Levels of Organisation (AQA GCSE Biology): Revision Note
Exam code: 8461
Producers
Photosynthetic organisms are the producers of biomass for life on Earth
They produce their own food using energy from the Sun
A producer has the following characteristics:
They are at the start of every food chain (the first trophic level, which is always the biggest)
They can photosynthesise (producers are normally green plants or algae)
They make glucose by photosynthesis
They use this glucose to produce other biological molecules, which then make up the producer’s biomass (some of the glucose produced is also used in respiration to release energy for the cell)
In extreme environments (such as underwater volcanic vents) the producers are not photoautotrophs but chemoautotrophs (who produce organic molecules without using energy from the Sun)
Food chains
A simple way to show the feeding interactions between the organisms in a community is with a food chain
A food chain shows the transfer of energy from one organism to the next
The source of all energy in a food chain is light energy from the Sun
The arrows in a food chain show the transfer of energy from one level of the food chain to the next
Levels in a food chain
Producer: food chains always begin with a producer
Primary consumer: producers are eaten by primary consumers (herbivores/omnivores)
Secondary consumer: primary consumers are eaten by secondary consumers (carnivores/omnivores)
Tertiary consumer: secondary consumers are eaten by tertiary consumers (carnivores/omnivores)
An example food chain
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, the interactions between species, and the interactions between species and their abiotic environment
Ecologists are biologists that study these interactions by investigating ecosystems
Ecologists determine the distribution and abundance of species in an ecosystem using quadrats and transects
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 e.g. 0.25 m2 or 1 m2
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
Quadrats can be used to measure abundance by recording:
The number of an individual species: the total number of individuals of a single species (e.g.. 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 e.g. grass or moss)
Diagram of how to estimate percentage cover using a quadrat
Transects
A transect is used to measure how the abundance of a species changes along the habitat, e.g. 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 to be investigated with a tape measure
A quadrat is then placed at regular intervals (e.g. 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)
Diagram of how to use a transect
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.
Abundance of organisms
Worked Example
A student is investigating whether buttercups are more abundant in an open field or in woodland. In each habitat, the student randomly places 10 quadrats and records 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:
Calculating 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
The mean number of buttercups per quadrat in the open field is 4.5.
The mean number of buttercups per quadrat in the woodland is 2.7.
Calculating the median
For each habitat, place the set of numbers in increasing order of size.
number of buttercups in open field: 1, 2, 3, 4, 5, 5, 5, 6, 7, 7
number of buttercups in 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
Calculate the mode
The mode is the value that occurs most often.
The mode number of buttercups per quadrat in the open field is 5, as this value was recorded 3 times
The mode number of buttercups per quadrat in the woodland is 2, as this value was recorded 3 times.
Worked Example
A student is investigating how the abundance of buttercups changes with increasing altitude. They set up a transect on the side of a hill and placed a quadrat at 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:
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.
A model a predator-prey cycle between the Canadian lynx and the snowshoe hare
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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