GCSE Biology revision

Communities

Adaptations, interdependence and competition

AQA 4.1.1.1

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Identify and describe What is a Community?, Biotic & Abiotic Factors and Competition & Interdependence in the context of communities.

MOST 🎯🎯

Explain the relationships between What is a Community?, Biotic & Abiotic Factors and Competition & Interdependence and how they contribute to communities.

SOME 🎯🎯🎯

Apply and analyse knowledge of What is a Community?, Biotic & Abiotic Factors and Competition & Interdependence to interpret unfamiliar information about communities.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is a Community?

A community is all the populations of different species living in the same habitat at the same time.
An ecosystem includes both the community of living organisms (biotic factors) and the non- living environment (abiotic factors) they interact with.
Organisms within a community depend on each other for food, shelter, and other resources, creating a network of interdependence.

1. What is a Community?AQA

Notes

A community is all the populations of different species living in the same habitat at the same time.
An ecosystem includes both the community of living organisms (biotic factors) and the non- living environment (abiotic factors) they interact with.
Organisms within a community depend on each other for food, shelter, and other resources, creating a network of interdependence.

Biotic & Abiotic Factors

Biotic factors are living influences on a community, such as predators, prey, pathogens, and competition for food.
Abiotic factors are non-living influences, including temperature, light intensity, water availability, soil pH, and carbon dioxide concentration.
Changes in either biotic or abiotic factors can have significant knock-on effects on the size and distribution of populations within a community.

2. Biotic & Abiotic FactorsAQA

Notes

Biotic factors are living influences on a community, such as predators, prey, pathogens, and competition for food.
Abiotic factors are non-living influences, including temperature, light intensity, water availability, soil pH, and carbon dioxide concentration.
Changes in either biotic or abiotic factors can have significant knock-on effects on the size and distribution of populations within a community.

Competition & Interdependence

Organisms compete for resources; plants compete for light, water, and mineral ions, whilst animals compete for food, water, mates, and territory.
Interspecific competition occurs between different species, whereas intraspecific competition occurs between individuals of the same species.
A stable community is one where all the species and environmental factors are in balance, so population sizes remain roughly constant.
Mutualism is a relationship where both organisms benefit, such as the relationship between bees and flowering plants.

3. Competition & InterdependenceAQA

Notes

Organisms compete for resources; plants compete for light, water, and mineral ions, whilst animals compete for food, water, mates, and territory.
Interspecific competition occurs between different species, whereas intraspecific competition occurs between individuals of the same species.
A stable community is one where all the species and environmental factors are in balance, so population sizes remain roughly constant.
Mutualism is a relationship where both organisms benefit, such as the relationship between bees and flowering plants.

Predator–Prey Relationships

In a predator–prey relationship, the population sizes of predators and prey are closely linked and cycle in a predictable pattern.
When prey numbers increase, predator numbers rise due to greater food availability, which then causes prey numbers to fall again.
These cyclical changes in population size can be shown on a predator–prey graph, where the predator curve always lags slightly behind the prey curve.

4. Predator–Prey RelationshipsAQA

Notes

In a predator–prey relationship, the population sizes of predators and prey are closely linked and cycle in a predictable pattern.
When prey numbers increase, predator numbers rise due to greater food availability, which then causes prey numbers to fall again.
These cyclical changes in population size can be shown on a predator–prey graph, where the predator curve always lags slightly behind the prey curve.

Sampling & Studying Communities

Quadrats are used to sample the abundance and distribution of plants or slow-moving animals within a habitat.
A transect is a line along which quadrats are placed at regular intervals to investigate how species distribution changes across a habitat.
It is important to take a large number of random samples to ensure results are representative and to reduce the effect of chance.
Mean population size can be estimated using the capture–mark–recapture method, calculated as: Population size = (number in first sample × number in second sample) / (number in second sample that are marked)

5. Sampling & Studying CommunitiesAQA

Notes

Quadrats are used to sample the abundance and distribution of plants or slow-moving animals within a habitat.
A transect is a line along which quadrats are placed at regular intervals to investigate how species distribution changes across a habitat.
It is important to take a large number of random samples to ensure results are representative and to reduce the effect of chance.
Mean population size can be estimated using the capture–mark–recapture method, calculated as: Population size = (number in first sample × number in second sample) / (number in second sample that are marked)