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Biotic factors are living factors such as food availability, predation, new pathogens and competition between species. Free notes and 12 questions answered.
Living Factors Affecting Communities is part of Adaptations, interdependence and competition in GCSE AQA Higher Biology.
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Biotic factors are living factors such as food availability, predation, new pathogens and competition between species. Free notes and 12 questions answered.
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Use this lesson to stay inside Adaptations, interdependence and competition while connecting the detail back to the wider Ecology topic.
Read the core explanations from Living Factors Affecting Communities before testing yourself from memory.
Picture a woodland where deer numbers suddenly crash, or a pond where fish populations explode. What's controlling these changes? It's not just weather or temperature - it's the living world itself.
Biotic factors are the living components of an ecosystem that directly influence which organisms survive, reproduce, and thrive. Unlike abiotic factors (temperature, rainfall), these involve interactions between living things.
Trap: Many students think biotic factors only include obvious predators. Actually, they include any living influence - from bacteria causing disease to plants competing for sunlight.
Food availability is often the primary limiting factor for population size. When food is abundant, populations grow. When it becomes scarce, populations crash.
This works through a simple mechanism: more food means better survival rates and more successful reproduction. Fewer offspring die from starvation, and adults have more energy to invest in breeding.
Worked example: How Food Shortage Affects Population Size
A rabbit population of 200 experiences a drought that reduces plant food by 60%. Predict the population changes.
Answer: Population would likely crash to 60-80 rabbits due to starvation and reduced breeding success
Real ecosystems show this pattern clearly. Lynx populations in Canada closely follow snowshoe hare numbers - when hares are abundant, lynx thrive and reproduce more successfully.
When a new predator arrives in an ecosystem, it can devastate prey populations that have no evolved defences. This is because the prey species hasn't had time to develop behavioural or physical adaptations.
The classic example is cats introduced to islands. Native bird populations, which evolved without mammalian predators, have no instinct to avoid ground-dwelling hunters. The result? Population crashes of 90% or more.
New predators affect communities through two mechanisms: direct predation (killing prey) and indirect effects (prey changing behaviour, spending more time hiding, less time feeding).
Pathogens (disease-causing organisms) can spread rapidly through populations that lack immunity. Unlike predators, which are limited by their own population size, pathogens can multiply exponentially within their hosts.
New pathogens are particularly devastating because the host population has no evolved resistance. Think of how European diseases decimated Native American populations, or how chestnut blight fungus eliminated American chestnut trees.
The impact depends on transmission rate and mortality rate. Highly contagious diseases with high death rates can eliminate entire populations within months.
When two species compete for the same resources, one usually wins. The losing species doesn't just struggle - it can be reduced to numbers so low that breeding becomes impossible.
The critical point is the breeding threshold. If a population drops below a certain size, individuals can't find mates, or there isn't enough genetic diversity for healthy offspring. This leads to local extinction even if some individuals survive.
Grey squirrels outcompeting red squirrels in Britain shows this perfectly. Grey squirrels are better at digesting acorns and reproduce faster. Red squirrel populations become so fragmented that breeding success plummets.
Biotic factor questions often ask you to predict population changes or explain why populations have changed. The key is linking the living factor to its specific effect on survival or reproduction.
Command word focus: 'Explain' questions want you to link cause and effect with 'because'. 'Describe' questions want you to state what happens without explaining why.
Remember: biotic factors work together. A population weakened by disease becomes more vulnerable to predation. Reduced food availability makes animals more susceptible to pathogens. Understanding these interactions is key to explaining complex population changes in exam questions.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Lynx numbers in Canada closely follow snowshoe hare numbers because when hares are abundant, lynx have more food, survive better and reproduce more successfully. Food availability is often the main limiting factor for population size.
Biotic factors are living factors that affect the distribution and abundance of organisms, including food availability, predation, new pathogens (disease) and competition between species. They involve interactions between living things, unlike abiotic factors such as temperature and rainfall.
Food availability is often the primary limiting factor for population size: more food means better survival and more successful reproduction. If drought cut plant food for 200 rabbits by 60%, competition would increase, malnourished females would produce fewer offspring and the population would likely drop to 60-80.
A new predator can devastate prey that have no evolved defences, because the prey has not had time to develop behavioural or physical adaptations. Cats introduced to islands have caused native bird population crashes of 90% or more, through direct predation and by changing prey behaviour.
New pathogens are particularly devastating because the host population has no evolved resistance, and pathogens can multiply exponentially within their hosts. Chestnut blight fungus eliminated American chestnut trees; the impact depends on the disease's transmission rate and mortality rate.
Interspecific competition can reduce the losing species to numbers so low that breeding becomes impossible: individuals cannot find mates or there is not enough genetic diversity for healthy offspring. Grey squirrels outcompeting red squirrels in Britain leaves red populations so fragmented that breeding success plummets.
Cats introduced to islands harm native birds because those birds evolved without mammalian predators and have no instinct to avoid ground-dwelling hunters. The result can be population crashes of 90% or more.
In a drought that reduces plant food by 60%, a rabbit population of 200 would face more competition for food, and malnourished females would produce fewer offspring. The population would likely crash to 60-80 rabbits.
A breeding threshold is the population size under which individuals cannot find mates or there is not enough genetic diversity for healthy offspring. Once a population is that small, local extinction follows even if some individuals survive.
New predators affect prey through direct predation, killing them, and indirect effects, where prey change their behaviour, spending more time hiding and less time feeding.
Biotic factors work together: a population weakened by disease becomes more vulnerable to predation, and reduced food availability makes animals more susceptible to pathogens. These interactions explain complex population changes.
Examples of new pathogens devastating populations include European diseases devastating Native American populations and chestnut blight fungus eliminating American chestnut trees. Host populations had no evolved resistance.