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Food and Energy


Food Chains

A food chain should be very simple, something you have been studying since primary school; but now we'll take it to the next level.

A typical food chain chain looks like this:

a food chain involving grass, a rabbit and fox

A food chain shows how energy is transfered between organisms, in this case the rabbit is eating grass and the fox is eating the rabbit.

The initial energy comes as light from the sun that plants use to convert into chemical energy via photosynthesis. The herbivores eat the plants, taking some of the energy and if they are prey the energy is transfered to the predator.

But some of this energy is lost at each stage (or trophic level). In the above example, the grass looses some energy by respiration. And then the rabbit loses energy by heat and waste. And so by the time it gets to the fox a fraction of the original energy gets to it.

Energy Transfer

In an ecosystem there is energy, and this is what allows the organisms to live. This energy mainly comes from one original source: photosynthesis. The plants use this solar energy to produce carbohydrates which are then consumed by other organisms: transfering the energy.

Not all of the energy from sunlight, however, is used by the plants; they are far from efficient. Much sunlight misses the plant, is in the wrong wavelength or lost in the inefficiencies of photosynthesis. We use the term gross primary production to refer to the total energy in the molecules of the plant; and net primary production is the surplus energy not used by the plant itself.

At each level of the food chain energy is lost because it is used by the organism itself for respiration. This limits the number of steps there can be on a food chain.

diagram representing energy transfer in a food chain

The diagram above represents quantitatively (in numbers) the efficiency of energy transfer in a food chain. Notice how only about 8% of the energy is transfered from one stage to the next.

Ecological Pyramids

A food chain can be represented quantatively (with numbers) in the form of a pyramid of numbers, below is one for the previous food chain. From this graph we can see there are fewer foxes than rabbits; which makes sense because a fox must eat several rabbits to get enough energy in order to survive.

An ecological pyramid shows the relative sizes of different components at the various trophic levels of a food chain. A trophic level refers to each stage (shown as a horizontal bar on ecological pyramids). There are three types of ecological pyramid we use: numbers, biomass and energy.

pyramids of energy, biomass and numbers for two different food chains

The pyramids of numbers shows the raw number of each species at each trophic level. The top example is a typical food chain with a large number of producers but diminishing numbers of consumers. However, if the producer was a tree, followed by insects, then the bottom bar would appear small as many organisms feed on one tree. In this instance the pyramid of biomass is more useful as the tree is much larger.

In the lower example, both the pyramid of numbers and biomass show a smaller producer bar; given what was discussed under the previous heading - this does not make sense. This is because the phytoplankton reproduce very quickly. However, when we represent this information in a pyramid of energy we get a true pyramid.

Plotting the energy will always give a true pyramid because it is impossible to create new energy so a trophic level will always be smaller than the one below it and as discussed above there is a great deal of energy loss at each level.