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: on the Serengeti plains of Tanzania, a wildebeest gets its energy from grass, a lion from the wildebeest, and a vulture from what the lion leaves behind.
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
- Wildebeest grazing grass
Predator
- Lions hunting wildebeest
Parasite
- Ticks sucking wildebeest blood
Real example: about 1.3 million wildebeest follow the rains across the Serengeti each year, grazing as they go; lions and hyenas follow the herds, and ticks ride on them.
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.
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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: on the Serengeti, vultures strip a carcass in hours, dung beetles roll away and bury the herds' dung, and fungi and bacteria break down what is left.
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.
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: Australia's native dung beetles could not deal with cattle dung, so pats piled up on the pastures. From 1965 to 1985 scientists brought in dung beetles from Africa and Europe; about 23 species settled, burying dung and returning its nutrients to the soil.
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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: an English oak woodland drops tonnes of leaves each autumn, yet by the next summer most have gone: earthworms, woodlice, fungi and bacteria have turned them into soil, feeding the trees and the shrews, beetles and birds that eat the decomposers.
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'.
How this comes up: Paper 2, Section B (b): explain how decomposers contribute to the stability of an ecosystem [7].
Every autumn an oak woodland in England drops a thick layer of dead leaves.
Explain how detritivores and decomposers contribute to the stability of the oak woodland ecosystem.
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