Energy flow

Except for some deep-sea ecosystems around hydrothermal vents, every ecosystem runs on energy from the sun.

• Of the sunlight that falls on the earth, less than 50% is photosynthetically active radiation (PAR), the part plants can use.
• Plants capture only 2-10% of that PAR, and this small amount keeps the whole living world going.

Energy flows in one direction: from the sun to producers, then to consumers. It is never recycled. Ecosystems obey the laws of thermodynamics: energy cannot be created, and at every transfer some is lost as heat. That is why an ecosystem needs a constant supply of energy from the sun.

Food chains show who eats whom:

• The grazing food chain (GFC) begins with living plants: grass → goat → man.
• The detritus food chain (DFC) begins with dead organic matter, eaten by decomposers, also called saprotrophs (fungi and bacteria), which release digestive enzymes onto the detritus and absorb the products.

In aquatic ecosystems, the GFC carries most of the energy; in terrestrial ones, much more energy flows through the DFC. The two chains are linked, since animals of the DFC are eaten by animals of the GFC. Real food chains interconnect into food webs.

Each step in a food chain is a trophic level:

• First: the producers (plants).
• Second: the herbivores, or primary consumers.
• Third: the primary carnivores, or secondary consumers, which eat herbivores.
• Fourth: secondary carnivores, which eat other carnivores.

The amount of living matter at each level at a given time is its standing crop.

The 10 per cent law. Only about 10% of the energy at one trophic level is passed on to the next; the rest is used for life processes and lost as heat. So if producers fix 10,000 J, herbivores get about 1000 J, primary carnivores 100 J, and secondary carnivores just 10 J. That is why food chains are short, rarely more than four or five steps: there is too little energy left to support more.