05 April 2014

#142 Food chain and energy efficiency

In term of conversations of energy, there is an increased efficiency in supplying green plants  as human food and a relative inefficiency in feeding crop plants  to animals.

Short food chains are more efficient than long ones in providing energy to the top consumer. Below are two food chains and energy values for each level in them. Both food chains have a human being as the top consumer. 

                             maize cow human
unit of energy          100       10        1
                               maize human
unit of energy            100     10      

Ten times more energy is available to the human in the second food chain than in the first. In the second food chain, the human is a herbivore (vegetarian). But eating parts of a cow provide humans with other nutrients, as well as those we gain energy from – it would be very difficult to persuade everyone to become vegetarian for the sake of energy efficiency.

Some farmers try to maximize meat production by reducing movement of their animals (keeping them in pens or cages with a food supply) and keeping them warm in winter. This means less stored energy is wasted by the animals. 

Why food chains  usually have fewer than 5 trophic levels? 

As the energy is passed along the chain, each organism uses some of it. So the further along the chain you go, the less energy there is. The loose of energy along the food chain limits the length of it. There rarely more than 5 links in a chain, because there is not enough energy left to supply the next link. Many food chains only have 3 links. 

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