Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

7.1.3Ecology

Biotic Factors

5 min read 9 parts Lesson 4 of 14 Unit quiz

0%read

Section 1 of 9What are biotic factors?

Biotic factors are the living features of the environment that affect other organisms. They include the availability of food, new predators or pathogens, competition between species and between individuals of one species, and the activities of humans.

Section 2 of 9Competition

Organisms compete when they need the same resource and there is not enough for all. Competition can be interspecific (between different species) or intraspecific (within one species).

Plants
Plants compete for light, water, space (for roots and leaves) and minerals from the soil.
Animals
Animals compete for food, water, shelter, territory and mates.

The red squirrel in Britain is being replaced in many areas by the introduced grey squirrel, which competes for food and also carries a virus that harms reds. Competition is most intense between organisms with similar needs, so individuals of the same species compete most.

Section 3 of 9Predation and food

Predators reduce prey numbers; the prey population in turn affects the number of predators.

Predation and food
The loss of a food supply,
  • such as a drop in plankton, reduces the number of fish that feed on it and consequently the number of birds that eat the fish.
A new predator introduced into a habitat can cause prey with no defences to decline sharply.

Section 4 of 9Disease and other factors

A new pathogen can reduce a population quickly, as with Dutch elm disease and ash dieback. Dense populations suffer disease outbreaks more easily because the pathogen spreads between close neighbours. Other biotic effects include pollinators, which affect plant reproduction, and humans who clear land, hunt or introduce species.

Section 5 of 9Interpreting population data

When describing a graph, quote the data, state the trend, and offer an explanation using the biotic factor.

Interpreting population data
For example, if numbers of one species fall while another rises, competition or predation is a likely explanation.
Using a table of compensating changes (a fall in food followed by a fall in consumers) shows understanding of how species are linked in a community.
Biotic factorExample of effect
CompetitionSeedlings in shade grow poorly under a tree canopy
PredationFox numbers rise after a rabbit population increase
DiseaseMyxomatosis lowered rabbit numbers
Food supplyFewer fish when plankton decline

Section 6 of 9Predator-prey cycles and population graphs

The study of biotic factors often involves reading a graph with two lines showing the numbers of predators and prey against time.

1
A classic example is the snowshoe hare and the Canada lynx in northern Canada,
  • where records of furs traded show repeating cycles of roughly ten years.
2
The prey line peaks first.
3
The predator line peaks later, because the lynx need time to find extra food, breed and raise young.
4
When the lynx numbers are high, hare numbers fall rapidly, and then the lynx decline again.

When asked to describe a graph, give a sequence of statements with numbers.

Predator-prey cycles and population graphs
For example: hare numbers rose from 20 000 in year 1 to a maximum of 150 000 in year 4, then fell to 40 000 in year 7.
  • The lynx peak of 60 000 occurred in year 5, one year after the hare peak.
Then offer explanations using biotic ideas: increased food supply, increased predation, competition and starvation.

It is also important to realise that prey numbers are affected by other things, such as the weather and availability of plants, and predators may eat other species when one is scarce. In real communities this makes the cycles less regular than the graph of a simple model.

Other biotic relationships can be mentioned.

1Mutualism
In mutualism both species benefit,
  • as when bees feed on nectar while pollinating flowers,
  • or when bacteria in the root nodules of legumes fix nitrogen in return for sugars.
2Parasitism
In parasitism one organism, the parasite, benefits at the expense of the host, as with tapeworms in the gut or fleas on a dog.
3
In both, the numbers of the two species are linked, because the parasite population depends on the number of hosts.

Remember that predator-prey cycles are an example of a stable community: the numbers swing but average out, showing balance between the species rather than collapse.

Section 7 of 9Exam-style questions

  1. Explain how a rise in the number of rabbits in a field could affect the number of foxes and the number of dandelions. [4]4 marks

  2. Describe what plants compete for and explain why competition between plants of the same species is greater than between different species. [3]3 marks

  3. Grey squirrel numbers rose from 400 to 1000 while red squirrel numbers fell from 600 to 150. Calculate the percentage fall in red squirrels and suggest a reason. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Biotic factorA living factor in the environment that affects other organisms.
CompetitionOrganisms using the same limited resource.
PredatorAn animal that kills and eats other animals.
PreyAn animal that is eaten by a predator.
PathogenA microorganism that causes disease.
Interspecific competitionCompetition between different species.
Intraspecific competitionCompetition within the same species.

Section 9 of 9Exam technique

1
Calling competition 'fighting'. In ecology it means using the same limited resource.
2
Forgetting that competition happens within a species as well as between species.
3
Saying that predators reduce prey permanently; the numbers rise and fall in cycles.
4
Describing the effect without linking to a mechanism, such as less food leading to starvation.
\n