7.4.4
Investigating Population Size
Quadrats
Quadrats
The population size of immobile (e.g. plants) or slow-moving organisms (e.g. limpets) can be estimated using random samples of quadrats.


Quadrat
Quadrat
- Quadrats are normally 1m2 square frame divided into 100 squares.
- Each square represents 1% of the total area of the quadrat.
- If an individual covers more than half of a square, it is counted as representing 1% of the quadrat.
- Quadrats are used to count the population size within a large area.


Sampling
Sampling
- If you wanted to investigate the size of a population, it would take hours to count every individual.
- Taking samples (a small section of the total area) allows the population size to be estimated.
- It is important that the samples accurately represent the total population size.


Random sampling
Random sampling
- Random sampling can be applied by dividing the area of interest into a grid and labelling the grid with coordinates.
- A random number generator provides a set of random coordinates.
- The quadrats can be placed using the coordinates.


Transects
Transects
- Quadrats could also be placed at regular intervals along a transect.
- A transect is a line across a habitat, usually placed using a tape measure.
- Transects are used when there is a change, or gradient, in abiotic conditions across the habitat.
- Transects are used when looking for changes in the abundance or distribution of a species across the environment (e.g. how abundance changes from open field to forest).


Line transect
Line transect
- A line transect records the species that makes contact with the tape measure at regular intervals along the transect.


Belt transect
Belt transect
- A belt transect uses quadrats.
- Quadrats are placed at regular intervals along the transect.
- An abundance of different species in the quadrat can be measured by:
- Percentage cover – estimate how much of the quadrat is covered by a particular species
- Frequency – count how many individuals of a particular species are present in the quadrat.
Mark-Release-Recapture
Mark-Release-Recapture
Mark-release-recapture is a technique used to investigate population size of mobile organisms (e.g. a tree shrew). The steps involved are:


1) Capture
1) Capture
- The first step in mark-release-recapture involves the capturing of animals.
- Small mammals can be captured using a Sherman trap (a box with a trap door) and ground insects can be captured using a pitfall trap.


2) Mark
2) Mark
- Captured animals are marked in some way (e.g. using tags, bands, paint, or other body markings).
- Marking the animals allows you to identify which animals have already been counted and which animals have not.


3) Release
3) Release
- The marked animals are released back into the environment and the traps are set up again.
- Releasing the animals back into the environment allows them to mix with the rest of the population.


4) Recapture
4) Recapture
- A new sample is collected from the traps. The unmarked individuals that are captured are counted, marked and released.


5) Calculate population size
5) Calculate population size
- Total population size can be calculated using the following equation:
- population size =
Effect of Environmental Factors
Effect of Environmental Factors
Belt transects can be used to investigate the effect of abiotic factors on species distribution. The steps involved are:


1) Marking the belt transect
1) Marking the belt transect
- A belt transect is used to investigate how population size varies across an environment.
- In this investigation, the belt transect should be marked along an area where one environmental factor changes.
- E.g. Light exposure will vary along a transect that is marked across an open field and into dense tree cover.
- E.g. Soil pH will vary along a transect in a coastal ecosystem.


2) Place the quadrat
2) Place the quadrat
- Place a 1m2 quadrat at regular intervals along the belt transect.
- Use the quadrat to identify which species are present and count the percentage cover of the different species.
- If a species covers over half of one square on the quadrat, it has 1% cover of the quadrat.


3) Measure environmental factor
3) Measure environmental factor
- Record how the environmental factor you are investigating varies at each sample point along the belt transect.
- If you are investigating light intensity, use a light meter.
- If you are investigating soil pH, use a pH probe.


4) Record results
4) Record results
- Plot a graph that shows how the distribution of species correlates to the environmental factor.
1Biological Molecules
1.1Monomers & Polymers
1.2Carbohydrates
1.3Lipids
1.4Proteins
1.4.1The Peptide Chain
1.4.2Investigating Proteins
1.4.3Primary & Secondary Protein Structure
1.4.4Tertiary & Quaternary Protein Structure
1.4.5Enzymes
1.4.6Factors Affecting Enzyme Activity
1.4.7Enzyme-Controlled Reactions
1.4.8End of Topic Test - Lipids & Proteins
1.4.9A-A* (AO3/4) - Enzymes
1.4.10A-A* (AO3/4) - Proteins
1.5Nucleic Acids
1.6ATP
1.7Water
1.8Inorganic Ions
2Cells
2.1Cell Structure
2.2Mitosis & Cancer
2.3Transport Across Cell Membrane
2.4Cell Recognition & the Immune System
3Substance Exchange
3.1Surface Area to Volume Ratio
3.2Gas Exchange
3.3Digestion & Absorption
4Genetic Information & Variation
4.1DNA, Genes & Chromosomes
4.2DNA & Protein Synthesis
4.3Mutations & Meiosis
4.4Genetic Diversity & Adaptation
4.5Species & Taxonomy
4.6Biodiversity Within a Community
4.7Investigating Diversity
5Energy Transfers (A2 only)
5.1Photosynthesis
5.1.1Overview of Photosynthesis
5.1.2Photoionisation of Chlorophyll
5.1.3Production of ATP & Reduced NADP
5.1.4Cyclic Photophosphorylation
5.1.5Light-Independent Reaction
5.1.6A-A* (AO3/4) - Photosynthesis Reactions
5.1.7Limiting Factors
5.1.8Photosynthesis Experiments
5.1.9End of Topic Test - Photosynthesis
5.1.10A-A* (AO3/4) - Photosynthesis
5.2Respiration
5.3Energy & Ecosystems
6Responding to Change (A2 only)
6.1Nervous Communication
6.2Nervous Coordination
6.3Muscle Contraction
6.4Homeostasis
6.4.1Overview of Homeostasis
6.4.2Blood Glucose Concentration
6.4.3Controlling Blood Glucose Concentration
6.4.4End of Topic Test - Blood Glucose
6.4.5Primary & Secondary Messengers
6.4.6Diabetes Mellitus
6.4.7Measuring Glucose Concentration
6.4.8Osmoregulation
6.4.9Controlling Blood Water Potential
6.4.10ADH
6.4.11End of Topic Test - Diabetes & Osmoregulation
6.4.12A-A* (AO3/4) - Homeostasis
7Genetics & Ecosystems (A2 only)
7.1Genetics
7.2Populations
7.3Evolution
7.3.1Variation
7.3.2Natural Selection & Evolution
7.3.3End of Topic Test - Populations & Evolution
7.3.4Types of Selection
7.3.5Types of Selection Summary
7.3.6Overview of Speciation
7.3.7Causes of Speciation
7.3.8Diversity
7.3.9End of Topic Test - Selection & Speciation
7.3.10A-A* (AO3/4) - Populations & Evolution
8The Control of Gene Expression (A2 only)
8.2Gene Expression
8.2.1Stem Cells
8.2.2Stem Cells in Disease
8.2.3End of Topic Test - Mutation & Gene Epression
8.2.4A-A* (AO3/4) - Mutation & Stem Cells
8.2.5Regulating Transcription
8.2.6Epigenetics
8.2.7Epigenetics & Disease
8.2.8Regulating Translation
8.2.9Experimental Data
8.2.10End of Topic Test - Transcription & Translation
8.2.11Tumours
8.2.12Correlations & Causes
8.2.13Prevention & Treatment
8.2.14End of Topic Test - Cancer
8.2.15A-A* (AO3/4) - Gene Expression & Cancer
8.3Genome Projects
Jump to other topics
1Biological Molecules
1.1Monomers & Polymers
1.2Carbohydrates
1.3Lipids
1.4Proteins
1.4.1The Peptide Chain
1.4.2Investigating Proteins
1.4.3Primary & Secondary Protein Structure
1.4.4Tertiary & Quaternary Protein Structure
1.4.5Enzymes
1.4.6Factors Affecting Enzyme Activity
1.4.7Enzyme-Controlled Reactions
1.4.8End of Topic Test - Lipids & Proteins
1.4.9A-A* (AO3/4) - Enzymes
1.4.10A-A* (AO3/4) - Proteins
1.5Nucleic Acids
1.6ATP
1.7Water
1.8Inorganic Ions
2Cells
2.1Cell Structure
2.2Mitosis & Cancer
2.3Transport Across Cell Membrane
2.4Cell Recognition & the Immune System
3Substance Exchange
3.1Surface Area to Volume Ratio
3.2Gas Exchange
3.3Digestion & Absorption
4Genetic Information & Variation
4.1DNA, Genes & Chromosomes
4.2DNA & Protein Synthesis
4.3Mutations & Meiosis
4.4Genetic Diversity & Adaptation
4.5Species & Taxonomy
4.6Biodiversity Within a Community
4.7Investigating Diversity
5Energy Transfers (A2 only)
5.1Photosynthesis
5.1.1Overview of Photosynthesis
5.1.2Photoionisation of Chlorophyll
5.1.3Production of ATP & Reduced NADP
5.1.4Cyclic Photophosphorylation
5.1.5Light-Independent Reaction
5.1.6A-A* (AO3/4) - Photosynthesis Reactions
5.1.7Limiting Factors
5.1.8Photosynthesis Experiments
5.1.9End of Topic Test - Photosynthesis
5.1.10A-A* (AO3/4) - Photosynthesis
5.2Respiration
5.3Energy & Ecosystems
6Responding to Change (A2 only)
6.1Nervous Communication
6.2Nervous Coordination
6.3Muscle Contraction
6.4Homeostasis
6.4.1Overview of Homeostasis
6.4.2Blood Glucose Concentration
6.4.3Controlling Blood Glucose Concentration
6.4.4End of Topic Test - Blood Glucose
6.4.5Primary & Secondary Messengers
6.4.6Diabetes Mellitus
6.4.7Measuring Glucose Concentration
6.4.8Osmoregulation
6.4.9Controlling Blood Water Potential
6.4.10ADH
6.4.11End of Topic Test - Diabetes & Osmoregulation
6.4.12A-A* (AO3/4) - Homeostasis
7Genetics & Ecosystems (A2 only)
7.1Genetics
7.2Populations
7.3Evolution
7.3.1Variation
7.3.2Natural Selection & Evolution
7.3.3End of Topic Test - Populations & Evolution
7.3.4Types of Selection
7.3.5Types of Selection Summary
7.3.6Overview of Speciation
7.3.7Causes of Speciation
7.3.8Diversity
7.3.9End of Topic Test - Selection & Speciation
7.3.10A-A* (AO3/4) - Populations & Evolution
8The Control of Gene Expression (A2 only)
8.2Gene Expression
8.2.1Stem Cells
8.2.2Stem Cells in Disease
8.2.3End of Topic Test - Mutation & Gene Epression
8.2.4A-A* (AO3/4) - Mutation & Stem Cells
8.2.5Regulating Transcription
8.2.6Epigenetics
8.2.7Epigenetics & Disease
8.2.8Regulating Translation
8.2.9Experimental Data
8.2.10End of Topic Test - Transcription & Translation
8.2.11Tumours
8.2.12Correlations & Causes
8.2.13Prevention & Treatment
8.2.14End of Topic Test - Cancer
8.2.15A-A* (AO3/4) - Gene Expression & Cancer
8.3Genome Projects

Unlock your full potential with GoStudent tutoring
Affordable 1:1 tutoring from the comfort of your home
Tutors are matched to your specific learning needs
30+ school subjects covered