AQA A-level Biology: Populations in Ecosystems and Succession (7402) – Revision Notes

These are condensed notes for section 3.7.4, Populations in ecosystems (A-level only), in the AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015. The content is assessed in Paper 2 and Paper 3. For full explanations and longer worked examples, read the study guide, then test yourself with the practice questions. Course pages: AQA A-level Biology hub and printable checklist. If you are unsure where to start, take the free diagnostics.

Key terms

Term	Meaning
Population	Every individual of a species living in an area
Community	Populations of different species living together
Ecosystem	A community plus the non-living components of its environment; any size, from very small to very large
Habitat	Where a species lives
Niche	How a species fits into its habitat, governed by adaptation to abiotic and biotic conditions
Abiotic factor	Non-living: temperature, light, pH, water, mineral ions
Biotic factor	From living things: food, competition, predation, parasites, disease
Carrying capacity	The size of population of a species that an ecosystem can support
Pioneer species	The first species to colonise bare ground
Climax community	The relatively stable community at the end of succession
Sustainability	Using a natural resource no faster than it is replaced

What changes carrying capacity

• Abiotic factors: a drought, frost or loss of light changes food or space, so the carrying capacity changes.
• Interspecific competition: different species, same limited resource.
• Intraspecific competition: same species. Competition intensifies as numbers rise, so growth slows and levels off.
• Predation: prey and predator numbers rise and fall in linked cycles. The predator peak comes after the prey peak, because predators need time to respond.

Formulas and calculations

Quantity	Calculation
Mean per quadrat	total count ÷ number of quadrats
Density per m²	mean per quadrat ÷ quadrat area in m²
Population estimate (quadrats)	density × total area in m²
Frequency	quadrats containing the species ÷ total quadrats × 100
Mean percentage cover	sum of cover values ÷ number of quadrats
Population estimate (mark-release-recapture)	N = (n₁ × n₂) ÷ m
Percentage change	(new − old) ÷ old × 100
Doubling time	time elapsed ÷ number of doublings

In the mark-release-recapture row, n₁ is the number marked and released, n₂ the total in the second sample and m the number marked in the second sample.

Worked reminder (quadrats). Ten quadrats of 0.25 m² on a 900 m² slope give 4, 7, 2, 0, 5, 6, 3, 1, 8 and 4 plants. Total = 40, mean = 4.0 per quadrat, density = 4.0 ÷ 0.25 = 16 per m², estimate = 16 × 900 = 14 400.

Worked reminder (mark-release-recapture). 36 marked, second sample 40 with 9 marked: N = (36 × 40) ÷ 9 = 160.

Sampling methods: which to use

Method	Use for	Key point
Random quadrats	Plants and slow-moving or non-motile animals in an even area	Random coordinates remove bias
Belt transect with quadrats	Change in abundance along a gradient	Systematic sampling; measure an abiotic factor at each quadrat
Mark-release-recapture	Motile animals	Relies on assumptions

Percentage cover and frequency are both measures of abundance for sessile species. Cover shows how much space the species occupies. Frequency shows how widely it is spread. A lichen present in every quadrat but covering only 2% has high frequency and low cover.

Method in steps: random quadrat survey

1. Lay two tapes at right angles along the edges of the area.
2. Generate random coordinates with a random number table.
3. Position the quadrat at each coordinate; count or score the species.
4. Calculate the mean, then scale up to the whole area.

Mark-release-recapture: assumptions and their effects

Problem	Effect on m	Effect on N
Marks lost or faded	m too low	Overestimate
Marked animals eaten more often because they are conspicuous	m too low	Overestimate
Marked animals trap-happy	m too high	Underestimate
Marked animals not yet mixed	m wrong, either way	Unreliable
Immigration or births between samples	m too low	Overestimate

The other assumptions: marking is harmless, no emigration or deaths between samples, and each animal has an equal chance of capture.

Primary succession

Stage	Typical organisms	Change in environment
Pioneer	Lichens, mosses, cyanobacteria	Slight weathering; dead organic matter begins to build up
Early soil	Small herbaceous plants, grasses	Soil deepens; more water and nutrients are held
Shrubs	Shrubs and small trees	More shade; deeper roots; more shelter
Climax	Trees	Stable community; mostly determined by climate; little light at ground level

Key ideas to state:

• Each stage changes the abiotic environment (soil depth, water, nutrients, light), making it less hostile and more suitable for new species with different adaptations.
• The new species may make the environment less suitable for earlier species, which are out-competed (shaded, for example).
• Biodiversity usually rises as the environment becomes less hostile.
• Conservation often involves managing succession, for example by grazing, mowing or coppicing to stop a later stage taking over.

Conservation and human needs

• The conflict: people need food, timber, fish and land, and conservation can restrict them.
• Management aims at sustainability, such as quotas, mesh sizes and replanting.
• Compromise works best when local people gain, and when the evidence supports the rules.

Method in steps: evaluate conservation data

1. State the trend with figures (and a percentage change).
2. Check the sample size, the number of sites and the length of the study.
3. Look for a control or comparison group.
4. Ask whether the data shows a cause or only a correlation.
5. Consider the source and any possible bias.
6. If two sources conflict, compare their methods, sites and timescales, then reach a balanced conclusion.

Required practical 12

Investigate the effect of a named environmental factor on the distribution of a species.

• Independent variable: the factor, such as light intensity or soil moisture, measured at each quadrat.
• Dependent variable: percentage cover or frequency of the species.
• Controls: quadrat size, observer, time of year, other conditions where possible.
• Use random placement across an area, or a belt transect along a gradient.
• Process: scatter graph, then a correlation coefficient. A strong correlation does not prove cause.
• Risk assessment for the site, for example uneven ground, water and insect bites.

Microbial growth

Turbidity (cloudiness) of a broth culture can estimate growth rate. A log scale makes growth over several orders of magnitude easy to plot, and a straight line shows steady doubling. Turbidity does not distinguish living from dead cells.

Quick self-test

1. Define a niche.
2. Name two abiotic and two biotic factors that affect a population.
3. Ten quadrats of 0.25 m² contain 4, 7, 2, 0, 5, 6, 3, 1, 8 and 4 plants on a 1200 m² site. Estimate the population.
4. A species is present in 9 of 12 quadrats. Calculate its frequency.
5. 30 snails are marked. A second sample has 25 snails, 5 marked. Estimate the population.
6. Explain why fading marks give an overestimate.
7. Why is a belt transect more suitable than random quadrats for studying a change across a shore?
8. Name a pioneer species and say how pioneers change the environment.
9. Explain why mosses decline when shrubs establish.
10. A bird count rises from 48 to 62 pairs. Calculate the percentage increase.
11. A culture grows from 2.0 × 10⁴ to 3.2 × 10⁵ cells per cm³ in 10 hours. Calculate the doubling time.
12. Give one way in which management of succession helps conservation.

Answers

1. The role of a species in its habitat, governed by its adaptation to abiotic and biotic conditions.
2. Abiotic: any two of temperature, light, pH, water, mineral ions. Biotic: any two of food, competition, predation, disease.
3. Mean = 40 ÷ 10 = 4; density = 16 per m²; estimate = 16 × 1200 = 19 200.
4. 9 ÷ 12 × 100 = 75%.
5. N = (30 × 25) ÷ 5 = 150.
6. Fewer marked animals are recognised, so m is too small. N = n₁n₂ ÷ m is then too large.
7. The factors change in order along the shore, so systematic sampling along a line shows the pattern. Random quadrats would not.
8. Lichen or moss. It weathers the rock slightly and adds dead organic matter, which forms soil that holds water and nutrients.
9. The shrubs shade the moss. It is out-competed for light as the environment changes.
10. (62 − 48) ÷ 48 × 100 = 29.2%.
11. The ratio is 16 = 2⁴, so 4 doublings. 10 ÷ 4 = 2.5 hours (150 minutes).
12. Grazing or mowing prevents scrub invading, so a species-rich grassland is kept.

Where marks are usually lost

• Giving “habitat” when the question asks for “niche”.
• Forgetting to scale a quadrat mean up to the area, or scaling by quadrat area in cm² instead of m².
• Stating mark-release-recapture assumptions in vague words (“the animals must be unharmed”) without saying what it does to the estimate.
• Writing that frequency and percentage cover are the same measure.
• Describing succession as a list of species, with no change in soil, water or light.
• Saying earlier species “die” without the reason: they are out-competed after the environment changes.
• Calling a correlation proof of cause.
• Evaluating data with generic comments, such as “more repeats”, and not tying them to the study described.
• Ignoring a time lag in predator and prey graphs.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), for first teaching 2015: section 3.7.4, Populations in ecosystems (A-level only), and required practical 12.