The essentials
Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy for all ecosystems on earth. Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria (autotrophs) fix this energy, and plants capture only 2–10 per cent of the PAR; this small amount sustains the entire living world. Energy flow is unidirectional: from the sun to producers and then to consumers. Ecosystems obey the second law of thermodynamics; they need a constant supply of energy to synthesise molecules and counteract the universal tendency towards disorder.
Producers are green plants; all animals depend on plants directly or indirectly for food and are consumers (heterotrophs). Herbivores feeding on plants are primary consumers; carnivores feeding on herbivores are secondary consumers; those feeding on them are tertiary consumers.
Food chains:
- Grazing food chain (GFC): grass → goat → man. It starts with producers.
- Detritus food chain (DFC): begins with dead organic matter; made of decomposers (saprotrophs) such as fungi and bacteria, which secrete digestive enzymes and absorb the simple products.
In aquatic ecosystems the GFC is the major conduit of energy flow; in terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain. The two chains are linked, since some DFC organisms are eaten by GFC animals. Natural interconnection of food chains forms a food web.
Trophic levels: Organisms occupy a specific place in a food chain called a trophic level: producers first, herbivores second, primary carnivores third, secondary carnivores fourth. The amount of living material at each trophic level at a particular time is the standing crop, measured as biomass or number per unit area.
The 10 per cent law (Lindeman): Only about 10 per cent of the energy at one trophic level is transferred to the next. This is why food chains are short, with a limited number of trophic levels.
Exam tips
- For 10 per cent questions, multiply by 0.1 for each step between levels.
- Deep-sea hydrothermal ecosystems are the textbook exception to solar dependence.