Organisms and their environment: ecosystems and food webs

What you'll learn

This topic examines how organisms interact within their environment and how energy flows through ecosystems. You'll learn to construct and interpret food chains and webs, understand trophic levels, and explain key ecological relationships including predator-prey dynamics, competition, and nutrient cycling.

Key terms and definitions

Producer — an organism that makes its own organic nutrients, usually through photosynthesis (e.g. green plants, algae)

Consumer — an organism that obtains energy by feeding on other organisms; includes herbivores, carnivores, and omnivores

Trophic level — the position of an organism in a food chain, food web, or pyramid; representing the number of energy transfers from producers

Food chain — a diagram showing the transfer of energy from one organism to the next, beginning with a producer

Food web — a network of interconnected food chains showing the feeding relationships within an ecosystem

Decomposer — an organism that breaks down dead organic material and waste products, releasing nutrients back into the environment (e.g. bacteria, fungi)

Biomass — the total mass of living material in an organism or at a trophic level

Pyramid of numbers — a diagram showing the number of organisms at each trophic level in a food chain at a particular time

Core concepts

Producers and the basis of food chains

All food chains begin with producers. These are typically green plants or algae that convert light energy into chemical energy through photosynthesis. The equation for photosynthesis is:

carbon dioxide + water → glucose + oxygen

Producers form the first trophic level in every ecosystem. Examples include:

• Grass in a meadow ecosystem
• Phytoplankton in marine ecosystems
• Trees in woodland ecosystems
• Seaweed on rocky shores

Producers are autotrophs — they manufacture their own food from inorganic substances. Without producers, no other life could exist because they are the original source of all food and energy in an ecosystem.

Consumers and trophic levels

Consumers are heterotrophs — they cannot make their own food and must obtain organic nutrients by consuming other organisms. Consumers are classified by their position in the food chain:

Primary consumers (second trophic level):

• Herbivores that feed directly on producers
• Examples: rabbits eating grass, caterpillars eating leaves, zooplankton feeding on phytoplankton

Secondary consumers (third trophic level):

• Carnivores that feed on primary consumers
• Examples: foxes eating rabbits, small birds eating caterpillars, small fish eating zooplankton

Tertiary consumers (fourth trophic level):

• Carnivores that feed on secondary consumers
• Examples: hawks eating small birds, large fish eating small fish

Quaternary consumers (fifth trophic level):

• Top predators with few or no natural predators
• Examples: orcas, polar bears, eagles

Omnivores feed on both plants and animals, so they can occupy different trophic levels depending on what they eat. Humans are omnivores.

Food chains and food webs

A food chain shows a single pathway of energy transfer. Arrows show the direction of energy flow (from the organism being eaten to the organism doing the eating).

Example food chain: grass → grasshopper → frog → snake → hawk

Each arrow means "is eaten by" or "provides energy for".

A food web is more realistic than a food chain because most organisms eat more than one type of food and are eaten by more than one predator. Food webs show multiple interconnected food chains within an ecosystem.

In a food web:

• Organisms can appear at different trophic levels
• Removing one species affects multiple pathways
• The system is more stable than a single food chain

For example, in a pond ecosystem, a small fish might eat both insect larvae and algae, placing it at different trophic levels depending on its food source.

Energy transfer and pyramids

Energy enters ecosystems through producers (via photosynthesis) and flows through trophic levels. However, energy transfer between levels is inefficient:

Energy losses occur because:

• Not all parts of organisms are eaten (e.g. bones, roots)
• Not all eaten material is absorbed (some passes out as faeces)
• Energy is released as heat during respiration
• Energy is used for movement and other life processes

Typically, only about 10% of energy is transferred from one trophic level to the next. This is why:

• Food chains rarely exceed 4-5 trophic levels
• There are fewer organisms at higher trophic levels
• Top predators are relatively rare

Pyramids of numbers show the number of organisms at each trophic level. They usually have a pyramid shape (wide base, narrow top), but not always:

• One tree can support many caterpillars (inverted at the base)
• Parasites are numerous but small (inverted at the top)

Pyramids of biomass show the total mass of organisms at each level. These are more reliable and almost always show a true pyramid shape because they account for organism size.

Pyramids of energy show energy flow through trophic levels. These always form true pyramids because energy cannot increase as you move up levels (it's always lost).

Decomposers and nutrient cycling

Decomposers play a crucial role by breaking down dead organic matter and waste products. Main decomposer groups include:

• Bacteria (break down all types of organic matter)
• Fungi (especially important for breaking down plant material like wood)

Decomposition returns nutrients to the soil, where they can be absorbed by plant roots and reused. This creates nutrient cycles.

Conditions affecting decomposition rate:

• Temperature: decomposers work faster in warm conditions (enzymes work faster)
• Oxygen: many decomposers need oxygen for respiration
• Moisture: decomposers need water for metabolic processes

Without decomposers:

• Dead organisms and waste would accumulate
• Nutrients would become locked up in dead matter
• Producers would run out of mineral nutrients
• Ecosystems would collapse

Ecological relationships

Predator-prey relationships: Predators hunt and kill prey for food. Population sizes of predators and prey are interconnected:

• When prey numbers increase, predators have more food, so predator numbers increase
• When predator numbers increase, more prey are eaten, so prey numbers decrease
• When prey numbers fall, predators have less food, so predator numbers decrease
• When predator numbers fall, fewer prey are eaten, so prey numbers recover

This creates cyclical population changes, with predator peaks lagging behind prey peaks.

Competition: Organisms compete for limited resources.

Intraspecific competition (within the same species):

• Competition between members of the same species
• Usually more intense because they need identical resources
• Examples: plants competing for light, water, minerals; animals competing for food, mates, territory

Interspecific competition (between different species):

• Competition between members of different species
• For shared resources like food, water, space
• Example: different bird species competing for nesting sites in the same trees

The better competitor will be more successful, potentially excluding the other from that habitat.

Worked examples

Example 1: Constructing and interpreting a food web

Question: The diagram shows a food web in a woodland ecosystem.

Oak tree → Caterpillar → Blue tit → Sparrowhawk Oak tree → Aphid → Ladybird → Spider → Blue tit Oak tree → Woodlouse → Shrew → Owl

(a) Name a producer in this food web. [1] (b) Which organism is a primary consumer of oak trees? Name two. [2] (c) At what trophic level is the blue tit when it eats caterpillars? [1] (d) Explain what would happen to the aphid population if all ladybirds were removed from this ecosystem. [2]

Mark scheme answers:

(a) Oak tree [1 mark]

• Only the oak tree is a producer (makes its own food by photosynthesis)

(b) Caterpillar, aphid, woodlouse [2 marks for any two]

• Primary consumers feed directly on producers
• All three feed on the oak tree

(c) Third trophic level / secondary consumer [1 mark]

• Oak tree (producer, 1st) → caterpillar (2nd) → blue tit (3rd)

(d) The aphid population would increase [1 mark] because there would be fewer/no predators eating them [1 mark]

• Removing a predator reduces predation pressure on prey
• More aphids survive and reproduce

Example 2: Energy transfer and pyramids

Question: A pyramid of biomass is shown below for a grassland ecosystem:

        Hawks (40 kg)
      Snakes (400 kg)
    Frogs (3000 kg)
  Grasshoppers (8000 kg)
Grass (20,000 kg)

(a) Calculate the percentage of energy transferred from grass to grasshoppers. [2] (b) Explain why energy transfer between trophic levels is inefficient. [3] (c) Why are pyramids of biomass more useful than pyramids of numbers? [2]

Mark scheme answers:

(a) (8000 ÷ 20,000) × 100 [1 mark] = 40% [1 mark]

• Show working for method mark
• Correct calculation for second mark

(b) Any three from:

• Not all parts of organisms are eaten / some parts indigestible [1]
• Some energy lost in faeces / egestion [1]
• Energy used in respiration / released as heat [1]
• Energy used for movement / life processes [1]

(c) Pyramids of biomass account for the size of organisms [1], whereas pyramids of numbers only count individuals which can give misleading results [1]

• Alternative: One large organism (like a tree) supports many small organisms, which would invert a pyramid of numbers but not biomass

Example 3: Decomposers and nutrient cycling

Question: An investigation compared decomposition rates at different temperatures. Dead leaves were placed in mesh bags and buried in soil at 10°C, 20°C, and 30°C. After four weeks, the percentage mass loss was recorded.

Results: 10°C = 15% loss; 20°C = 35% loss; 30°C = 62% loss

(a) Explain why decomposition was faster at higher temperatures. [3] (b) Describe the role of decomposers in an ecosystem. [3]

Mark scheme answers:

(a) Decomposers / bacteria / fungi work faster at higher temperatures [1] because their enzymes work faster [1] so they break down dead material more quickly / respire faster [1]

(b) Decomposers break down dead organisms and waste products [1] They release / recycle nutrients back into the soil [1] These nutrients can be absorbed by plant roots / used by producers [1]

Common mistakes and how to avoid them

• Reversing food chain arrows: Arrows show energy flow from food source to consumer (what is eaten → what eats it), not the direction of eating. Remember: the arrow points to where the energy goes.

• Confusing trophic levels: Students often miscalculate trophic levels in complex food webs. Always count from the producer: producer = 1st, primary consumer = 2nd, secondary consumer = 3rd, etc.

• Saying energy is "lost" or "destroyed": Energy is not destroyed (law of conservation of energy). It is transferred to less useful forms (mainly heat) or used for life processes. Use precise language: "energy is transferred" or "energy is released as heat."

• Forgetting decomposers: Many students omit decomposers from ecosystem descriptions. Remember they are essential for nutrient recycling and should be included when describing complete ecosystems.

• Assuming all pyramids are the same shape: Pyramids of numbers can be inverted; pyramids of biomass rarely are; pyramids of energy never are. Know when and why each type might have an unusual shape.

• Oversimplifying predator-prey relationships: Population changes are cyclical, with predator peaks lagging behind prey peaks. Don't just say "when one goes up, the other goes down" — explain the time delay and the complete cycle.

Exam technique for "Organisms and their environment: ecosystems and food webs"

• Command words matter: "State" requires a simple answer; "Explain" requires reasons; "Describe" requires characteristics or changes but not reasons. For a 3-mark "Explain" question, give three distinct points with reasoning.

• Drawing food chains and webs: Always use arrows correctly (pointing to the consumer). Start with a producer. Write organism names, not just trophic levels. Keep your diagram clear and well-spaced for web questions.

• Calculation questions: Show your working for energy transfer or percentage calculations. This allows method marks even if your final answer is incorrect. Include units where appropriate.

• Extended response questions: Structure answers logically. For ecosystem impact questions, consider: immediate effect → consequence → further consequence. Use ecological terminology precisely (producers, consumers, trophic levels, predation, competition).

Quick revision summary

Ecosystems consist of producers (make own food via photosynthesis), consumers (herbivores, carnivores, omnivores at different trophic levels), and decomposers (recycle nutrients). Food chains show single energy pathways; food webs show interconnected relationships. Only ~10% of energy transfers between levels due to heat loss, respiration, and incomplete consumption. This limits food chains to 4-5 levels. Pyramids of numbers, biomass, and energy represent ecosystem structure differently. Predator-prey populations cycle in linked patterns. Competition occurs within and between species. Decomposers are essential for nutrient cycling, returning minerals to soil for producer uptake.