Ecosystems and Communities

This lesson covers the key concepts of ecosystems and communities as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the organisation of life within ecosystems, the difference between biotic and abiotic factors, and how organisms within communities are interdependent.

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What Is an Ecosystem?

An ecosystem is a community of living organisms interacting with each other and with the non-living (physical) components of their environment. Ecosystems can be large (e.g. tropical rainforest, ocean) or small (e.g. a pond, a rotting log).

Every ecosystem contains two types of component:

Component	Meaning	Examples
Biotic	Living factors	Plants, animals, fungi, bacteria, competition, predation
Abiotic	Non-living factors	Temperature, light, water availability, soil pH, wind

Exam Tip: The examiner may describe a scenario and ask you to identify biotic and abiotic factors. Always check — if something is alive or produced by a living organism, it is biotic.

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Key Ecological Terms

You must be confident with the following definitions:

Term	Definition
Habitat	The place where an organism lives, e.g. a pond, a woodland, a rock pool
Population	All the organisms of one species living in a habitat at a given time
Community	All the populations of different species living and interacting in an area
Ecosystem	A community of organisms plus the abiotic (non-living) conditions in which they live
Niche	The role an organism plays within its ecosystem, including what it eats and how it interacts with other species

Hierarchy of Organisation

graph LR
    A["Individual organism"] --> B["Population (one species)"]
    B --> C["Community (all species)"]
    C --> D["Ecosystem (community + abiotic environment)"]

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Interdependence

Organisms within a community are interdependent — they depend on each other for survival. Changes in one species can affect others.

Examples of interdependence:

• Bees pollinate flowers; without bees, many plants cannot reproduce, which in turn affects animals that eat those plants.
• Decomposers break down dead material and recycle nutrients back into the soil, which plants need to grow.
• Predator-prey relationships — if the population of a prey species falls, predators may also decline because of reduced food supply.

Exam Tip: If asked to explain the effect of removing one species from a food web, trace the knock-on effects both up (predators lose food) and down (prey populations may increase).

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Stable Communities

A stable community is one where all the species and environmental factors are in balance so that population sizes remain relatively constant over time. Examples include:

• Tropical rainforests — high biodiversity, stable temperature and rainfall year-round
• Ancient oak woodlands — established over centuries with balanced predator-prey relationships

In a stable community:

• Resources are sufficient for all species
• Predation, competition and disease keep populations in check
• Physical conditions remain relatively constant

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Competition

Organisms in a community compete for limited resources. Competition can be:

Intraspecific Competition

Competition between organisms of the same species, e.g. two oak trees competing for light.

Interspecific Competition

Competition between organisms of different species, e.g. red squirrels and grey squirrels competing for food.

Organisms compete for...	Plants	Animals
Light	Yes	—
Water	Yes	Yes
Minerals / nutrients	Yes	—
Space / territory	Yes	Yes
Food	—	Yes
Mates	—	Yes

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Producers, Consumers and Trophic Levels

Every ecosystem relies on energy flow through trophic levels:

Trophic Level	Name	Description
1	Producer	Organisms that make their own food by photosynthesis (e.g. plants, algae)
2	Primary consumer	Herbivores that eat producers (e.g. rabbit, caterpillar)
3	Secondary consumer	Carnivores or omnivores that eat primary consumers (e.g. frog, robin)
4	Tertiary consumer	Top predators that eat secondary consumers (e.g. hawk, fox)

Decomposers (bacteria and fungi) break down dead organisms at every trophic level and recycle nutrients.

graph TD
    A["Sun (energy source)"] --> B["Producers (plants, algae)"]
    B --> C["Primary consumers (herbivores)"]
    C --> D["Secondary consumers (carnivores)"]
    D --> E["Tertiary consumers (top predators)"]
    B --> F["Decomposers"]
    C --> F
    D --> F
    E --> F
    F --> G["Nutrients returned to soil"]
    G --> B

Exam Tip: Always remember that the Sun is the ultimate source of energy for almost all ecosystems. Producers transfer light energy into chemical energy via photosynthesis.

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Summary

• An ecosystem includes all the living organisms (community) and non-living factors in an area.
• A population is one species in an area; a community is all species together.
• Organisms are interdependent — changes to one species affect others.
• Abiotic factors are non-living; biotic factors are living.
• Stable communities have balanced populations and constant environmental conditions.
• Organisms compete for resources — intraspecific (same species) and interspecific (different species).
• Energy flows from producers through consumers to decomposers.

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Worked Example: Estimating Population Density from Quadrat Data

A field biologist uses a 0.25 m² quadrat to estimate the density of dandelions in a meadow measuring 40 m × 25 m. Ten randomly placed quadrats contain 2, 5, 3, 6, 4, 0, 7, 3, 5 and 5 dandelions respectively.

Step 1 — Mean per quadrat.

Total dandelions = 2 + 5 + 3 + 6 + 4 + 0 + 7 + 3 + 5 + 5 = 40. Mean = 40 / 10 = 4 dandelions per 0.25 m².

Step 2 — Density per square metre.

Density = mean / quadrat area = 4 / 0.25 = 16 dandelions per m².

Step 3 — Scale up to the full meadow.

Total area = 40 x 25 = 1,000 m². Estimated population = 16 x 1,000 = 16,000 dandelions.

Common mistake callout: Students often forget to divide by quadrat area. If you calculate "mean x total area" without first converting to density per m², you will undercount by a factor of four (for a 0.25 m² quadrat). Always state units at every step.

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Comparison Table: Levels of Organisation

Level	Contains	Example	Biotic only?
Organism	A single individual	One oak tree	Yes
Population	All individuals of one species in an area	All oak trees in Sherwood Forest	Yes
Community	All populations of all species in an area	Oaks, squirrels, fungi, beetles in Sherwood	Yes
Ecosystem	Community + abiotic environment	Sherwood Forest plus soil, rainfall, sunlight	No (includes abiotic)
Biome	Large region with a characteristic climate and community	Temperate deciduous forest	No

Exam Tip: The word "ecosystem" is the only level that explicitly includes abiotic factors. A community is purely biological.

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Mermaid Diagram: Interdependence in a Meadow Community

graph TD
    A["Wildflowers"] -->|"nectar / pollen"| B["Bees"]
    B -->|"pollination"| A
    A -->|"food"| C["Rabbits"]
    C -->|"faeces (nutrients)"| D["Soil + decomposers"]
    D -->|"mineral ions"| A
    C -->|"prey"| E["Foxes"]
    E -->|"carcass"| D

Every arrow is a relationship of interdependence. Removing any node (for example, through disease) changes resource availability for every other node, illustrating why biodiversity contributes to stability.

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Common-Mistake Callouts

Mistake 1: Writing that "a pond is a community." A pond is a habitat or, if abiotic factors are included, an ecosystem. The community is the set of living populations within it.

Mistake 2: Confusing niche with habitat. The habitat is where an organism lives; the niche is what it does (feeding method, activity time, reproductive strategy). Two species cannot share an identical niche in the long term — one will outcompete the other (the competitive exclusion principle).

Mistake 3: Listing "sunlight" as biotic. Sunlight is abiotic energy; the organisms that capture it (producers) are biotic.

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Exam-Style Practice Question

Q. A woodland contains oak trees, grey squirrels, red squirrels, foxes and soil bacteria. Explain why the community is described as stable, and predict two effects of removing the foxes. (6 marks)

Model points:

1. A stable community has relatively constant population sizes over time.
2. Populations are balanced by predation, competition and disease.
3. Removing foxes (a predator) reduces predation pressure on squirrels.
4. Squirrel populations may increase, leading to greater interspecific competition between red and grey squirrels.
5. Increased grazing on acorns may reduce oak regeneration.
6. Fewer fox carcasses reduces nutrients available to decomposers, affecting soil fertility.

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Answering at different grade levels

Grade band	Expected response to "Explain how abiotic factors shape a community"
Grades 1–3	States one abiotic factor (e.g. "light") and gives a simple example ("plants need light"). Limited use of specification terminology.
Grades 4–5	Identifies several abiotic and biotic factors and links them to distribution. Uses terms such as community, population and interdependence correctly.
Grades 6–7	Explains how changes in abiotic factors cause measurable shifts in population sizes and compositions. Correctly distinguishes biotic factor from abiotic factor, and references trophic levels.
Grades 8–9	Constructs a fully linked argument showing how abiotic change propagates through the biomass pyramid via interdependence, references the carbon cycle, water cycle and decomposition, and evaluates impact on biodiversity.

To reach grades 8–9 you must move beyond listing factors and instead chain reasoning: abiotic change -> producer biomass change -> trophic level knock-on -> decomposition rate -> nutrient cycling -> biodiversity.

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Extended Case Study: A British Pond Ecosystem

A small garden pond offers a compact example of everything in this lesson. Producers include pondweed and algae; primary consumers include tadpoles, water fleas and pond snails; secondary consumers include dragonfly larvae and small fish; and decomposers include bacteria and fungi breaking down leaf litter at the bottom. Abiotic factors — water temperature, dissolved oxygen, pH, light penetration — vary with season and depth. In summer, warm surface water supports rapid algal growth, but at night respiration drops oxygen sharply, showing how abiotic conditions interact with biotic activity. When autumn leaves fall, decomposition releases mineral ions that fuel next spring's producers, linking the carbon and water cycles to ecosystem productivity.

Checklist of Key Terms

Term	Meaning	Biotic or abiotic?
Biotic factor	Any living influence on a community	Biotic
Abiotic factor	Any non-living influence	Abiotic
Population	All organisms of one species in an area	Biotic
Community	All populations in an area	Biotic
Ecosystem	Community plus abiotic environment	Both
Interdependence	Mutual reliance between species	Biotic
Trophic level	Position in a food chain	Biotic concept
Biomass pyramid	Mass at each trophic level	Biotic measurement
Decomposition	Breakdown of dead matter by decomposers	Biotic process
Biodiversity	Variety of life within an area	Biotic measure
Carbon cycle	Movement of carbon between reservoirs	Both
Water cycle	Movement of water between reservoirs	Both

Knowing which side of the biotic/abiotic line each term sits on is worth easy marks on short-answer questions.

Six-Mark Model Answer Framework

When a six-mark question asks you to describe relationships within a community, structure your answer as follows:

1. Define the ecosystem scope (named habitat, geographic location).
2. Identify producers and at least one consumer at each trophic level.
3. Identify one abiotic factor and one biotic factor limiting the community.
4. Describe an interdependence relationship (for example, pollinator and plant).
5. Explain how disturbance would propagate through the biomass pyramid.
6. Conclude with implications for biodiversity and material cycles.

Hitting all six bullet points reliably secures full marks.

Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Biology Topic 9 Ecosystems and material cycles — specifically CB9.1 Communities and ecosystems, CB9.2 Abiotic and biotic factors, and CB9.4 Biodiversity. Assessed on Biology Paper 2.