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B15 RP Required Practical: Field Investigations

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

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: required practical – measuring the population size of a common species with quadrats, and investigating distribution with a transect. 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

  • Estimate the population size of a common species using random quadrats
  • Use a transect to investigate the effect of an abiotic factor on the distribution of a species
  • Identify the variables, hazards and precautions in fieldwork
  • Process field data: mean, percentage cover, population estimates, running means and kite graphs
  • Evaluate field methods, including resolution and sources of error, and suggest improvements

Key words

  • Random sampling: Placing quadrats at coordinates chosen by chance, so that the sample is not biased.
  • Represent- ative: A sample that is typical of the whole area.
  • Percentage cover: The percentage of a quadrat’s area covered by one species.
  • Running mean: The mean recalculated after each extra quadrat; when it levels off, the sample is big enough.
  • Kite graph: A graph showing how the abundance of several species changes along a transect.
  • Resolution: The smallest change a measuring instrument can show, e.g. 10 lux on a light meter.

Practical A: estimating population size

Aim: to estimate the number of daisies on a school field. Equipment: 1 m × 1 m quadrat, two 50 m tape measures, random number generator (calculator or app), clipboard and tally chart, identification key.

  • 1; Method Lay two tape measures at right angles along two edges of the area to make axes.
  • 2; Method Generate two random numbers to give a coordinate, e.g. (23, 11).
  • 3; Method Place the bottom-left corner of the quadrat at that coordinate.
  • 4; Method Count the daisies inside. Use a rule for plants on the edge, e.g. count them only if more than half is inside.
  • 5; Method Repeat for at least 10 quadrats – keep going until the running mean levels off.
  • 6; Method Calculate the mean per quadrat, then per m², and multiply by the total area.

Worked example: estimating the daisy population

Counts in twelve 1 m² quadrats: 6, 3, 9, 5, 0, 7, 8, 4, 10, 6, 7, 7. Total = 72.
Mean = 72 ÷ 12 = 6 daisies per m². The field is 60 m × 40 m = 2400 m².
Estimated population = 6 × 2400 = 14 400.

Answer: about 14 400 daisies

Variables: there is no independent variable – this is an estimate. The dependent variable is the number of plants per quadrat. Control: same quadrat size, same counting rule, same person identifying the species, all quadrats on the same day.

Practical B: distribution along a transect

Aim: to investigate how light intensity affects the distribution of plants from a hedge into a field. Extra equipment: 0.5 m × 0.5 m quadrat divided into 100 squares, light meter, soil moisture meter.

  • 1; Method Run a tape measure from the base of the hedge out into the field, at right angles to the hedge.
  • 2; Method Place a quadrat at regular intervals, e.g. every 2 m (a belt transect).
  • 3; Method In each quadrat, record the percentage cover of each species (count squares more than half covered) or count the plants.
  • 4; Method Measure the light intensity at ground level in each quadrat, at about the same time and in the same weather.
  • 5; Method Repeat the transect in two or three other places along the hedge and calculate means.
  • independent; In this investigation distance from the hedge (which changes the light intensity)
  • dependent; In this investigation percentage cover or number of each plant species
  • control; In this investigation quadrat size; same intervals; same time of day and weather for light readings; same person judging cover

Exam tip: Measuring light alongside the counts lets you look for a correlation. But soil moisture, nutrients and trampling also change along a transect, so you cannot be sure light is the only cause.

Risk assessment

  • uneven ground, holes, slopes; Risk trips and falls; Precaution wear sturdy shoes; walk, do not run
  • nettles, thorns, brambles; Risk stings and scratches; Precaution wear gloves and long sleeves
  • soil and animal droppings; Risk infection; Precaution cover cuts; wash hands after fieldwork; do not eat while working
  • ponds, streams, the shore; Risk drowning; being cut off by the tide; Precaution keep away from deep water and steep banks; check tide times; work in groups
  • insect stings, pollen; Risk allergic reaction; Precaution tell the teacher about allergies; carry medication

Errors, resolution and improvements

  • quadrats not placed randomly; Effect biased sample; estimate too high or too low; Improvement use random coordinates
  • too few quadrats; Effect not representative; Improvement use more; check the running mean has levelled off
  • plants hard to tell apart; Effect miscounting; Improvement use an identification key; same person counts
  • estimating percentage cover by eye; Effect different people give different values; Improvement use a gridded quadrat and count squares
  • light changes as clouds pass; Effect readings not comparable (random error); Improvement take readings quickly at the same time; repeat and average

The resolution of an instrument is the smallest change it can show: a light meter that reads to the nearest 10 lux has a resolution of 10 lux. Repeating readings and calculating a mean reduces the effect of random errors; results are precise when repeat readings are close together.

Common mistakes

  • Forgetting that there is no independent variable when estimating a population size.
  • Not dividing by the quadrat area before multiplying by the total area (needed whenever the quadrat is not 1 m²).
  • Saying “light causes the change” when the data show only a correlation.
  • Writing a hazard with no precaution, or a vague precaution such as “be careful”.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the best way to decide where to place quadrats when estimating a population? (Choose from: Use a random number generator to choose coordinates, Place them where the plants are thickest, Throw them over your shoulder, Place them all along one edge of the field.)

Answer: Use a random number generator to choose coordinates

Working: Random coordinates on a grid made with two tapes give an unbiased sample. Throwing quadrats is not truly random and can be unsafe.

Question 2. The mean number of dandelions in 0.5 m × 0.5 m quadrats was 4.2. The lawn has an area of 600 m². Estimate the number of dandelions on the lawn.

Answer: 10080

Working: 4.2 ÷ 0.25 = 16.8 per m²; 16.8 × 600 = 10 080 dandelions (about 10 000).

Question 3. A student uses a transect to investigate how light intensity affects the distribution of ferns in a wood. What is the independent variable? (Choose from: Light intensity, The number of ferns, The size of the quadrat, The time of day.)

Answer: Light intensity

Working: Light intensity (which changes with distance along the transect). The fern count or cover is the dependent variable.

Related lessons

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This lesson is section B15 RP (AGCSC1509) 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.

 

 

 

 

 

 

 

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