February 23, 2012

Energy Pyramid Part 1

An energy pyramid is a graphical model of energy flow in a community. The different levels represent different groups of organisms that might compose a food chain. From the bottom-up, they are as follows:
  • Producers — bring energy from nonliving sources into the community
  • Primary consumers — eat the producers, which makes them herbivores in most communities
  • Secondary consumers — eat the primary consumers, which makes them carnivores
  • Tertiary consumers — eat the secondary consumers
In some food chains, there is a fourth consumer level, and rarely, a fifth. Have you ever wondered why there are limits to the lengths of food chains?

Why are energy pyramids shaped the way they are?

An energy pyramid’s shape shows how the amount of useful energy that enters each level — chemical energy in the form of food — decreases as it is used by the organisms in that level. How does this happen?
Recall that cell respiration “burns” food to release its energy, and in doing so, produces ATP, which carries some of the energy as well as heat, which carries the rest. ATP is then used to fuel countless life processes. The consequence is that even though a lot of energy may be taken in at any level, the energy that ends up being stored there – which is the food available to the next level — is far less. Scientists have calculated that an average of 90% of the energy entering each step of the food chain is “lost” this way (although the total amount in the system remains unchanged).
The consumers at the top of a food pyramid, as a group, thus have much less energy available to support them than those closer to the bottom. That’s why their numbers are relatively few in most communities. Eventually, the amount of useful energy left can’t support another level. That’s why energy flow is depicted in the shape of a pyramid. The energy that enters a community is ultimately lost to the living world as heat.

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