What you need to know
- 5.12Numbers and distribution in a habitatThe place where an organism lives. are controlled by bioticLiving. Biotic factors include competition, predation and disease. (and abioticNon-living. Abiotic factors include light, temperature, water and soil pH.) factors.
Niche overlap as an explanation of coexistence is developed in 5B 5. Disease case studies in Topic 6 are not required here. Intraspecific and interspecific competition both count as biotic.
By the end, you should be able to…
- Distinguish intraspecific from interspecific competitionCompetition between individuals of different species for the same resources..
- Explain predation, grazing, parasitism and mutualism as controls on numbers.
- Combine a biotic factor with an abiotic one in a short explanation.
Build your understanding
Living neighbours
bioticLiving. Biotic factors include competition, predation and disease. factors are the effects of other organisms: competition, predation, grazing, parasitism, disease, mutualism and human harvesting. They can raise or lower numbers and can exclude a speciesA group of organisms that can interbreed to produce fertile offspring. from an otherwise suitable abioticNon-living. Abiotic factors include light, temperature, water and soil pH. habitatThe place where an organism lives..
intraspecific competitionCompetition between individuals of the same species for the same resources. is within one species for the same resource — nest holes, light in a crowded canopy, mates. It is density-dependent: as the populationAll the individuals of one species living in one area at the same time. rises, each individual gets less, so birth rates fall or death rates rise, which can stabilise numbers. interspecific competitionCompetition between individuals of different species for the same resources. is between species, for example two limpets grazing the same algae.
Eat, infect, or help
Predation and grazing remove individuals or biomassThe mass of living material, often measured as dry mass per unit area.. Predators can hold prey below the level abiotic resources would allow; prey can also limit predator numbers. Parasites and pathogens reduce survival or fecundity. Mutualism can increase numbers of both partners, as in mycorrhizas supplying mineral ions to plants.
A biotic factor often interacts with an abiotic one. Shade from a competitor is light as well as competition. Drought kills seedlings that a grazer would otherwise have eaten. Write both when the question gives both.
Distribution as well as numbers
A species may be absent from a habitat that looks abiotic-friendly because a competitor, predator or disease is present. Introduction or loss of one species can therefore redraw maps of others. That is still 5.12, not a claim that every absence is competitive exclusionWhen two species compete for the same limited resources, the better competitor eventually removes the other from the habitat. — you need evidence of the interaction.
Humans are biotic factors: fishing, trampling, weeding, conservationProtecting and managing species and habitats to maintain biodiversity. grazing. Treat “people” as a named interaction (trampling on a dune path) rather than a moral speech.
NOT: biotic factors do not only reduce numbers. Mutualism can increase them, and they also change distribution.
- Intraspecific competition can stabilise numbers
- Shade from a competitor is both light and competition
- Parasites reduce survival or fecundity
- Humans count when the interaction is named, such as trampling
- Absence still needs evidence; it is not automatic exclusion
Two traps this diagram blocks
- Do not say every biotic factor decreases abundance.
- Do not call competition between two species intraspecific.
From the diagram alone
Question
Distinguish intraspecific competition from interspecific competition, and give one biotic interaction that can increase abundance. [3]
Show a model answer
Intraspecific competition is between members of the same species. Interspecific competition is between different species. Mutualism can increase abundance, because both partners benefit.
| Interaction | Who is affected | Effect on numbers |
|---|---|---|
| Intraspecific competition | Same species | Density-dependent check on one population |
| Interspecific competition | Two or more species | Can reduce one or both, or exclude one |
| Predation / grazing | Consumer and consumed | Can hold prey down; predators depend on prey |
| Parasitism / disease | Host and parasite | Often lowers host survival or breeding |
| Mutualism | Both partners | Can raise numbers of both where the partner occurs |
Make your reasoning visible
Say who competes for what
“Competition” alone is weak. Name the speciesA group of organisms that can interbreed to produce fertile offspring. and the resource.
Density-dependent is not a synonym for biotic
Some bioticLiving. Biotic factors include competition, predation and disease. effects are patchy or seasonal rather than a smooth function of density.
Avoid this misconception
The claim: Biotic factors only reduce numbers; abioticNon-living. Abiotic factors include light, temperature, water and soil pH. factors only set the place.
The correction: Mutualism can increase numbers. Competition and predation also change distribution. Both kinds of factor can do both jobs.
Why does a population stop growing, even when nothing is hunting it?
Other organisms are biotic factors: members of the same species, competitors, predators, pathogens and partners all change birth and death rates.
What the model shows
A grassland population over 40 years, with each biotic factor added in turn, and its births and deaths per individual.
What to look for
Where the population levels off, how the birth and death bars change as it grows, and the lag between predator and prey peaks.
What it tells us
Intraspecific competition gives a carrying capacity. Competitors, predators and disease lower numbers; mutualists can raise them. Predator–prey numbers cycle.
Be careful what you claim
Fictional species and smooth model equations: no chance events, migration or seasons.
Try it in the model: press Play with every factor off and watch the birth and death bars meet.
Biotic factors: competition, predation and more
Grow a fictional grassland population over 40 years, then add a competitor, a predator, a disease or gut mutualists and see where numbers settle. Rates are illustrative.
What you see
- Abiotic: a belt transect from woodland shade to open meadow. Each quadrat shows the plants counted and bars for light, soil water and soil nitrate. The graph is the species’ tolerance curve for one factor, with the quadrats placed on it.
- Biotic: a grassland with species A, and if you add them, a competitor, a predator, a disease or gut mutualists. The graph shows numbers against time.
How to use it
- Abiotic: choose a species, click a quadrat, turn watering on or off, drag soil nitrate and choose which tolerance curve to plot.
- Biotic: turn biotic factors on or off, then press Play, or Skip to year 40.
What it shows
- How abiotic factors such as light, water and soil nutrients affect the size and distribution of a population (5B 3).
- How intraspecific and interspecific competition, predation, disease and mutualism affect population size (5B 4).
- The limiting-factor idea and the shape of a carrying-capacity curve and a predator–prey cycle.
Limits of the model
- All species, numbers and rates are fictional and illustrative. The plant count uses only the most limiting factor; in real habitats factors interact.
- The populations are modelled with simple equations, with no random events, migration or seasons. Real predator–prey cycles are rarely this regular.
- Each quadrat is drawn with at most 24 plants, and each animal icon may stand for two individuals.
Guided tour · step 1 of 8
- In Grassland, just below these steps, keep every factor on Off.
- Press Play and let it run past year 30.
Numbers rise quickly, then level off at the carrying capacity. As A gets crowded, births per individual fall and deaths rise: intraspecific competition for food and space.
Grassland
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Distinguish intraspecific and interspecific competition.
Show suggested answer
Intraspecific is between individuals of the same species; interspecific is between different species. Both are for limited resources.
Practice question
Retrieval and explanation · suggested answer
How can predation change distribution as well as abundance?
Show suggested answer
Prey may be excluded from patches where predators hunt, even if food and abiotic conditions would allow them to live there, so the map of the prey shrinks as well as its numbers.
Practice question
Retrieval and explanation · suggested answer
Give one biotic and one abiotic reason a woodland seedling might fail.
Show suggested answer
Biotic: grazing or shade from competing plants. Abiotic: drought, frost, or soil pH outside its range.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Species A and B eat the same seeds. A is better at finding seeds; B is better at avoiding a shared predator. Predict a pattern of coexistence, stating an assumption.
Show a hint
Two different limiting factors can prevent simple exclusion.
Explore the reasoning
They may coexist if seed competition and predation weigh differently in different patches or years, so neither has a complete advantage everywhere. Assume the predator really uses both and seeds really are shared. If predation is removed, A might exclude B.
Recall this lesson
Lesson recall questions
Question 1 of 5: What is meant by a biotic factor?
Biotic and abiotic control of distribution
Practice question
Retrieval and explanation · suggested answer
What is meant by a biotic factor?
Lesson: 5B 4: The Effect of Biotic Factors on Populations
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Competition is one interaction among several
5B · The Effect of Biotic Factors on PopulationsNature Education’s Scitable essay on species interactions distinguishes interference from exploitation competition and notes that a superior competitor can exclude another (Hardin’s competitive exclusion). It also notes that…Specification 5.12Read & use the scienceClose article
Why this matters
Nature Education’s Scitable essay on species interactions distinguishes interference from exploitation competition and notes that a superior competitor can exclude another (Hardin’s competitive exclusion). It also notes that disturbance and different strategies can prevent that outcome. Use it as context, not as extra named models you must recall. Lotka–Volterra equations are enrichment. A field study cannot infer competition solely from two species occurring in different places. To test that explanation, researchers also need evidence about the shared limiting resource and what happens when one competitor changes.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.12 understand that the numbers and distribution of organisms in a habitat are controlled by biotic and abiotic factors
Use the science
Why is “the better competitor always wins everywhere” too strong for a field habitat?
Explore a possible response
Abiotic patches, predators, disease and disturbance can favour the weaker resource competitor in some places or times, so exclusion is not automatic.
Explore the source
- Scitable: species interactions and competitionNature Education
Disturbance changes the invasion story
5B · The Effect of Biotic Factors on PopulationsA US Geological Survey study compared native and introduced plants across blue oak savanna, chaparral and coniferous forest in the southern Sierra Nevada. Grazing changed alien plant cover…Specification 5.12Read & use the scienceClose article
Why this matters
A US Geological Survey study compared native and introduced plants across blue oak savanna, chaparral and coniferous forest in the southern Sierra Nevada. Grazing changed alien plant cover only slightly in the savanna, while fire was followed by more aliens in chaparral. The researchers also found no support for the simple idea that more native species always prevent invasion. Competition for light, water or nutrients is plausible, but disturbance, seed arrival and other conditions can change the outcome. A careful ecologist compares plots while measuring those conditions. An association between alien cover and native cover alone does not prove competitive exclusion.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.12 understand that the numbers and distribution of organisms in a habitat are controlled by biotic and abiotic factors
Use the science
If invasive cover rises after a fire, what additional comparison would help test whether competition caused the native decline?
Explore a possible response
Compare plots with similar fire and grazing history but different invasive cover, or manipulate the competitor while monitoring native growth and abiotic conditions.
Explore the source
- Fire and grazing impacts on plant diversityUS Geological Survey