The Sun as the Source of Energy

With one striking exception, the sun is the only source of energy for every ecosystem on Earth. That exception is the deep-sea hydrothermal ecosystem, where life runs on chemical energy rather than sunlight. Everywhere else, the story of an ecosystem is really the story of what happens to solar energy once it arrives.

Of the sunlight that reaches the Earth, only a part is useful to plants. Less than 50 per cent of the incident solar radiation is photosynthetically active radiation (PAR) — the wavelengths that photosynthesis can actually use. Autotrophs — the plants and photosynthetic bacteria — fix this radiant energy to make food from simple inorganic materials. Remarkably, plants capture only 2-10 per cent of the PAR, and yet this small fraction is enough to sustain the entire living world.

Energy Flow Is Unidirectional

Every organism depends on producers for food, either directly or indirectly. This dependence sets the direction of energy movement: energy flows from the sun to the producers, and then on to the consumers. It never flows back — a consumer cannot return energy to the plant it ate, and no organism sends energy back to the sun.

This one-way movement is fully consistent with the first law of thermodynamics, which tells us that energy is neither created nor destroyed but only transformed and transferred from one form or store to another. In an ecosystem, sunlight is transformed into the chemical energy of food and then handed along the chain.

Why Ecosystems Obey the Second Law

Ecosystems are not exempt from the second law of thermodynamics either. Living systems are highly ordered, and the universe tends constantly toward greater disorder, or entropy. To build and maintain their complex molecules against this pull, organisms need a constant supply of energy.

This is exactly why an ecosystem cannot run once and then coast. The sun's energy has to keep pouring in, because a steady input is what allows producers and consumers to keep synthesising the molecules they need and to counteract the universal drift toward disorder. Cut off the energy supply and the ordered structure of life quickly breaks down.

Producers and Consumers

The green plants of an ecosystem are its producers. On land the major producers are the herbaceous and woody plants; in water they are species such as phytoplankton, algae and higher plants. Producers alone can convert inorganic materials into the organic food that everything else lives on.

Every animal depends on plants, directly or indirectly, so animals are called consumers, and because they cannot make their own food they are also heterotrophs. Those that feed on producers are primary consumers, and since they eat plants they are herbivores — insects, birds and mammals on land, and molluscs in water. Animals that eat these herbivores are secondary consumers (primary carnivores), and animals that feed on the primary carnivores are the tertiary consumers, or secondary carnivores. A simple grazing food chain runs Grass (producer) -> Goat (primary consumer) -> Man (secondary consumer).

One principle underlies all of this: no energy trapped in an organism stays there forever. The energy captured by a producer is either passed on to a consumer, or the organism dies — and death is the beginning of the detritus food chain.

Quick Recap

  • Except for the deep-sea hydrothermal ecosystem, the sun is the only source of energy for all ecosystems.
  • Less than 50 per cent of incident solar radiation is photosynthetically active radiation (PAR); plants capture only 2-10 per cent of the PAR, and this small amount sustains all life.
  • Autotrophs (plants and photosynthetic bacteria) fix the sun's energy into food.
  • Energy flow is unidirectional — sun to producers to consumers, never back — in keeping with the first law of thermodynamics.
  • Ecosystems obey the second law of thermodynamics: a constant energy supply is needed to build molecules and counteract rising entropy.
  • Producers are green plants; consumers (heterotrophs) are primary (herbivores), secondary (primary carnivores) and tertiary. A grazing food chain: Grass -> Goat -> Man.

Solved Examples — Section 5

Q1. Which ecosystem is the exception to the sun being the only source of energy?

Answer: The deep-sea hydrothermal ecosystem, which runs on chemical energy rather than sunlight.

Q2. What fraction of incident solar radiation is photosynthetically active radiation (PAR)?

Answer: Less than 50 per cent of the incident solar radiation is PAR.

Q3. How much of the PAR do plants actually capture?

Answer: Only about 2-10 per cent of the PAR, and this small amount sustains the entire living world.

Q4. Energy flow in an ecosystem is unidirectional. Which law of thermodynamics is this consistent with?

Answer: The first law of thermodynamics.

Q5. Why do ecosystems need a constant supply of energy?

Answer: To synthesise the molecules they require and to counteract the universal tendency toward increasing disorder (entropy), as required by the second law of thermodynamics.

Q6. Write a simple grazing food chain and label each organism.

Answer: Grass (producer) -> Goat (primary consumer) -> Man (secondary consumer).