Interaction of biotic and abiotic components result in a physical structure that is characteristic for each type of ecosystem. One can derive species composition of an ecosystem by identifying and enumerating plant and animal species Vertical distribution of different species occupying different levels is called stratification. For example, trees occupy top vertical strata or layer of a forest, shrubs the second and herbs and grasses occupy the bottom layers.

The components of the ecosystem are seen to function as a unit, when you consider the following aspects:

(i) Productivity

(ii) Decomposition

(iii) Energy flow

(iv) Nutrient cycling

PRODUCTIVITY

• Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight or energy.
• Productivity is the rate of biomass production. It is expressed in terms of to compare the productivity of different ecosystems.
• Productivity can be divided into gross primary productivity (GPP) and net primary productivity (NPP).
• Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration.
• Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers).
• Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).
• GPP – R = NPP
• Secondary productivity is defined as the rate of formation of new organic matter by consumers.
• Primary productivity depends on the plant species inhabiting a particular area.

 It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems.

Decomposers

They break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition.

Dead plant remains such as leaves, bark, flowers and dead remains of animals, including faecal matter, constitute detritus, which is the raw material for decomposition.

• The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.
• Fragmentation is a process in which detritivores (e.g., earthworm) break down detritus into smaller particles.
• Leaching is a process in which-water inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
• Catabolism is a process in which bacterial and fungal enzymes degrade detritus into simpler inorganic substances. It is important to note that all the above steps in decomposition operate simultaneously on the detritus (Figure).
• Humification leads to accumulation of a dark-coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients.
• Mineralisation is a process in which humus is further degraded by some microbes and release of inorganic nutrients occur.
•  Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors.
  •  In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.
  • Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build-up of organic materials.

Detritus food chain (DFC)

• It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.
• They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs (sapro: to decompose).
• Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.
• In an aquatic ecosystem, Grazing Food Chain (GFC) is the major conduit for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC.
• Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level.
• Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third.

• It refers to the movement of energy through an ecosystem, from one organism to another. It is the process by which energy from the sun is converted into usable energy by plants through photosynthesis, and then transferred to other organisms through feeding relationships.
• In an ecosystem, energy flows from producers (plants) to consumers (animals), and from one trophic level to another.
• Trophic levels are different levels in a food chain or food web, which represent the position of an organism in the chain.
  • The first trophic level consists of primary producers (plants), which convert sunlight into usable energy through photosynthesis.
  • The second trophic level consists of herbivores, which eat the plants.
  • The third trophic level consists of carnivores, which eat the herbivores. There can be multiple levels in a food chain or web, with each level representing a different feeding relationship.
• One must remember that the trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time;
  • for example, a sparrow is a primary consumer when it eats seeds, fruits, peas, and a secondary consumer when it eats insects and worms.
• Each trophic level has a certain mass of living material at a particular time called as the standing crop. The Standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
• Energy flow is not 100% efficient, as energy is lost at each trophic level due to various factors such as respiration, movement, and waste. This means that the higher up an organism is in a food chain or web, the less energy it receives.
• The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
• Overall, energy flow is an important process in ecosystems, as it determines the amount of energy available to support life and the interactions between organisms.

Except for the deep-sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth.

Ecological Pyramids

Ecological pyramids are graphical representations of the structure of ecosystems, showing the relative amounts of different components such as biomass, energy or numbers of individuals at each trophic level.The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer.

There are three main types of ecological pyramids:

Pyramid of Numbers

• This type of ecological pyramid shows the number of individuals at each trophic level in an ecosystem. The pyramid of numbers is usually widest at the base, where the producers are located, and narrows at higher trophic levels.

Pyramid of Biomass

• This type of ecological pyramid shows the total amount of living matter, or biomass, at each trophic level in an ecosystem. Biomass is a more accurate measure of the amount of energy available to support life, as it takes into account the mass of living tissue at each trophic level.
• The pyramid of biomass is usually widest at the base, where the producers are located, and narrows at higher trophic levels.

Pyramid of Energy

• This type of ecological pyramid shows the amount of energy that flows through each trophic level in an ecosystem. The pyramid of energy is always upright, as the amount of energy available decreases as you move up the trophic levels.

• It’s important to note that the shape of the ecological pyramid can vary depending on the ecosystem being studied. For example, in some aquatic ecosystems, the pyramid of biomass may be inverted, with the biomass of phytoplankton (producers) being less than the biomass of zooplankton (consumers) due to the rapid turnover of phytoplankton.
• Additionally, in some ecosystems, such as grasslands, the pyramid of biomass may not match the pyramid of numbers due to differences in the size and mass of organisms at different trophic levels.

However, there are certain limitations of ecological pyramids such as it does not take into account the same species belonging to two or more trophic levels. It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web. Moreover, saprophytes are not given any place in ecological pyramids even though they play a vital role in the ecosystem