National 5 Biology | Unit 3 | Key Area 4: Energy in Ecosystems

Name two factors that are of great importance for living things in any balanced ecosystem:

- The flow of energy

- The cycle of nutrients

Food chain

A food chain represents the flow of energy in ecosystems. These rarely go past four trophic levels. The arrows show the direction in which energy flows (food → consumer, ie. leaves → caterpillar)

Food web

A food web shows the inter-relationships between different members of organisms of an ecosystem, composed of many interlinking food chains.

What often happens to organisms as you go along a food chain?

Organisms get larger and therefore fewer.

Why does the number of organisms decrease with each successive trophic level?

Energy is lost at each link, so fewer and fewer animals can be supported.

Materials enter a food chain when plants take in what? (3)

- Carbon as carbon dioxide in photosynthesis

- Nitrogen as nitrate

- Water and minerals from soil

Producer

An organism that produces its own food using light energy from the sun, ie. plants. Also known as autotrophs. All food chains will start with producers (plants) and is considered as the first trophic level in any food chain.

Consumer

An organism that must consume food to gain energy.

Herbivore

An animal that only eats plants for energy. Also known as primary consumers.

Carnivore

An animal that only eats other animals for energy.

Omnivore

An animal that eats both plant and animal for energy.

Predator

An animal that hunts and kills other animals for food.

Prey

An animal that is hunted and killed by other animals for food.

Trophic level

A hierarchical stage in a food chain in an ecosystem.

Primary consumer

An organism that feeds on producers (ie. it feeds on plants). Also known as herbivores and make up the second trophic level.

Secondary consumer

An organism that feeds on primary consumers (ie. it feeds on other animals). These are carnivores or omnivores and make up the third trophic level.

Tertiary consumer

An organism that feeds on secondary consumers (ie. it feeds on other animals). These are carnivores or omnivores. Typically the top of the food chain, making up the fourth trophic level unless there are quaternary consumers.

Quaternary consumer

An organism that feeds on tertiary consumers (ie. it feeds on other animals). These are typically carnivores and at the top of the food chain, making up the fifth trophic level.

Describe the transfer of energy in ecosystems in terms of efficiency.

The transfer of energy is not efficient. About 90% of the energy acquired is lost in various ways, so only 10% is used to make new cells and is available for the next consumer.

In what ways is energy lost during the transfer of energy?

- Most energy is used by the consumer for movement, body heat and metabolism.

- Some energy remains in undigested food and passes through the organism as undigested material.

What happens to energy in food chains in the last case?

Energy is passed to decomposer organisms instead of another consumer.

Name types of decomposer organisms.

Bacteria, fungi

Explain the role of decomposer organisms within a food chain.

They break down the remains of other organisms (carcasses and excreta), allowing nutrients to be recycled and returned to the food chain, such as ammonium compounds from proteins and nitrogenous wastes.

Describe how a pyramid is usually laid out.

Producers are always at the base of the pyramid, with each successive feeding level (trophic level) above it. Final carnivores (the apex predators) are at the top of the pyramid.

Pyramid of numbers

A pyramid of numbers is a diagram which describes how numbers of organisms vary at each trophic level in a food chain.

Pyramid of biomass

A pyramid of biomass is a diagram which illustrates the decrease in mass of organisms with each successive trophic level in a food chain, by showing the mass of living organisms at each level. These are almost always true pyramids.

Pyramid of energy

A pyramid of energy is a diagram which illustrates the decrease in energy content of organisms within each successive trophic level in a food chain, by showing the energy content of living organisms at each level. These are always true pyramids.

Which trophic level has the largest biomass?

Producers

Why does the biomass of each trophic level decrease as the food chain progresses?

Energy is lost at each link, so smaller masses of organisms can survive.

Between pyramids of numbers and biomass, which provides a more accurate representation of trophic levels?

Pyramids of biomass. However these are more time consuming and demanding to gather data for.

What is the most refined pyramid to use?

Pyramid of energy, as it is the most accurate and takes into account seasonal changes in organism mass and number.

What are practical issues associated with collecting data for pyramids of energy?

Samples must be taken at each trophic level and their energy content measured (eg. by burning it in a calorimeter). These values are then used to estimate (by calculation) the energy content of the entire trophic level.

Name common materials that humans release by natural process and are returned to the environment: (2)

- Carbon released as carbon dioxide produced in respiration.

- Nitrogen released as urea in urine