Feeding roles in ecosystems

Organisms obtain energy and organic nutrients in different ways. Their feeding role describes the source they use.

A producer makes its own organic nutrients from inorganic substances. Most producers, such as plants and algae, use energy from sunlight to do this by photosynthesis.

A consumer obtains energy and organic nutrients by feeding on other organisms. A herbivore is a consumer that eats plants; a carnivore is a consumer that eats other animals.

A decomposer obtains energy from dead organic material or waste. Decomposers, such as many bacteria and fungi, break down this material and absorb the resulting nutrients.

A feeding role describes what an organism obtains its energy from, but it does not by itself give the organism’s exact position in a particular food chain. For example, a carnivore may eat a herbivore or another carnivore.

Food chains

A food chain shows a sequence of energy transfers between organisms as they feed. It begins with a producer, such as a green plant, which uses energy from sunlight to make its own food. The organisms that feed after it are consumers: they obtain energy by eating other organisms.

Each arrow points from the organism being eaten to the organism that eats it. The arrow therefore shows the direction in which energy is transferred, not the direction in which the consumer is looking.

Constructing and interpreting a food chain

To construct a food chain, place the producer first. Then place each consumer after the organism it eats, with an arrow pointing towards the consumer. For example:

The grass is the producer. The grasshopper eats the grass, so the first arrow points from the grass to the grasshopper. The frog eats the grasshopper, so the second arrow points from the grasshopper to the frog. Following the arrows shows the direction of energy transfer through the chain.

Food webs

A food web shows how several food chains are connected. Unlike a single food chain, it can show that an organism eats more than one kind of food or is eaten by more than one kind of organism. Energy can therefore pass through the web along different routes.

An arrow points from the food to the organism that eats it. It shows the direction in which energy is transferred, not the direction in which the feeder looks for food.

Tracing a food web

These connected food chains form a simple food web:

• Grass → rabbit → fox
• Grass → mouse → fox
• Blackberry plant → mouse → owl

To find what the mouse eats, follow the arrows pointing towards it: grass and the blackberry plant. To find what eats the mouse, follow the arrows pointing away from it: the fox and the owl.

The connections also reveal different routes for energy transfer. Energy from grass can pass to the fox through the rabbit or through the mouse. Energy from the blackberry plant can pass through the mouse to the owl or to the fox.

Trophic levels

A trophic level is an organism’s feeding position in a food chain, food web or ecological pyramid. The position depends on what the organism feeds on in the feeding relationship being considered.

Producers form the first trophic level. They make their own food rather than feeding on other organisms. Each successive consumer level is named for the level on which it feeds:

1. A primary consumer feeds on a producer.
2. A secondary consumer feeds on a primary consumer.
3. A tertiary consumer feeds on a secondary consumer.
4. A quaternary consumer feeds on a tertiary consumer.

Trophic levels in a food web

A food web contains different feeding routes. An organism can occupy different trophic levels in different routes because it may feed on organisms at different levels.

For example, the following routes both include a small bird that eats caterpillars and plant material. Each arrow means ‘is eaten by’:

• Plant → caterpillar → small bird → snake → hawk
• Plant → small bird → snake → hawk

In the first route, the small bird is a secondary consumer because it eats the caterpillar, a primary consumer. The snake is a tertiary consumer and the hawk is a quaternary consumer.

In the second route, the small bird is a primary consumer because it eats the plant. The snake is therefore a secondary consumer and the hawk is a tertiary consumer. The labels describe each organism’s position in the route, not a fixed category for that organism.

Human disruption of food webs

Organisms in a habitat are connected by feeding relationships. If people change the population of one species, the effects can extend to organisms that eat it, organisms it eats, and others connected to them. A food chain shows one sequence of feeding relationships; a food web shows how several feeding relationships connect.

Overharvesting a food species

Overharvesting occurs when people remove individuals of a species faster than its population can replace them. As that population falls, less food is available to its consumers. Those consumers may also decrease if they cannot obtain enough food elsewhere.

At the same time, the reduced population eats fewer of its own prey. This lowers the feeding pressure on the prey, whose population may increase. For example, if people overharvest small fish that eat aquatic insects, larger fish that eat the small fish have less food available, while the insects are eaten less often.

Introducing a foreign species

When people introduce a foreign species to a habitat, it can form new feeding relationships. If it eats a species already living there, it adds feeding pressure and may reduce that species’ population. It may also use the same food as an existing species, leaving less food available to that competitor.

For example, an introduced fish that eats aquatic insects could reduce the insect population. This could leave less food for the small fish that also eat those insects, potentially affecting the larger fish that feed on them.

A food web helps trace these possible effects from one organism to another. It does not guarantee a particular outcome: the result depends on the feeding relationships involved, including whether consumers have other food sources.

Pyramids of biomass

A pyramid of biomass shows the total mass of living material at each feeding stage in a food chain. Each feeding stage is called a trophic level.

The pyramid is made of bars. The producers form the base, with the consumer levels stacked above them in the order of the food chain. The width of each bar represents the total biomass of the organisms at that level: a wider bar means a greater biomass.

Drawing and interpreting a pyramid of biomass

To draw a pyramid, place each trophic level in the correct order and draw its bar to a width that represents its biomass. Use the same scale for every bar so that their widths can be compared.

To interpret a pyramid, match each bar to its trophic level, then compare the widths. For example, if the producer bar is twice as wide as the primary consumer bar, the producers have twice the total biomass of the primary consumers.

Biomass compared with numbers

A pyramid of numbers counts each organism as one, regardless of its size. A pyramid of biomass instead accounts for the mass of the organisms, so it gives a better indication of how much living material is present at each level.

For example, one large tree might support many insects. A count could show more insects than trees, but the single tree could still have a much greater total biomass than all the insects combined.

Pyramids of energy

A pyramid of energy shows the energy available at each feeding level in a food chain over a specified period of time. Each feeding level is called a trophic level. The width of each bar represents the amount of energy available at that level: a wider bar represents more energy.

To draw or interpret the pyramid, place the levels in feeding order. Producers form the base, primary consumers are above them, and further consumer levels follow towards the top. Less energy is available at each successive level, so the bars become narrower towards the top. Some energy is lost between levels, for example through respiration and waste.

Energy compared with numbers and biomass

A pyramid of energy represents energy directly. A pyramid of numbers instead counts the organisms at each level, regardless of their size. A pyramid of biomass measures the living material present at each level, but does not show how much energy passes through that level over a period of time. A pyramid of energy therefore gives a clearer representation of the energy available through the food chain.

Energy transfer between trophic levels

A trophic level is a feeding position in a food chain. Energy passes from one trophic level to the next when organisms are eaten, but only part of the energy available at one level reaches the next.

Energy does not all reach the next consumer for several reasons:

• Not all material is eaten. Some organisms are not eaten, and consumers may leave parts of the organisms they eat.
• Not all eaten material is digested. Undigested material is egested, so its energy does not become available to the consumer.
• Organisms respire. Respiration releases energy for life processes, with energy transferred to the surroundings as heat rather than remaining available to the next consumer.

Energy in dead material and waste may pass to decomposers instead of to the next consumer in the food chain.

Energy transfer and food-chain length

Each successive trophic level has less energy available than the level before it. Eventually, too little energy is available to support another consumer level. This is why food chains usually have fewer than five trophic levels.

Energy efficiency of eating crops directly

When humans eat crop plants, energy stored in the plants’ biomass passes from the producer to the human consumer. When livestock eat the crops and humans then eat the livestock, the energy must pass through an additional trophic level: the livestock.

The livestock use some of the energy obtained from the crops in respiration. This energy is not all stored in livestock biomass; some is transferred to the surroundings as heat. Of the energy stored in livestock biomass, only some can reach humans: not all parts of the animal are eaten, and not all the material eaten is digested.

Humans also do not necessarily eat or digest all of a crop plant. Nevertheless, feeding crops to livestock adds another stage at which energy is used or remains unavailable to humans. As a result, a smaller proportion of the energy originally available in the crops reaches humans through livestock than when the crops are eaten directly. Eating crop plants directly is therefore more energy efficient.