Communities — AQA GCSE Biology
Test yourself on Communities with AQA GCSE practice questions.
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Communities explained
Ecology is studied at nested levels. A single organism is one individual; a population is all the individuals of one species in a habitat; a community is all the populations of different species living together; an ecosystem adds the abiotic (non-living) surroundings, such as light, water, temperature and soil. Interdependence means organisms depend on each other: plants supply food and oxygen, animals pollinate flowers and disperse seeds, decomposers return mineral ions to the soil. Competition occurs when resources are limited. Organisms compete for light, space, water and mineral ions (plants) or food, mates and territory (animals). Successful competitors survive and reproduce, so their characteristics become more common. For example, in a woodland, oak trees, insects, birds and fungi form a community; the whole ecosystem includes rainfall, soil minerals and sunlight.
Students should be able to, when provided with appropriate information: • suggest the factors for which organisms are competing in a given habitat • suggest how organisms are adapted to the conditions in which they live.
When data or a description of a habitat is supplied, you must reason from the evidence rather than recall a list. To suggest competition factors, identify limited resources: plants compete for light, space, water and mineral ions; animals compete for food, water, mates, shelter and territory. For example, in a dense wood, tall trees shade smaller plants, so light is a key factor. To suggest adaptations, link a feature to its advantage in that habitat. A camel has a humped fat store and concentrated urine for dry conditions; a polar bear has thick fur, a white coat and a layer of fat for cold and camouflage. Structural adaptations are body features, behavioural adaptations are actions, and functional adaptations are internal processes. Always name the feature, then explain how it helps the organism survive and reproduce there.
An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.
An ecosystem is not just a place or a list of species; it is the interaction between the living community and the non-living surroundings. The biotic part is all the organisms: plants, animals, fungi and bacteria, including their relationships such as feeding, pollination and decomposition. The abiotic part is everything non-living: light intensity, temperature, moisture, soil pH and mineral content, wind and oxygen availability. These parts interact constantly. For example, in a pond, water temperature and oxygen concentration (abiotic) affect which fish and plants (biotic) can survive, while the plants add oxygen and the fish release carbon dioxide. Changes to one part ripple through the whole system: a drought lowers water availability, reducing plant growth, which reduces food for herbivores. Always name both components and state how they affect each other.
To survive and reproduce, organisms require a supply of materials from their surroundings and from the other living organisms there.
Every organism needs matter and energy to stay alive and produce offspring. Materials come from two sources: the non-living surroundings and other living organisms. Plants take carbon dioxide from the air and water and mineral ions from the soil, using light energy to make glucose by photosynthesis. Animals cannot photosynthesise, so they obtain energy and building materials by eating other organisms; they also take in oxygen for respiration and water for transport. These materials are used for growth, repair, movement and reproduction. For example, a caterpillar eats leaves for energy and materials, then uses them to grow and eventually reproduce as a butterfly. If any required material is in short supply, growth and reproduction are limited, which is why competition matters.
Plants in a community or habitat often compete with each other for light and space, and for water and mineral ions from the soil.
Green plants are producers, so their growth sets the energy budget of a habitat. Because light, space, water and mineral ions are finite, individual plants that capture more of a resource leave more offspring. Light and space are contested above ground: a taller shoot or a wider leaf canopy shades neighbours, while a spreading root or stem system claims ground. Below ground, root networks draw water and dissolved mineral ions such as nitrate and magnesium from the same soil volume. A gardener thinning carrot seedlings sees the result: crowded seedlings stay pale and thin, whereas spaced plants grow larger because each has more light, water and ions. Competition is therefore a density-dependent limit on growth, and it shapes which species and individuals persist.
Animals often compete with each other for food, mates and territory.
Animals cannot photosynthesise, so they must obtain energy by eating other organisms. Where food is limited, individuals that find and secure more of it survive longer and breed more. Competition also occurs for mates, because an individual that attracts or defends a partner passes on its alleles, and for territory, because a defended area supplies food, shelter and a safe place to raise young. Competition may be between members of the same species, which is intraspecific, or between different species, which is interspecific. A male robin singing to warn rivals away from its patch is defending territory; two lions disputing a carcass are competing for food. The outcome is not random: better competitors are more likely to survive and reproduce, so competition drives natural selection.
Within a community each species depends on other species for food, shelter, pollination, seed dispersal etc.
A community is all the populations of different species living together in a habitat, and its members are linked by many dependencies. Food webs show feeding links: a consumer depends on its prey, and a predator depends on prey numbers. Shelter may be provided by another species, such as a tree canopy or a badger sett. Flowering plants often depend on insects, birds or mammals for pollination, and many plants rely on animals or wind for seed dispersal. Some species depend on others for nutrients, as decomposers do when they break down dead material. These links mean a community is a network rather than a list. Removing one species can therefore change food supply, shelter, pollination or dispersal for several others, so dependencies are best studied as connections, not isolated pairs.
If one species is removed it can affect the whole community.
Because species in a community are linked, removing one can change many others. If a predator disappears, its prey may increase, then overgraze the plants below it, so plant numbers fall and other herbivores lose food. If a plant disappears, animals that fed on it, sheltered in it or depended on it for pollination or seed dispersal are affected. Effects can cascade through a food web, and a keystone species has an unusually large influence relative to its abundance. A practical example is removing a top predator from a lake, after which smaller fish multiply and the invertebrates they eat decline. The size of the effect depends on how many links the species had and whether another species can take its place.
This is called interdependence.
Interdependence means that organisms in a community depend on each other for survival. Within a community, each species relies on others for resources such as food, shelter, pollination and seed dispersal. A change in one population can therefore affect many others. For example, if a plant species declines, herbivores that feed on it may decline, and predators that eat those herbivores may also be affected. Interdependence also includes relationships such as mutualism, where both species benefit, and parasitism, where one benefits at the expense of the other. Understanding interdependence helps explain why removing or adding a species can destabilise a community and alter population sizes.
A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant.
A stable community has all species and environmental factors in balance, so population sizes stay fairly constant over time. Stability does not mean nothing changes; it means changes are small or self-correcting. For example, if a predator's population rises, prey numbers may fall, which reduces food for the predator and its numbers fall again. Environmental factors such as temperature, water and light also remain suitable. A stable community therefore resists small disturbances and maintains biodiversity. If a major factor changes, such as a new predator or a drought, the balance can be lost and populations may change significantly.
Students should be able to extract and interpret information from charts, graphs and tables relating to the interaction of organisms within a community.
This skill requires reading data from charts, graphs and tables about organisms in a community and using it to describe or explain interactions. You may be given a line graph showing predator and prey populations over time, a bar chart of species numbers, or a table of environmental measurements. To extract information, read the axes, units, labels and key carefully, then identify values or trends. To interpret, link patterns to biological ideas such as predation, competition or interdependence. For example, a graph may show prey numbers falling after predator numbers rise, which you can explain as increased predation. Always quote data to support your answer.
Your focus
- Sequence the levels of organisation from individual organism to population, community and ecosystem.
- Define population, community and ecosystem accurately using biotic and abiotic terms.
- Explain, with a named example, how interdependence and competition affect which organisms survive and reproduce.
Show all 33 objectives
- Identify, from supplied information, the resources for which organisms compete in a habitat.
- Describe adaptations of named organisms and classify them as structural, behavioural or functional.
- Explain how a named adaptation increases an organism's chance of survival and reproduction in its habitat.
- Define an ecosystem as the interaction of a community with the abiotic environment.
- Distinguish biotic from abiotic components and give correct examples of each.
- Explain how a change in one component of an ecosystem can affect other organisms within it.
- State that organisms require materials from their surroundings and from other living organisms.
- Give named examples of materials obtained from abiotic surroundings and from other organisms.
- Explain how a shortage of a required material can limit survival, growth or reproduction.
- Identify light, space, water and mineral ions as resources for which plants compete.
- Explain how a shortage of each named resource reduces plant growth.
- Apply the idea of competition to interpret data from crowded and spaced plantings.
- Name food, mates and territory as resources for which animals compete.
- Distinguish intraspecific from interspecific competition using examples.
- Explain how successful competition affects survival and reproduction.
- Define a community as all the populations of different species in a habitat.
- Give examples of species depending on other species for food, shelter, pollination and seed dispersal.
- Interpret food webs to identify feeding dependencies between species.
- Describe how removing one species can change populations of other species.
- Trace the knock-on effects of a removal through a food web.
- Explain why the size of the effect depends on the links a species has.
- Define interdependence and identify examples within a community.
- Explain how a change in one population can affect another population.
- Apply the concept of interdependence to interpret a food web or community scenario.
- Describe the features of a stable community.
- Explain how balance between species and environmental factors keeps population sizes fairly constant.
- Evaluate whether data from a community shows stability or change.
- Extract relevant values and trends from charts, graphs and tables about communities.
- Interpret data to explain interactions such as predation or competition.
- Support conclusions with numerical evidence from the provided resource.
Communities exam tips
Marking Points
- Correctly sequence the levels: individual organism → population → community → ecosystem.
- Define a population as all the individuals of one species in a habitat, and a community as all the populations of different species living together.
- State that an ecosystem includes both the living community and the non-living (abiotic) parts of the environment.
- Explain interdependence with a named example, such as plants providing food and oxygen while animals pollinate flowers and disperse seeds.
- Explain that competition happens when resources are limited, and identify resources plants and animals compete for.
- Link successful competition to survival and reproduction, so that advantageous characteristics become more common in the population.
- Identify competition factors from the information given, such as light, space, water and mineral ions for plants, or food, mates, shelter and territory for animals.
- Explain why the named factor is limited in that habitat, using evidence from the data or description.
- Name a specific adaptation of the organism rather than describing it vaguely.
- Classify the adaptation as structural, behavioural or functional where the question requires it.
- Explain how the adaptation gives an advantage in the stated conditions, linking it to survival and reproduction.
- Use comparative language, such as more likely to survive, when explaining the benefit of an adaptation.
- Define the biotic component as the community of living organisms, including their interactions.
- Define the abiotic component as the non-living parts of the environment, such as light, temperature, water, soil pH and oxygen.
- State that an ecosystem involves interaction between the biotic and abiotic components, not merely their coexistence.
- Give a named example of an interaction, such as plants affecting oxygen concentration or temperature affecting which species survive.
- Explain that a change in one component can affect other components of the ecosystem.
- State that organisms need materials for survival, growth, repair and reproduction.
- Identify the surroundings as a source of materials, such as carbon dioxide, water, oxygen and mineral ions.
- Identify other living organisms as a source of materials, such as food for animals and hosts for parasites.
- Give a named example of an organism obtaining materials from both sources.
- Explain that a shortage of a required material limits growth or reproduction.
- Link the need for materials to the processes of photosynthesis, respiration and growth.
- Competition occurs when a resource is in limited supply and more than one plant requires it.
- Light and space are contested mainly by shoots and leaves, so shading and crowding reduce photosynthesis and growth.
- Water and mineral ions are absorbed by roots from soil, so overlapping root systems deplete the same supply.
- Mineral ions such as nitrate and magnesium are needed for healthy growth, so shortage causes poor growth and yellowing.
- Successful competitors survive and reproduce, so competition acts as a selection pressure within the community.
- Thinning or spacing crop plants reduces competition and increases yield, which shows the effect in practice.
- Animals compete because food, mates and territory are limited resources.
- Food supplies energy and nutrients, so competition for food affects survival and growth.
- Mates are needed for reproduction, so competition for mates affects how many offspring an individual leaves.
- Territory provides food, shelter and breeding space, so defending it improves an animal's chances of success.
- Competition can be intraspecific, within one species, or interspecific, between different species.
- Individuals that compete successfully are more likely to survive and reproduce, so competition is a selection pressure.
- A community consists of all the populations of different species in a habitat.
- Food webs show feeding dependencies, with energy transferred from producers to consumers.
- Shelter can be provided by another species, for example a tree canopy or a burrow.
- Pollination is a dependency in which animals transfer pollen between flowers as they feed.
- Seed dispersal can depend on animals that carry, bury or eat and drop seeds.
- Decomposers depend on dead organisms and waste, returning mineral ions to the soil.
- Species in a community are linked by feeding and other dependencies, so a change in one population can change others.
- Removing a predator can allow its prey population to increase.
- An increased prey population may overgraze producers, reducing plant numbers and affecting other herbivores.
- Removing a plant affects animals that used it for food, shelter, pollination or seed dispersal.
- Effects can spread through a food web as a cascade rather than staying with one pair of species.
- A keystone species has a disproportionately large effect on the community relative to its abundance.
- Defines interdependence as the dependence of organisms within a community on one another for survival.
- Identifies ways organisms depend on each other, such as food, shelter, pollination and seed dispersal.
- Explains that a change in one population can affect other populations in the community.
- Uses a named example, such as a plant and its herbivore or a predator and its prey, to show interdependence.
- Distinguishes interdependence from simple competition by emphasising mutual reliance within a community.
- States that a stable community has species and environmental factors in balance.
- Explains that population sizes remain fairly constant rather than changing dramatically.
- Recognises that stability involves small fluctuations that are corrected by negative feedback.
- Gives an example of a factor, such as food supply or predation, that helps maintain balance.
- Explains that a major environmental change can destabilise a community and alter population sizes.
- Reads axes, labels, units and keys accurately to extract values from charts, graphs or tables.
- Identifies trends or patterns, such as increases, decreases, peaks or cycles, in the data.
- Links patterns in the data to interactions between organisms, such as predation, competition or interdependence.
- Uses numerical evidence from the resource to support an explanation.
- Compares data for two or more species or factors to explain a relationship within the community.
Examiner Tips
- 💡When asked to describe levels of organisation, work upwards from individual organism to ecosystem and give a brief definition at each level.
- 💡Use the words biotic and abiotic correctly: biotic means living, abiotic means non-living.
- 💡For interdependence questions, name two organisms and state what each provides for the other rather than writing a general statement.
- 💡Read the habitat description carefully and quote or refer to the data when suggesting competition factors.
- 💡Use the structure: feature → how it works → advantage in this habitat.
- 💡If the question says suggest, use the information provided and apply your biological knowledge rather than repeating the stem.
- 💡In definitions, include both the living community and the non-living environment, and use the words biotic and abiotic.
- 💡When giving an example, state the direction of the effect, such as how temperature affects enzyme activity or growth.
- 💡Avoid writing that an ecosystem is just a habitat; add the idea of interaction.
- 💡Name the specific material and its source, for example carbon dioxide from the air or mineral ions from the soil.
- 💡Link the material to its use, such as glucose for respiration or amino acids for growth.
- 💡Use examples from both plants and animals to show the two sources of materials.
- 💡Name the resource and say why it is limiting, for example light is blocked by a taller neighbour.
- 💡Link each resource to the plant part that obtains it: leaves for light, roots for water and ions.
- 💡Use a before-and-after comparison, such as crowded seedlings against spaced seedlings, to show the effect of competition.
- 💡State whether the competition described is intraspecific or interspecific and justify your choice.
- 💡Link each resource to its benefit, for example territory gives shelter and feeding grounds.
- 💡When interpreting graphs, look for a fall in survival or offspring number as population density rises.
- 💡Use the phrase depends on and name the specific benefit, such as food, shelter or pollination.
- 💡When asked to complete a food web, check that arrows point in the direction of energy transfer.
- 💡Give named examples, such as bees pollinating flowers or birds dispersing seeds, to make dependencies concrete.
- 💡Trace the effect step by step through the food web, naming each population that changes.
- 💡State the direction of each change, such as prey numbers rise and plant numbers fall.
- 💡Use the phrase may or can, because the outcome depends on the rest of the community.
- 💡Use the word community when explaining interdependence, because it refers to all the populations living together in a habitat.
- 💡Give a specific example and state the direction of the effect, such as fewer plants leading to fewer herbivores.
- 💡Link interdependence to food webs or pollination when answering questions about changes in population size.
- 💡Use the phrase fairly constant when describing population sizes in a stable community.
- 💡Refer to both species and environmental factors to cover all parts of the definition.
- 💡When interpreting data, look for small fluctuations around a mean rather than a perfectly flat line.
- 💡Quote specific figures from the graph or table, including units, to support each point you make.
- 💡Use comparative language such as higher than, lower than or increases while to describe relationships.
- 💡Check whether the question asks you to extract, interpret or both, and make sure your answer does what is asked.
Common Mistakes
- Confusing a population with a community: a population is one species, whereas a community includes all species.
- Describing an ecosystem as only the living organisms; the correction is that an ecosystem also includes the abiotic parts, such as light, water, temperature and soil.
- Treating interdependence as one-way, for example saying only that animals depend on plants; the correction is that the relationship works in both directions, such as plants depending on animals for pollination and seed dispersal.
- Listing competition factors without linking them to the habitat; the correction is to state why that resource is scarce in the given conditions.
- Naming an adaptation without explaining its advantage; the correction is to add how the feature helps the organism survive or reproduce.
- Mixing up structural and behavioural adaptations; the correction is to check whether the feature is a body part or an action.
- Defining an ecosystem as only the organisms present; the correction is that it also includes the non-living environment and the interactions between them.
- Treating abiotic factors as unimportant background; the correction is that factors such as temperature and light directly affect which organisms survive.
- Giving a habitat as the definition of an ecosystem; the correction is that a habitat is where an organism lives, whereas an ecosystem includes the community and its interactions with the abiotic environment.
- Saying organisms need food only for energy; the correction is that food also supplies materials for growth and repair.
- Forgetting that plants obtain materials from both the air and the soil; the correction is to name carbon dioxide from the air and water and mineral ions from the soil.
- Assuming all organisms obtain materials in the same way; the correction is that plants photosynthesise, whereas animals and fungi obtain materials from other organisms.
- Treating competition as fighting or deliberate harm; correct this by describing it as two or more organisms needing a resource that is in short supply.
- Assuming plants compete only above ground; correct this by naming water and mineral ions taken in by roots as below-ground resources.
- Confusing a mineral ion shortage with a disease; correct this by linking poor growth or yellowing to a lack of ions such as nitrate or magnesium.
- Listing only food and ignoring mates and territory; correct this by naming all three resources in the statement.
- Describing competition as always involving physical fighting; correct this by including non-contact competition such as calling, scent marking or displaying.
- Confusing a predator-prey relationship with competition; correct this by noting that competitors need the same resource, whereas a predator consumes the prey itself.
- Treating a community as a single species; correct this by defining it as all the populations of different species in a habitat.
- Assuming every dependency is a feeding link; correct this by including shelter, pollination and seed dispersal.
- Confusing a community with a habitat; correct this by describing the habitat as the place and the community as the organisms living there.
- Assuming only the direct predator and prey are affected; correct this by tracing effects along further links in the food web.
- Claiming the community always collapses; correct this by saying the effect depends on the number of links and whether another species can replace the lost one.
- Ignoring plant dependencies; correct this by considering food, shelter, pollination and seed dispersal provided by plants.
- Treating interdependence as only competition for resources; correct by explaining that organisms also rely on each other, for example for pollination or food.
- Assuming a change affects only one species; correct by tracing the effect through a food web to other populations.
- Confusing interdependence with a single organism's needs; correct by describing relationships between different species in a community.
- Thinking stable means no change at all; correct by explaining that populations fluctuate slightly but remain fairly constant.
- Assuming only living factors matter; correct by including environmental factors such as temperature, light and water.
- Believing a stable community cannot be affected by change; correct by stating that large disturbances can upset the balance.
- Ignoring the units or scale on an axis; correct by checking units and using the correct scale when reading values.
- Describing a trend without linking it to a biological interaction; correct by explaining what the pattern shows about organisms in the community.
- Quoting data without comparing it to another point or species; correct by making a direct comparison to support the interpretation.