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Biotic and Abiotic Factors in Ecosystems — Answers

Answer sheet with model answers and success criteria for the Year 8 ecosystems & food chains worksheet.

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Success TutoringWorksheetsAnswer sheetYear 8 · Science · Ecosystems & Food Chains

Biotic and Abiotic Factors in Ecosystems — Answers

Part A: Living or non-living?

Circle biotic or abiotic.

An ecosystem is made up of all the organisms in an area together with their non-living surroundings.

Biotic factors are the living parts of an ecosystem and their effects: food sources, predators, competitors, parasites and disease. Abiotic factors are the non-living parts: light, temperature, water, wind, soil type, soil pH and saltiness.

Both kinds of factor affect where an organism can live (its distribution) and how many there are (its population size).

1.The amount of sunlight reaching the forest floor is a(n) … factor.
  1. Abiotic
  2. Babiotic

Answer: abiotic

Sunlight is non-living, so it is abiotic.

2.Rabbits eating the grass in a field is a(n) … factor.
  1. Abiotic
  2. Babiotic

Answer: biotic

Rabbits are living things, so this is a biotic factor affecting the grass.

3.The saltiness of the water in a river mouth is a(n) … factor.
  1. Abiotic
  2. Babiotic

Answer: abiotic

Salt concentration is a non-living, physical or chemical condition.

4.A disease spreading through a frog population is a(n) … factor.
  1. Abiotic
  2. Babiotic

Answer: biotic

Diseases are caused by living microorganisms, so they are biotic.

5.Soil temperature is a(n) … factor.
  1. Abiotic
  2. Babiotic

Answer: abiotic

Temperature is a non-living condition.

6.All the living things and non-living surroundings in an area, interacting together, make up an (answer space).

Answer: ecosystem

Part B: Measuring the environment

7.Match each abiotic factor to the instrument used to measure it.
  • light intensity
  • air or soil temperature
  • soil pH
  • soil moisture
  • wind speed
  • pH test kit or pH meter
  • anemometer
  • moisture meter
  • thermometer or temperature probe
  • light meter

Answer: light intensity → light meter; air or soil temperature → thermometer or temperature probe; soil pH → pH test kit or pH meter; soil moisture → moisture meter; wind speed → anemometer

8.Moss grows thickly on the shady side of a wall but hardly at all on the sunny side. Suggest one abiotic factor that could explain this.

Answer: Any one of: less light, cooler temperature or more moisture on the shady side. (Moss needs damp conditions and dries out in strong sun.)

9.A new predator arriving in an ecosystem is an example of a change in an abiotic factor.

Answer: False

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

Part C: Sampling with quadrats

Show your working.

It is usually impossible to count every plant in a large area, so ecologists take samples. A quadrat is a square frame, often 1 m × 1 m, placed on the ground. The organisms inside it are counted.

Estimated population = mean number per quadrat × (total area ÷ area of one quadrat).

Field study: A class wanted to estimate how many clover plants grow in a grassy field with an area of 200 m². They placed a 1 m × 1 m quadrat at five random spots and counted the clover plants inside each one.

Quadrat 1: 4 · Quadrat 2: 6 · Quadrat 3: 3 · Quadrat 4: 7 · Quadrat 5: 5

10.What is the mean number of clover plants per quadrat?

Answer: 5 plants per m²

  1. 4 + 6 + 3 + 7 + 5 = 25
  2. 25 ÷ 5 = 5 per quadrat (5 per m²)
11.Estimate the total number of clover plants in the 200 m² field.

Answer: About 1000 plants

  1. 5 per m² × 200 m² = 1000
12.Why were the quadrats placed randomly?
  1. Ato save time
  2. Bso the class did not choose only the areas with the most (or fewest) plants, which would bias the results
  3. Cbecause random places have more clover
  4. Dso they could skip counting

Answer: so the class did not choose only the areas with the most (or fewest) plants, which would bias the results

Random placement avoids bias, so the sample better represents the whole field.

13.Using more quadrats would give a more reliable estimate.

Answer: True

A larger sample reduces the effect of any one unusual quadrat.

14.The class wants to find out whether the number of clover plants changes from a shady area under trees to an open sunny area. Describe how they could use quadrats and instruments to investigate this, and explain how they would make their results reliable.

Model answer: They could stretch a long tape measure (a transect) from the shady area under the trees out into the open sunny area. Every 2 m along the tape they would place a quadrat and count the clover plants inside it. At each quadrat they would also measure light intensity with a light meter and soil moisture with a moisture meter, so they can see which abiotic factors change. To make the results reliable they would repeat the transect in at least two more places, use the same size quadrat each time, count carefully in the same way, and calculate the average number at each distance before looking for a pattern.

Success criteria:

  • Uses quadrats placed along a line from shade to sun
  • Measures at least one relevant abiotic factor with a suitable instrument
  • Repeats sampling / averages for reliability
  • Keeps quadrat size and method the same
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