Biotic factors — AQA GCSE Combined Science
Test yourself on Biotic factors with AQA GCSE practice questions.
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Biotic factors explained
A community is all the interacting populations in a habitat.
Read the full explanation
Biotic factors are living influences: food availability, new predators, new pathogens, and competition between species. To explain a change, identify the factor, name the affected population, then trace the effect through feeding relationships and numbers. For example, if a new predator arrives, prey numbers fall; predators may then compete, and plants the prey ate may grow more. If a pathogen spreads, susceptible species decline while resistant ones may increase. If a competitor is removed, the remaining species may gain resources and increase. Always link cause to effect using data such as population counts, then state the resulting change in the community.
Biotic (living) factors which can affect a community are:
Biotic factors are living influences on a community. The main ones are food availability, the arrival of a new predator, the arrival of a new pathogen, and one species outcompeting another. Food availability affects how many organisms can survive and reproduce. A new predator reduces prey numbers and may also affect plants or animals the prey fed on. A new pathogen can spread through a population, reducing the population size of susceptible species. Competition occurs when organisms need the same food, water or space; the better-adapted species may survive and reproduce more, outcompeting the other so its numbers are no longer sufficient to breed. These factors interact, so a change in one can ripple through the whole community.
availability of food
Food availability is a biotic factor: the amount of suitable food in a habitat that a population can actually obtain. It sets a limit on how many individuals an area can support, because every organism needs energy and matter from food for respiration, growth, repair and reproduction. If food is plentiful, more offspring survive and the population can rise; if food becomes scarce, competition within and between species intensifies, so some individuals starve, emigrate or fail to breed, and numbers fall. Availability changes with season, weather, disease and human activity. For example, oak trees produce mast years with many acorns, and grey squirrel numbers often rise afterwards. Ecologists sample populations and food supply to link the two.
new predators arriving
A new predator arriving is a biotic factor because it is a living organism that eats other organisms. When a predator enters an ecosystem where prey have not evolved defences against it, it can quickly reduce prey numbers. The predator may be introduced accidentally, for example in ballast water, or deliberately for pest control. Its impact depends on how many predators arrive, how many prey they eat, whether the prey can hide, flee or reproduce fast enough, and whether other food is available. Prey numbers may fall, then predator numbers may fall as food becomes scarce, giving a cyclical relationship. The change can also affect other species, for example plants that the prey used to eat may grow more.
new pathogens
A pathogen is a microorganism that causes disease, such as a bacterium, virus, fungus or protist. When a new pathogen arrives in a population, it is a biotic factor because it is a living (or virus) influence from another organism. If individuals have no inherited or acquired immunity, the pathogen can spread rapidly, cause illness and reduce population numbers. For example, if a new virus enters a rabbit population, many rabbits may die, so fewer compete for food and the population size falls. The effect depends on how easily the pathogen is transmitted, how harmful it is and how many individuals are resistant. New pathogens therefore change population sizes and can also change the distribution of species in a habitat.
one species outcompeting another so the numbers are no longer sufficient to breed.
Competition is a biotic factor because it involves living organisms using the same limited resources. When one species outcompetes another, it uses resources such as food, water, light or space more successfully. The weaker competitor may survive in smaller numbers. If its population becomes too small, individuals may not meet often enough to find a mate within their own species, so breeding stops or becomes very rare. For example, if grey squirrels outcompete red squirrels for food and habitat, red squirrel numbers can fall until the remaining individuals are too scattered to breed successfully. The population may then decline further and could become locally extinct.
Students should be able to extract and interpret information from charts, graphs and tables relating to the effect of biotic factors on organisms within a community.
Biotic factors are living influences on organisms in a community, such as food availability, predators, pathogens and competition. To extract information, read the title, axis labels, units and key first, then locate the relevant data point or trend. To interpret, link the pattern to a biological effect: for example, a graph showing increasing predator numbers followed by falling prey numbers suggests predation reduces prey population. In a table, compare values across rows or columns to identify which factor changes most. Always quote figures with units and state the direction of change. Explain the effect using cause and effect, not just description, and consider that other biotic or abiotic factors may also act.
Your focus
- Identify the biotic factor that has changed in a given context or data set.
- Describe the effect of that change on a named population.
- Explain how the effect transfers to other populations and changes the community.
Show all 21 objectives
- Recall the main biotic factors that affect a community.
- Distinguish biotic factors from abiotic factors.
- Describe how each named biotic factor can change a population.
- Define food availability as a biotic factor and explain why it is biotic.
- Explain how changes in food availability affect survival, reproduction and population size.
- Apply the concept to a named habitat and organism, using competition and energy transfer ideas.
- Identify a new predator as a biotic factor and explain why it is biotic.
- Explain how a new predator can change prey numbers and then its own numbers.
- Apply predator–prey reasoning to a named example and describe an indirect effect on another species.
- Define a pathogen and classify it as a biotic factor.
- Explain how a new pathogen can reduce population size when immunity is low.
- Apply the concept of a new pathogen to a named population and predict the effect on numbers.
- Describe how one species can outcompete another for limited resources.
- Explain why a very small population may no longer be sufficient to breed.
- Predict the effect of competitive exclusion on population numbers and community structure.
- Read values accurately from charts, graphs and tables about biotic factors.
- Describe trends in data and link them to named biotic factors.
- Explain how a biotic factor affects organisms in a community using data as evidence.
Biotic factors exam tips
Marking Points
- Identifies the specific biotic factor that has changed, such as a new predator, pathogen, competitor or change in food availability.
- Names the population directly affected and states whether its numbers rise or fall.
- Explains the mechanism, for example predation reduces prey numbers or competition reduces shared resources.
- Traces the effect to at least one other population through a feeding relationship or shared resource.
- Uses data or context provided, such as population sizes or dates, to support the explanation.
- Concludes with the overall change to the community, such as a fall in biodiversity or a shift in species present.
- States that biotic factors are living factors affecting a community.
- Names food availability as a biotic factor and links it to survival and reproduction.
- Names a new predator and explains that it reduces prey numbers.
- Names a new pathogen and explains that it causes disease and reduces population size.
- Names competition between species and identifies a shared resource such as food, water or space.
- Explains that one species can outcompete another so the numbers are no longer sufficient to breed, altering community composition.
- States that food is a biotic factor because it comes from other living organisms, including plants, animals and decaying material.
- Explains that food supplies energy and matter for respiration, growth, repair and reproduction, so it limits population size.
- Describes how increased food availability can raise survival of offspring and allow population growth.
- Describes how reduced food availability causes starvation, emigration, reduced breeding or death, lowering population size.
- Uses the idea of competition: individuals compete for limited food, and only some obtain enough to survive and reproduce.
- Applies the factor to a named example, such as acorns affecting squirrel numbers or aphids affecting ladybird numbers.
- Recognises that food availability varies with season, weather, disease and human activities such as farming or deforestation.
- States that a new predator is a biotic factor because it is a living organism that feeds on other organisms.
- Explains that predation transfers energy from prey to predator and reduces the number of prey.
- Describes how prey without suitable defences may decline rapidly when a new predator arrives.
- Explains that predator numbers may later fall if prey become scarce, linking the two populations.
- Recognises that the effect depends on factors such as predator numbers, prey reproduction rate and availability of refuges.
- Describes indirect effects on other species, such as increased plant growth when herbivorous prey decline.
- Uses a named example, such as introduced rats eating native bird eggs or cane toads affecting native prey.
- A pathogen is a disease-causing microorganism, including bacteria, viruses, fungi and protists.
- A new pathogen is a biotic factor because it comes from another living organism or virus and affects the population.
- If the population has little or no immunity, the pathogen can spread quickly and cause many individuals to become ill or die.
- A fall in the number of surviving individuals reduces population size and may change the age structure of the population.
- The size of the effect depends on transmission rate, virulence and the proportion of resistant individuals.
- New pathogens can also affect other species in the community, for example predators or competitors, so the whole ecosystem may change.
- Competition occurs when organisms need the same limited resource, such as food, water, light or space.
- One species outcompetes another when it obtains the resource more successfully, so the weaker competitor gains less.
- A reduced population may have too few individuals to find mates within their own species, so breeding becomes unsuccessful or stops.
- If breeding stops, the population cannot replace individuals that die, so numbers continue to fall.
- This can lead to local extinction of the weaker competitor and a change in the community.
- Competition is a biotic factor because it depends on the presence and activity of other living organisms.
- Identify the independent variable, dependent variable and any control or comparison group from the chart, graph or table.
- Read values accurately from axes or table cells, including units and any stated uncertainty or sample size.
- Describe the trend or pattern, such as increase, decrease, plateau or cyclical change, using comparative language.
- Interpret the pattern biologically, linking a named biotic factor to a named effect on organisms in the community.
- Use data as evidence, quoting specific figures with units to support the interpretation.
- Recognise limitations, such as correlation not proving causation or other factors affecting the community.
Examiner Tips
- 💡Underline the factor named in the question and the population it acts on before writing.
- 💡Use connectives such as therefore and as a result to show a clear chain of reasoning.
- 💡Refer to figures from the data, for example a fall from 200 to 120, rather than saying numbers decreased.
- 💡Learn the four named biotic factors and give a one-line effect for each.
- 💡When asked to name factors, give the factor and a brief consequence to show understanding.
- 💡Use examples from the question context rather than generic ones where possible.
- 💡Name the factor as biotic and justify it by linking food to living organisms.
- 💡Use a chain of reasoning: food changes → competition or survival changes → population changes.
- 💡Give a named organism and a specific food source to make the answer concrete.
- 💡Use comparative words such as more, fewer, increase and decrease to show the direction of change.
- 💡Identify the predator as a biotic factor and name the prey species.
- 💡Describe the short-term effect on prey numbers, then the longer-term effect on predator numbers.
- 💡Use food web language such as producer, consumer, prey and predator accurately.
- 💡Mention one indirect effect to show understanding of interdependence.
- 💡Link the pathogen to a named population and state the effect on numbers, for example fewer rabbits survive to breed.
- 💡Use the term immunity when explaining why a new pathogen can spread quickly through a population.
- 💡If asked to suggest a biotic factor, name the pathogen and explain its effect rather than just writing disease.
- 💡Name the resource being competed for and state which species obtains it more successfully.
- 💡Explain the link between low numbers and failure to breed, rather than just writing that the species dies out.
- 💡Use the phrase sufficient to breed when describing the point at which the population can no longer replace itself.
- 💡Underline the command word, such as describe or explain, and match your answer to it.
- 💡Quote at least one pair of figures with units from the chart, graph or table as evidence.
- 💡Use comparative connectives such as whereas, therefore and as a result to make the link between data and biology clear.
Common Mistakes
- Treating a biotic factor as abiotic, for example calling temperature a biotic factor. Correction: biotic factors are living influences such as predators, pathogens and competitors.
- Stating only that numbers change without explaining why. Correction: give the mechanism, such as predation or competition for food.
- Assuming every population in the community changes in the same direction. Correction: trace each effect separately, as some populations may rise while others fall.
- Listing abiotic factors such as light, temperature or water when asked for biotic factors. Correction: biotic factors are living, such as predators, pathogens and competitors.
- Confusing a pathogen with a predator. Correction: a pathogen causes disease inside an organism, while a predator kills and eats prey.
- Saying competition always kills the weaker species immediately. Correction: competition reduces resources, so the less well-adapted species may decline until numbers are no longer sufficient to breed.
- Treating food as an abiotic factor: correct this by stressing that food is produced by living organisms, so it is biotic.
- Assuming more food always means unlimited population growth: correct this by noting that predators, disease, space and other factors also limit populations.
- Confusing food availability with territory or shelter: correct this by defining food as material eaten for energy and matter, while shelter is a different resource.
- Calling a predator abiotic: correct this by stating that a predator is a living organism, so it is biotic.
- Assuming the predator always wipes out the prey: correct this by explaining that prey may survive in refuges, reproduce, or the predator may switch prey.
- Ignoring indirect effects: correct this by considering how reduced prey numbers affect plants or other animals in the food web.
- Calling a pathogen an abiotic factor: correct this by stating that pathogens are living organisms or viruses and are therefore biotic.
- Assuming all pathogens kill their host: correct this by explaining that some cause illness but the host survives, so the effect on population size varies.
- Confusing a pathogen with a non-infectious cause of disease such as a vitamin deficiency: correct this by defining a pathogen as a disease-causing microorganism.
- Saying the weaker species dies immediately: correct this by explaining that numbers fall gradually and breeding may fail before all individuals die.
- Treating competition as an abiotic factor: correct this by stating that competition involves living organisms and is therefore biotic.
- Assuming the two species compete only for food: correct this by mentioning other resources such as water, light, shelter or territory.
- Describing the graph shape without interpreting it biologically; correct by linking each trend to a named biotic factor and its effect on organisms.
- Reading the wrong axis or ignoring units; correct by checking axis labels, scales and units before quoting any value.
- Treating a correlation as proof of cause; correct by stating that the data suggest a relationship and that other factors may also be involved.