Populations, habitats and ecological studyEdexcel International A Level Biology: Revision notes
Section 1
Ecological terms
- Ecosystem: all the living organisms (biotic) and the non-living (abiotic) components in an area, and their interactions.
- Community: all the populations of different species living in the same place at the same time.
- Population: all the organisms of one species living in a habitat at the same time.
- Habitat: the place where an organism lives.
- Niche: the role of a species in its ecosystem: its feeding relationships, its interactions with other species and the abiotic conditions it tolerates.
The abundance of a species is the number of individuals (or percentage cover) in an area. The distribution is where in the habitat the species is found.
A niche is not a place. The place is the habitat. The niche is the role and the conditions tolerated.
Section 2
Biotic and abiotic factors
Abiotic factors are the non-living factors, such as temperature, light intensity, soil pH, water availability, salinity, wind speed and oxygen concentration.
Biotic factors are the effects of other living organisms: interspecific competition (between species), intraspecific competition (within a species), predation, grazing, parasitism and disease.
The distribution and abundance of a species is set by its niche. A species is found where abiotic conditions are within its tolerance and where it can compete successfully with other species. Two species with the same niche compete, and one is likely to be excluded.
Section 3
Sampling with quadrats
It is impossible to count every organism, so a sample is taken using a quadrat (a square frame, often 0.25 m² or 1 m²).
- Random sampling is used to estimate the abundance in a uniform area. The quadrat positions are chosen using random coordinates or a random number generator, to avoid bias.
- Count the individuals (density) or estimate the percentage cover, and the frequency (the proportion of quadrats in which the species is found).
- Larger samples are more representative and more reliable.
Population estimate = mean number per quadrat × (total area ÷ area of one quadrat).
Worked example: 85 daisies in 20 quadrats of 0.25 m² in a field of 1 000 m². Mean = 4.25 per quadrat. 1 000 ÷ 0.25 = 4 000 quadrat areas, so the population is 4.25 × 4 000 = 17 000.
Remember to divide by the quadrat area as well as multiplying by the area of the habitat.
Section 4
Transects and abiotic factors (Core Practical 11)
Where conditions change gradually, such as from the sea up a shore or from a hedge into a field, use a systematic sample along a transect.
- A line transect records the species touching the line at intervals.
- A belt transect uses a quadrat placed at regular intervals along the line (interrupted belt transect), or a continuous strip.
In Core Practical 11 you sample abundance and measure abiotic factors at each sampling point, then relate them: for example, light intensity (light meter), temperature (thermometer or probe), soil moisture (moisture meter), soil pH (pH probe or indicator) or time covered by the tide.
To make the data valid and reliable: use the same sized quadrat, measure abiotic factors at the same time and place as the counts, use the same instrument, take repeat transects and calculate means. Plot the data to look for a pattern.
Explain why a transect: it samples along an environmental gradient so changes in distribution can be linked to changes in abiotic factors.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Populations, habitats and ecological study
- A coastal sand dune system contains marram grass, rabbits, beetles, bacteria and many other species. The sand is salty, dry and low in nutrients, and the wind speed is high.Distinguish between the habitat and the niche of marram grass in the dune system.2 marks
- A student estimates the population of daisies in a rectangular field measuring 40 m by 25 m. She places 20 quadrats, each 0.5 m by 0.5 m, at positions chosen using random coordinates. She counts a total of 85 daisies in the 20 quadrats.Suggest two ways in which the student could make her estimate of the population more reliable.2 marks
- A student lays a 30 m line transect running from the lower shore to the upper shore of a rocky beach and places a quadrat at 2 m intervals along it. The percentage cover of the seaweed Fucus falls from 90% at the lowest position to 5% at the highest position. Going up the shore, the length of time each position is covered by seawater each day falls and the temperature of the rock surface on sunny days rises.Explain why a line transect is more suitable than random sampling for this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).