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Energy flow in ecosystems

A food chain represents the transfer of energy between organisms in an ecosystem.

Energy in food chains refers to the organic molecules that are consumed by organisms. The arrows represent the direction of energy transfer. They only flow in one direction.

Each different stage of the food chain is called a trophic level. The trophic level of an organism can be measured by counting how many stages it is from the start of the food chain.

The second organism in a food chain is in the second trophic level.

  • A food chain always starts with a producer .
  • Producers are usually green plants like trees.

  • Producers are eaten by consumers .
  • The organism at the top of a food chain is a top predator (apex predator).
  • Top predators such as lions are rarely hunted.

Energy is lost from food chains at each trophic level. The amount of energy provided by an organism is always less than the energy it consumed.

There are many ways energy is lost in a food chain:

  • Breathing : heat lost in exhaled air
  • Heat radiation : heat lost from the body
  • Energy released by respiration during exercise is lost as heat via breathing and radiation!

  • Waste material : not all ingested food is fully digested
Diagram of energy loss in a horse. Only 10 units out of 100 are made available for the next trophic level.
Diagram of energy loss in a horse. Only 10 units out of 100 are made available for the next trophic level.

The remaining energy is used for growth and repair of the organism. This energy is roughly 10% of the total consumed energy.

Additionally, energy is lost between trophic levels. Organisms are never able to extract all of the available energy from their food.

A lion does not eat the bones of its prey. As a result, some of the energy used for growth and repair of the prey is lost.

Producers (also known as autotrophs) produce their own organic food from inorganic matter.

Every food chain begins with a producer that occupies the first trophic level. Energy enters the rest of the food chain through the organic molecules they produce.

The energy that is stored in the producer is the maximum energy in the food chain.

Green plants are the most common producers. They use light energy from the sun to make sugars using photosynthesis.

In aquatic food chains, the producers can be photosynthetic algae!

Remember: plants are living organisms too. In fact, they are the most important organisms in most food chains!
Remember: plants are living organisms too. In fact, they are the most important organisms in most food chains!

Consumers (also known as heterotrophs) must consume other organisms to get energy.

Primary consumers eat producers. They always occupy the second trophic level. Primary consumers are herbivores - they eat plants.

Secondary consumers eat primary consumers. They are carnivores - they eat other animals.

An organism can be classed as two types of consumer. An animal that eats both meat and plants is called an omnivore.

A sparrow feeds on seeds and insects. This means it is both a primary and secondary consumer.

A food web is a map of interactions between all the food chains in an ecosystem.

Linear food chains are usually overly simplified. In reality, ecosystems involve multiple food chains that interact with each other. Food webs illustrate this.

Food webs have the same core features of a food chain. However, there may be multiple organisms associated with each trophic level.

Removing or adding organisms to a food web can have large effects on the whole web.

When rats were introduced to the island of Mauritius, they began eating the eggs of the native Dodo bird. The Dodo soon died out.

A pyramid of numbers shows the total number of organisms at each trophic level in a food chain.

In a food chain, each trophic level represents a group of individuals of the same species. In a pyramid of numbers, each bar represents a single species group.

A pyramid of numbers based on a simple food chain of four trophic levels.
A pyramid of numbers based on a simple food chain of four trophic levels.

1 falcon must eat 20 sparrows per year to survive. 1 sparrow must eat 500 snails.

Roughly 90% of the energy is lost at each stage. This means that only a few top predators can be supported by the energy in a food chain.

Important! Pyramids of numbers do not account for the size of organisms. As a result, pyramids of numbers are often not pyramid shaped.

A pyramid of biomass shows the total biomass of organisms at each trophic level in a food chain.

Biomass is the mass of biological material in an organism. Biomass can be estimated by calculating the dry mass (dried weight) of an organism. The biomass of a trophic level is:

$$$\text{Biomass of organism } \times \text{ Number of organisms}$$$

Unlike the number of organisms, the biomass of each trophic level always decreases along a food chain.

Pyramids of biomass are always pyramid shaped because biomass decreases down the food chain.

Pyramids of biomass give an estimate of how much energy (in biological molecules) is transferred between each trophic level. The more energy is passed on as food, the more of that energy will be incorporated into biomass.

A pyramid of biomass based on a simple food chain of four trophic levels. Not to scale!
A pyramid of biomass based on a simple food chain of four trophic levels. Not to scale!