Ecosystem, ecology and trophic levels

Ecology is the branch of science that studies the relationships of living organisms with each other and with their physical environment (the term was coined by Ernst Haeckel, 1866).

An ecosystem is a functional unit of nature in which a community of living things (biotic components) interacts with the non-living environment (abiotic components) through flow of energy and cycling of materials. Examples: forest, pond, grassland, river, Phewa Lake.

Components of an ecosystem

• Abiotic: sunlight, water, air, soil, temperature, minerals.
• Biotic:
  • Producers (autotrophs) – green plants, algae; make food by photosynthesis.
  • Consumers (heterotrophs) – primary (herbivores), secondary (carnivores), tertiary (top carnivores).
  • Decomposers – bacteria and fungi that break dead matter into nutrients.

Flow of energy and materials through trophic levels

A trophic level is the position of an organism in a food chain.

Sun
 |
 v
Producers (grass)        10,000 kJ   T1
 v
Herbivores (deer)         1,000 kJ   T2
 v
Carnivores (fox)            100 kJ   T3
 v
Top carnivores (tiger)       10 kJ   T4
     \    dead matter    /
      --> Decomposers --> nutrients back to soil

• Energy flow is one-way (unidirectional): sunlight → producers → consumers, and is finally lost as heat. Only about 10% of energy passes from one level to the next (Lindeman's 10% law); the rest is used in respiration and lost as heat. That is why food chains are usually short (4–5 levels) and the energy pyramid is always upright.
• Materials (nutrients) cycle: carbon, nitrogen, phosphorus and water move from the environment into producers, through consumers, and back to soil and air through decomposers (biogeochemical cycles). Matter is reused; energy is not.