What you need to know
- 5.12Numbers and distribution in a habitatThe place where an organism lives. are controlled by abioticNon-living. Abiotic factors include light, temperature, water and soil pH. (and bioticLiving. Biotic factors include competition, predation and disease.) factors.
This lesson is the abiotic half of 5.12. Biotic interactions are 5B 4. Q10 and climate-change physiology are 5.21 in 5C; temperature is still a valid abiotic factor here without that calculation.
By the end, you should be able to…
- Name abiotic factors that affect distribution and abundance.
- Link a named factor to a physiologicalDescribes an adaptation of body processes, such as producing concentrated urine. or physical reason.
- Explain why a factor can limit numbers without killing every individual.
Build your understanding
Non-living conditions
abioticNon-living. Abiotic factors include light, temperature, water and soil pH. factors are the non-living conditions of the habitat: light intensityThe amount of light energy reaching a given area in a given time. and wavelength, temperature, water availability, humidity, wind, soil or water pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline., mineral ions, salinity, oxygen concentration and, in aquatic habitatThe place where an organism lives., current or wave action. They set where a speciesA group of organisms that can interbreed to produce fertile offspring. can live and how large its populationAll the individuals of one species living in one area at the same time. can grow.
Distribution is where organisms are found. Abundance is how many, or how much cover, in that place. A plant may be present only in the splash zone (distribution) and still be sparse there (abundance). Both can change with an abiotic gradient.
Mechanisms, not lists
Light limits photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen., so shade-intolerant seedlings fail under a closed canopy. Temperature affects enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. activity and therefore growth, germination and animal activity; extremes kill. water potentialThe tendency of water to move out of a solution. Pure water is 0; adding solutes makes it negative. Water moves from higher to lower water potential. and rainfall control stomatal opening, germination and who can live in a dune slack versus a dry ridge. Low oxygen in still water limits fish and many invertebrates. Mineral ions such as nitrateNO₃⁻, a mineral ion taken up by plant roots and used to make amino acids, proteins and nucleic acids. and magnesiumA mineral ion (Mg²⁺) that plants need to make chlorophyll., met in 4A, limit plant growth and therefore the animals that eat those plants.
A factor is limiting when changing it would change rate or numbers. At saturating light, extra lamps do little if the soil is dry. Always say which process is affected. “Temperature is important” is not an explanation.
Gradients and ranges
Few species tolerate the full range of a factor. Each has a range in which survival is possible and a narrower range in which reproduction is good. At the edge of the range, numbers fall long before the last individual dies. That is why a rocky-shore species can be abundant at one height and absent a metre away.
Measure the factor when you claim it. A “wet wood” guess is weaker than a soil-moisture reading next to a quadratA square frame of known area used to sample organisms.. How to take those readings is 5B 6 and Core Practical 11.
NOT: presence does not prove a factor is not limiting. Name the process the factor affects.
- A factor is limiting when changing it would change the rate or the numbers
- At saturating light, water may be the limit instead
- Mineral ions and pH are abiotic too
- Each species has a survival range and a narrower breeding range
- Measure the factor beside the quadrat; do not only guess
Two traps this diagram blocks
- Do not list abiotic factors with no link to a process.
- Do not treat presence as proof that nothing is limiting.
From the diagram alone
Question
Explain how light and soil water could produce different plant distributions in one habitat. [3]
Show a model answer
High light allows photosynthesis in shade-intolerant plants, which fail under a closed canopy. Soil water allows germination and survival in damp hollows, while the same species may be absent from a dry ridge. Each factor changes a named process, so the maps differ.
| Abiotic factor | A process it affects | Possible field pattern |
|---|---|---|
| Light | Photosynthesis, seedling survival | Species A only in canopy gaps |
| Temperature | Enzyme activity, germination | Species B missing from north-facing snow hollows |
| Water / salinity | Osmoregulation, germination | Saltmarsh plants zoned by height |
| Oxygen or mineral ions | Respiration or plant growth | Few fish in a stagnant pond; pale plants on poor soil |
Make your reasoning visible
Distribution and abundance
Say whether the factor changes where organisms occur, how many there are, or both.
One named mechanism
Link light to photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen., oxygen to respiration, nitrateNO₃⁻, a mineral ion taken up by plant roots and used to make amino acids, proteins and nucleic acids. to protein synthesis.
Key explanations
- Distribution is the places occupied. Abundance is the number or cover in those places.
- Water availability controls germination and which plants occupy wet and dry microsites.
Avoid this misconception
The claim: If a speciesA group of organisms that can interbreed to produce fertile offspring. is present, abioticNon-living. Abiotic factors include light, temperature, water and soil pH. factors cannot be limiting.
The correction: Presence shows the factor is within the survival range. Numbers can still be held down, and reproduction may fail in some years.
Why does a plant grow in one part of a field but not ten metres away?
Every species has a range of tolerance for each abiotic factor. Where one factor falls outside it, numbers fall, or the species is absent.
What the model shows
A belt transect from shade to open meadow, with light, soil water and nitrate measured in each quadrat and plants counted.
What to look for
Which factor is limiting in each quadrat, and what happens to the count when you change that factor or a different one.
What it tells us
Population size is set by the factor in shortest supply (or beyond the optimum). Changing a non-limiting factor makes no difference.
Be careful what you claim
Fictional plants and readings. Real factors interact, and biotic factors also act along a transect.
Try it in the model: click quadrat 1, then quadrat 10, and compare which bar is outlined.
Abiotic factors along a transect
Walk a belt transect from woodland shade into open meadow, read light, soil water and nitrate in each quadrat, and find the factor that limits a fictional plant. Readings 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 9
Click quadrat 1, 2 or 3 in the woodland shade in the simulator, or in Transect, just below these steps, set Quadrat (1–10) to 1, 2 or 3.
There are no meadow herbs here. Light is below the lowest level this species can survive, so it is absent, whatever the water or nitrate.
Transect
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
What is an abiotic factor? Give two examples that could limit a pond community.
Show suggested answer
A non-living condition. Examples: temperature, dissolved oxygen, light, pH or mineral ion concentration.
Practice question
Retrieval and explanation · suggested answer
Explain how low light can change both distribution and abundance of a woodland herb.
Show suggested answer
The herb may be absent from deep shade (distribution) and only sparse at the shade edge (abundance) because photosynthesis, and therefore growth and seed set, are reduced.
Practice question
Retrieval and explanation · suggested answer
Why might extra watering not increase a plant population if soil nitrate is very low?
Show suggested answer
Water is not the limiting factor. Nitrate needed for amino acids and proteins still caps growth, so numbers stay low.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two slopes have the same rainfall. Slope P is bare rock; slope Q has deep soil. Predict a difference in plant abundance and name the abiotic factor that actually differs, stating an assumption.
Show a hint
Rainfall is not the same as water available to roots.
Explore the reasoning
Q should support greater abundance if deeper soil stores more water and ions. The differing abiotic factor is water and mineral availability in the rooting zone, not rainfall. Assume similar seed rain and no extreme grazing difference.
Recall this lesson
Lesson recall questions
Question 1 of 5: List four abiotic factors that can control the distribution of organisms in a habitat.
Biotic and abiotic control of distribution
Practice question
Retrieval and explanation · suggested answer
List four abiotic factors that can control the distribution of organisms in a habitat.
Lesson: 5B 3: The Effect of Abiotic 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.
Why field ecologists still carry meters
5B · The Effect of Abiotic Factors on PopulationsOpen University ecosystem materials treat energy, water and nutrients as part of what makes a living system, not as scenery. School abiotic readings are a local version of…Specification 5.12Read & use the scienceClose article
Why this matters
Open University ecosystem materials treat energy, water and nutrients as part of what makes a living system, not as scenery. School abiotic readings are a local version of that idea. A single temperature or pH value is a snapshot, not a climate record — those longer records belong with 5C evidence. The same site can differ between morning and afternoon, or after rain. Repeated readings and paired organism counts let a researcher test whether an abiotic gradient plausibly explains a distribution instead of merely coinciding with it.
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 one midday pH reading a weak claim that “the stream is always acidic”?
Explore a possible response
pH can change with flow, photosynthesis and rainfall. A distribution explanation needs a pattern, or repeated measurements, not one unrepeated number.
Explore the source
- OpenLearn: ecosystems as living communitiesThe Open University
A reef turns white under stress
5B · The Effect of Abiotic Factors on PopulationsNOAA explains that reef-building corals host photosynthetic algae. When temperature, light or water quality moves outside tolerable conditions, the partnership can break down and the coral becomes pale.…Specification 5.12Read & use the scienceClose article
Why this matters
NOAA explains that reef-building corals host photosynthetic algae. When temperature, light or water quality moves outside tolerable conditions, the partnership can break down and the coral becomes pale. A bleached coral is stressed, not necessarily dead; recovery depends on the duration and severity of exposure. This is a precise example of an abiotic factor changing the distribution and abundance of living organisms, with a biotic interaction layered on top. A field biologist would measure water conditions and repeated coral cover rather than treating one white photograph as proof of permanent reef loss.
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 would a single photograph of a pale reef be insufficient evidence that high temperature alone caused a permanent fall in coral abundance?
Explore a possible response
Other stressors can bleach coral, and bleaching may be reversible. Repeated temperature and coral-survival measurements are needed.