Ecosystem

In this chapter: ecosystem structure and function; the pond as an example; productivity; decomposition; energy flow and food chains; ecological pyramids and their limitations. In the rationalised NCERT, ecological succession, nutrient cycling and ecosystem services have been removed; this page covers what is in the current syllabus.

Ecosystem: structure and function

An ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment. It can be as large as a forest or as small as a pond; even an aquarium or a crop field is an ecosystem. Ecosystems are broadly terrestrial (forest, grassland, desert) or aquatic (pond, lake, wetland, river, estuary); crop fields and aquaria are man-made ecosystems.

• The interaction of biotic and abiotic components results in a physical structure characteristic of each ecosystem.
• Species composition and stratification are its key structural features. 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.
• The components function as a unit through four processes: (1) productivity, (2) decomposition, (3) energy flow, and (4) nutrient cycling.

The pond as an example

• Abiotic: water with dissolved inorganic and organic substances, and the rich soil deposit at the bottom. Solar input, temperature cycle, day length and other climatic conditions regulate the pond's function.
• Autotrophs (producers): phytoplankton, some algae, and floating, submerged and marginal plants at the edges.
• Consumers: zooplankton, free-swimming and bottom-dwelling forms.
• Decomposers: fungi, bacteria and flagellates, especially abundant at the bottom.
• The pond shows all the functions: conversion of inorganic into organic material using radiant energy by autotrophs; consumption by heterotrophs; decomposition and mineralisation of dead matter to release minerals for reuse; and unidirectional movement of energy towards higher trophic levels with dissipation and loss as heat.

Productivity

• Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed as weight (g m⁻²) or energy (kcal m⁻²).
• Productivity is the rate of biomass production, expressed as g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹, used to compare the productivity of different ecosystems.
• Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis. A considerable part is used by plants in respiration (R).
• Net primary productivity: NPP = GPP − R. NPP is the biomass available for consumption by heterotrophs (herbivores and decomposers).
• Secondary productivity is the rate of formation of new organic matter by consumers.
• Primary productivity depends on the plant species in an area, environmental factors, availability of nutrients and the photosynthetic capacity of plants, so it varies between ecosystems.
• Annual net primary productivity of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Despite occupying about 70 per cent of the surface, the productivity of the oceans is only 55 billion tons.

Decomposition

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

Fragmentation, leaching and catabolism go on simultaneously on the detritus.

Factors affecting the rate of decomposition

• Chemical composition of detritus: slower if rich in lignin and chitin; quicker if rich in nitrogen and water-soluble substances like sugars.
• Climate: warm and moist conditions favour decomposition; low temperature and anaerobiosis inhibit it, resulting in the build-up of organic material.
• Decomposition is largely an oxygen-requiring process.

Energy flow

• Except in 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 per cent is photosynthetically active radiation (PAR).
• Plants and photosynthetic bacteria (autotrophs) fix solar energy; plants capture only 2 to 10 per cent of the PAR, and this small amount sustains the entire living world.
• Energy flow is unidirectional, from the sun to producers and then to consumers. Ecosystems obey the second law of thermodynamics: they need a constant supply of energy to synthesise the molecules they require and to counteract the universal tendency towards increasing disorderliness.

Food chains and trophic levels

• Grazing food chain (GFC): Grass → Goat → Man. Producers are consumed by herbivores (primary consumers), which are consumed by carnivores (secondary consumers), and so on.
• Detritus food chain (DFC): begins with dead organic matter. It is made up of decomposers, which are heterotrophic organisms, mainly fungi and bacteria (saprotrophs); they secrete digestive enzymes that break down dead and waste materials into simple inorganic materials, which they absorb.
• In an aquatic ecosystem, GFC is the major conduit for energy flow. In a terrestrial ecosystem, a much larger fraction of energy flows through the DFC than through the GFC.
• The DFC may be connected with the GFC at some levels. Natural interconnection of food chains makes a food web.
• Organisms occupy a place in the ecosystem based on their source of nutrition: their trophic level. Producers are the first trophic level, herbivores the second, carnivores the third.
• Each trophic level has a certain mass of living material at a particular time, called the standing crop, measured as biomass or number per unit area.
• The number of trophic levels in a GFC is restricted because energy transfer follows the 10 per cent law: only 10 per cent of the energy is transferred to each trophic level from the lower one (Lindeman). For example, 1000 J at producers leaves 100 J for herbivores, 10 J for primary carnivores, and 1 J for secondary carnivores.

Ecological pyramids

The base of each pyramid represents producers (first trophic level); the apex represents tertiary or top-level consumers. There are three types: the pyramid of number, of biomass and of energy. For any calculation of energy content, biomass or numbers, all organisms at a trophic level must be included.

Limitations of ecological pyramids

• They 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; they do not accommodate a food web.
• Saprophytes are not given any place in ecological pyramids, even though they play a vital role in the ecosystem.

Common traps: (1) Oceans cover 70 per cent of the surface but give only 55 of the 170 billion tons of NPP. (2) Plants capture 2 to 10 per cent of PAR, and PAR is less than 50 per cent of incident radiation. (3) In terrestrial ecosystems, more energy flows through the DFC, not the GFC. (4) Lignin and chitin slow decomposition; nitrogen and sugars speed it up. (5) Humus is resistant to microbial action, and it is mineralisation (not humification) that releases nutrients. (6) The pyramid of energy can never be inverted.

NEET focus

• GPP, NPP, secondary productivity and their units; the 170 and 55 billion tons figures.
• Steps and factors of decomposition.
• PAR figures; GFC vs DFC; 10 per cent law.
• Shapes of pyramids, inverted cases, limitations.

Practice questions