AQA GCSE Combined Science Foundation (8464), Foundation tier · Biology › Ecology › Adaptations, interdependence and competition
Practise Biotic factors. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Biotic factors are the living parts of the environment that affect a community: food, predators, pathogens and competition. Expect questions asking you to explain how a change in one of these affects population sizes, often with a food web, a graph of population changes or an example such as red and grey squirrels.
Key facts
Biotic = living
Biotic factors: food availability, new predators, new pathogens, being outcompeted
More food → more survive and breed → population rises
New predator → prey not adapted to escape → prey numbers fall
New pathogen → no resistance → many die
Outcompeted → numbers too low to breed → species can disappear from the area
Notes
The biotic factors
Biotic factors are living factors that can affect a community:
availability of food;
new predators arriving;
new pathogens (microorganisms that cause disease);
one species outcompeting another, so the numbers of the losing species are no longer large enough to breed.
How they change populations
Food: when food is plentiful, more animals survive and reproduce, so the population grows. When food is scarce, competition increases and some animals starve or breed less.
New predators: prey may not be adapted to escape a predator they have never met, so their numbers can fall sharply.
New pathogens: a population with no resistance to a new disease can be badly affected, and the species that depend on it are then affected too.
Competition: if a new species is better at getting the same food or space, the original species may decline. If its numbers fall too low, individuals rarely meet to mate and it can die out in that area.
Example: red and grey squirrels
diagram
Grey squirrels were brought to Britain from North America. They compete with native red squirrels for food and habitat.
Grey squirrels are larger and feed well on acorns, so in many areas they get more of the food than red squirrels do.
Grey squirrels also carry a virus (squirrelpox) that rarely harms them but kills red squirrels, so a new pathogen adds to the decline.
Red squirrels now survive mainly in areas with few or no grey squirrels.
Grey squirrels outcompete red squirrels and spread squirrelpox, so as greys increase, reds decline.
Linking factors together
Changes often combine: a cold winter (abiotic) can reduce the food available (biotic), which then affects the predators.
Always give the reason for each effect, not just 'the numbers go down'.
How to answer each type of question
Recall: identify biotic factors
1 to 2 marksGrade 3
A biotic factor is an organism or is caused by one: food, predators, pathogens, competitors.
Check each option in a list: is it living, or a physical or chemical condition?
Example. Which two of the following are biotic factors? soil pH / a new species of predator / wind speed / a new fungal disease / temperature
Show the model answerHide the model answer
A new species of predator (1); a new fungal disease (1)
Interpret data on population changes
2 to 3 marksGrade 5
Describe what happens to each population, with dates or values.
Look for cause and effect: which change came first?
Suggest a biotic reason, e.g. more food, a new disease or more predators.
Example. Ecologists counted the rabbits and foxes in an area each spring. In summer 2020 a new disease that kills rabbits arrived in the area.
Year
Number of rabbits
Number of foxes
2019
810
40
2020
800
40
2021
150
33
2022
170
25
Suggest why the number of foxes fell between 2020 and 2022. (2 marks)Show the model answerHide the model answer
The disease killed many rabbits, so the foxes had less food (1). So fewer foxes survived or reproduced (1).
Don’t lose marks
Giving abiotic factors (temperature, light) when biotic factors are asked for.
Writing 'they die' without saying why: eaten, starved or killed by disease.
Missing the last step for competition: numbers become too low to breed, so the species can disappear from the area.
Calling a pathogen a predator. Pathogens are microorganisms that cause disease.
Ash dieback is a disease of ash trees caused by a fungus. The disease was first found in the UK in 2012. In some woodlands, most of the ash trees have now died.
(a) Is ash dieback a biotic factor or an abiotic factor? Give a reason for your answer.[1]
(b) Some species of insect and lichen live only on ash trees. Explain why these species could become extinct in the UK.[2]
(c) When ash trees die, more light reaches the woodland floor. Suggest how this could affect the plants growing on the woodland floor.[2]
(d) A small number of ash trees are not affected by the disease. Suggest how scientists could use these trees to help restore ash woodlands.[2]
Show the answer and mark scheme
(a)Answer: Biotic — caused by a living organism (a fungus)
biotic, because it is caused by a living organism / pathogen / fungus
(b)
ash trees provide food / shelter / a habitat for these species
(when the ash trees die) these species lose their only food / habitat, so they cannot survive or breed
(c)
more photosynthesis, so some plants grow more
(so) some species may increase in number / new species may grow / competition between ground plants may change
(d)
collect seeds from / breed from the unaffected trees (selective breeding)
plant their offspring, which are likely to inherit the alleles for resistance