ECOSYSTEM

An ecosystem is a functional unit in which organisms interact with each other and with the physical environment. Its key processes are productivity, decomposition, energy flow and nutrient cycling.

Ecosystems may be terrestrial (forest, grassland, desert), aquatic (pond, lake, wetland, river, estuary) or human-made (crop field, aquarium). Biotic and abiotic interactions create each ecosystem's characteristic physical structure.

• Species composition is identification and enumeration of the species. Stratification is vertical layering, such as trees, shrubs, herbs and grasses in forest.
• A pond shows the key components: water, bottom soil and dissolved substances; producers, consumers and decomposers.
• Function is understood through productivity, decomposition, energy flow and nutrient cycling.

Primary productivity is the amount of biomass/organic matter produced by plants per unit area per unit time. The rate is expressed as g m^-2 yr^-1 or kcal m^-2 yr^-1.

• GPP is total organic matter formed during photosynthesis. Plant respiration uses part of it: GPP - R = NPP.
• NPP is biomass available to heterotrophs, including herbivores and decomposers. Secondary productivity is new organic matter formed by consumers.
• Primary productivity depends on plant species, nutrient availability, environmental conditions and photosynthetic capacity.

Decomposers break complex detritus into carbon dioxide, water and inorganic nutrients. The main steps are fragmentation, leaching, catabolism, humification and mineralisation.

• Fragmentation is detritivores breaking detritus into smaller particles; leaching carries water-soluble inorganic nutrients downward into soil.
• Microbial enzymes cause catabolism. Humification forms slow-decomposing humus, while mineralisation releases inorganic nutrients.
• Warm, moist and oxygen-rich conditions favour decomposition. Low temperature, anaerobiosis, and detritus rich in lignin or chitin slow it.

Energy flow is unidirectional: sunlight to producers and then to consumers and decomposers. Less than half of incident solar radiation is PAR, and plants capture only about 2-10% of PAR.

• Grazing food chains begin with living producers and dominate aquatic ecosystems. Detritus food chains begin with dead organic matter and dominate terrestrial energy flow.
• Food chains are interconnected into food webs. A trophic level is a feeding position, not a species label; a sparrow can be primary consumer on seeds and secondary consumer on insects.
• Only 10% of energy is transferred to the next trophic level, restricting chain length.

Ecological pyramids show trophic relationships using number, biomass or energy. Producers form the base and top consumers the apex.

• Pyramids of number and biomass are generally upright, but number can be inverted in a tree ecosystem and biomass is often inverted in sea.
• The energy pyramid is always upright because energy at a lower trophic level is always greater than at a higher one.
• All organisms at a trophic level must be included. An organism may occupy more than one trophic level simultaneously.

• An ecosystem has biotic and abiotic components, species composition and stratification.
• GPP is total photosynthetic production; NPP is GPP minus respiration.
• Decomposition returns inorganic nutrients through mineralisation.
• Energy flows one way; matter cycles; only about 10% energy reaches the next level.
• Energy pyramids are always upright; biomass can be inverted in aquatic systems.