Ecosystems and Population Size — AQA A-level Biology

SPEC 3.7.4 - GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS

Populations in Ecosystems

Carrying capacity, interspecific and intraspecific competition, predator–prey cycles, primary succession and conservation management

Ecosystems, Communities & Niches

Start with the key terminology — examiners reward precise use of these definitions.

Key Definitions

TermDefinitionEcosystemA community of organisms and the non-living (abiotic) environment in which they live, interacting togetherCommunityAll the populations of different species living in the same area at the same timeHabitatThe place where an organism lives, characterised by physical and biological featuresNicheThe role and position a species occupies in its ecosystem, defined by its biotic and abiotic requirements and interactions

The Niche Concept

A niche is governed by adaptation to both abiotic conditions (temperature, pH, light) and biotic conditions (predators, competitors, food sources). No two species can occupy exactly the same niche in the same habitat — if they do, the better-adapted species will outcompete and exclude the other through interspecific competition.

Exam Tip

Examiners distinguish habitat from niche. Habitat = where; niche = how (role, requirements, interactions). Two species can share a habitat but not a niche.

Carrying Capacity & Population Growth

Definition

Carrying capacity (K) — the maximum population size that an environment can sustainably support, limited by available resources.

Population growth typically follows an S-shaped (logistic) curve:

Factors Limiting Population Size

Abiotic Factors

Temperature

Water availability

Light intensity

Mineral/nutrient availability

pH

Biotic Factors

Intraspecific competition

Interspecific competition

Predation

Disease

Parasitism

Competition: Intra- vs Interspecific

Intraspecific

Competition between individuals of the same species for the same resources (food, mates, territory, nesting sites).

As population size increases → resources become scarcer → more intense intraspecific competition → death rate rises / birth rate falls → population decreases back toward K.

This is the main mechanism that keeps population size at K.

Interspecific

Competition between individuals of different species for the same resources.

If two species occupy the same niche, the better-adapted species outcompetes the other. The less successful species declines in numbers and may eventually be excluded from the habitat.

Predator–Prey Cycles

Predation is a key biotic factor that regulates population size. The relationship produces cyclical oscillations:

Prey population increases → more food for predators

Predator population increases (with a time lag)

Increased predation → prey population decreases

Less food → predator population decreases (with a time lag)

Prey population recovers → cycle repeats

Exam Tip

Always state the time lag: predator numbers peak after prey numbers peak. This is because it takes time for increased food availability to translate into increased predator reproduction.

Primary Succession

Primary succession is the sequential colonisation of a bare, previously uncolonised habitat by species communities, leading eventually to a stable climax community.

1

Pioneer stage: Bare rock/sand with no soil and no organic matter. Pioneer species (e.g. lichens) colonise. They tolerate extreme abiotic conditions: drought, UV exposure, nutrient poverty. They fix nitrogen, weather rock, and deposit organic matter as they die.

2

Early intermediate: Thin soil develops. Mosses colonise. More organic matter trapped. Soil deepens and water-holding capacity increases. Small herbaceous plants can now establish.

3

Late intermediate: Grasses, then shrubs, then small trees colonise. Each community modifies the abiotic environment (shade, soil depth, humidity) — making it less suitable for the previous species and more suitable for the next species.

4

Climax community: The final, stable community in equilibrium with the environment. In temperate UK: oak woodland. High biomass; no further directional change unless the community is disturbed.

Synoptic Link — Topic 3

The improvements in abiotic conditions during succession (increased soil organic matter, reduced temperature extremes) link to gas exchange and mineral nutrition in plants — richer soils support greater plant diversity (Topic 3.3).

Conservation & Managing Succession

Many of the UK's most biodiverse habitats are intermediate successional stages, not the climax community.

Why Manage Succession?

Heathland (e.g. Dorset heath) — maintained by grazing/burning; without management, succession → scrub → woodland → heathland species lost

Chalk grassland — rich in orchids and butterflies; maintained by rabbit grazing / sheep grazing; trees invade if grazing ceases

Sand dunes — early successional dunes support rare plant species lost if succession proceeds unchecked

Conservation Management Techniques

TechniqueEffectControlled burning (prescribed burning)Removes mature vegetation; resets to earlier successional stageGrazing by livestock / rabbitsPrevents scrub and tree establishment; maintains open grasslandMowing / cuttingRemoves dominant tall grasses; increases light to ground level; favours low-growing speciesCoppicingPeriodic cutting of trees; promotes diverse woodland structure with light gapsRe-introduction of grazing animalsRestores natural grazing pressure; can re-establish lost habitats (rewilding)

Human Needs vs Conservation

The AQA spec requires you to evaluate the conflict between human needs and conservation. This appears as extended response / essay questions.

Arguments for Conservation

Biodiversity has intrinsic value — species have a right to exist

Ecosystem services : pollination, water purification, flood prevention, soil formation

Genetic resources : wild species are sources of future medicines, crops, materials

Habitat loss is often irreversible

Ecotourism : conserved habitats generate economic income

Human Competing Needs

Agricultural land — feeding growing human population

Urbanisation and infrastructure

Forestry and timber production

Fishing industries — livelihoods of coastal communities

Economic development in low-income countries

Sustainability

AQA expects you to understand the need to manage the conflict in order to maintain the sustainability of natural resources. Sustainable management means meeting present human needs without compromising the ability of future generations to meet their own needs.

Exam Tip

When evaluating conservation evidence, consider: sample size, duration of study, confounding variables, whether data is from a controlled experiment or field observation, and whether conflicting studies exist.

Examiner Intelligence

Examiner Alert — Most Common Errors

Niche ≠ habitat: Many students write "niche is where an organism lives." Wrong — that is the habitat. Niche is the organism's role and all its biotic/abiotic requirements.

Succession mechanism: Students often say "environment improves" without specifying how. State: pioneer species add organic matter, fix nitrogen, weather rock → soil develops → more species can colonise.

Intraspecific vs interspecific: "Intra" = same species; "inter" = different species. A common mix-up costing easy marks.

Predator–prey time lag: Must state that predator numbers peak after prey numbers peak, and explain the reproductive delay.

Conservation management: State the purpose is to arrest succession at a desired sub-climax stage, not simply to "protect wildlife."

DEEP Chain — Succession Mechanism

D — Define: Primary succession is the sequential colonisation of bare, previously uncolonised habitat, from pioneer species to a stable climax community.

E — Explain mechanism: Pioneer species (e.g. lichens) tolerate extreme abiotic conditions. They fix nitrogen and deposit organic matter as they die → thin soil forms → mosses colonise → soil deepens → herbaceous plants → shrubs → trees. At each stage, species modify abiotic conditions, making the environment less suitable for pioneers and more suitable for the next seral community.

E — Evidence/Example: UK climax community is typically oak woodland. Chalk grassland is maintained at a sub-climax stage by sheep grazing; cessation of grazing leads to hawthorn scrub invasion within approximately 10 years.

P — Practical/Quantitative: Species diversity (measured using Simpson's diversity index) increases through intermediate seral stages. Managed chalk grassland maintained by grazing supports substantially higher plant species richness than the equivalent ungrazed scrub, demonstrating that conservation management maintains biodiversity above the levels found in either bare ground or dense scrub.

Practice Question

A conservation organisation is managing a heathland habitat in southern England.

The heathland supports several rare invertebrate species. Since grazing by rabbits declined following a disease outbreak, scrub vegetation has been encroaching on the open heathland. The organisation is considering controlled burning as a management strategy.

(a) Explain why the decline of rabbit grazing has led to an increase in scrub vegetation. [3 marks]

(b) Explain how controlled burning could arrest succession and help maintain the heathland community. [3 marks]

(c) Suggest one conflict that might arise between conservation of this heathland and the needs of a local farming community. [2 marks]

(a) Rabbits graze on grasses and low-growing plants, preventing taller shrub species from establishing (1). Without grazing pressure, interspecific competition favours shrubs: they outcompete heathland plants for light and space (1). Succession proceeds to a later seral stage with scrub species (e.g. hawthorn, gorse) dominating (1).

(b) Controlled burning removes the above-ground biomass of scrub and rank grasses (1). This resets the habitat to an earlier successional stage by removing the dominant species (1). Pioneer and heathland species can re-establish from seeds or buried root stock as succession restarts from this earlier stage (1).

(c) e.g. Farmers may wish to use the heathland for grazing livestock / extending agricultural land (1). Conservation measures such as controlled burning or fencing to exclude livestock could prevent agricultural use of the land, reducing farm income / restricting food production (1).

Synoptic Links & Summary

Synoptic Link — Paper 3 Essay Alert

This topic regularly appears in synoptic essay titles. Common examples: "The importance of biodiversity", "The role of natural selection in evolution", "How ecosystems are maintained". Link succession → biodiversity → conservation → human impact on ecosystems. Also connect to:

Topic 4 (Biodiversity) — species diversity index (Simpson's D); in-situ vs ex-situ conservation

Topic 5 (Energy transfers) — productivity decreases along food chains; energy available limits carrying capacity

Topic 7.06 — population sampling (quadrats, transects, MRC) to monitor succession and conservation outcomes

Key Points Checklist

Ecosystem = community + abiotic environment; niche = role + biotic & abiotic requirements

Carrying capacity (K) = max sustainable population; S-shaped (logistic) growth curve

Intraspecific competition = same species; interspecific = different species

Predator–prey cycles: predator peak follows prey peak due to time lag in reproduction

Primary succession: bare habitat → pioneer species → intermediate seral stages → climax community

At each stage: species modify abiotic environment; less suitable for previous species, more suitable for next

Conservation often involves arresting succession at a sub-climax stage (e.g. burning, grazing, mowing)

Conflict: human needs (agriculture, development) vs conservation of habitats; sustainability requires managing both