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
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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
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
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Quick check
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Revision flashcards
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Species
A group of organisms that share common characteristics and can interbreed to produce fertile offspring. For example, all humans are one species (Homo sapiens).
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
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Species: A single type of organism (e.g., the African lion, Panthera leo).
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Population: A group of African lions in the Serengeti National Park.
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Community: The lions, zebras, acacia trees, grasses, and vultures in the Serengeti.
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Ecosystem: The entire Serengeti community plus the sunlight, water, soil, and climate.
Practice — then mark it
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Test your knowledge of ecosystems and ecology
Test your knowledge of ecosystems and ecology
Extra simulations & links
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Frequently asked
Checkpoint
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