Abiotic and biotic factors affecting communities — AQA GCSE Biology Revision Notes

What you'll learn

This topic explores how both living and non-living factors shape ecosystems and determine which organisms can survive in different habitats. You'll learn to identify and explain how various environmental conditions and species interactions affect the distribution and abundance of organisms in communities. Understanding these concepts is essential for answering questions about ecosystems, populations and adaptations in your AQA GCSE Biology exam.

Key terms and definitions

Community — all the populations of different species living in the same habitat at the same time

Population — all the organisms of one species living in a habitat

Ecosystem — the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment

Abiotic factors — non-living factors that affect organisms and communities, such as light intensity, temperature and moisture levels

Biotic factors — living factors that affect organisms and communities, such as availability of food, new predators and pathogens

Interdependence — the reliance of organisms on each other for resources such as food, shelter and pollination

Abundance — the number of individuals of a species in a given area

Distribution — where particular species are found within an ecosystem

Core concepts

Understanding communities and ecosystems

A community consists of all the different species living together in one area. For example, a pond community includes fish, frogs, water plants, algae, bacteria and invertebrates. Each species within the community exists as a population of individuals.

Communities exist within ecosystems, which include both:

The living components (biotic factors) — all the organisms in the community
The non-living components (abiotic factors) — soil, water, air, light and minerals

Organisms within communities are interdependent. This means they rely on each other for:

Food sources (herbivores depend on plants; carnivores depend on other animals)
Shelter and habitat (birds nesting in trees; small fish hiding in coral reefs)
Pollination and seed dispersal (insects pollinating flowering plants)
Nutrient cycling (decomposers breaking down dead material)

If one species is removed from a community, it affects other species directly or indirectly. For instance, if all the bees disappeared from a meadow ecosystem, flowering plants wouldn't be pollinated, seeds wouldn't form, herbivores would lose food sources, and predators would then struggle to find prey.

Abiotic factors affecting communities

Abiotic factors are non-living conditions that influence where organisms can survive and how well they thrive. The key abiotic factors you need to know are:

Light intensity

Plants require light for photosynthesis to produce glucose for growth
In shaded areas beneath forest canopies, only shade-tolerant plants can survive
Light intensity affects the distribution of plant species
Animals are indirectly affected because they depend on plants for food and habitat
Example: woodland floor plants like bluebells flower in spring before tree leaves block sunlight

Temperature

Affects the rate of enzyme-controlled reactions in organisms
Temperature influences metabolic rates and therefore growth and reproduction
Many species can only survive within specific temperature ranges
Cold-blooded animals (ectotherms) like reptiles are particularly sensitive to temperature
Example: tropical rainforest communities differ greatly from Arctic tundra communities due to temperature

Moisture levels

Water is essential for all living organisms
Plants need water for photosynthesis, support and transport
Animals need water for metabolic reactions and temperature regulation
Affects the distribution of species across habitats
Example: cacti and camels are adapted to survive in low moisture environments, whilst mosses and amphibians require high moisture levels

Soil pH and mineral content

Soil pH affects the availability of mineral ions for plant uptake
Different plant species are adapted to different pH levels
Acidic soils (pH <7) suit heathers and rhododendrons
Alkaline soils (pH >7) suit plants like clematis
Mineral content affects plant growth — nitrogen for proteins, magnesium for chlorophyll
Example: hydrangea flower colour changes with soil pH (blue in acidic, pink in alkaline)

Wind intensity and direction

Affects the rate of transpiration in plants
Strong winds increase water loss, causing water stress
Shapes plant growth patterns (windswept trees on exposed hillsides)
Influences seed and spore dispersal
Affects temperature by removing warm air layers around organisms

Carbon dioxide levels (for plants)

Required as a raw material for photosynthesis
Higher CO₂ concentrations can increase the rate of photosynthesis
Affects plant growth and productivity
Atmospheric CO₂ levels impact global plant distribution patterns

Oxygen levels (for aquatic animals)

Fish and other aquatic organisms require dissolved oxygen for aerobic respiration
Warm water holds less dissolved oxygen than cold water
Polluted water often has reduced oxygen levels
Affects the abundance and distribution of aquatic species
Example: stonefly larvae only survive in well-oxygenated streams

Biotic factors affecting communities

Biotic factors are living components that influence populations and communities. The main biotic factors are:

Availability of food

Limits population sizes of animals
More food available allows populations to grow
Food scarcity reduces population growth and survival rates
Affects competition between and within species
Example: red squirrel populations declined in the UK partly due to food competition with introduced grey squirrels

New predators arriving

Can dramatically reduce prey populations
May cause local extinction of prey species if they have no adaptations to escape
Changes the balance within existing communities
Example: introduction of cats to island ecosystems has caused bird extinctions

New pathogens

Disease-causing microorganisms (bacteria, viruses, fungi)
Can rapidly reduce population sizes
Species with no evolutionary resistance are particularly vulnerable
Spread can be affected by population density
Example: ash dieback disease (caused by fungal pathogen) killing ash trees across the UK

Competition

Organisms compete for limited resources
Interspecific competition — competition between different species for the same resources
Intraspecific competition — competition between members of the same species
Competition for food, water, territory, mates and light (plants)
The better-adapted organisms are more successful and out-compete others
Example: in Caribbean coral reefs, fast-growing coral species compete with slower species for space and light

Adaptations and stable communities

Stable communities are those where all the species and environmental factors are in balance, so population sizes remain relatively constant. Examples include:

Tropical rainforests
Ancient oak woodlands
Coral reefs (though increasingly threatened)
Mature river systems

In stable communities:

Birth rates and death rates are balanced
Resources are sufficient for existing populations
Predator and prey populations fluctuate but remain within sustainable ranges
Abiotic factors remain relatively constant

Organisms are adapted to both abiotic and biotic factors in their environment through:

Structural adaptations — physical features (thick fur for cold climates, large surface area ears for heat loss)
Behavioural adaptations — actions organisms take (migration, hibernation, nocturnal activity)
Functional adaptations — internal processes (producing venom, antifreeze proteins in Arctic fish)

How abiotic and biotic factors interact

Abiotic and biotic factors don't work in isolation — they interact to determine community structure:

Temperature (abiotic) affects decomposition rates, which influences nutrient availability for plants (biotic)
Light intensity (abiotic) affects plant growth, which determines food availability for herbivores (biotic)
Rainfall (abiotic) affects plant distribution, which determines habitat availability for animals (biotic)
Predator populations (biotic) are limited by prey availability, which depends on plant productivity determined by abiotic factors

Example interaction in a UK woodland:

Increased spring temperatures (abiotic) cause earlier leaf emergence
Earlier leaves provide more food for caterpillars (biotic)
Caterpillar populations increase
More food available for birds feeding chicks (biotic)
Bird populations may increase if timing matches their breeding season

Measuring the effects of factors on distribution

Scientists investigate how abiotic and biotic factors affect distribution using:

Quadrats — square frames used to sample plant populations

Random placement to avoid bias
Count organisms or estimate percentage cover within each quadrat
Calculate mean number of organisms per quadrat
Determine how common (abundant) a species is

Transects — lines across a habitat to measure how distribution changes

Belt transect — quadrats placed at regular intervals along a line
Shows how communities change along an environmental gradient
Example: from seashore to clifftop shows how salt tolerance affects plant distribution

Measuring abiotic factors

Light meters measure light intensity
Temperature probes measure soil and air temperature
pH meters or indicator solutions measure soil pH
Moisture meters measure soil water content

Worked examples

Example 1: A student investigated the effect of light intensity on dandelion distribution in a school field. They placed a 0.5 m² quadrat at 10 random positions in sunny areas and 10 random positions in shaded areas. In sunny areas, they found a mean of 12 dandelions per quadrat. In shaded areas, they found a mean of 3 dandelions per quadrat.

(a) State whether light intensity is an abiotic or biotic factor. [1 mark]

(b) Suggest why there are more dandelions in sunny areas. [2 marks]

Mark scheme answers:

(a) Abiotic (factor) [1 mark]

(b) Award 1 mark each for two points:

More light allows more/faster photosynthesis [1 mark]
So dandelions grow better/produce more seeds/compete more successfully in sunny areas [1 mark]

Alternative: Dandelions in shade cannot photosynthesise as much, so grow poorly/cannot compete

Example 2: Red and grey squirrels both live in UK woodlands. Grey squirrels were introduced from North America. Red squirrel populations have declined dramatically where grey squirrels are present.

(a) Name the biotic factor affecting red squirrel populations. [1 mark]

(b) Suggest two reasons why grey squirrels have caused red squirrel populations to decline. [2 marks]

Mark scheme answers:

(a) Competition (for food/resources) OR new predator OR disease/pathogen [1 mark]

(b) Award 1 mark each for two points:

Grey squirrels out-compete red squirrels for food/nuts/habitat [1 mark]
Grey squirrels carry squirrelpox virus which kills red squirrels / grey squirrels are larger and stronger [1 mark]

Cold winters/low temperature [1 mark]
Could reduce squirrel populations as food is scarce/more energy needed for warmth [1 mark]

Alternative: Lack of rain affects tree seed production, reducing food availability for squirrels

Example 3: A group of students investigated how distance from a path affects the number of daisy plants. They used a belt transect with quadrats every 2 metres from the path edge to 10 metres away. Near the path (0-2 m), they found many more daisies than further away (8-10 m).

Explain this pattern. [3 marks]

Mark scheme answer:

Award up to 3 marks:

Trampling near the path prevents tall plants from growing [1 mark]
This reduces competition for light / allows low-growing daisies to receive more light [1 mark]
Further from the path, taller plants out-compete daisies (for light) / daisies are shaded [1 mark]

Common mistakes and how to avoid them

Confusing abiotic and biotic factors — Remember: abiotic = non-living (temperature, light), biotic = living (predators, disease). Soil is abiotic even though it contains living organisms; the soil itself is non-living material.
Saying "light energy" instead of "light intensity" — Use the correct term. Light intensity (how bright it is) is the abiotic factor, though light does provide energy for photosynthesis.
Not linking factors to specific effects — Don't just state a factor; explain the mechanism. For example, "high temperature increases enzyme activity in metabolic reactions" is better than "high temperature helps organisms grow."
Forgetting indirect effects — Abiotic factors can affect animals indirectly through plants. For example, low rainfall reduces plant growth, which then reduces food for herbivores, which then affects predator populations.
Writing vague statements about "survival" — Be specific about why a factor affects survival. Link to photosynthesis, enzyme activity, reproduction rates, or competition rather than just saying organisms "can't survive."
Mixing up distribution and abundance — Distribution is where organisms are found (their location); abundance is how many there are (population size) in a given area.

Exam technique for "Abiotic and biotic factors affecting communities"

Command word awareness — "State" requires a simple answer (1 mark). "Explain" requires reasoning with because/so/therefore (usually 2+ marks). "Suggest" means apply your knowledge to unfamiliar contexts.
Use correct biological terminology — Write "photosynthesis" not "making food," "competition" not "fighting," and "interdependent" not "need each other." Precision earns marks.
Link factors to mechanisms — Always connect environmental factors to biological processes: photosynthesis rates, enzyme activity, reproduction success, or competition intensity. Show the examiner you understand how factors work, not just that they have an effect.
Refer to data when provided — Quote figures from tables or graphs to support your answer. Calculate means or identify patterns, then explain them using biological knowledge.

Quick revision summary

Communities contain populations of different species living together in ecosystems. Abiotic factors (light, temperature, moisture, pH, wind, CO₂, oxygen) are non-living conditions affecting distribution and abundance. Biotic factors (food availability, predators, pathogens, competition) are living influences on populations. Organisms are interdependent and adapted to their environmental conditions. Stable communities maintain balanced populations. Scientists use quadrats and transects to measure how factors affect organism distribution across habitats.