Cambridge Lower Secondary CheckpointStage 7

Biology: Ecosystems and feeding relationships

Science Stage 7 Chapter Notes

What this chapter covers

Biology: Ecosystems and feeding relationships
ShareWhatsAppPost
Biology: Ecosystems and feeding relationships notes

Unable to load PDF

The notes viewer could not load. Please refresh the page.

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Biology: Ecosystems and feeding relationships notes as text: skim, search, and jump between subtopics.

~9 min read

In this chapter

  1. 1Ecosystems: The Basics
  2. 2Food Chains and Energy Flow
  3. 3Food Webs: Complex Interactions
  4. 4Decomposers and Nutrient Cycling

1. Ecosystems: The Basics

An ecosystem is the complex network of interactions between a community of living organisms (biotic factors) and their non-living physical environment (abiotic factors). Key terms to understand are:

  • Habitat: The specific place where an organism lives.
  • Population: All the organisms of a single species living in a habitat.
  • Community: All the different populations of different species living and interacting in a habitat.

An ecosystem can be as small as a pond or as large as a rainforest. It's all about the connection between life and the environment it depends on.

Key term

Ecosystem: A biological system of interacting organisms (the community) and their physical environment (the abiotic factors).

Examiner insight

Examiners look for the key idea that an ecosystem includes both the living organisms and their non-living environment.

Common pitfall

Confusing a 'community' (all the different species in an area) with a 'population' (all the individuals of just one species).

Worked example 13 marks

A student is studying a small woodland area. They list the following: oak tree, soil, squirrel, rainfall, fox, air temperature, and a patch of moss. Categorise each item as either a biotic or an abiotic factor.

  1. 1

    Step 1: Identify the definition of biotic and abiotic. Biotic factors are the living or once-living components of an ecosystem. Abiotic factors are the non-living physical and chemical components.

  2. 2

    Step 2: Categorise the living components. The oak tree, squirrel, fox, and moss are all living organisms. Therefore, they are biotic factors.

  3. 3

    Step 3: Categorise the non-living components. The soil, rainfall, and air temperature are all non-living parts of the environment. Therefore, they are abiotic factors.

  4. 4

    Answer: Biotic factors: oak tree, squirrel, fox, moss. Abiotic factors: soil, rainfall, air temperature.

Recap

  • An ecosystem includes both living (biotic) and non-living (abiotic) components.
  • A habitat is an organism's address.
  • A population is one species in a habitat.
  • A community is all the different species in a habitat.

Quick check

  1. What is the difference between a population and a community?2 marks

2. Food Chains and Energy Flow

A food chain is a simple model that shows how energy is transferred through an ecosystem. It always starts with a producer, an organism that makes its own food, usually through photosynthesis. The arrows in a food chain are crucial: they show the direction of energy flow. For example, in 'grass → rabbit', the energy flows from the grass to the rabbit when it is eaten. Organisms that eat other organisms are called consumers.

  • Producers: Make their own food (e.g., plants, algae).
  • Primary Consumers: Eat producers (herbivores).
  • Secondary Consumers: Eat primary consumers (carnivores or omnivores).
  • Tertiary Consumers: Eat secondary consumers.

Key term

Producer: An organism that produces its own food, usually using light energy from the sun in a process called photosynthesis.

Common pitfall

Drawing arrows pointing the wrong way. The arrow must point in the direction of energy flow, from the food to the feeder.

Fun fact

Only about 10% of the energy from one level of a food chain is passed on to the next. The rest is lost as heat, used for movement, or is not digested.

Worked example 14 marks

In a garden, lettuces are eaten by slugs, and the slugs are eaten by thrushes.a) Draw the food chain.b) Identify the producer, primary consumer, and secondary consumer.

  1. 1

    Step 1 (a): To draw the food chain, place the organisms in order of feeding. The arrows must point from the organism being eaten to the organism that eats it, showing the direction of energy transfer.

  2. 2

    Answer (a): Lettuce → Slug → Thrush

  3. 3

    Step 2 (b): Identify the role of each organism. The producer is the plant that makes its own food. The primary consumer eats the producer. The secondary consumer eats the primary consumer.

  4. 4

    Answer (b): Producer: Lettuce; Primary Consumer: Slug; Secondary Consumer: Thrush.

Recap

  • A food chain shows the flow of energy in an ecosystem.
  • Arrows in a food chain point from the organism being eaten to the organism that eats it.
  • Food chains always start with a producer (usually a plant).
  • Organisms that eat plants are primary consumers (herbivores).
  • Organisms that eat other animals are secondary or tertiary consumers (carnivores/omnivores).

Quick check

  1. What is the ultimate source of energy for most food chains on Earth?1 mark

3. Food Webs: Complex Interactions

In any real habitat, organisms rarely eat only one thing. Several food chains overlap and interconnect to form a food web. A food web is a more realistic and complex model of feeding relationships. It shows how a change in one population can affect many other populations. For example, if the population of a key prey animal decreases, it can cause a decline in the populations of several different predators that rely on it for food. Ecosystems with more complex food webs are generally more stable, as organisms have alternative food sources.

Key term

Food Web: A model representing the network of interconnected food chains within an ecosystem.

Examiner insight

Marks are often awarded for explaining the consequences of a change in a food web, such as the population of a predator decreasing if its main prey is removed.

Worked example 15 marks

Consider the following food web: Grass is eaten by voles and grasshoppers. Voles are eaten by owls and stoats. Grasshoppers are eaten by spiders and stoats. Spiders are eaten by shrews.a) Draw this food web.b) What would likely happen to the owl population if a disease wiped out most of the voles? Explain your answer.

  1. 1

    Step 1 (a): Start by placing the producer (Grass) at the bottom. Draw arrows to the primary consumers that eat it (Voles, Grasshoppers). Then draw arrows from these to the secondary consumers, and so on. An organism can have multiple arrows pointing away from it and multiple arrows pointing towards it.

  2. 2

    Answer (a): A diagram showing: Grass → Vole → Owl; Grass → Vole → Stoat; Grass → Grasshopper → Spider → Shrew; Grass → Grasshopper → Stoat.

  3. 3

    Step 2 (b): Identify the owl's food source from the web. The owl only eats voles. If the vole population is wiped out, the owl's food source is gone.

  4. 4

    Step 3 (b): Predict the consequence. With no food, the owls would either have to migrate, starve, or fail to reproduce. Therefore, the owl population would likely decrease significantly.

  5. 5

    Answer (b): The owl population would decrease. This is because voles are their only food source in this food web, so they would have nothing to eat.

Recap

  • A food web consists of multiple, interconnected food chains.
  • Food webs are a more realistic representation of feeding in an ecosystem than food chains.
  • A change in one population can have knock-on effects on many others in the web.
  • Organisms in a food web can be both predator and prey.

Quick check

  1. Why are complex food webs considered more stable than simple ones?2 marks

4. Decomposers and Nutrient Cycling

What happens to organisms when they die? They are broken down by decomposers. Decomposers are microorganisms, mainly bacteria and fungi, that feed on dead organic matter (like dead plants and animals) and waste products (like faeces). This process is called decomposition or decay. As they feed, decomposers release the simple mineral ions (nutrients) that were locked up in the dead bodies back into the soil and water. These nutrients, such as nitrates and phosphates, are then absorbed by producers (plants) and used for growth. This essential process is known as nutrient cycling. Without decomposers, nutrients would remain locked up in dead organisms, and life would eventually grind to a halt.

Key term

Decomposer: An organism, such as a bacterium or fungus, that breaks down dead organic material and returns nutrients to the environment.

Examiner insight

Top marks are given for linking decomposition to the recycling of nutrients, which are then made available again for producers. Simply stating 'they break down dead things' is often not enough for full credit.

Common pitfall

Forgetting that decomposers act on all trophic levels, not just the 'top' of the food chain. They break down dead producers, consumers, and other decomposers.

Worked example 14 marks

A simple food chain is: Rose bush → Aphid → Ladybird → Spider.a) Where would you add decomposers to a diagram of this food chain?b) Explain the role of the decomposers in this ecosystem.

  1. 1

    Step 1 (a): Decomposers break down all dead organic matter. This means they act on every organism in the food chain when it dies. You would draw arrows from the rose bush, aphid, ladybird, and spider all pointing to a box labelled 'Decomposers'.

  2. 2

    Step 2 (b): Define the role of decomposers. They break down dead material. Explain what this breakdown achieves: the release of nutrients.

  3. 3

    Step 3 (b): Link the released nutrients back to the start of the chain. The nutrients are absorbed by the producer (the rose bush).

  4. 4

    Answer (b): Decomposers (bacteria and fungi) break down the dead bodies of the rose bush, aphids, ladybirds, and spiders. This process of decay releases mineral nutrients back into the soil. These nutrients are then absorbed by the roots of the rose bush, allowing it to grow, thus recycling the nutrients within the ecosystem.

Recap

  • Decomposers are mainly bacteria and fungi.
  • They feed on dead organic matter and waste products from all trophic levels.
  • Decomposition releases mineral nutrients back into the environment.
  • These nutrients are then used by producers for growth.
  • This process is called nutrient cycling and is vital for all ecosystems.

Quick check

  1. Name two types of microorganism that act as decomposers.1 mark
  2. What key substances do decomposers return to the soil?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the terms 'community' and 'habitat' as used in ecology.2 marks
  2. A food chain is shown: Algae → Water Flea → Minnow → Perch. Identify the producer, the primary consumer, and the secondary consumer in this food chain.3 marks
  3. Use the following information to construct a food web: Grass is eaten by rabbits and voles. Foxes eat rabbits. Weasels eat voles. Owls eat voles and weasels.4 marks
  4. Explain why a pyramid of biomass is usually a better representation of energy transfer in a food chain than a pyramid of numbers.3 marks
  5. Describe the complete role of decomposers in a woodland ecosystem, explaining why they are essential for the survival of the producers.4 marks
  6. A farmer sprays a field with a persistent pesticide to kill insects. The food chain is: Cabbage → Cabbage White Butterfly → Small Bird → Sparrowhawk. Predict which organism will contain the highest concentration of the pesticide, and explain why this happens.4 marks
  7. From the food web you constructed in question 3, identify one organism that is both a predator and a prey. Justify your answer.2 marks
  8. Explain the difference between how energy and nutrients move through an ecosystem. Use the terms 'flow' and 'cycle' in your answer.5 marks
  9. If a disease were to kill all the foxes in the food web from question 3, predict two likely effects on other populations in the food web. Explain your reasoning for each effect.4 marks
  10. In some deep-sea vents, the ecosystem is based on bacteria that use chemical energy from the vents to make food, rather than sunlight. What term would be used to describe these bacteria in their food chain?1 mark

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Thinking and Working Scientifically: Models and representationsThinking and Working Scientifically: Scientific enquiryThinking and Working Scientifically: Carrying out scientific enquiryThinking and Working Scientifically: Analysis, evaluation and conclusionsBiology: Structure and functionBiology: Life processes