16.9 Required Practicals: Adaptations, Interdependence, and Competition

A free GCSE Biology lesson for AQA (8461) from OBA School: required practical 9 – field investigations: estimating population size 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 field work
• Process field data: mean, percentage cover, population estimates and kite graphs
• Evaluate field methods 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.
• Correlation: A link between two variables. It does not prove that one causes the other.

Practical A: estimating population size

Aim: to estimate the number of buttercups in a meadow. Equipment: 1 m × 1 m quadrat, two 30 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. (17, 6).
• 3; Method Place the bottom-left corner of the quadrat at that coordinate.
• 4; Method Count the buttercups inside. Decide a rule for plants on the edge, e.g. count them 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 buttercup population

Counts in twelve 1 m² quadrats: 9, 4, 11, 7, 0, 8, 10, 6, 12, 5, 9, 9. Total = 90.
Mean = 90 ÷ 12 = 7.5 buttercups per m². The meadow is 80 m × 50 m = 4000 m².
Estimated population = 7.5 × 4000 = 30 000.

Answer: about 30 000 buttercups

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 sampled 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 or pH probe.

• 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 individual plants.
• 4; Method Measure light intensity at ground level in each quadrat. Take all readings at about the same time, 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, trampling and nutrients 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; keep away from steep banks
• 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, rivers, the shore; Risk drowning; being cut off by the tide; Precaution stay away from deep water; check tide times; work in groups
• insect stings, pollen; Risk allergic reaction; Precaution tell the teacher about allergies; carry medication
• sun, rain or cold; Risk sunburn, hypothermia; Precaution suitable clothing, sun cream and water

Errors and improvements

• quadrats not placed randomly; Effect biased sample; estimate too high or 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 when clouds pass; Effect readings not comparable; Improvement take readings quickly / at the same time; repeat and average

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 (only needed if 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: Lay out two tapes at right angles and use random numbers as coordinates. Throwing quadrats is not truly random and can be unsafe.

Question 2. The mean number of plantains in 0.5 m × 0.5 m quadrats was 3.5. The lawn is 400 m². Estimate the number of plantains on the lawn.

Answer: 5600

Working: 3.5 ÷ 0.25 = 14 per m²; 14 × 400 = 5600 plantains.

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) is the independent variable. The number of ferns is the dependent variable; quadrat size and time are controls.

Related lessons

• Previous lesson: 16.8 Adaptations in Plants
• Next lesson: 17.1 Feeding Relationships
• In this chapter: 16.1 The Importance of Communities
• In this chapter: 16.2 Organisms in Their Environment
• In this chapter: 16.3 Distribution and Abundance

Continue in the full course

This lesson is section 16.9 (ARPBH1609) 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.