top of page

B15.2 Organisms in Their Environment

GCSE Combined Science · AQA · Chapter B15: Adaptations, Interdependence, and Competition

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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
  • Explain how invasive species can damage a community
  • Read data and graphs that link a factor to the number or distribution 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.
  • Pathogen: A microorganism that causes disease, such as a bacterium, virus or fungus.
  • Invasive species: A species that is not native to an area and harms the organisms already living there.
  • 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 (physical and chemical) 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. Near the poles, light is too weak for trees beyond the tree line. Shade plants often have large, dark green leaves full of chlorophyll.
  • temperature; Why it matters Affects enzymes, so it affects photosynthesis, growth and survival of plants and animals.
  • moisture levels; Why it matters All organisms need water. Cacti survive very dry deserts but die if over-watered; mosses need damp places.
  • soil pH and mineral content; Why it matters Some plants grow only in acidic soil, others only in alkaline soil. Low mineral ions (e.g. nitrate) slow plant growth. Farmers add lime to make acidic soil less acidic.
  • wind intensity and direction; Why it matters Strong wind increases water loss from plants and makes trees on exposed coasts grow bent and stunted.
  • carbon dioxide level; Why it matters Plants need CO₂ for photosynthesis, so it can limit plant growth.
  • oxygen level in water; Why it matters Aquatic animals need dissolved oxygen for aerobic respiration. Some, like stonefly nymphs, need a lot.

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 nymphs increases: none at 1 mg/dm³ but 28 at 11 mg/dm³.
Explain: nymphs need oxygen for aerobic respiration to release energy. Where there is little oxygen, few can survive.
Calculate: from 5 to 9 mg/dm³ the number rises from 7 to 24, an increase of 17.

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:

  • availability of food; Example of its effect A poor beech-nut year means fewer young mice and squirrels 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, is killing 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.

An invasive species often does well because it has no natural predators or diseases in its new home. Because species are interdependent, one change rarely affects only one species: if ash trees die, the insects and lichens that live on them lose their habitat, and more light reaches the woodland floor. This is why bringing plants and animals into a country is controlled by law.

Common mistakes

  • Calling a predator or pathogen “abiotic”. If it is (or was) alive, it is biotic; physical and chemical conditions are abiotic.
  • Saying “the weather” or “the environment”. Name the exact factor: temperature, light intensity, wind…
  • Describing a graph with no numbers. Quote values from the data.
  • Saying a correlation proves cause. Other factors may also be changing.

Revision questions

Try these, then check your answers below each one.

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

Answer: A new disease-causing pathogen

Working: A pathogen is a living organism, so it is biotic. The others are abiotic (non-living) factors.

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. The bar chart shows heather cover and soil pH in five fields. Which conclusion is best supported by the data? (Choose from: Heather grows best in acidic soil, Heather grows best in alkaline soil, Soil pH has no effect on heather, Heather makes the soil acidic.)

Answer: Heather grows best in acidic soil

Working: Heather cover is highest (72%) at pH 4.0 and lowest (2%) at pH 7.5, so it grows best in acidic soil. The data show a correlation; they do not prove what causes what.

Related lessons

Continue in the full course

This lesson is section B15.2 (AGCSC1502) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

bottom of page