Ecosystem — Notes

An ecosystem is a functional unit of nature in which living organisms interact among themselves and with their surrounding physical environment. Its size ranges from a small pond to a large forest or a sea; the entire biosphere is regarded as a global ecosystem.

Types: terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary); man-made ecosystems include crop fields and an aquarium.

Structure. Biotic and abiotic components interact and function as a unit, giving a physical structure characteristic of each ecosystem. Identification and enumeration of plant and animal species gives the species composition. Stratification is the vertical distribution of different species occupying different levels: in a forest, trees occupy the top layer, shrubs the second, and herbs and grasses the bottom layers.

Pond ecosystem.

• Abiotic: water with dissolved inorganic and organic substances, and the rich soil deposit at the bottom. Solar input, the cycle of temperature, day length and other climatic conditions regulate the rate of function.
• Autotrophs: phytoplankton, some algae, and floating, submerged and marginal plants.
• Consumers: zooplankton, and free-swimming and bottom-dwelling forms.
• Decomposers: fungi, bacteria and flagellates, especially abundant at the bottom.

Functions (pond and biosphere alike): conversion of inorganic into organic material by autotrophs using the sun’s radiant energy; consumption of autotrophs by heterotrophs; decomposition and mineralisation of dead organic matter, releasing minerals for reuse; unidirectional movement of energy towards higher trophic levels and its dissipation and loss as heat.

Four important components of ecosystem function: productivity, decomposition, energy flow and nutrient cycling.

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.

• Primary production: the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis; expressed as weight (g m⁻²) or energy (kcal m⁻²).
• Productivity: the rate of biomass production, expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹, so that different ecosystems can be compared.
• Gross primary productivity (GPP): the rate of production of organic matter during photosynthesis. A considerable amount of GPP is used by plants in respiration (R).
• Net primary productivity (NPP) = GPP − R: the biomass available for consumption by heterotrophs (herbivores and decomposers).
• Secondary productivity: the rate of formation of new organic matter by consumers.

Primary productivity depends on the plant species inhabiting an area, environmental factors, availability of nutrients and the photosynthetic capacity of plants. The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Although oceans occupy about 70 percent of the surface, their productivity is only 55 billion tons; the rest is on land.

Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients. The raw material, detritus, is dead plant remains such as leaves, bark and flowers, and dead remains of animals, including fecal matter.

Steps:

1. Fragmentation: detritivores (e.g. earthworm) break detritus into smaller particles.
2. Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification: accumulation of a dark coloured amorphous substance, humus, that is highly resistant to microbial action and decomposes at an extremely slow rate. Being colloidal, it serves as a reservoir of nutrients.
5. Mineralisation: humus is further degraded by some microbes, releasing inorganic nutrients.

Fragmentation, leaching and catabolism operate simultaneously on the detritus.

Factors. Decomposition is largely an oxygen-requiring process. Its rate is controlled by the chemical composition of detritus and climatic factors. It is slower if detritus is rich in lignin and chitin, and quicker if it is rich in nitrogen and water-soluble substances like sugars. Warm and moist conditions favour it; low temperature and anaerobiosis inhibit it, resulting in a build-up of organic material.

Except for the deep-sea hydrothermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50 percent is photosynthetically active radiation (PAR); plants capture only 2–10 percent of PAR, and this small amount sustains the entire living world.

There is a unidirectional flow of energy from the sun to producers and then to consumers. Ecosystems are not exempt from the second law of thermodynamics: they need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency towards increasing disorderliness.

• Producers: green plants. Consumers: all animals depend on plants directly or indirectly. Primary consumers (herbivores: insects, birds and mammals on land; molluscs in water); secondary consumers (carnivores) and so on.
• Grazing food chain (GFC): grass → goat → man.
• Detritus food chain (DFC): begins with dead organic matter; made of decomposers (heterotrophs, mainly fungi and bacteria — saprotrophs) that secrete digestive enzymes and absorb the products.
• In an aquatic ecosystem the GFC is the major conduit of energy flow; in a terrestrial ecosystem a much larger fraction of energy flows through the DFC. The DFC may be connected with the GFC at some levels; the natural interconnection of food chains is a food web.
• Trophic levels: producers (first), herbivores (second), carnivores (third). Each level has a certain mass of living material at a particular time, the standing crop, measured as biomass or number per unit area.
• The number of trophic levels in a grazing food chain is restricted because transfer of energy follows the 10 percent law: only about 10 percent of the energy is transferred to each trophic level from the lower one.

The relationship between producers and consumers can be shown as a pyramid of number, biomass or energy, with the producers at the base and top consumers at the apex.

• Pyramid of number: usually upright (many grass plants, fewer herbivores, very few top carnivores); inverted when one large tree supports many insects.
• Pyramid of biomass: usually upright on land; inverted in the sea, because the biomass of fishes (and zooplankton) far exceeds that of phytoplankton at any time.
• Pyramid of energy: always upright, never inverted, because when energy flows from one trophic level to the next some energy is always lost as heat.

Limitations: pyramids do not take into account the same species belonging to two or more trophic levels; they assume a simple food chain, which almost never exists in nature, and do not accommodate a food web; and saprophytes are given no place in them, even though they play a vital role in the ecosystem.