Ecosystems, Food Webs and Energy Flow

In short: An ecosystem is a community of living things together with its non-living environment. Energy flows one way through food chains and webs, with only about 10% passing to each next level, while matter such as carbon and nitrogen is recycled by decomposers.

Starting point: what you already know

Producers make their own food by photosynthesis. Consumers eat other organisms (Year 7 science).
Energy enters food chains from the sun and is released by respiration (see Photosynthesis and Cellular Respiration).
A habitat is the place an organism lives.

Parts of an ecosystem

An ecosystem is all the organisms in an area and the non-living environment they interact with.

Term	Meaning
Population	All the members of one species in an area
Community	All the populations of different species in an area
Biotic factors	Living things and their effects: predators, competitors, disease, food
Abiotic factors	Non-living conditions: light, temperature, water, soil, pH, salinity, fire

Food chains and food webs

A food chain shows what eats what. Arrows point in the direction energy flows, from the eaten to the eater.

grass → kangaroo → dingo

A food web is many linked food chains, which is more realistic.

Trophic level	Role	Example
Producer	Makes food by photosynthesis	Grass, eucalyptus, algae
Primary consumer	Herbivore, eats producers	Kangaroo, koala, grasshopper
Secondary consumer	Eats primary consumers	Kookaburra, fox
Tertiary consumer	Eats secondary consumers	Wedge-tailed eagle, dingo
Decomposers	Break down dead matter and return nutrients to soil	Bacteria, fungi

Energy flow and pyramids

Only about 10% of the energy at one level passes to the next. The rest is used in respiration (lost as heat), passed out in faeces, or never eaten.

Each level of an energy pyramid has about one tenth of the energy of the level below.

An energy pyramid is always this shape. A biomass pyramid (the dry mass of living things at each level) is usually this shape too. A pyramid of numbers can be odd: one large tree can feed thousands of insects.

Cycling of matter

Energy flows through an ecosystem and is lost as heat. Matter is recycled.

The carbon cycle.

Photosynthesis takes $C O_{2}$ out of the air into plants.
Feeding passes carbon to animals.
Respiration by plants, animals and decomposers returns $C O_{2}$ to the air.
Burning fossil fuels and forests adds extra $C O_{2}$ .

The nitrogen cycle. Plants need nitrogen to make proteins, but cannot use nitrogen gas from the air.

Nitrogen-fixing bacteria in soil and in root nodules of legumes (like wattles) turn $N_{2}$ into compounds plants can use.
Decomposers turn dead matter into ammonia, and nitrifying bacteria turn it into nitrates.
Plants absorb nitrates. Denitrifying bacteria return $N_{2}$ to the air.

Ecosystems

Parts

Population and community
Biotic: living
Abiotic: non-living

Feeding

Food chains and webs
Producers and consumers
Decomposers

Energy

About 10% passes on
Lost as heat
Pyramids

Matter recycles

Carbon cycle
Nitrogen cycle

Year 11Relationships between species

Relationship	Effect on each species	Example
Predation	+ for predator, − for prey	Quoll eats a bandicoot
Competition	− for both	Foxes and quolls competing for prey
Mutualism	+ for both	Clownfish and sea anemone. Corals and the algae in them
Commensalism	+ for one, 0 for the other	Orchids growing on tree branches
Parasitism	+ for parasite, − for host	Ticks on a dog
Allelopathy	One plant releases chemicals that harm others	Some eucalypts release chemicals that stop seeds near them germinating

A keystone species has a much bigger effect on its ecosystem than its numbers suggest, like the dingo in the story. A niche is the role of a species: what it eats, where it lives and when it is active. Two species cannot share exactly the same niche for long (competitive exclusion).

Quick summary

An ecosystem is a community of organisms plus its abiotic environment.
Food chain arrows show the direction of energy flow. Food webs link many chains.
About 10% of energy passes to the next trophic level. The rest is lost mostly as heat.
Energy flows one way. Matter, like carbon and nitrogen, is recycled.
Decomposers and bacteria are vital for recycling nutrients.

Quiz

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Which is an abiotic factor?
1. A Predators
2. B Disease
3. C Soil pH
4. D Competition
Show answer
C: Soil pH
Soil pH is a non-living condition.
In a food chain, the arrows show
1. A who is bigger
2. B the direction energy flows
3. C the order animals were born
4. D where animals live
Show answer
B: the direction energy flows
Arrows point from the eaten to the eater.
A koala eats eucalyptus leaves. The koala is a
1. A producer
2. B primary consumer
3. C secondary consumer
4. D decomposer
Show answer
B: primary consumer
It eats a producer.