OxfordAQA International AS and A-level Biology: Populations, communities and ecological succession (9610)

This guide teaches section 3.3.1, The effect of biotic and abiotic factors on populations, from the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. It covers 3.3.1.1 to 3.3.1.3 and required practical 9. Unit 3 (Populations and genes) is listed in the specification as International A-level only. Course pages: OxfordAQA A-level Biology hub and printable checklist. Companions: revision notes and practice questions. Try the free diagnostics too.

What the section asks you to do

Section	What you must be able to do
3.3.1.1	Define population, community, ecosystem and niche
3.3.1.2	Explain how abiotic factors, competition and predation change population size. Interpret data on numbers and distribution
3.3.1.3	Describe primary succession, its effect on the environment and biodiversity, and how conservation manages succession
Required practical 9	A laboratory-based investigation of the effect of competition on seedling growth

Populations, communities and ecosystems

• A population is every organism of a single species living in one habitat.
• Populations of different species in the same place form a community.
• A community together with the non-living parts of its surroundings makes up an ecosystem.

Take a garden pond. Its newts are one population. The newts, snails, pondweed and damselfly larvae together are a community. Add water, dissolved oxygen, light and sediment for the ecosystem.

A habitat is where an organism lives. Within a habitat each species occupies a niche, and the specification links the niche to how well the species is adapted to both biotic and abiotic conditions. Biotic conditions come from other living things: food, competitors, predators, parasites. Abiotic conditions are the non-living ones: temperature, light intensity, pH, water availability, salinity and mineral ions.

A niche is a role, not an address. One woodland bird may have a short, thick beak that cracks seeds. Another has a long, thin beak that probes leaf litter for insects. They share a habitat but have different niches, so they compete little. Species with very similar niches compete strongly, and one may decline.

Variation in population size

Population size changes because of abiotic factors and because of interactions between organisms: interspecific competition, intraspecific competition and predation.

Abiotic factors

Abiotic factors act on survival, growth and reproduction. A marine alga may tolerate only a narrow salinity range, so it declines after rain dilutes a rock pool. Explain the chain, not just the factor: low temperature slows enzyme-controlled reactions, so growth is slower, fewer individuals reach reproductive age, and the population falls.

Intraspecific competition

Intraspecific competition is competition between individuals of the same species for limited supplies of food, light, water, mineral ions, nesting sites or mates. Individuals have identical niches, so they compete most strongly. As numbers rise, each individual gets a smaller share. Growth slows, fewer offspring are produced and more individuals die, so the population levels off. Intraspecific competition therefore tends to stabilise a population.

Interspecific competition

Interspecific competition is competition between individuals of different species for a resource they both need. If one species uses the resource more effectively, the other has lower survival or reproduction and its population falls. Where the niche overlap is large, the weaker competitor may disappear from that habitat. Where it is small, both persist.

Predation

A predator kills and eats prey, so each population affects the other.

1. Prey numbers rise, so predators have more food.
2. Predator numbers rise after a delay, because they need time to reproduce.
3. More predators eat more prey, so prey numbers fall.
4. Predators now have less food, so they decline after a delay and prey recover.

The predator curve peaks later than the prey curve. Prey may also be limited by food or disease, so predation is a likely cause, not a proven one.

Analysing data on numbers and distribution

The specification asks you to analyse and interpret data on the numbers and distribution of organisms, recognising correlations and causal relationships. Describe the pattern with figures, calculate something, suggest explanations, then judge what the data can prove.

Worked example 1: a predator-prey table. Water fleas (Daphnia) per litre and perch fry per 100 m³ were counted in a lake for eight years.

Year	1	2	3	4	5	6	7	8
Water fleas per litre	40	85	150	120	60	35	90	160
Perch fry per 100 m³	6	5	8	14	12	7	5	9

• Water fleas peak in year 3 (150 per litre); perch fry peak in year 4 (14 per 100 m³). The predator peak follows the prey peak by one year.
• Water fleas fall from 150 to 35 between years 3 and 6: (35 − 150) / 150 × 100 = −76.7%.
• Explanation: more water fleas meant more food, so more perch fry survived. Heavier predation and falling food then reduced both.
• Limit: eight years and one lake, so temperature could also play a part.

Worked example 2: a correlation along a gradient. Along a transect from a woodland path into dense canopy, light intensity (arbitrary units) and dog’s mercury shoots per m² were recorded.

Station	1	2	3	4	5	6	7	8
Light	82	64	55	47	31	28	15	9
Shoots per m²	4	9	7	15	12	20	18	26

A scatter diagram shows shoots rising as light falls: a negative correlation. The specification’s mathematical requirements list the Spearman rank correlation test. Rank each variable from 1 for the smallest value, then find d, the difference between each pair of ranks.

• Light ranks (stations 1 to 8): 8, 7, 6, 5, 4, 3, 2, 1.
• Shoot ranks: 1, 3, 2, 5, 4, 7, 6, 8.
• d: 7, 4, 4, 0, 0, −4, −4, −7, so Σd² = 49 + 16 + 16 + 0 + 0 + 16 + 16 + 49 = 162.
• rₛ = 1 − (6 × Σd²) / (n(n² − 1)) = 1 − (6 × 162) / (8 × 63) = 1 − 972/504 = −0.929.

If a table gives the critical value for n = 8 at p = 0.05 as 0.738, then 0.929 is larger, so the correlation is significant and you reject the null hypothesis of no correlation.

Correlation is not causation. The test shows that light and shoot number change together. It does not show that light causes the change. Soil moisture and leaf litter depth may also change along the transect. Causation is more convincing with a plausible mechanism, repeated results at several sites, and an experiment in which one factor is changed and the others are controlled.

The standard error is the standard deviation divided by √n. The 95% confidence limits are roughly the mean ± 2 × standard error. If two means have limits that do not overlap, the difference is unlikely to be chance. Random sampling and a suitable sample size keep the data representative.

Required practical 9: competition and seedling growth

The specification requires a laboratory-based investigation of the effect of competition on seedling growth. A typical design uses fast-growing seeds such as cress or mustard.

Intraspecific version. Sow one species at several densities, for example 5, 10, 20 and 40 seeds per tray.

Interspecific version. Sow two species together in different proportions, keeping total seeds constant, and compare with each grown alone.

Method outline

1. Fill identical trays with the same compost.
2. Sow seeds evenly at the planned densities and the same depth.
3. Keep temperature, light and watering the same.
4. After a set time (for example 14 days), measure shoot height, or harvest the shoots, dry them until the mass stops changing, and weigh them. Dry mass ignores water content.
5. Divide total dry mass by the number of seedlings to get the mean mass per seedling.
6. Use at least three replicate trays per density, to calculate a mean and spot anomalies.

Control variables: species and seed batch, compost, tray area, sowing depth, watering volume, light, temperature and harvest time. Tray area matters most, because density only means something if the area is the same.

Worked example 3: interpreting the data. Mean dry mass per seedling after 14 days:

Seedlings per tray	5	10	20	40
Mean dry mass per seedling (mg)	62	54	41	27
Total dry mass per tray (mg)	310	540	820	1080

• Mean mass per seedling falls as density rises: from 5 to 40 per tray it falls by (62 − 27) / 62 × 100 = 56.5%.
• This is intraspecific competition for light, water and mineral ions, so each plant gets less.
• Total mass per tray rises from 310 mg to 1080 mg, but density rose eightfold and the total only about 3.5-fold.

Five replicate trays gave mean masses per seedling of 56, 52, 58, 51 and 53 mg at density 10 and 29, 26, 24, 30 and 26 mg at density 40.

• Density 10: mean 54, standard deviation 2.92, standard error 2.92 / √5 = 1.30, so limits of about 51.4 to 56.6 mg.
• Density 40: mean 27, standard deviation 2.45, standard error 1.10, so limits of about 24.8 to 29.2 mg.
• The ranges do not overlap, so the difference is unlikely to be chance.

Evaluating: seedlings at the tray edge may get more light, and one harvest time shows nothing about how competition develops.

Ecological succession

Succession is the directional change in a community over time. Primary succession begins on newly formed or exposed land that has no soil, for example bare rock exposed by a retreating glacier, or new sand or volcanic ash. It proceeds from the first colonisers, the pioneer species, through intermediate stages to a climax community.

1. Pioneers such as lichens, mosses and cyanobacteria colonise first. They tolerate extreme conditions and need no soil. They weather the rock slightly and their dead remains add organic matter.
2. Soil begins to form. Fragments of rock mix with organic matter. The soil holds more water and mineral ions and the environment becomes less hostile.
3. Colonists with different adaptations arrive: small herbaceous plants, then grasses and shrubs. Their roots go deeper and they grow taller. They shade out and out-compete the pioneers.
4. The climax community is relatively stable, is determined mainly by climate and in many places is woodland.

The central idea is that the species present at one stage alter the environment in ways that favour newcomers with different adaptations. Those newcomers may then change conditions so that the earlier species no longer thrive.

Worked example 4: succession data. Floors of abandoned limestone quarries were sampled at four sites of known age.

Site age (years)	5	20	50	120
Dominant vegetation	Lichens, mosses	Grasses, herbs	Willow, alder shrubs	Mixed woodland
Soil depth (cm)	0.5	4	10	25
Plant species per 100 m²	3	14	22	19
Light reaching ground (% of open)	100	95	60	8

• Soil depth rises as rock weathers and organic matter accumulates, so the environment is less hostile.
• Species number rises from 3 to 22 by year 50 as more niches appear.
• Between years 50 and 120 species number falls by (19 − 22) / 22 × 100 = −13.6%. The likely cause is shading: light at ground level drops to 8%, so few light-demanding species survive beneath the canopy.
• This fits early species changing the environment for their successors, but four sites of different ages only substitute for watching one site for 120 years.

Conservation and managing succession

Many valued habitats are stages in succession. Left alone, grassland would be invaded by shrubs and then woodland. Conservation of habitats frequently involves management of succession: preventing a habitat from moving on to the next stage.

• Grazing or mowing stops shrubs establishing and keeps grassland open.
• Coppicing woodland lets light reach the ground, which favours ground-layer plants.

Management has costs: a landowner may lose income, and the unit introduction stresses managing the conflict between human needs and conservation. In an answer, describe the management, say which stage it holds the habitat at, and give a benefit and a cost.

Common errors

• Defining a population without “one species”.
• Treating a correlation as proof.
• Saying biodiversity always rises. It may fall near the climax.
• Forgetting tray area and replicates in the seedling practical.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), specification Version 5.1, for International A-level exams May/June 2018 onwards, published by OxfordAQA. Section 3.3.1 and required practical 9 in Unit 3.