Practise Biotic factors. 14 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.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define a biotic factorA living factor, such as another organism, that can affect a community.
4
List the four biotic factors in the specificationAvailability of food, new predators arriving, new pathogens, and one species outcompeting another.
5
Explain the effect of food availabilityMore food means more organisms survive and breed, so the population grows; less food has the opposite effect.
6
Explain effects of new predators or pathogensPrey may not be adapted to escape a new predator and organisms may have no resistance to a new pathogen, so numbers can fall fast.
7
Explain how competition can make a species declineIf one species outcompetes another, the other's numbers can fall until too few are left to breed.
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
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.
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'.
Cheatsheet
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
How to answer each type of question
Recall: identify biotic factors
1 to 2 marks3
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 answer
A new species of predator (1); a new fungal disease (1)
Interpret a graph of population changes
2 to 3 marks5
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. A graph shows that about 800 rabbits lived in an area until a new disease arrived in 2020. By 2021 there were 150 rabbits. The number of foxes in the area fell from 40 to 25 between 2020 and 2022. Suggest why the number of foxes fell. (2 marks)
Show the model answer
The disease killed many rabbits, so the foxes had less food (1). So fewer foxes survived or reproduced (1).
Explain the effect of a new predator or pathogen
2 to 3 marks6
State the effect on the population (e.g. it decreases).
Explain why: many are eaten or killed, because the prey are not adapted to escape or have no resistance.
Add a knock-on effect on another species if the question asks about the community.
Example. American mink escaped from fur farms and now live wild along British rivers. Mink are predators of water voles. Explain why the number of water voles fell after mink arrived. (3 marks)
Show the model answer
Mink were a new predator, so many water voles were eaten (1). The water voles were not adapted to escape mink, e.g. mink can swim and can get into their burrows (1). So fewer water voles survived to breed and the population fell (1).
Explain how one species outcompetes another
3 to 4 marks7
Name the resource they compete for (food, space, territory, mates, light, water).
Say which species is better at getting it and why, using the information given.
Explain the effect: fewer of the other species survive and reproduce.
If numbers fall very low, too few are left to breed, so the species can disappear from the area.
Example. A non-native plant grows rapidly and covers the surface of a pond. Explain how this could affect the native plants that grow underwater. (4 marks)
Show the model answer
The non-native plant absorbs most of the light at the surface / shades the water (1). The underwater plants get less light, so they photosynthesise less (1). So they grow less, and fewer survive and reproduce (1). The non-native plant outcompetes them for light and space, so the native plants may disappear from the pond (1).
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
Question 3Hard6 marks
A non-native shrub was planted in gardens and escaped into a nearby woodland. The shrub:
grows quickly, forming dense thickets up to 4 m tall
keeps its leaves all year round
has leaves that are not eaten by any native animals
produces flowers that provide nectar for bees in spring.
Explain how the arrival of the shrub could affect the woodland community.[6]
Show the answer and mark scheme
the shrub is a new competitor (a biotic factor)
its dense leaves shade the woodland floor all year, so native plants get less light and photosynthesise less
it also competes with native plants for space, water and mineral ions, so some native plants may die out locally
no native animals eat it, so nothing controls its spread
animals that feed on the native plants, e.g. insects, lose food and shelter, so their numbers fall
predators that feed on those animals then have less food
tree seedlings are shaded, so the woodland may not regenerate
bees gain nectar in spring, so some species may benefit
overall, biodiversity falls and the community becomes less stable
Marked with levels of response: the full level descriptors are in the app.