Energy and living things
Joanna George-Johnson, Contributor

Students at Wolmer's Girls' School pay attention during their lab session. - Contributed

Most students are familiar with grouping organisms by structural features.

These include number of legs, presence or absence of a backbone, etc.

However, when looking at organisms in a community, it is better to group things by way of feeding relationships.

Grouping of organisms by feeding relationships

• Producers: These organisms are normally plants. This is because they can photosynthesise. Please note that there are other organisms that can make their own food, such as certain bacteria. However, they do not use photosynthesis.
  
  
• Consumers: These are all animals. Since no animal can synthesise food, they must then rely on other organisms for their nutrition.
  
  
• Primary consumers: These are animals that feed on plants. They are also called herbivores.
  
  
• Secondary consumers: These animals feed on the herbivores. These are also called carnivores.
  
  
• Tertiary consumers: These are animals that feed on the secondary consumers. Some may even feed on plants. Animals that feed on both plants and animals are called omnivores.
  
  
• Decomposers: This group was not placed into the flow chart. The reason for this is that these feed on dead organisms, whether plants or animals. They are usually bacteria or fungi.

All of the groups or categories are called trophic levels. This refers to their feeding or energy levels. This becomes very important when looking at food chains and food webs.

A Closer Look at Feeding Relationships

1st trophic level 2nd trophic level 3rd throphic level 4th trophic level

Producers 1° Consumers 2° Consumers 3° Consumers

Leaf caterpillar bird Cat

algae Water beetles Small fish Bigger fish

mango Fruit fly lizard Cat

A simple way of looking at feeding relationships is by what is called a food chain. A food chain is a simple, direct demonstration of the feeding relationship among a group of organisms. A complete food chain will contain all of the categories of organisms as mentioned before.

Generally, most organisms tend to have more than one food source. So, a food chain is too simple to completely study a community of organisms. For a more comprehensive look at feeding relationships within a community, food webs are used instead. A food web is a combination of many food chains that demonstrate the complex feeding relationships within a community.

Diagram showing an example of a food web

As shown in the diagram above, a more complex feeding relationship is demonstrated. For example, we can see that the fly is food to more than one organisms in the community. Also, much information can be gleaned from a food web, such as, the possibilities that exist if a specific organism (for example, the snake) were removed from the community. (Examiners occasionally take a few analytical questions from food webs, so beware.)

Points to remember:

• Always draw the arrows in the food web and chain pointing toward the organism that is doing the eating. In any other direction, they will be wrong!
  
  
• When asked to construct a food chain, use plausible examples of organisms. For example, do not have one showing that a dog is eating grass or a cat. These are not usual cases.
  
  
• Memorise an example of a terrestrial and an aquatic food chain for the exam. Make sure that it has at least four organisms in it.
  
  
• Always start your food chain with the producers.

Joanna George-Johnson teaches at Ardenne High School.