In simple terms
A friendly intro before the formal notes — no formulas yet.
Nature's Levels of Organisation
Ecology is like studying a city, from individual people to whole neighbourhoods and the city infrastructure. We start with a single organism and zoom out to see how it fits into its population, community, and the entire ecosystem including its physical surroundings.
Think of a school. A single student is an 'individual'. All the Year 12 students together form a 'population'. All students from all years, plus all the teachers and staff, make up the 'community'. The entire school, including the buildings, playing fields, and weather (the non-living parts), is the 'ecosystem'. Each person has a place they hang out (habitat) and a specific role they play (niche).
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First, identify the levels of organisation in a given scenario: start with a single species, then its population, the wider community of different species, and finally the whole ecosystem including non-living factors.
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Next, classify the components as either biotic (living or from living organisms, like predation or a fallen log) or abiotic (non-living, physical factors like sunlight, pH, or temperature).
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Then, analyse the interactions between organisms. Determine if it's competition, predation, or a symbiotic relationship like mutualism or parasitism.
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Finally, define an organism's role by describing its niche. This includes its habitat, its food sources, its predators, and its tolerance to abiotic factors.
Explore the concept
Use the live diagram and synced steps — play it or tap a step card to walk through.
2 more simulations for this topic — run them in the Simulations section below
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
2 simulations
- GeoGebraCoreIB 2.1
Rabbits and Foxes: predator–prey model
Four sliders set how fast rabbits breed, how fast foxes die and how strongly each affects the other; two boxes set the starting numbers.
Try this
- Read the default run: blue dots are rabbits, red dots are foxes. Note which population peaks first.
- Raise a (how often a fox catches a rabbit) and watch the rabbit troughs deepen.
- Type 0 into the F₀ box: with no predator the rabbit line climbs off the chart.
- Drag l further below zero (foxes die faster): the rabbits boom before the foxes catch up.
Look for Predator numbers track prey numbers with a lag: the fox peak always arrives after the rabbit peak. That is predation (+/−) acting as negative feedback on both populations.
jorj.kowszun · GeoGebra · GeoGebra Terms of Service
- GeoGebraIB 2.1
Exponential and logistic growth
Drag the r slider and the carrying capacity point to compare unlimited growth (a J-curve) with growth that levels off (an S-curve).
Try this
- Drag r up and down: the two curves start together, then split.
- Drag the carrying capacity point higher up the vertical axis and watch where the logistic curve now flattens.
- Find the stretch where the curves are almost identical and explain why limiting factors hardly matter there.
Look for Limiting factors only bite as a population nears carrying capacity; that is what bends a J-curve into an S-curve.
Arthur Lee · GeoGebra · GeoGebra Terms of Service
Full topic notes
Formal explanation with the rigour you need for the exam.
Levels of Ecological Organisation
To make sense of the complexity of the natural world, ecologists use a hierarchical system. At the most basic level is the species, a group of organisms that can interbreed to produce fertile offspring. A group of individuals of the same species in a specific area forms a population. When you consider all the different populations of different species living and interacting together, you have a community. Finally, an ecosystem encompasses the entire community plus its interactions with the abiotic (non-living) environment.
Species: A single type of organism (e.g., the African lion, Panthera leo).
Population: A group of African lions in the Serengeti National Park.
Community: The lions, zebras, acacia trees, grasses, and vultures in the Serengeti.
Ecosystem: The entire Serengeti community plus the sunlight, water, soil, and climate.
Biotic and Abiotic Factors
Every ecosystem is defined by the interplay between its living and non-living parts. Biotic factors are all the living or once-living components, including plants, animals, fungi, bacteria, and their interactions such as predation, disease, and competition. Abiotic factors are the non-living physical and chemical elements of the environment, such as temperature, sunlight, pH, salinity, and soil composition. These factors determine which species can survive and thrive in a particular ecosystem.
Interactions and Niches
Organisms within a community constantly interact. These interactions can be categorised based on their effect on the participants. Competition (-/-) occurs when two or more organisms require the same limited resource. Predation (+/-) is when a predator feeds on its prey. Symbiosis describes close, long-term interactions, and includes parasitism (+/-), where one organism benefits at the other's expense, and mutualism (+/+), where both benefit.
These interactions help define an organism's niche - its functional role in the ecosystem. This is distinct from its habitat, which is simply the physical place it lives. A niche includes its feeding habits, its position in the food web, its reproductive strategy, and its tolerance to all biotic and abiotic factors. The principle of competitive exclusion states that two species cannot occupy the exact same niche in the same habitat indefinitely.
When asked to describe a species' niche, don't just state its habitat. A complete answer must include its relationships with other species (e.g., what it eats, what eats it) and its use of abiotic resources (e.g., its temperature tolerance, its need for sunlight).
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A student uses a 0.5m x 0.5m quadrat to sample daisies on a school field. They count an average of 5 daisy plants per quadrat over 10 samples. The total area of the field is 2000 m². During the study, the student records the soil pH as 6.5 and notes that the field is south-facing, receiving high light intensity.
(a) Estimate the total population of daisies on the field. [2] (b) Identify one biotic and one abiotic factor mentioned or implied in the text. [2]
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Calculate the area of the quadrat: . [1]
In a British woodland, the grey squirrel (Sciurus carolinensis), an invasive species, has largely replaced the native red squirrel (Sciurus vulgaris). Both species feed on nuts like acorns and hazelnuts. However, the grey squirrel can digest acorns more efficiently than the red squirrel.
(a) Identify the type of interaction between the red and grey squirrels. [1] (b) Using the concept of the niche, explain why the red squirrel population has declined. [3]
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The red and grey squirrels share a similar habitat (woodlands) and have overlapping fundamental niches, as both consume nuts. [1]
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- species
At the most basic level is the species, a group of organisms that can interbreed to produce fertile offspring.
- Biotic factors
Every ecosystem is defined by the interplay between its living and non-living parts. Biotic factors are all the living or once-living components, including plants, animals, fungi, bacteria, and their interactions such as predation, disease, and competition. Abiotic factors are the non-living physical and chemical elements of the environment, such as temperature, sunlight, pH, salinity, and soil composition.
- Competition
These interactions can be categorised based on their effect on the participants. Competition (-/-) occurs when two or more organisms require the same limited resource. Predation (+/-) is when a predator feeds on its prey. Symbiosis describes close, long-term interactions, and includes parasitism (+/-), where one organism benefits at the other's expense, and mutualism (+/+), where both benefit.
- niche
These interactions help define an organism's niche - its functional role in the ecosystem. This is distinct from its habitat, which is simply the physical place it lives.
- habitat
This is distinct from its habitat, which is simply the physical place it lives.
Name it
Read the meaning, then pick which of this lesson’s terms it describes. Miss one and you see what your choice really means.
A group of organisms that share common characteristics and can interbreed to produce fertile offspring. For example, all humans are one(Homo sapiens).
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
If you can explain it simply, you own it — gaps here are marks you’d lose.
Teach it back
Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Species: A single type of organism (e.g., the African lion, Panthera leo).
Population: A group of African lions in the Serengeti National Park.
Community: The lions, zebras, acacia trees, grasses, and vultures in the Serengeti.
Ecosystem: The entire Serengeti community plus the sunlight, water, soil, and climate.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Test your knowledge of ecosystems and ecology
Test your knowledge of ecosystems and ecology
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Test your knowledge of ecosystems and ecology on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
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