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

This guide teaches section 3.7.4, Populations in ecosystems, from the AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015. The section is A-level only, so it is not part of the AS course. It sits in topic 7, which the specification assesses in Paper 2 and in Paper 3. Course pages: AQA A-level Biology hub and printable checklist. Companion pages: revision notes and practice questions. To see where you stand, try the free diagnostics.

What the section asks you to do

Specification idea	What you must be able to do
Community, ecosystem, niche	Define the terms and explain niche in terms of abiotic and biotic conditions
Carrying capacity	Explain how abiotic factors, competition and predation change it
Estimating size	Quadrats and belt transects for slow-moving or non-motile organisms; mark-release-recapture for motile ones, with its assumptions and calculation
Succession	Describe primary succession from pioneer species to climax community
Conservation	Manage the conflict with human needs; evaluate data and conflicting evidence
Required practical 12	Effect of a named environmental factor on the distribution of a species

Populations, communities and ecosystems

A population is all the individuals of one species living in one place at one time. Populations of different species form a community. A community and the non-living components of its environment form an ecosystem.

A habitat is the place where a species lives. Within it, a species occupies a niche: the way it fits into the ecosystem, governed by its adaptations to abiotic and biotic conditions. Abiotic factors are non-living: temperature, light intensity, pH, water availability and mineral ion content. Biotic factors come from other living things: food, competitors, predators, parasites and disease.

A niche is more than an address. Two bird species can share a wood by feeding on different insects or in different layers of the canopy. If two species have very similar niches, they compete and one often declines.

Carrying capacity and what changes it

An ecosystem supports a certain size of population of a species. This is the carrying capacity. It is not fixed, because the factors behind it vary.

• Abiotic factors. A cold, dry year lowers plant growth, so less food is available and the carrying capacity for herbivores falls.
• Interspecific competition. Members of different species compete for a shared resource such as light or nesting sites. The better competitor lowers the carrying capacity of the other.
• Intraspecific competition. Members of the same species compete for limited resources. As the population rises, each individual gets less, so birth rates fall or death rates rise until the population levels off.
• Predation. When prey numbers rise, predators have more food and increase after a delay. More predators then reduce the prey, and predator numbers fall because food is short.

A population graph that rises and then levels off shows the population reaching carrying capacity.

Estimating population size

You cannot count every organism, so you sample. The method depends on whether the organism moves.

Randomly placed quadrats

Use quadrats for plants and for slow-moving or non-motile animals, such as limpets. A frame quadrat is often 0.5 m by 0.5 m, giving 0.25 m².

1. Mark out the study area with two tape measures at right angles.
2. Use a random number table or generator to produce pairs of coordinates.
3. Put the quadrat down at each coordinate and count or score the species.
4. Use enough quadrats for a reliable mean.

Random placement avoids bias.

Worked example. A student sampled plantain in a lawn of 640 m² using ten quadrats of 0.25 m². The counts were 5, 1, 7, 3, 0, 6, 2, 4, 8 and 2.

• Total = 38, so the mean = 38 ÷ 10 = 3.8 per quadrat.
• Density = 3.8 ÷ 0.25 = 15.2 per m².
• Estimate for the lawn = 15.2 × 640 = 9728, which is about 9700 plants.

Ten small quadrats on an uneven lawn give only a rough answer.

Percentage cover and frequency

For sessile organisms, or those you cannot count as individuals (grasses, lichens, algae), use two measures of abundance. Percentage cover is the proportion of the quadrat that the species covers, often judged on a grid of 100 squares. Frequency is the percentage of quadrats in which the species occurs.

If a species is present in 17 of 25 quadrats, its frequency is 17 ÷ 25 × 100 = 68%. If five quadrats show cover of 34, 12, 0, 45 and 9 per cent, mean cover = 100 ÷ 5 = 20%.

Frequency ignores how much of the quadrat is covered, so a species can have high frequency but low cover. Cover is a judgement and can differ between observers.

Belt transects

A transect shows how abundance changes along a gradient, such as from a path into woodland or down a shore. Stretch a tape along the gradient. In a belt transect, you record organisms in a strip of fixed width, using quadrats at regular intervals (for example every 5 m) or end to end. The sampling is systematic, not random, which suits a gradient. Measure an abiotic factor at each quadrat.

Mark-release-recapture

For motile animals, such as beetles, use mark-release-recapture.

1. Capture a sample, for example with pitfall traps. Count it (n₁).
2. Mark each animal harmlessly and inconspicuously. Release them where they were caught.
3. Leave time for the marked animals to mix.
4. Capture a second sample. Count the total (n₂) and the number marked (m).

The proportion of marked animals in the second sample is taken to equal the proportion marked in the whole population:

m / n₂ = n₁ / N, so N = (n₁ × n₂) / m

Worked example. 64 ground beetles are marked and released. A later sample has 52 beetles, 13 of them marked. N = (64 × 52) ÷ 13 = 3328 ÷ 13 = 256 beetles.

The calculation relies on assumptions:

• no immigration, emigration, births or deaths between samples
• marked animals mix fully and randomly with the rest
• marking does not change an animal’s survival, behaviour or chance of being caught
• the mark is not lost
• every animal has the same chance of being caught

If these fail, the estimate is wrong. If marks come off, so only 10 of the 13 recaptured beetles are recognised, N = 3328 ÷ 10 = 332.8, which is an overestimate. If marked animals are easier to catch the second time, the estimate is too low.

Succession

Ecosystems are dynamic systems. Primary succession is the change in a community that begins on newly formed or exposed land with no soil, such as bare rock after a lava flow or a retreating glacier. It runs from colonisation by pioneer species to a climax community.

1. Pioneers. Lichens, mosses and cyanobacteria can survive bare rock with little water and few nutrients. They weather the rock slightly and, when they die and decay, add organic matter.
2. Soil forms. Rock fragments and dead organic matter build a thin soil that holds more water and nutrients. The environment becomes less hostile.
3. New colonisers. Small herbaceous plants, then grasses and shrubs, grow. They have different adaptations, such as deeper roots. They shade and out-compete the pioneers.
4. Climax. Taller plants and trees arrive. The community becomes relatively stable, with a greater biodiversity than the first stage. The climax is determined mainly by climate.

At each stage, certain species change the environment so that it becomes more suitable for other species with different adaptations. The new species may then make it less suitable for the previous species. Changes to the abiotic environment, such as soil depth, water retention, nutrients and light, produce a less hostile environment and change biodiversity.

Biodiversity usually rises but can fall at the end, because a dense canopy lets little light through.

Conservation and human needs

Many habitats that conservationists value are stages in succession. Left alone, a species-rich grassland would be invaded by shrubs, and then woodland would replace it. Conservation of habitats therefore often involves management of succession. Examples:

• Grazing or mowing stops scrub from growing and keeps the grassland.
• Coppicing woodland (cutting trees to a stump so they regrow) lets light reach the woodland floor, favouring ground flora.
• Removing invading scrub from heathland.

These actions hold the community at an earlier stage.

People need food, timber, fuel, fish and land, and conservation can limit access or profit. You must understand the need to manage the conflict between human needs and conservation to maintain the sustainability of natural resources: using a resource no faster than it can be replaced. Examples include fishing quotas and larger net mesh sizes, so young fish survive and breed.

Evaluating data and conflicting evidence

You may be given data on a species or habitat and asked to evaluate it. Work through these points.

• What does the data show? Quote figures and calculate a percentage change.
• Is the sample large enough? Check the number of sites, quadrats, years and repeats.
• Is there a control? Compare managed with unmanaged plots.
• Is a cause shown, or only a correlation?
• Is the source reliable? Consider who funded it.
• Do two studies disagree? Different sites, periods, methods or measures can explain it.

Worked example. A reserve is grazed from year 0. After five years, the mean number of plant species per m² is 14.2 in grazed plots and 8.6 in ungrazed plots. Grazed plots have 65% more species ((14.2 − 8.6) ÷ 8.6 × 100 = 65.1). If the plots started alike, the data supports grazing. A farmer may reply that the study ran for only five years in one reserve. A fair conclusion supports grazing and says what extra evidence would help.

Required practical 12: environmental factor and distribution

You must investigate the effect of a named environmental factor on the distribution of a given species. Example: how soil moisture affects the distribution of creeping buttercup along a transect from a drainage ditch.

1. Choose a transect that runs along the gradient of the factor.
2. Place a quadrat at regular intervals and estimate percentage cover or frequency of the species.
3. At each point, measure the factor with a suitable meter and record other conditions.
4. Repeat on a second transect and take means.
5. Plot a scatter graph and calculate a correlation coefficient.

Worked analysis. Eight stations have soil moisture of 12, 18, 25, 31, 38, 44, 52 and 60% and buttercup cover of 4, 9, 14, 30, 33, 52, 61 and 70%. The correlation coefficient is r = 0.99, a strong positive correlation. For eight pairs, the 5% critical value is 0.707, so the correlation is significant. This does not prove that moisture causes the distribution, because shade, for example, may also change along the transect.

Carry out a risk assessment for the site, state the quadrat size and take enough quadrats.

Microbial growth and turbidity (optional skills)

You could investigate the growth rate of a broth culture, such as yeast, using turbidity: the cloudier the culture, the more cells. Growth over several orders of magnitude is plotted on a logarithmic scale.

Worked example. A yeast culture grows from 4.0 × 10³ to 2.56 × 10⁵ cells per cm³ in 9 hours. The ratio is 64 = 2⁶, so there were six doublings. Doubling time = 9 ÷ 6 = 1.5 hours. On a log₁₀ scale the points are 3.60 and 5.41, a slope of 0.20 per hour. Growth later levels off as nutrients fall. Turbidity cannot tell living cells from dead ones.

Common errors

• Writing “niche” as if it meant “habitat”.
• Giving the quadrat total as the population without scaling up to the whole area.
• Forgetting that the mark must not change survival or behaviour.
• Describing succession as a list of species, with no change in soil, light or water.
• Treating a correlation as proof of cause.

Related: biodiversity, energy transfer and nutrient cycles.

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.