Ecosystems and Food Chains

Ecosystems and the levels of organisation

An ecosystem is the interaction of a community of living organisms (biotic parts) with the non-living (abiotic) parts of their environment. Levels of organisation:

• Organism → population (all of one species in an area) → community (all the populations) → ecosystem (community + environment).

Organisms compete for resources and depend on each other — this is interdependence. If one species is removed, it can affect many others.

Competition

• Plants compete for light, water, space and mineral ions.
• Animals compete for food, water, mates and territory.

Abiotic and biotic factors

• Abiotic (non-living) factors: light intensity, temperature, moisture, soil pH, CO₂ and oxygen levels.
• Biotic (living) factors: availability of food, new predators, new pathogens, and competition between species.

Food chains and food webs

A food chain shows the feeding relationships and the transfer of energy:

producer → primary consumer → secondary consumer → tertiary consumer

• Producers (usually green plants/algae) make their own food by photosynthesis — they are the start of nearly every food chain.
• Primary consumers (herbivores) eat producers.
• Secondary/tertiary consumers are carnivores that eat other consumers.
• Predators eat prey. Arrows point in the direction energy flows (from the eaten to the eater).

A food web is many interlinked food chains, showing that most animals eat more than one thing.

Predator–prey cycles

The numbers of predators and prey rise and fall in linked cycles:

• More prey → predators have more food → predator numbers rise.
• More predators → more prey eaten → prey numbers fall → then predator numbers fall → prey recover. The cycles are out of phase (the predator peak lags behind the prey peak).

The carbon and water cycles

• Carbon cycle: carbon moves between the air (CO₂), living things and the ground. Photosynthesis removes CO₂; respiration, combustion and decay return it.
• Decomposers (bacteria and fungi) break down dead organisms and waste, recycling nutrients back into the ecosystem.

Biodiversity and human impact

Biodiversity is the variety of species in an ecosystem — high biodiversity makes ecosystems more stable. Humans reduce it through deforestation, pollution, and habitat destruction, and by increasing greenhouse gases. Conservation measures include protecting habitats, replanting and reducing waste.

Worked example

In the chain grass → rabbit → fox, what are the roles?

• Grass = producer, rabbit = primary consumer (herbivore), fox = secondary consumer (predator). Arrows show energy flowing from grass to rabbit to fox. ✓

Common mistakes

• Drawing food-chain arrows the wrong way — they point towards the eater (energy flow).
• Confusing abiotic (non-living) and biotic (living) factors.
• Forgetting producers start the chain and make food by photosynthesis.

Exam tips

• Learn the levels of organisation and the food-chain roles (producer, primary/secondary consumer, predator/prey).
• Explain predator–prey graphs: predator peaks lag behind prey peaks.
• Mention decomposers when asked how nutrients are recycled.

Key facts to remember

• Ecosystem = community + abiotic environment; organisms show interdependence and compete for resources.
• Food chains start with producers (photosynthesis); arrows show energy flow to the eater; food webs link many chains.
• Decomposers recycle nutrients; biodiversity gives stability and is reduced by human activity.