Energy Flow through an Ecosystem

A food chain shows how each living thing gets its food. Some animals eat plants and some animals eat other animals. For example, a simple food chain links the trees & shrubs, the giraffes (that eat trees & shrubs), and the lions (that eat the giraffes). Each link in this chain is food for the next link. A food chain always starts with plant life and ends with an animal.

1. Plants are called producers because they are able to use light energy from the Sun to produce food (sugar) from carbon dioxide and water.  You already know that plants can convert energy from the sun into chemical energy through photosynthesis.
    
2. Animals cannot make their own food so they must eat plants and/or other animals. They are called consumers. There are three groups of consumers.
   1. Animals that eat ONLY PLANTS are called herbivores (or primary consumers).
       
   2. Animals that eat OTHER ANIMALS are called carnivores.
      • carnivores that eat herbivores are called secondary consumers
         
      • carnivores that eat other carnivores are called tertiary consumers
        e.g., killer whales in an ocean food web ... phytoplankton → small fishes → seals → killer whales
3. Animals and people who eat BOTH animals and plants are called omnivores.
    
4. Then there are decomposers (bacteria, mushrooms, earthworms) which feed on decaying matter. 
   
   These decomposers speed up the decaying process that releases mineral salts back into the food chain for absorption by plants as nutrients.
    

Do you know why there are more herbivores than carnivores?

In a food chain, energy is passed from one link to another. When a herbivore eats, only a fraction of the energy (that it gets from the plant food) becomes new body mass; the rest of the energy is lost as waste or used up by the herbivore to carry out its life processes (e.g., movement, digestion, reproduction). Therefore, when the herbivore is eaten by a carnivore, it passes only a small amount of total energy (that it has received) to the carnivore. Of the energy transferred from the herbivore to the carnivore, some energy will be "wasted" or "used up" by the carnivore. The carnivore then has to eat many herbivores to get enough energy to grow.

Because of the large amount of energy that is lost at each link, the amount of energy that is transferred gets lesser and lesser ...

The further along the food chain you go, the less food (and hence energy) remains available.

The energy pyramid below shows many shrubs & grass providing food and energy to zebras. Note that as we go up, there are fewer zebras than shrubs & grass and even fewer lions than zebras ... as we go further along a food chain, there are fewer and fewer consumers. In other words, a large mass of living things at the base is required to support a few at the top ... many herbivores are needed to support a few carnivores.