7.1.2 Abiotic Factors · 7. Ecology | Aqa GCSE Combined Science Trilogy 8464: Biology

Section 1 of 9What are abiotic factors?

Table of abiotic factors and how they affect a community

Abiotic factors are the non-living features of the environment that can affect organisms and communities. They help determine which species can live in an area and how many of them there are.

Section 2 of 9The main abiotic factors

Factor	Effect on organisms
Light intensity	Limits the rate of photosynthesis in plants, so affects growth and the food available to herbivores
Temperature	Affects enzyme activity, so growth and reproduction; extremes may kill organisms
Moisture level	Plants need water for photosynthesis and support; animals need water to survive. Dry places hold fewer species
Soil pH and mineral content	Affects which plants can grow; for example nitrate shortage limits growth
Carbon dioxide (plants) and oxygen (animals) levels	Limit photosynthesis and respiration respectively; low oxygen in polluted water stops fish surviving
Wind intensity and direction	Increases water loss by transpiration, damages plants, affects insect flight

Section 3 of 9Examples in nature

In a woodland, ground plants such as bluebells grow and flower in early spring before the tree canopy leaves block the light.

Examples in nature

On a rocky shore, organisms higher up face longer periods without water and greater changes in temperature than those lower down.

In a pond, the oxygen concentration falls when there is pollution that increases the decomposers, and only species tolerant of low oxygen, such as bloodworms, survive.

Section 4 of 9Measuring abiotic factors

Table of abiotic factors and how each is measured

Light

Light light meter.

Temperature

Temperature thermometer or temperature probe.

Soil moisture

Soil moisture moisture meter, or by mass loss when a soil sample is dried.

pH

pH pH probe or indicator paper.

Oxygen in water

Oxygen in water oxygen probe or data logger.

Taking several readings at each location and finding the mean improves reliability. Data loggers can record continuously over a day or season.

Section 5 of 9Interpreting data

To analyse the effect of an abiotic factor, compare the abundance of a species at sites with different values of the factor. A transect that shows the number of organisms falling as light decreases suggests (but does not prove) that light limits the species. Other factors might vary at the same time, so conclusions should be cautious.

Section 6 of 9Abiotic factors in a woodland and a rocky shore

It is useful to know a few detailed examples.

1

In a deciduous woodland the light reaching the ground changes through the year.

2

In winter the trees are bare, so enough light reaches the floor, but little photosynthesis is possible because it is cold.

3

In early spring, the temperature rises while the canopy is still open, and plants such as bluebells and wood anemone grow and flower rapidly.

4

By summer the full canopy blocks most of the light, so low-growing species are shade-tolerant, such as ferns, and the ground is dominated by plants with large, thin leaves that absorb as much of the weak light as possible.

5

Soil moisture and mineral content are also higher near the surface where leaves decay.

On a rocky shore, the abiotic factors change from the top of the beach to the bottom.

1

At the upper level organisms are covered by water only at the highest tides, so they must withstand drying out, temperature extremes and changes in salinity after rain.

2

Species such as small periwinkles and some lichens live here.

3

Lower down, organisms are submerged for longer, so conditions are more stable, and species such as kelp and sea anemones are common.

4

The result is that the species change in distinct bands, a pattern called zonation, which is investigated using a belt transect.

Abiotic factors often interact.

Abiotic factors in a woodland and a rocky shore

Water availability and temperature together affect how much a plant loses by transpiration.

Wind increases water loss and cools the leaf.

When explaining effects, name the factor, describe the change, and then state the effect on a process in the organism: for example less light means a slower rate of photosynthesis, less glucose, less growth and fewer plants, which means less food for herbivores.

Gathering reliable data requires measurements to be made at the same time of day, using the same instrument at each site and taking repeats.

Some organisms are used as indicators of abiotic conditions. For example, lichens show sulfur dioxide levels, and some aquatic invertebrates show how much oxygen is dissolved in water, so their distribution gives information about the abiotic environment without expensive instruments.

Section 7 of 9Exam-style questions

Pitfall trap and line transect sampling methods

Name three abiotic factors that could affect the distribution of plants in a woodland. [3]3 marks
Describe how you could investigate the effect of light intensity on the abundance of daisies on a lawn. [4]4 marks
The mean soil moisture was 24% at site A and 12% at site B. Calculate how many times greater the moisture was at A, and suggest an effect on plants. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Abiotic factor	A non-living factor of the environment.
Light intensity	The brightness of light, affecting photosynthesis.
Distribution	Where organisms are found in an area.
Abundance	How many individuals of a species there are.
Data logger	An electronic device that records readings automatically.
Transect	A line along which sampling is carried out.

Section 9 of 9Exam technique

1

Listing biotic factors such as predation as abiotic. Abiotic means non-living.

2

Forgetting to explain the mechanismsay why light or temperature changes the organism.

3

Claiming that correlation proves cause when data show a species is fewer in a dark area.

4

Measuring at only one point; repeat readings are required.