Food Chains

A food chain is a sequence indicating what each living organism in an ecosystem uses as food.

In other words, a food chain represents the trophic relationships between different living organisms.

Here is a simple example of a food chain.

Here, the plants (producers) are eaten by the hare (primary consumer). The hare is then eaten by the wolf (secondary consumer).

A food web forms when all the food chains in an ecosystem are connected.

In this food web, the arrow means is eaten by. Phytoplankton is eaten by bacteria and copepods. The bacteria are then eaten by ciliated eukaryotes. This results in two food chains, because there is no link between ciliated eukaryotes and copepods.

Food Pyramids 

The pyramid model is common in ecology. It is mainly used to illustrate the following concepts:

1. Pyramid of numbers

2. Pyramid of biomass

3. Pyramid of energy

Each of these models has advantages and, sometimes, disadvantages.

 

Pyramid of numbers

The base of this pyramid usually shows the lower trophic level of any food chain–the producers (plants)–with the higher trophic levels stacked on top. The bases of number pyramids are not always wider than the apex. Pyramids can be inverted with an apex wider than the base or have a peculiar shape, such as in the second example below. Regardless, in each case, the width of each trophic level indicates the number of organisms belonging to these levels.

Here are two examples of pyramids of numbers.

This example could represent the following food chain: grass - grasshopper - frog - hawk. This type of pyramid clearly indicates that there are fewer apex predators (the hawks) than there are primary carnivores (the frogs). Similarly, the population of primary carnivores is smaller than that of herbivores (the grasshoppers). The width of the base indicates that a significant number of organisms is needed to meet the food requirements of organisms at a higher trophic level in the food chain.

This example could represent the following food chain: grass - rabbit - fox - fleas. This type of pyramid indicates that there are more apex predators (the fleas) than primary carnivores (the foxes). Multiple fleas, which are insects, will feed on the blood of a single fox. Still, the primary carnivore population is smaller than that of herbivores (the rabbits).

Pyramids of numbers only show how many individuals are required to feed an organism on a higher trophic level. With that in mind, what information is really obtained when a ratio of 1 000 grass plants to 10 rabbits is established? This model has limitations. It is better to use pyramids of biomass. Unlike pyramids of numbers, pyramids of biomass take the difference between the sizes of the individuals in a food chain into consideration.

Pyramid of biomass

Pyramids of biomass can be used to compare the trophic levels of a food chain in terms of the total mass of the organisms. This ecological consideration takes into account that a hare is a better meal than a mouse for a hungry fox.

The shape of a pyramid of biomass is relatively constant: the higher the trophic level, the smaller the biomass.

Here is an example of a pyramid of biomass.

Pyramid of energy

A hungry fox will not get the same energy value for its meal if it eats |1\ \text{kg}| of chicken or |1\ \text{kg}| of mice. Similarly, |500\ \text{g}| of grass does not have the same energy value as |500\ \text{g}| of meat.

Neither the number nor the pyramid of biomass provides information on the energy associated with a food source, although this aspect is important to consider in a food chain. The pyramids of energy are used to get a more accurate representation.

Here is an example of a pyramid of energy.