Energy Flow

The sun is the only source of energy for virtually all ecosystems on Earth, with the exception of certain deep-sea ecosystems. Of the total solar radiation reaching the Earth, less than 50% qualifies as photosynthetically active radiation (PAR) — the portion of the spectrum usable for photosynthesis. Plants and photosynthetic bacteria (autotrophs) are able to capture only 2–10% of this PAR, and this small captured fraction is what sustains virtually the entire living world. Energy flow through an ecosystem is strictly unidirectional: it moves from the sun to producers and then on to consumers, never in reverse, and once used it cannot be reused in the same form.

Autotrophs need a constant supply of energy to synthesise the organic molecules they require, and are accordingly called producers — herbaceous and woody plants in terrestrial ecosystems, and phytoplankton and algae in aquatic ones. All animals depend, directly or indirectly, on plants for food, and are therefore called consumers (heterotrophs). Animals that feed directly on plants are primary consumers (herbivores); those that eat the herbivores are secondary consumers (carnivores); and animals that in turn eat those carnivores are tertiary consumers. Each such feeding position is called a trophic level: producers occupy the first trophic level, herbivores the second, and successive carnivores the third and beyond. …