Ecosystem — Notes

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their surrounding physical environment. Ecosystems vary greatly in size — from a small pond to a large forest or sea; the whole biosphere may be regarded as a global ecosystem.

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

Components: biotic — producers (autotrophs), consumers (heterotrophs) and decomposers; abiotic — water, soil, minerals, light, temperature and other physical factors. Interaction of biotic and abiotic components gives a physical structure to an ecosystem, characterised by species composition and stratification (vertical distribution of species at different levels, for example trees, shrubs and herbs in a forest).

The pond ecosystem: a shallow water body in which all the basic components can be studied.

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

Functions: (i) conversion of inorganic into organic material with radiant energy by autotrophs; (ii) consumption of autotrophs by heterotrophs; (iii) decomposition and mineralisation of dead organic matter, releasing minerals for reuse; (iv) unidirectional movement of energy towards higher trophic levels and its dissipation and loss as heat.

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

• Primary production is 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). The rate of biomass production is productivity (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹).
• Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis. A considerable amount is used by plants in respiration (R).
• Net primary productivity (NPP) = GPP − R: the biomass available for consumption by heterotrophs (herbivores and decomposers). Example: GPP 25000 − R 9000 = NPP 16000 kcal m⁻² yr⁻¹.
• Secondary productivity is the rate of formation of new organic matter by consumers. Primary productivity depends on the plant species, environmental factors, availability of nutrients and photosynthetic capacity, so it varies between ecosystems. The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter; oceans, despite covering about 70 percent of the surface, produce only about 55 billion tons.

Photosynthetically active radiation (PAR) is about 50 percent of the incident solar radiation; plants capture only 2–10 percent of PAR, and this small amount sustains the entire living world.

Standing crop — the mass of living material (biomass) at each trophic level at a particular time (as fresh or dry weight per unit area). Standing state — the amount of nutrients such as carbon, nitrogen, phosphorus and calcium present in the soil at any given time.

Decomposers break down complex organic matter into inorganic substances such as carbon dioxide, water and nutrients; this process is decomposition. Dead plant remains (leaves, bark, flowers) and dead remains of animals, including faecal matter, form detritus, the raw material.

Steps (operating simultaneously on the detritus):

1. Fragmentation — detritivores such as earthworms break down 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 called humus, which is highly resistant to microbial action and decomposes extremely slowly; being colloidal, it serves as a reservoir of nutrients.
5. Mineralisation — humus is further degraded by some microbes, releasing inorganic nutrients.

Factors affecting the rate: decomposition is largely an oxygen-requiring process. The rate is controlled by the chemical composition of the detritus — slower if it is rich in lignin and chitin, quicker if rich in nitrogen and water-soluble substances such as sugars — and by climatic factors: warm and moist environments favour it, whereas low temperature and anaerobiosis inhibit it, resulting in a build-up of organic material.

Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy for all ecosystems. Of the incident solar radiation, less than 50 percent is PAR, and plants capture only 2–10 percent of it. Energy flow is unidirectional: from the sun to producers and then to consumers; it obeys the laws of thermodynamics, and at every transfer a great deal is lost as heat.

Food chains:

• Grazing food chain (GFC) — starts with living producers: grass → goat → man. In aquatic ecosystems the GFC is the major conduit for energy flow.
• Detritus food chain (DFC) — begins with dead organic matter; made up of decomposers (heterotrophic saprotrophs: fungi and bacteria) and detritivores. In terrestrial ecosystems a much larger fraction of energy flows through the DFC than through the GFC. The DFC may be connected with the GFC, as some DFC organisms are prey to GFC animals.
• Food web — natural interconnection of food chains.

Trophic levels: organisms occupy a place in the food chain according to their feeding relationship: producers (first), herbivores or primary consumers (second), primary carnivores or secondary consumers (third), secondary carnivores or tertiary consumers (fourth). A species may occupy more than one trophic level in the same ecosystem at the same time.

10 percent law (Lindeman): only about 10 percent of the energy is transferred to each higher trophic level from the lower one (for example 20000 → 2000 → 200 → 20 J). Hence the number of trophic levels is limited.

The relationship between organisms at different trophic levels, in terms of number, biomass or energy, can be shown as a pyramid. The base represents the producers (first trophic level) and the apex the top-level consumers.

• Pyramid of numbers — usually upright (grassland: many grasses, fewer herbivores, few carnivores). It can be inverted: for example, a single tree supports a large number of insects, which support fewer birds.
• Pyramid of biomass — usually upright on land. In the sea it is generally inverted, because the biomass of fishes (and zooplankton) far exceeds that of the fast-reproducing phytoplankton at any one time.
• Pyramid of energy — always upright and can never be inverted, because when energy flows from one trophic level to the next, some is always lost as heat at each step.

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 not given any place, although they play a vital role.