Abiotic factors, tolerance and biomes (Biology HL)

The big idea: Every organism lives in an environment made of two kinds of factor.

Abiotic factors are the non-living physical and chemical parts of the environment — things like temperature, light, water, pH, oxygen and salinity.

Biotic factors are the living parts — other organisms acting as predators, competitors, parasites or food.

An organism can only survive where every abiotic factor stays inside its range of tolerance. Across the whole planet, regions that share a characteristic climate (and so a characteristic community) are called biomes.

Abiotic factor

A non-living, physical or chemical feature of the environment — for example temperature, light intensity, water availability, pH, oxygen concentration or salinity.

Biotic factor

A living feature of the environment — the effect of other organisms, such as predators, competitors, parasites, disease or food supply.

Range of tolerance

The range of an abiotic factor over which an organism can survive. It performs best in an optimum range in the middle and is absent beyond its upper and lower limits.

Limiting factor

The single abiotic factor that is furthest from an organism's optimum, and so the one that restricts where it can live.

Biome

A large geographical region with a characteristic climate (its abiotic conditions) and a characteristic community of organisms — for example hot desert, tropical rainforest or tundra.

Living or non-living — that is the whole test: To sort a factor: ask is it alive?

If it is a non-living physical or chemical condition (heat, light, water, pH), it is abiotic.

If it is the effect of another organism (a predator, a competitor, a parasite), it is biotic.

A species is not equally happy at every level of an abiotic factor.

For something like temperature, there is an optimum range in the middle where the organism performs best. As you move away from the optimum in either direction, it passes through a zone of stress where it survives but does poorly, and finally reaches a limit of tolerance beyond which it cannot survive at all and is absent.

Range of tolerance — read it off the curve: Plotting an organism's performance (or abundance) against an abiotic factor gives a bell-shaped tolerance curve.

The peak is the optimum range; the shoulders are zones of stress; the points where the curve reaches zero are the lower and upper limits of tolerance.

Beyond those limits the organism is absent — so the curve tells you exactly where a species can and cannot live for that one factor.

The limiting factor decides the niche: An organism must be inside its tolerance range for every abiotic factor at once.

The factor that is furthest from its optimum is the limiting factor — it is the one holding the organism back, even if every other condition is fine.

This is why a species often grows faster in a controlled tank or mesocosm than in the wild: in the lab the limiting factor (say, low light or a competitor) has been removed, so conditions sit closer to its optimum.

Biomes — characteristic abiotic conditions: A biome is a large region defined by its climate — mainly its temperature and its rainfall (water availability).

A hot desert is characterised by very high daytime temperatures and very low, scarce rainfall. A tropical rainforest has high temperature and very high rainfall all year. A tundra is very cold with little available water.

The abiotic conditions of a biome decide which organisms can tolerate living there.

A memory hook: Tolerance = the range you can take; limit = where it kills you. The limiting factor is the one nearest a limit.

For a biome, lead with its two headline abiotic conditions: how hot and how wet.

How this is tested: On Paper 1A a one-mark question often gives a list of examples and asks you to select the abiotic factor — pick the non-living physical/chemical one (temperature, light, pH), not a predator or competitor.

On Paper 2 a 2-mark State question asks for abiotic factors that characterise a named biome (for example a hot desert: high temperature and low rainfall).

Data questions (a Paper-1B / Paper-2 favourite) give tolerance ranges or a graph and ask you to deduce where a species can live, or suggest why it grows faster in a controlled mesocosm than in its natural environment.

Why this scores full marks: Both answers are abiotic (non-living) and both are characteristic of the named biome.

A common slip is to name cacti or camels — those are organisms, not abiotic factors, so they would score zero on a 'state the abiotic factors' question.