Ecosystems and Communities

This lesson covers the key concepts of ecosystems and communities as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the organisation of life within ecosystems, the difference between biotic and abiotic factors, and how organisms within communities are interdependent.

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What Is an Ecosystem?

An ecosystem is a community of living organisms interacting with each other and with the non-living (physical) components of their environment. Ecosystems can be large (e.g. tropical rainforest, ocean) or small (e.g. a pond, a rotting log).

Every ecosystem contains two types of component:

Component	Meaning	Examples
Biotic	Living factors	Plants, animals, fungi, bacteria, competition, predation
Abiotic	Non-living factors	Temperature, light, water availability, soil pH, wind

Exam Tip: The examiner may describe a scenario and ask you to identify biotic and abiotic factors. Always check — if something is alive or produced by a living organism, it is biotic.

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Key Ecological Terms

You must be confident with the following definitions:

Term	Definition
Habitat	The place where an organism lives, e.g. a pond, a woodland, a rock pool
Population	All the organisms of one species living in a habitat at a given time
Community	All the populations of different species living and interacting in an area
Ecosystem	A community of organisms plus the abiotic (non-living) conditions in which they live
Niche	The role an organism plays within its ecosystem, including what it eats and how it interacts with other species

Hierarchy of Organisation

graph LR
    A["Individual organism"] --> B["Population (one species)"]
    B --> C["Community (all species)"]
    C --> D["Ecosystem (community + abiotic environment)"]

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Interdependence

Organisms within a community are interdependent — they depend on each other for survival. Changes in one species can affect others.

Examples of interdependence:

• Bees pollinate flowers; without bees, many plants cannot reproduce, which in turn affects animals that eat those plants.
• Decomposers break down dead material and recycle nutrients back into the soil, which plants need to grow.
• Predator-prey relationships — if the population of a prey species falls, predators may also decline because of reduced food supply.

Exam Tip: If asked to explain the effect of removing one species from a food web, trace the knock-on effects both up (predators lose food) and down (prey populations may increase).

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Stable Communities

A stable community is one where all the species and environmental factors are in balance so that population sizes remain relatively constant over time. Examples include:

• Tropical rainforests — high biodiversity, stable temperature and rainfall year-round
• Ancient oak woodlands — established over centuries with balanced predator-prey relationships

In a stable community:

• Resources are sufficient for all species
• Predation, competition and disease keep populations in check
• Physical conditions remain relatively constant

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Competition

Organisms in a community compete for limited resources. Competition can be:

Intraspecific Competition

Competition between organisms of the same species, e.g. two oak trees competing for light.

Interspecific Competition

Competition between organisms of different species, e.g. red squirrels and grey squirrels competing for food.

Organisms compete for...	Plants	Animals
Light	Yes	—
Water	Yes	Yes
Minerals / nutrients	Yes	—
Space / territory	Yes	Yes
Food	—	Yes
Mates	—	Yes

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Producers, Consumers and Trophic Levels

Every ecosystem relies on energy flow through trophic levels:

Trophic Level	Name	Description
1	Producer	Organisms that make their own food by photosynthesis (e.g. plants, algae)
2	Primary consumer	Herbivores that eat producers (e.g. rabbit, caterpillar)
3	Secondary consumer	Carnivores or omnivores that eat primary consumers (e.g. frog, robin)
4	Tertiary consumer	Top predators that eat secondary consumers (e.g. hawk, fox)

Decomposers (bacteria and fungi) break down dead organisms at every trophic level and recycle nutrients.

graph TD
    A["Sun (energy source)"] --> B["Producers (plants, algae)"]
    B --> C["Primary consumers (herbivores)"]
    C --> D["Secondary consumers (carnivores)"]
    D --> E["Tertiary consumers (top predators)"]
    B --> F["Decomposers"]
    C --> F
    D --> F
    E --> F
    F --> G["Nutrients returned to soil"]
    G --> B

Exam Tip: Always remember that the Sun is the ultimate source of energy for almost all ecosystems. Producers transfer light energy into chemical energy via photosynthesis.

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Summary

• An ecosystem includes all the living organisms (community) and non-living factors in an area.
• A population is one species in an area; a community is all species together.
• Organisms are interdependent — changes to one species affect others.
• Abiotic factors are non-living; biotic factors are living.
• Stable communities have balanced populations and constant environmental conditions.
• Organisms compete for resources — intraspecific (same species) and interspecific (different species).
• Energy flows from producers through consumers to decomposers.