Feeding relationships
Monacia Williams, Contributor

Last week, as we began to look at feeding relationships, you were introduced to some words which we will use as we continue to look at the feeding relationships that exist between organisms in the environment.

An excellent way of doing this is to go on a nature walk. Many of you have small trees and shrubs, either around your home or in the schoolyard. These trees can provide you with a wealth of knowledge that can help you to understand how these interactions occur. A good tree to examine is the West Indian cherry or garden cherry tree.

Here are some of the organisms that you may observe: lizards, wasps, honey bees, stink bugs - sometimes called bishop's hat - aphids and scale insects, ladybird beetles, spiders and ants. Stay a while and observe the behaviour of these organisms. You will learn something about how they feed. For example, you should notice that lizards, spiders and ladybird beetles are hunters, therefore, carnivores.

Scale insects and aphids

If you turn over a leaf and look at the underside, next to the veins, you will find scale insects and aphids. These feed on the food material being transported in the veins and are, therefore, herbivores. The bees will be seen visiting the flowers of the tree, collecting nectar, so they are also herbivores. The stink bugs are also associated with leaves, so they, too, are herbivores.

We can use this knowledge to construct a food chain. A food chain is the sequence by which energy in the form of food passes from plant to animal and then to other animals. Let us revisit our cherry tree. What is the organism here that converts the sun's energy to the chemical energy in food? It is, of course, the cherry tree! Hence, it is the producer. There are several organisms that feed directly on the cherry tree: the honey bees, scale insects, stink bugs and the aphids. These can, therefore, be described as primary consumers or herbivores.

Secondary consumers

The ladybird beetles which feed on other small, herbivorous insects on the tree are classified as secondary consumers because they feed on the primary consumers. Any unwary insects which spiders entrap in their webs could be either primary or secondary consumers. If they were secondary consumers, the spider would then become a tertiary consumer. Lizards would be similar to spiders.

How do we use this information to construct a food chain? The food chain always begins with the producer. For example:

Cherry tree -----> aphid ------> ladybird beetle -------> spider

Cherry tree -----> moth -----> lizard ------> bird

Trophic or feeding level

Each organism in the food chain represents a trophic or feeding level. In both examples, there are four trophic levels: the cherry tree is at trophic level number one; the aphid and the moth are at trophic level number two; the ladybird beetle and the lizard are at trophic level three; while the spider and the bird are at trophic level four. You will find that most food chains do not have more than four trophic levels. Can you think of a reason why this might be so? Did I hear someone mention the word, energy? If you did, you are on the right track! Do you remember the definition for food chain? It is the transfer of energy from one organism to another. This energy is transferred when a part or all of the organism is eaten.

Remember that not all of what is eaten is digested and, hence, converted to energy. Parts which are not eaten, such as hairs, skins and teeth, represent energy that is lost from one trophic level to another. Additionally, each organism uses some of the consumed energy to carry out its own activities, so this energy is no longer available to the next trophic level.

Hence, as you would have deduced by now, energy decreases from one trophic level to the next, so that by trophic level number four, the energy remaining in the organisms is not sufficient to support another level.

Next week, we will look at food webs. Wouldn't it be great if you were to find out what these were before we meet again?

Hervine Gibbons, visual arts teacher, prepares his fifth formers for their school-based assessment. - Photo by Michael Lee

Monacia Williams teaches at Glenmuir High School.