Pyramids of feeding relationships
Joanna George-Johnson, Contributor

Students of Vauxhall Comprehensive High School perform a skit during World Wetlands Day 2006 celebrations at the UWI marine lab at Port Royal on February 2. - Andrew Smith/Photography Editor

Previously, we went into depth about food chains and webs. Although they may be accurate in showing the feeding relationship at each trophic level, they do not account for the number of organisms feeding on each other, or the relative numbers at each trophic level.

For this reason, ecologists use a diagram known as the pyramid of numbers. This pyramid takes into account the number of organisms at each level within the community being studied.

Pyramid of Numbers

For example, let's look at a simple food chain:

Leaves Caterpillars Small Bird Vulture

If we were to consider a small community, we would notice that, for example, there will be more leaves available than caterpillars. If we were to construct a pyramid of numbers, we would get one similar to the one below.

Pyramid of Numbers for a small backyard

In a pyramid of numbers, the area of each box shows approximately how many living things are at each level. It should also be noted that usually as one ascends a pyramid of numbers the relative size of the organism increases. This is not always the case, however, and a pyramid of numbers does not take this into account. Therefore, it has some drawbacks. For example, a single blade of grass is given the same importance as a single tamarind tree. You can appreciate that one tamarind tree can feed a lot more organism than a single blade of grass. Therefore, the pyramid may not look as uniformed as the one above.

For this reason ecologists sometimes use a next pyramid called the pyramid of biomass.

Pyramid of biomass

Biomass is the mass of the living material, so a pyramid of biomass shows the mass of the living material at each trophic level per unit area or volume. This type of pyramid is not without its faults. It only takes into account one instant in time. It cannot be used to study what takes place in various seasons. This is because some organisms lose or gain mass at different times of the year. It also does not take into account the speed at which some organisms grow. So, when a biomass pyramid is constructed it may show some organisms being or deceptively low or high mass. The best way to represent a feeding relationship within a community is with a pyramid of energy. This show the amount of energy transferred from one trophic level to a next.

Pyramid of energy

An energy pyramid of an efficient ecosystem will show that a maximum of 10 per cent of energy from one trophic level is available to the next. Consider the diagram below:

A pyramid of energy shows that less energy is available to higher trophic levels in a food chain. As shown in the diagram above, the available energy to the next trophic level is always less than the previous. The reason for this is because energy is 'lost' on its way to the next trophic level. The energy in all ecosystems comes from the sun. Of course, only a small amount of this energy is transferred to the producers during photosynthesis. The chemical energy converted by the plant is used for respiration, growth, etc. Some may even be transferred to decomposers. Therefore, only the energy the plant stores as it increases in biomass is made available for the next trophic level.

Although there is quite a large biomass available, the herbivores do not receive all of it. The reason for this is because:

• Not all of the food may be eaten.
• Some of the food may pass through the herbivore without being digested.

Similar 'loss' of energy occurs between other trophic levels. These include:

• Losses through respiration of the animal
• Heat loss
• Loss to decomposers (faeces, dead remains).

However, usually a higher percentage of energy conversion efficiency may be achieved by some carnivores since proteins are more efficiently digested than carbohydrates.

Tips

• An energy pyramid can never be inverted, since it only takes into consideration the amount of energy which is passed on. And some is always lost on its way to the next trophic level.
  
  
• Although they are better to show the efficiency of an ecosystem, they are harder to gather data for.
  
  
• The pyramid of biomass and numbers may be irregularly shaped.

Joanna George-Johnson teaches at Ardenne High School.