Levels of Organisation (AQA GCSE Biology): Revision Note
Exam code: 8461
Producers
Photosynthetic organisms are the producers of biomass for life on Earth
Examples of photosynthetic organisms include:
plants
algae
Producers are organisms that transfer light energy from the sun in order to make their own biological molecules, such as glucose
The biological molecules are used to build biomass, which can then be passed to other organisms during feeding or decomposition
Food chains
A food chain shows the transfer of energy from one organism to the next within feeding relationships
The levels in a food chain are:
producers: organisms that transfer energy, e.g. light energy from the sun, into chemical energy
primary consumers: organisms that consume producers
secondary consumers: organisms that consume primary consumers
tertiary consumers: organisms that consume secondary consumers
For the food chain:
grass → mouse → owl
Grass:
Producer
Makes its own food using energy from sunlight in photosynthesis
Mouse:
Primary consumer
Eats the producer
Owl:
Secondary consumer
Eats the primary consumer
Investigating ecosystems
Ecology is the branch of biology that studies the:
distribution and abundance of species
interactions between species
interactions between species and their abiotic environment
Ecologists are biologists that study these interactions by investigating ecosystems
Ecologists can determine the distribution and abundance of species in an ecosystem using quadrats and transects
Quadrats
Quadrats are square frames with a fixed area, e.g. 0.25 m2 or 1 m2
They are placed on the ground and the organisms within them are recorded
Quadrats should be placed at random so the sample is representative and avoids bias: mark out the area as a grid and use a random number generator (or random number table) to pick coordinates for each quadrat – do not throw the quadrat
Use a large number of quadrats (e.g. 10 or more) to get a more accurate mean
They can be used to measure the abundance of plants or slow-moving animals
Quadrats can be used to measure abundance by recording:
the number of individuals of a particular species
species richness: the total number of different species
percentage cover: the approximate percentage of the quadrat area in which an individual species is found
This method is used when it is difficult to count individuals of the species being recorded, e.g. grass or moss
Worked Example
Determine the percentage cover of moss in the quadrat below.
Examiner Tips and Tricks
When estimating percentage cover, you can count a square (and include it in your calculations) if more than half of it is covered by the species.
Transects
A transect is a line used to measure how the abundance of a species changes with an environmental gradient, e.g. changing altitude on a hillside, or from open field to dense woodland, or along the shoreline at the coast
A transect can be set up as follows:
A tape measure is set up through the changing habitat to be investigated
The distribution of species along the transect can be assessed in a range of ways. e.g.:
the species touching the line at set intervals are recorded
quadrats are placed at regular intervals (e.g. every 5 metres) along the transect and the species within recorded
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
Abundance of organisms
Worked Example
A student investigated whether buttercups were more abundant in an open field or in woodland. In each habitat, the student randomly placed 10 quadrats and recorded the number of buttercups in each quadrat.
Their results table is shown below.
Quadrat number | Open field | Woodland |
|---|---|---|
1 | 5 | 3 |
2 | 5 | 2 |
3 | 7 | 4 |
4 | 1 | 5 |
5 | 2 | 1 |
6 | 5 | 2 |
7 | 3 | 5 |
8 | 4 | 2 |
9 | 7 | 3 |
10 | 6 | 0 |
Calculate the mean, median and mode buttercup abundance in each habitat.
Answer:
Step one: calculate the mean
Find the total number of buttercups in each habitat (add the quadrat findings together).
open field: 5 + 5 + 7 + 1 + 2 + 5 + 3 + 4 + 7 + 6 = 45
woodland: 3 + 2 + 4 + 5 + 1 + 2 + 5 + 2 + 3 + 0 = 27
Divide each total by the number of quadrats (in this example there are 10 quadrats). This value is the arithmetic mean.
open field: 45 ÷ 10 = 4.5
woodland: 27 ÷ 10 = 2.7
Step two: calculate the median
For each habitat, place the set of numbers in increasing order of size.
open field: 1, 2, 3, 4, 5, 5, 5, 6, 7, 7
woodland: 0, 1, 2, 2, 2, 3, 3, 4, 5, 5
The median is the number that is in the middle. If you have an even number of values, calculate the average of the two middle numbers by adding them together and dividing by two.
open field: (5 + 5) ÷ 2 = 5
woodland: (2 + 3) ÷ 2 = 2.5
Step three: determine the mode
The mode is the value that occurs most often.
open field = 5, as this value was recorded 3 times
woodland = 2, as this value was recorded 3 times
Worked Example
A student investigated how the abundance of buttercups changed with increasing altitude. They set up a transect on the side of a hill and placed a quadrat every 10 metres of altitude gained. They then recorded the number of buttercups found in each quadrat.
Their results table is shown below.
Altitude (m) | Number of Buttercups Recorded in Quadrat |
|---|---|
0 | 84 |
10 | 66 |
20 | 62 |
30 | 45 |
40 | 30 |
50 | 30 |
60 | 13 |
70 | 0 |
Draw a graph to show how the abundance of buttercups changes with increasing altitude. Draw a line of best fit.
Answer:
Ensure that your graph has the following features:
Dependent variable on the y axis and independent variable on the x axis
Labelled axes with units and an even scale
Graph takes up at least half of the available space
Accurately plotted data points
A line that matches the requirement of the question
If the question does not specify a line type, then join the points if appropriate
This should not be done for a scatter graph
If the question asks for a line of best fit, do not join the points
If the data form a straight line then the line of best fit should be straight
If the data points form a curve then the line of best fit should be curved
A line of best fit should have a roughly even number of points above and below it, and should not extend beyond the points
Examiner Tips and Tricks
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
Feeding relationships
Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers who are themselves eaten by tertiary consumers
Consumers that kill and eat other animals are predators, and those eaten are prey
In a stable community the numbers of predators and prey rise and fall in cycles
Predator-prey cycles are always out-of-phase with each other as it takes time for one population to respond to a change in the other population.
For example, the peak in the Canadian lynx population occurs after the peak in the snowshoe hare population, as it takes time for the lynx to reproduce and for their numbers to increase.
The graph above demonstrates some of the key patterns of predator-prey cycles:
The number of predators increases as there is more prey available
The number of prey then decreases as there are now more predators
The number of predators decreases as there is now less prey available
The number of prey increases as there are now fewer predators
The cycle now repeats
Examiner Tips and Tricks
You should be able to interpret graphs used to model predator-prey cycles
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