Ecosystems can be seen as a flow of power – from the Sun, to plants and subsequently to herbivores and carnivores. In these complex food webs and foods chains, it is reassuring to know that humans aren’t the only creatures that should count their calories.

The ability of plants to convert solar power into chemical energy forms the basis for all ecosystems. It links plants, animals and their environment in a knotty system of foods chains and food webs. Each level within a foods chain or web is known like a trophic level. Most ecosystems have only three or four trophic levels, but some have six or seven. The number of levels is limited by the energy obtainable to support them.

Plant power

The power source for most organisms is the Sun. Solar power travels from the Sun towards the Earth’s surface by radiation. Only green plants are able to transform solar energy into chemical energy through the procedure of photosynthesis. This chemical energy may be the vital currency that the majority of life on Earth must deal in, for only in this chemical form can the power be utilized by other organisms. Of all the solar power to reach the Earth’s surface, only 5% is engaged by green plants in photosynthesis and so changed into usable chemical energy. Close to 80% is either absorbed through the ground or reflected away by dust, clouds and water. Close to 15% is absorbed by the ozone layer.

Energy transformation

Between the various stages in the foods chain energy decreases, but energy cannot actually be ‘lost’ or ‘gained’ – it is merely transformed from one form to one more. So energy said to be ‘lost’ via respiration or excretion is actually transformed from chemical power into heat power, which cannot continue within the foods chain in the same way as chemical power. Thus it may be the amount of available chemical energy that really decreases through the foods chain. The overall amount of power entering an ecosystem is precisely equal towards the quantity leaving it.