Field Investigations (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Investigating Ecosystems

• Ecology is the branch of biology that studies the distribution and abundance of species, the interactions between species, and the interactions between species and their abiotic environment

• Ecologists are biologists that study these interactions by investigating ecosystems

• You need to know how ecologists determine the distribution and abundance of species in an ecosystem using quadrats and transects

• You need to understand how they use quadrats and transects in order to do this

Quadrats

• Quadrats are square frames made of wood or wire

• They can be a variety of sizes eg. 0.25m² or 1m²

• They are placed on the ground and the organisms within them are recorded

• They can be used to measure the abundance of plants or slow-moving animals

Using a quadrat to investigate population size or distribution

• Quadrats can be used to measure abundance by recording:

  • The number of an individual species: the total number of individuals of a single species (eg. buttercups) is recorded

  • Species richness: the total number of different species (but not the number of individuals of each species) is recorded

  • Percentage cover: the approximate percentage of the quadrat area in which an individual species is found is recorded (this method is used when it is difficult to count individuals of the plant species being recorded eg. grass or moss

How to estimate percentage cover of one or more species using a quadrat

Transects

• A transect is used to measure how the abundance of a species changes as you move along a changing habitat, eg. changing altitude on a hillside, or from open field to dense woodland, or along the shoreline at the coast

  • A line called a transect is set up through the changing habitat you want to investigate with a tape measure

  • A quadrat is then placed at regular intervals (eg. every 5 metres) along the transect

  • At each interval, the quadrat can be used to record the number of an individual species, species richness or percentage cover

• Transects are useful for investigating how the abundance of a species is affected by an abiotic factor (e.g. light intensity, moisture levels, soil depth, altitude etc)

An example of a transect on a hillside. A quadrat is placed at regular intervals (every 10m of altitude gained) and counts the number of individuals (of the species being investigated eg. buttercups) in each quadrat

Abundance of Organisms

• When studying the abundance of organisms, you should be able to:

  • Understand the terms mean, mode and median

  • Calculate arithmetic means

  • Plot and draw appropriate graphs selecting appropriate scales for the axes