16.2 Organisms in Their Environment

A free GCSE Biology lesson for AQA (8461) from OBA School: abiotic (non-living) and biotic (living) factors and how they affect communities. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• List the abiotic factors that affect communities and explain their effects
• List the biotic factors that affect communities and explain their effects
• Read data and graphs that link a factor to the number of organisms
• Explain how a change in one factor can affect a whole community

Key words

• Abiotic factor: A non-living part of the environment, such as temperature, light or soil pH.
• Biotic factor: A living part of the environment, such as food supply, predators or pathogens.
• Distribution: Where organisms are found in a habitat.
• Abundance: How many organisms of a species there are in an area.
• Pathogen: A microorganism that causes disease, such as a bacterium, virus or fungus.
• Outcompete: When one species is better at getting a resource than another, so the other species declines.

Abiotic factors

Abiotic factors are the non-living conditions in a habitat. A change in any of them can change which species can live there and how many of each.

• light intensity; Why it matters Plants need light for photosynthesis. Low light under a dense canopy means few plants on the ground.
• temperature; Why it matters Affects enzymes, so it affects the rate of photosynthesis, growth and survival.
• moisture levels; Why it matters All organisms need water. Some species need damp soil; others survive in dry places.
• soil pH and mineral content; Why it matters Affects which plants can grow. Low mineral ions (e.g. nitrate) slow plant growth.
• wind intensity and direction; Why it matters Strong wind increases water loss from plants and can stunt or bend trees.
• carbon dioxide level; Why it matters Plants need CO₂ for photosynthesis.
• oxygen level in water; Why it matters Aquatic animals need dissolved oxygen for aerobic respiration. Some, like mayfly nymphs, need a lot.

Key fact: AQA lists seven abiotic factors: light intensity, temperature, moisture, soil pH and mineral content, wind intensity and direction, carbon dioxide (for plants) and oxygen (for aquatic animals).

Reading data on abiotic factors

Ecologists often measure an abiotic factor and count organisms at the same time. A graph then shows whether the two are linked.

Worked example: describe and explain the graph

Describe: as dissolved oxygen increases, the number of shrimps increases. There are none at 2 mg/dm³ and 30 at 10 mg/dm³.
Explain: shrimps need oxygen for aerobic respiration to release energy. Where there is little oxygen, few can survive.
Calculate: from 6 to 10 mg/dm³ the number rises from 11 to 30, an increase of 19.

Answer: positive correlation; oxygen is needed for respiration

Exam tip: “Describe” means say what the pattern is and quote numbers from the data. “Explain” means give the biological reason.

Biotic factors

Biotic factors are caused by other living organisms. AQA lists four:

• availability of food; Example of its effect A poor acorn crop means fewer young squirrels and mice survive the winter.
• new predators arriving; Example of its effect American mink escaped from fur farms and caused big falls in UK water vole numbers.
• new pathogens; Example of its effect Ash dieback, a fungal disease, has killed many ash trees in UK woodlands.
• one species outcompeting another; Example of its effect The losing species may fall to numbers too small to breed successfully.

Because species are interdependent, a biotic change rarely affects just one species. If ash trees die, the insects and lichens that live on them lose their habitat, and more light reaches the woodland floor.

Explaining a change in a community

Worked example: a cleared hedge

A farmer cuts down a tall hedge along the edge of a field. Suggest how this changes the abiotic factors and the community.
Abiotic: more wind reaches the field and the soil dries out faster; there is more light at ground level.
Biotic: birds lose nesting sites and food (berries, insects), so their numbers fall. Light-loving plants may grow where the hedge was.

Answer: link each factor to a named effect on living things

Higher-scoring answers link a cause to an effect and then to a further effect: “less oxygen → less aerobic respiration → fewer insects → less food for fish”.

Common mistakes

• Calling a predator or pathogen “abiotic”. If it is alive, it is biotic.
• Saying “weather” or “the environment”. Name the exact factor: temperature, light intensity, wind…
• Describing a graph with no numbers. Quote values from the data.
• Forgetting that oxygen in water matters for aquatic animals, and carbon dioxide for plants.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is a biotic factor? (Choose from: Soil pH, Wind direction, A new pathogen, Light intensity.)

Answer: A new pathogen

Working: A pathogen is a living microorganism, so it is a biotic factor. Soil pH, wind direction and light intensity are abiotic (non-living).

Question 2. True or false? A new predator arriving in a habitat is an abiotic factor.

Answer: False

Working: False. A predator is a living organism, so it is a biotic factor.

Question 3. After a drought, the number of freshwater shrimps in a river sample fell from 40 to 14. Calculate the percentage decrease.

Answer: 65 %

Working: (40 − 14) ÷ 40 × 100 = 26 ÷ 40 × 100 = 65%.

Related lessons

• Previous lesson: 16.1 The Importance of Communities
• Next lesson: 16.3 Distribution and Abundance
• In this chapter: 16.4 Competition in Animals
• In this chapter: 16.5 Competition in Plants
• In this chapter: 16.6 Adapt and Survive

Continue in the full course

This lesson is section 16.2 (AGCBH1602) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.