Ecosystem
An ecosystem is a structural and functional unit of nature where living organisms interact among themselves and with their physical environment. It comprises abiotic components (inorganic materials - air, water, soil) and biotic components (producers, consumers, decomposers). Ecosystems are broadly terrestrial (forest, grassland, desert) or aquatic (pond, lake, river, estuary); crop fields and aquaria are man-made ecosystems.
Structure
- Species composition: identification and enumeration of plant and animal species.
- Stratification: vertical distribution of species at different levels (trees on top, shrubs next, herbs and grasses at the bottom).
Four functional components
- Productivity - rate of biomass production.
- Decomposition - breakdown of detritus into inorganic matter.
- Energy flow - unidirectional flow from sun to producers to consumers.
- Nutrient cycling - storage and movement of nutrients through the ecosystem.
Productivity
- GPP (gross primary productivity) = rate of production of organic matter in photosynthesis.
- NPP = GPP - R (respiration losses); the biomass available to heterotrophs.
- Secondary productivity = rate of formation of new organic matter by consumers.
- Annual NPP of the biosphere is about 170 billion tons (oceans only 55).
Energy flow and pyramids
- Energy follows the 10 per cent law between successive trophic levels.
- Three ecological pyramids: of number, biomass and energy. The pyramid of energy is always upright.
Nutrient cycling
- Gaseous cycle (carbon) - reservoir is atmosphere/hydrosphere.
- Sedimentary cycle (phosphorus) - reservoir is Earth's crust.
Ecosystem services are products of ecosystem processes, e.g. purification of air and water by forests.