Abiotic factors: light intensity

The idea

Light intensity is the amount of light energy reaching a habitat per unit area per unit time, and it is a key abiotic factor because plants and algae need light for photosynthesis. In a woodland, the canopy receives the most light while the forest floor receives least, so plant distribution changes with distance from the canopy. Light intensity also affects animal behaviour, such as when some animals are active. It interacts with other abiotic factors, including temperature and water availability, so conclusions should consider more than one factor.

What earns the marks

• Light intensity is the amount of light energy per unit area per unit time reaching a habitat, commonly measured in lux with a light meter.
• Light is required for photosynthesis, so light intensity affects plant growth, biomass and the distribution of producers.
• In a woodland, light intensity decreases from the canopy to the forest floor, so shade-tolerant plants are found lower down while light-demanding plants grow higher up.
• Light intensity can affect animal behaviour and distribution indirectly, for example by influencing where plants grow or when animals are active.
• When investigating light intensity, readings should be taken at the same time of day and repeated to reduce the effect of changing conditions.
• Light intensity is one abiotic factor and should be considered alongside temperature, moisture and mineral availability when explaining distribution.

Common mistakes and how to put them right

• Thinking light intensity is the same as light quality or day length.

  Put it right: Light intensity is the amount of light energy per unit area per unit time, measured in lux.

• Assuming more light always increases plant growth without limit.

  Put it right: Other factors such as temperature, carbon dioxide concentration and water can become limiting.

• Recording a single light meter reading as representative of a whole habitat.

  Put it right: Take several readings at different points and calculate a mean to account for variation.

Exam tips

• Link light intensity to photosynthesis and then to plant distribution, rather than describing light intensity in isolation.
• When describing a method, state the instrument, the unit and how you would reduce random error, such as repeating readings and calculating a mean.
• Use comparative language such as higher light intensity, lower light intensity and greatest abundance to make your explanation precise.