Consumers at Higher Level: The same strategies as SL, with different examples: a European beehive, India's vultures and the Amazon rainforest. At HL, expect to explain in depth how decomposers keep a whole ecosystem stable.
Practise this as you read
- Name the strategy and an example for every consumer you meet.
- Link each decomposer point to stability or recovery.
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Food made by someone else: A consumer gets its energy from the carbon compounds it takes from other organisms, alive or dead.
The points to remember
- Consumers cannot make their own food: they take carbon compounds from other organisms.
- These carbon (organic) compounds, such as sugars, fats and proteins, hold chemical energy.
- Respiration in the consumer's cells then releases that energy for life (more in 2.2.6).
- Consumers have diverse strategies: some feed on living organisms, others on dead matter.
Remember it as: Can't make it, so take it.
Real example: in a European beehive, honeybees get their energy from the sugar in flower nectar, Asian hornets from the bees they catch, and tiny Varroa mites from the bees' own bodies.
Where this fits: Producers make their own carbon compounds and start every food chain (2.2.10). The words autotroph and heterotroph come later, in 2.2.21.
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Some consumers feed on organisms that are still alive. What they eat, and whether they kill it, decides their strategy.
Feeding on the living
- Herbivores eat producers: plants or algae (cows, rabbits, caterpillars).
- Carnivores eat other animals; most are predators, which kill and eat their prey (lions, hawks, spiders).
- Parasites live on or in a living host and feed on it, usually without killing it quickly.
- Omnivores eat both plants and animals (humans, bears, crows), so they feed at more than one level.
- Predators and parasites help control population sizes of the species they feed on.
Remember it as: Plants, prey or a host: three ways to eat the living.
Herbivore
- Honeybees feeding on nectar and pollen
Predator
- Asian hornets catching honeybees
Parasite
- Varroa mites feeding on bees
Real example: the Asian hornet reached France around 2004 and has spread across Europe, catching bees outside their hives, while Varroa mites, now found almost worldwide, weaken whole colonies.
A parasite is not a predator: A predator kills its prey and eats it. A parasite needs its host alive: it feeds on it, often for a long time.
Dead bodies, fallen leaves and dung still hold carbon compounds, and so energy. A whole group of consumers lives on them: detritus.
Feeding on the dead
- Scavengers eat the bodies of dead animals (vultures, hyenas, crabs).
- Detritivores eat dead or decaying plant or animal matter and dung, digesting it inside their bodies.
- Saprotrophs release enzymes onto dead matter and absorb the digested food: most fungi, many bacteria.
- Decomposers break dead matter down into inorganic nutrients: saprotrophic fungi and bacteria, helped by detritivores.
Remember it as: Scavengers take the bodies, detritivores eat the bits, saprotrophs dissolve the rest.
Detritivore
- Eats dead matter
- Digests it inside its body
- e.g. earthworms, woodlice, dung beetles
Saprotroph
- Grows into dead matter
- Digests it outside its body with enzymes
- e.g. fungi, many bacteria
Real example: India's vultures fell by more than 95% after they ate the bodies of cattle treated with the painkiller diclofenac. Carcasses rotted for longer, feral dogs fed on them instead, and cases of rabies rose.
Define it cleanly: A detritivore eats dead or decaying plant or animal matter. Keep extra facts out of the definition: 'eats carrion and live animals' contradicts it, unless you are describing one named species.
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Energy flows, nutrients cycle: Energy passes through an ecosystem once and is lost as heat. Nutrients are used again and again, and decomposers make that possible.
The points to remember
- Decomposers feed on dead matter, so leaves, dung and bodies do not pile up.
- Detritivores such as earthworms shred it first; fungi and bacteria finish the job.
- They turn organic matter into inorganic nutrients, such as nitrates and phosphates, in the soil or water.
- Plant roots absorb these nutrients, so the soil stays fertile and plants keep growing.
- So decomposers drive the carbon, nitrogen and phosphorus cycles.
- Earthworms also loosen the soil, letting in air and water for roots.
Real example: on the floor of the Amazon rainforest, fallen leaves are broken down within weeks, and shallow roots take the nutrients back almost at once. Most nutrients are held in the plants, so the soil itself is poor.
A stable ecosystem keeps itself going and recovers after a disturbance. Decomposers help in several separate ways, and each one can be a mark.
The points to remember
- More nutrients mean more plants, so more habitats and niches for other species.
- Decomposers are food too: the base of a detritus food web that feeds carnivores.
- Negative feedback: when decomposers boom, their predators eat more of them, so numbers settle.
- Decomposers compete with each other, which also keeps their numbers in check.
- When decomposers die or egest waste, other decomposers feed on it, so matter keeps cycling.
- They build humus, which holds water and nutrients in the soil.
- After a fire or flood they speed up and return nutrients fast: the ecosystem recovers (resilience).
Remember it as: Clean up, feed the soil, feed the web, bounce back.
Real example: when an area of Amazon rainforest is cleared and burned, decomposers quickly break down the ash and litter, but with no trees to take the nutrients back, rain washes them away: the loop only keeps the forest stable while it stays closed.
Finish every point with the effect: In an Explain [7], write each point as a cause and its effect on stability: 'decomposers release nitrates, so plants keep growing, so the ecosystem keeps its habitats'.
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How this comes up: Paper 2, Section B (b): explain how decomposers contribute to the stability of an ecosystem [7].
In the Amazon rainforest, leaves, fruit and dead animals fall to the forest floor all year round.
Explain how the community of decomposers contributes to the stability of the rainforest ecosystem.
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