The organism in the environment — Edexcel International GCSE Biology
Ecological terms, sampling populations with quadrats, biodiversity, and how abiotic and biotic factors affect where organisms live.
Revision notes
6 short notes, in the order of the specification. Each one in short:
A population is all the organisms of one species living in the same area, and a community is all the populations of different species living there. A habitat is the place where an organism lives. An ecosystem is a community together with the non-living, or abiotic, parts of its environment, such as soil, water, light and temperature.
A quadrat is a square frame used to sample the organisms in a small, known area. To compare population size in two areas, place at least 10 quadrats at random coordinates in each area, count the chosen species in each quadrat and calculate the mean number per quadrat. Random placement avoids bias, and more quadrats give a more reliable mean.
Population size is estimated by counting organisms in randomly placed quadrats and scaling up: estimated population = mean number per quadrat × (total area ÷ area of one quadrat). For example, a mean of 4 daisies in 0.25 m² quadrats across a 100 m² field gives 1600 daisies. Plants hard to count individually are measured by percentage cover.
Biodiversity
Biology onlyBiodiversity is the variety of all the different species of organisms in an ecosystem, a habitat or on Earth. It depends on the number of different species present, not the number of individual organisms. A field with a million plants of one species has low biodiversity, while a hedgerow with many plant and insect species has high biodiversity.
The distribution of an organism is where it is found. A belt transect investigates distribution: quadrats are placed at regular intervals along a tape measure, counting the species and measuring a factor such as light intensity at each. Biodiversity is measured by recording the number of different species in random quadrats and calculating a mean per quadrat.
Abiotic factors are non-living factors, such as temperature, light and water, and biotic factors are living factors, such as competition and predation. Both affect population size and distribution. Organisms compete for limited resources like food, water, light and space, and predator and prey numbers rise and fall in a cycle, with the predator peak coming after the prey peak.
Exam questions
7 exam-style questions (23 marks), each with its mark scheme.
Answer the questionsFlashcards
16 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecology and the environment on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheets
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
The organism in the environment revision notes7 pagesExam questions and mark scheme5 pagesFlashcards to print and cut out4 pagesEcology and the environment mind map1 pagePractical 12: Population size in two areas using quadrats9 pagesPractical 13: Distribution of organisms and biodiversity using quadrats9 pages
Quick check questions
What is a population?
Show answer
All the organisms of one species living in the same area.
What is an ecosystem?
Show answer
A community together with the non-living (abiotic) parts of its environment.
Why are quadrats placed randomly?
Show answer
To avoid bias.
Biology only What is biodiversity?
Show answer
The variety of different species in an ecosystem or habitat.
Name two biotic factors that affect population size.
Show answer
Competition and predation.
Frequently asked questions
What is the difference between a population and a community?
A population is all the organisms of one species living in the same area, such as all the badgers in a wood. A community is all the populations of different species living in that area, such as the badgers, foxes, oak trees, insects and fungi. If more than one species is mentioned, it is a community.
How do you estimate population size using quadrats?
Place quadrats randomly, count the organisms in each and calculate the mean number per quadrat. Then multiply the mean by the total area divided by the area of one quadrat. For example, a mean of 4 per 0.25 m² quadrat in a 100 m² field gives an estimate of 4 × 400 = 1600.
Why are quadrats placed randomly and how many should you use?
Quadrats are placed randomly, using coordinates from a random number generator, so the sample is not biased by the person choosing where to put them. You should use as many as possible, at least 10, because a bigger sample is more representative, reduces the effect of anomalies and gives a more reliable mean.
How do you investigate the distribution of organisms using a transect?
Biology only Lay a tape measure in a straight line across the habitat, such as from shade into open grass. Place a quadrat at regular intervals along it, count the species in each quadrat and measure an environmental factor like light intensity. Plotting both on a graph shows any correlation, but does not prove the cause.
How do abiotic and biotic factors affect populations?
Abiotic factors, such as temperature, light and water, affect growth and survival, so species live only where conditions suit them. Biotic factors include competition and predation. Organisms compete for limited food, water, light and space, and predator and prey numbers affect each other, rising and falling in a repeating cycle.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026