Abiotic variables and range of tolerance

Ecosystems (Form and function) · Adaptation to environment · note 2 of 5

Spec B4.1.3, B4.1.4Download all Adaptation to environment notes (PDF)Download all notes (PDF)9 pages

Abiotic variables and range of toleranceSpec B4.1.3, B4.1.4

In short

A range of tolerance is the span of values of an abiotic factor within which a species can survive. Abiotic variables such as temperature, light, soil pH, salinity and oxygen act as limiting factors: outside the range of tolerance a species cannot survive, so the distribution of plants and animals follows these variables, which transects can measure.

The distribution of a species depends on abiotic variables. Each species has adaptations that suit it to a certain set of conditions, and these give it a range of tolerance for each variable.

Examples of abiotic variables affecting distribution
Organisms	Abiotic variables
Plants	Light intensity, temperature, water availability, soil pH, mineral nutrients in the soil, salinity, wind exposure
Animals	Temperature, water availability, dissolved oxygen (aquatic animals), salinity, pH of water, shelter and substrate type

Range of tolerance of a limiting factor

Plotting how well a species does (for example population density) against an abiotic variable gives a bell-shaped tolerance curve.

Optimum range: conditions where the species grows and reproduces best, so it is most abundant.
Zones of physiological stress on either side: individuals survive but grow and reproduce less, so they are fewer.
Zones of intolerance at the extremes: the species cannot survive, so it is absent.

A limiting factor is the variable that restricts the distribution or abundance of a species because it lies closest to, or beyond, the limits of tolerance. For example, a plant may be absent from a shaded woodland floor because light intensity is below its tolerance, even though soil and water are suitable.

Tolerance curve: population size against an abiotic variable such as temperature, rising from zero at the lower limit of tolerance to a peak in the optimum range and falling to zero at the upper limit, with bands for the zones of intolerance (absent), zones of physiological stress (infrequent) and the optimum range (abundant). — A species is most abundant in its optimum range, infrequent in the zones of physiological stress and absent beyond its limits of tolerance.

Practical skill:

Use a line or belt transect from a natural or semi-natural habitat (influenced by humans but dominated by wild, not cultivated, species). Place quadrats at regular intervals, record abundance (percentage cover or counts) and measure the abiotic variable at each quadrat with sensors such as a light meter, temperature probe or soil pH probe.

A belt transect running from under a tree canopy into open grass, with quadrats every 2 m along a tape measure, and a graph showing that the number of plants per quadrat rises with distance from the tree as light intensity increases. — Quadrats placed at regular intervals along a tape across a light gradient (an interrupted belt transect). Abundance in each quadrat is plotted against distance; for IB, measure the abiotic variable (here light intensity, with a sensor) at each quadrat too.

Mean percentage cover along a transect

At 6 m along a transect, three replicate quadrats gave 40%, 35% and 30% cover of a grass species. Light intensity there was 5200 lux. At 2 m, the mean cover was 10% and light intensity was 1100 lux. Calculate the mean cover at 6 m and describe the relationship.

Mean = (40 + 35 + 30) ÷ 3
= 105 ÷ 3 = 35%
Higher light intensity (5200 lux) goes with higher cover (35%) than lower light intensity (1100 lux, 10%).

Answer: Mean cover is 35%. Cover is positively correlated with light intensity (practice data).

Maths skill:

A scatter graph of abundance against the abiotic variable shows correlation; a correlation coefficient (for example Spearman's rank) measures its strength. Correlation does not prove the variable causes the distribution.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between a habitat and a niche?

A habitat is the place where an organism, population, species or community lives, such as a sand dune. A niche is the role of a species in its ecosystem, including how it obtains food and all the biotic and abiotic interactions that affect its growth, survival and reproduction. Several species can share a habitat but not a niche.

How is marram grass adapted to sand dunes?

Marram grass rolls its leaves so the stomata, which sit in pits among hairs on the inner surface, are surrounded by humid air, reducing transpiration. A thick waxy cuticle covers the outer surface. Long roots and vertical rhizomes reach deep water and grow upwards when sand buries the plant.

What conditions do coral reefs need to form?

Coral reefs need shallow, clear water so light reaches the photosynthetic zooxanthellae in the corals. The water must also be warm, best at about 23 to 29 °C, of normal seawater salinity and slightly alkaline, so that carbonate ions are available for building calcium carbonate skeletons. That is why reefs are found mainly in shallow tropical seas.

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