Photosynthesis is the main route by which energy enters an ecosystem. Energy is transferred through the trophic levels in food chains and food webs and is dissipated. Quantitative consideration of the efficiency of energy transfer between trophic levels. Pyramids of numbers, biomass and energy and their relationship to their corresponding food chains and webs.

Ultimately, energy comes from the sun. So how does energy get into the food chain? from things than convert sunlight into stored chemical energy i.e. producers (things that carry out photosynthesis).

An animal will eat a plant to gain the energy, this animals is the primary consumer (first thing to eat) in a food chain. Some of the plant may not get eaten and some may not get digested meaning that the energy does not transfer from the plant to the animal.

Of the energy that is transferred to the animal some will be stored, but some will get used up for carrying out the processes of living (movement, homoeostasis*). This means that when a second animal eats this primary consumer it will only get a very small amount of energy from the original plant.

*a lot of energy is used up by warm blooded creatures (mammals and birds) because they use it to maintain their body temperature.

1 Some light energy fails to strike/is reflected/not of
 appropriate wavelength;
2 Efficiency of photosynthesis in plants is low/approximately
 2% efficient;
3 Respiratory loss / excretion / faeces / not eaten;
4 Loss as heat;
5 Efficiency of transfer to consumers greater than transfer to
 producers/approximately 10%;
6 Efficiency lower in older animals/herbivores/ primary
 consumers/warm blooded animals/homoiotherms;
7 Carnivores use more of their food than herbivores;

We can see how efficient an energy transfer is, or how much energy actually gets passed on, by looking at how much energy an animal took in and how much it gave out; this shows us how much energy is lost at that trophic level. So if you are given a food chain that asks you to calculate the efficiency of a transfer between a worm and a bird of prey, and the worm has 6500 (kjm-2year-1) of energy available and the bird has 1500 (kjm-2year-1) of energy avalible then:

Efficiency= energy after/energy before * 100
= 1500/6500 * 100
= 0.23 * 100
= 23%

Because energy is lost between levels, you might assume that there would always be fewer animals as you went up trophic levels as there is less energy. This would be the case in the food chain of algae, fish, bird:

Pyramid of number

However there are examples where this is not the case. If we look at a food chain surrounding a tree, often the organisms that feed off the tree are very small and the tree is very big, so one tree can feed many organisms:

Pyramid of number

A way to make sense of this is to use a pyramid of biomass. This is basically like saying the weight of the organisms on a level (this does not include the weight of water because it disrupts the figures; so dry mass). For this example if we weighed the tree, all the insects and all the birds, we would end up with something like this:

Pyramid of biomass

Pyramids of biomass are a much more reliable way of representing food chains, but in practice it is difficult to collect accurate data on the weight and number of organisms due to factors like seasonal change and movement.

Another problem with them is that there is a big difference between the energy stored in certain molecules; for example you may have a very heavy organism packed with carbohydrate that would have less energy than a lighter organism covered in fat. A way to get round this problem is to use a pyramid of energy which displays the amount of energy at each level:

Pyramid of energy