18.2.117-18 · Selection, Evolution, Classification and Conservation

Ecosystems and Niches — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 24 of 36 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

18.2.1 Ecosystems and Niches — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define ecosystem, habitat and niche and explain how niches relate to species interactions and biodiversity.Species interact with other species to form communities, and communities interact …

Biotic components
Fundamental niche
Biotic factor
Competitive exclusion
Carrying capacity
Correlation

Part 1 of 7Ecosystems

Ecosystems

Species interact with other species to form communities, and communities interact with their environment to form ecosystems.

Ecosystem

Ecosystem a relatively self-contained community of interacting organisms and the environment they live in.
It has living (biotic) and non-living (abiotic) components.
Energy flows through the ecosystem and nutrients are recycled within it.

Ecosystems vary in size, from a garden pond to the open ocean, and even a human is an ecosystem for thousands of bacterial species.

Ecosystems vary in complexity: a desert is simple and a tropical rainforest very complex.

No ecosystem is wholly self-contained, e.g. birds feed in several ecosystems.

Part 2 of 7A forest ecosystem

A forest ecosystem

Biotic components

Biotic components trees, birds, small and large mammals, insects and fungi.

Abiotic components

Abiotic components soil, dead leaves, rain and stream water, rocks and other physical or chemical factors.

The non-living components influence the community of organisms.

Abiotic factors include light intensity, temperature, pH, water availability, salinity and wind.

Biotic factors include predation, competition, disease and food supply.

Part 3 of 7Habitat and niche

Habitat

Habitat the place where a species lives within an ecosystem.

Niche

Niche the role a species plays in the ecosystem: where it is, how it gets energy and how it interacts with other species and its environment.

The niche describes how an organism fits into the ecosystem.

Dung beetle (Scarabaeus satyrus) niche:

Feeds on faeces, rolling dung into balls and storing them in underground burrows.
Burrows turn over and aerate the soil.
Buried dung releases nutrients that benefit plants.
Removing dung underground helps control fly populations.

Part 4 of 7Competition and niche partitioning

Competition and niche partitioning

No two species occupy exactly the same niche in the same place for long (competitive exclusion).

The better adapted species outcompetes the other, which declines or evolves into a different niche.
Red squirrels declined in Britain after grey squirrels were introduced.

Similar species can coexist if their niches differ in food, feeding time, depth or breeding season (niche partitioning).

Fundamental niche

Fundamental niche the conditions a species could use without competitors;
- realised niche | the narrower range actually used.

Intraspecific competition is usually stronger than interspecific because members of a species share the same niche.

Part 5 of 7Interactions between species

Interaction	Effect	Example
Predation	Predator benefits, prey harmed	Lynx and snowshoe hare
Interspecific competition	Both harmed	Two plants competing for light
Mutualism	Both benefit	Nitrogen-fixing bacteria in legume nodules; mycorrhizae
Parasitism	Parasite benefits, host harmed	Tapeworm; Plasmodium
Commensalism	One benefits, other unaffected	Epiphytic orchid on a tree

Part 6 of 7Niches, biodiversity and conservation

The number of species an ecosystem supports depends on how many niches it provides.

A layered rainforest has many niches and high species richness.

Removing a keystone species, e.g. sea otters controlling sea urchins, can collapse many niches.

An invasive species occupying a native species' niche can cause the native to decline.

Specialists such as the koala are more vulnerable to change than generalists such as the fox.

Populations are limited by abiotic and biotic factors and level off at the carrying capacity.

Density-dependent factors (competition, predation, disease) stabilise numbers;
- density-independent factors (fire, frost, flood) act regardless of density.

Part 7 of 7More from the specification

Key terms

Term	Meaning
Ecosystem	Self-contained community of organisms interacting with their environment.
Habitat	The place where a species lives within an ecosystem.
Niche	Role of a species in the ecosystem.
Biotic factor	A living component affecting organisms.
Abiotic factor	A non-living component affecting organisms.
Competitive exclusion	Better adapted species outcompetes another using the same niche.
Carrying capacity	Maximum population an environment can support indefinitely.

Avoid these mistakes

Using habitat and niche as the same — habitat is where an organism lives, niche is its role.

Describing an ecosystem as only the living organisms — it also includes the abiotic environment.

Saying nutrients flow through an ecosystem — energy flows and nutrients are recycled.

Forgetting that species interact with the abiotic environment as part of their niche.

Last updated: 18 October 2026