyramids of numbers can be inverted, and pyramids of biomass can occasionally appear inverted in aquatic systems where producers reproduce very rapidly. If a question asks which type of pyramid is always the correct shape, the answer is the pyramid of energy transfer.

Energy transfer along a food chain

The specification requires you to understand the transfer of substances and energy along a food chain, and to understand why only about 10% of energy is transferred from one trophic level to the next.

When a rabbit eats grass, it does not convert all the energy in the grass into rabbit biomass. Energy is lost at each transfer for three main reasons:

Respiration: organisms use a large proportion of the energy they absorb for cellular respiration, which releases energy as heat. This heat is lost to the surroundings and cannot be used by the next trophic level.
Excretion and egestion: not all the food an organism eats is digested. Faeces (egested material) still contain energy, which passes to decomposers rather than to the next consumer. Urine and other excretory products also carry some energy away.
Parts not eaten: predators may not eat every part of their prey. Bones, fur and feathers are often left behind, and the energy they contain is not transferred.

The result is that roughly 10% of the energy at one trophic level is available to the next. This explains why food chains rarely have more than four or five trophic levels: by the time energy has passed through four transfers, there is not enough left to support another level of consumer.

If you are given a calculation question, the working is straightforward. Suppose producers in an ecosystem fix 50,000 kJ of energy. Primary consumers receive about 10%, which is 5,000 kJ. Secondary consumers receive 10% of that, which is 500 kJ. Tertiary consumers receive 10% of 500 kJ, which is 50 kJ. Each step divides by ten.

Cycles within ecosystems

The carbon cycle

The specification requires you to describe the stages in the carbon cycle, including respiration, photosynthesis, decomposition and combustion. Carbon atoms move between the atmosphere (as carbon dioxide), living organisms (as organic compounds) and fossil fuels (as hydrocarbons), cycling continuously through these reservoirs.

Here are the four key processes and what they do to carbon:

Photosynthesis removes carbon dioxide from the atmosphere. Plants and algae absorb CO₂ and use it, along with water and light energy, to build glucose and other organic molecules. This is the main route by which carbon enters the living part of the cycle.
Respiration returns carbon dioxide to the atmosphere. All living organisms (plants, animals, fungi, bacteria) respire, breaking down organic molecules and releasing CO₂ as a waste product. Respiration happens continuously, day and night.
Decomposition returns carbon to the soil and atmosphere. When organisms die, decomposers (bacteria and fungi) break down their bodies. The decomposers respire as they do this, releasing CO₂. Some carbon is also incorporated into the soil as humus.
Combustion releases carbon that has been locked away for millions of years. Fossil fuels (coal, oil, natural gas) are the remains of organisms that died millions of years ago and were buried before they could fully decompose. Burning these fuels releases their stored carbon as CO₂.

In a balanced ecosystem, the carbon removed from the atmosphere by photosynthesis is roughly equal to the carbon returned by respiration and decomposition. However, the large-scale combustion of fossil fuels by human activity adds extra carbon dioxide to the atmosphere faster than photosynthesis can remove it. This is the basis of the greenhouse effect and climate change, a connection that links biology to the chemistry section of this single award course.

How the carbon cycle connects to the rest of the specification

One of the strengths of the Edexcel IGCSE Science Single Award is the way it encourages you to see connections across the three sciences. The carbon cycle links directly to photosynthesis and respiration (biology), to combustion and the properties of carbon dioxide (chemistry), and to energy transfers (physics). When you revise this topic, try to see it as a connecting thread rather than an isolated set of facts. In the exam, a question on the carbon cycle might reference greenhouse gases from the chemistry section, and the strongest answers will make that cross-disciplinary link explicit.

Self-check questions

Use these to test your understanding. Try answering each one from memory before checking your notes.

Define the terms population, community, habitat and ecosystem.
A student places fifteen 0.25 m² quadrats randomly in a park and counts an average of 6 clover plants per quadrat. The park has an area of 5,000 m². Estimate the total clover population in the park. Show your working.
Explain why a pyramid of numbers can be inverted but a pyramid of energy transfer cannot.
A producer fixes 80,000 kJ of energy per year. If only 10% is transferred at each trophic level, how much energy is available to a tertiary consumer?
Name four processes involved in the carbon cycle and state, for each one, whether carbon dioxide is released into or removed from the atmosphere.
Explain why food chains rarely have more than four or five trophic levels.
A student notices that there are more dandelion plants on the south-facing side of a school building than on the north-facing side. Suggest one abiotic factor that could explain this difference, and explain your reasoning.

Bringing it together

Ecology is a section where understanding the principles matters more than memorising isolated facts. If you understand why energy is lost at each trophic level, you can answer questions about food chain length, pyramid shapes, and energy calculations, even when the specific organisms in the question are unfamiliar. If you understand the four processes of the carbon cycle, you can trace carbon through any scenario the examiner puts in front of you.

For your biology: ecology and the environment edexcel igcse revision, make sure you can define every key term, carry out a quadrat calculation, draw and interpret all three types of pyramid, explain the 10% energy transfer rule, and describe the carbon cycle with all four named processes. These are the areas where marks are most reliably available in the igcse 4ss0 biology: ecology and the environment section of the exam.

When you feel confident with these edexcel igcse science single award notes, move on to edexcel igcse science single award practice questions using past papers. Mark your answers against the mark scheme, and pay close attention to any question where you lost marks on definitions or on the energy transfer calculation. Those are the areas where repeated practice produces the biggest improvement. The Green Bridge CBT platform provides topic-sorted questions and performance tracking so you can see exactly where your revision is paying off and where it still needs work.

Ecology is the section of the edexcel igcse science single award explained most naturally through examples. Every ecosystem you walk through, from a city park to a coastal rockpool, is a live demonstration of the principles in this unit. If you can see a food web in your local environment and trace the carbon moving through it, the exam questions will feel like familiar territory rather than abstract exercises.