3.7.4 Populations in ecosystems

This is where biology meets demography: how populations grow, what keeps them in check, how species fit into communities, how those communities change over time, and how careful management can hold a habitat at the stage where biodiversity is highest.

Ecosystems are shaped by abiotic factors, biotic factors, and the niche each species occupies.

An ecosystem comprises a community of organisms together with the abiotic features of the environment with which they interact. The distribution of any species within an ecosystem is set where its tolerance to the abiotic factors and its competitive position against the biotic factors both allow it to persist.

Write niche for the ecological role and habitat for the place. AQA rejects habitat as a substitute for niche.

Write community for the assemblage of species. Population and ecosystem are rejected as substitutes when the question asks for community.

Population size settles at the carrying capacity through intraspecific and interspecific competition.

Populations do not grow without limit. As numbers rise, resources per individual fall, intraspecific competition intensifies, and birth and death rates converge until the population stabilises near the carrying capacity — the environmental ceiling, not a y-axis label.

Write carrying capacity. AQA rejects maximum capacity. The credited term names the environment as the limiting factor; the rejected substitute names only a number.

Name the competition type explicitly: intraspecific or interspecific. The naming is itself a discrete mark in the mark scheme.

AQA rejects direct competition, allopatric competition, and spatial competition as competition-type labels.

1. Identify the competition type as interspecific — different species, shared resource.
2. State that the more competitive species reduces the availability of the resource to the other.
3. Trace the chain to the downstream molecular consequence: less light → less photosynthesis; less nitrate → less of a named nitrogen molecule (amino acid, protein, DNA, RNA, ATP); less phosphate → less of a named phosphorus molecule (DNA, RNA, ATP, phospholipid, RuBP, GP, TP, NADP).
4. State the consequence at the population level: less growth, slower reproduction, smaller population.

End the mechanism chain at photosynthesis or at a named molecule. Stopping at less light or less nitrate without naming what gets made less of forfeits the final mark — the molecular endpoint is what AQA credits.

The red and grey squirrel case is the standard worked example. Grey squirrels are larger and more generalist, extract more energy from acorns than red squirrels, and outcompete reds for food and nest sites where the two species share habitat. Red populations are reduced or eliminated in the overlap zones.

Predator and prey populations oscillate out of phase.

A predator captures and consumes a prey species; the two population sizes are coupled by a feedback loop with a lag, because a change in food availability takes time to translate into a change in predator birth rate.

The signature is the lag: on a graph, the predator peak follows the prey peak. The cycle is not the only mechanism regulating either population — disease, weather, and competition also act. In an answer, describe the feedback in directional terms (rises, falls, follows after a lag), not as a perpetual steady oscillation.

Sampling methods differ between sessile and mobile organisms.

Direct counting of every individual is rarely feasible. The technique is chosen by whether the organism moves. Sessile or slow-moving organisms — including all plants — are sampled by quadrat. Mobile animals are sampled by mark-release-recapture.

Plants are sampled by quadrat, not by mark-release-recapture. Plants cannot disperse after being marked, so marked individuals do not mix with the population and the Lincoln index assumption fails.

Write random number generator or random coordinates from a random number table for quadrat placement. AQA rejects throwing quadrats and randomly without a named method.

Write the calculation step in full: mean per quadrat multiplied by the number of quadrats in the total area. Scale up alone is rejected.

A precaution is what the investigator does (non-toxic mark, mark invisible to predators). An assumption is a condition that must hold (closed population, equal probability of capture). They are graded separately.

1. Capture a first sample, mark each individual in a way that does not affect survival or detectability, and release them into the same area.
2. Wait long enough for the marked individuals to disperse and mix randomly with the unmarked population.
3. Capture a second sample, count how many carry the mark, and apply the Lincoln index.

Write at least 10 quadrats or large sample. AQA rejects several as a sample-size description.

Pitfall — The three-mark sampling checklist

Every quadrat or MRR question allocates three independent marks. Drop any one and the answer caps below maximum.

1. Named random placement method — a random number generator or random table, not "randomly" without a named method.
2. Large sample size — at least 10 quadrats, or a stated number, not "several".
3. The calculation step in full — mean per quadrat × number of quadrats in the total area, or the Lincoln index written out. "Scale up" alone forfeits this mark.

The calculation step is the most consistently dropped. The answer is not complete until the multiplication or division is written.

Succession is a sequence of communities that change their own environment.

Succession is the directional replacement of one community by another over time. The mechanism is species-driven environmental change — each community modifies the abiotic conditions in ways that allow the next community to establish and that make conditions less suitable for itself.

1. Name a pioneer species from the figure (lichens or mosses on bare rock, for primary succession).
2. State an abiotic change with a named mechanism: forms soil; adds humus; stabilises the substrate; increases water-holding capacity.
3. State the consequence using the credited phrase: conditions are less hostile or more suitable for the next species.
4. Name the climax community from the figure — the stable end-point under the prevailing climate (woodland in the temperate UK).

Write pioneer species. AQA rejects primary species and coloniser.

Write climax community in full. Climax alone is not credited.

Write less hostile or more suitable. AQA rejects more habitable and more hospitable as substitutes.

Primary succession begins on bare substrate with no pre-existing soil — newly exposed rock or coastal sand. Secondary succession begins on previously inhabited ground where soil is already present, so it bypasses the early seral stages and proceeds more rapidly.

On a succession graph, time runs left to right: pioneer at the left, climax at the right. Reading the axis backwards — calling the trees pioneers and the grasses the climax — is the single most common error on this question type.

Conservation often holds a habitat at an earlier seral stage.

Many habitats of high conservation value — heathland, moorland, chalk grassland, reed beds — are not climax communities. Left unmanaged, each would progress through succession to its local climax, and the species characteristic of the open earlier-stage habitat would be lost. Conservation management arrests or resets the sequence.

1. Sterile males of the pest species are mass-released into the wild population and compete with fertile males for mates. The named mechanism is intraspecific competition.
2. Matings with sterile males produce no viable offspring.
3. Birth rate falls and the population declines over successive generations.

The sterile insect mechanism is intraspecific competition followed by failed reproduction. AQA rejects answers that explain the technique via the sterile males' inability to transmit disease — female mosquitoes, not males, bite, and the technique is about reproduction, not pathogenicity.

When sterile insect release fails or underperforms, the credited explanation is a fitness effect of the sterilising radiation — reduced lifespan, reduced courtship success, reduced attractiveness to females. Disease-transmission answers are rejected here too.

Key terms

• carrying capacity
• intraspecific competition
• interspecific competition
• niche
• quadrat
• mark-release-recapture
• Lincoln index
• succession
• pioneer species
• climax community