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Concept of Productivity:
The amount of organic matter i.e. the food prepared by a plant is known as productivity. When it is measured at any unit of time, it is known as the rate of productivity of that ecosystem.
Productivity is of the following types: A. PrimaryProductivity
B. SecondaryProductivity
C. NetProductivity
A. Primary Productivity: Productivity is the production capability of a plant. Thus,
primary productivity is always associated with autotrophs or photosynthetic organisms i.e. green plants. Some microorganisms also can carry out this process and are known as photosynthetic microorganisms.
Thus, primary productivity is the rate of fixation of solar energy from the sun by the photosynthesis activity of the organisms.
Primary Productivity are 2 types
1. Gross Primary Productivity (GPP)
2. Net Primary Productivity (NPP)
1. Gross Primary Productivity: It is the total rate of photosynthesis or food production by a photosynthetic organism. It depends on the chlorophyll content of a plant. Thus, it is calculated as the amount of CO2 fixed per gram of chlorophyll per hour. This can be represented as
GPP = amount of CO2 /gm Chl / hr.
2. Net Primary Productivity (NPP): We know, that energy is required for each activity they do, by living organisms. Thus, for respiration work, plants also need energy. So, the energy that was fixed during photosynthesis is being used here. As a result, the remaining energy after loss in respiration utilization is known as the Net Primary Productivity. In simple words, NPP = GPP – Respiration energy

B) Secondary Productivity: This is related to the heterotrophs in contrast to Primary Productivity. Secondary Productivity is the energy stored at the consumer level. Ecologist Odum (1971) prefers to use the term assimilation rather than production at this level. Secondary production is not a fixed level of energy utilization of production but rather moves from one consumer level to the other through the food chain.
C) Net Productivity: Net Productivity is the storage of energy by the consumers. That means it is the energy which remains in the body of the consumer after utilization in respiration or any other work done by the consumer. Thus, it can be measured as biomass. Net productivity can be expressed as the production of carbon mg/meter2/day. It can then be calculated how much energy per/biomass the consumer gets in a year.

Homeostasis

Homeostasis is defined as “The ability to maintain a constant internal environment in response to the environment changes”. This is a unique principle of biology. In the same way, natural ecosystems are also capable of maintaining their internal regulations i.e. self- regulations or self-maintenance at any point in time. This is called the stable steady state of an ecosystem (Homeo means same, and stasis means standing). Odum (1971) defined Homeostasis as the tendency of a natural ecosystem to resist change and to remain in the state of equilibrium, which implies that within an ecosystem there always stays a balance between the production, consumption, as well as decomposition and in all the living organisms within a particular ecosystem in a particular time, follow this kind of equilibrium.

According to the first law of thermodynamics, energy cannot be created nor destroyed, it can be transferred from one form to another. In an ecosystem, the energy gets fixed in other words, in presence of sunlight, the autotrophs or the producers prepare their food material. The heterotrophs get their food/energy from the autotrophs that is the plants. Energy is required by all living organisms to perform their work or metabolism.Energy utilization in an ecosystem occurs in two ways.
i) Quantity of solar energy the plants receive from the sun for photosynthesis
ii) Quantity of energy flow occurs from the plants to the consumers.
This behaviour of energy transaction in an ecosystem is known as energy flow.
Energy flow occurs in two models within the ecosystem
a) Single Channel Energy Flow Model
b) Y- Shaped Energy Flow Models a) Single Chain Energy Flow Model: This type of energy flow works as per the food chain of the ecosystem. For example, in a grassland ecosystem, grasses are the producers. They fix carbon dioxide2 from the atmosphere and produce carbohydrates as the gross productivity. It is a one-way direction of energy flow. This clearly indicates that, if the food chain is longer in length then the energy that reaches the top carnivores are less and if the food chain is a shorter one, more energy will be available to the topmost trophic level organisms.
b) Y-Shaped Energy Flow Model: In nature besides the single chain model, another way of energy transfer is found. The Food web shows a realistic picture of the flow of energy which is more complex with more combinations. The Y-model explains the connection between grazing & a detritus food chain

Ammonification: Ammonification is an important step in the nitrogen cycle. It is the process of production of ammonia (NH3) or ammonium (NH4) compounds from the decomposition action of bacteria on organic matter. Thus, on the death and decay of the plants as well as animals the complex organic compounds are released into the soil where they are again decomposed into simpler compounds by the microorganisms and release energy.
Examples of bacteria – Nitrosomonas bacteria, Nitrosococcus bacteria
C) Nitrification: When ammonium gets converted into nitrates, it is called nitrification. Nitrates can be directly absorbed by plants & incorporated into proteins, nucleic acids & other nitrogenous organic compounds. Some nitrates may be stored in the humus of the soil, immobilized by the bacteria & some may reach into the waterbodies with the runoffs.Nitrogen Cycle: It is a complex cycle that occurs in nature through various steps. About 79% of the atmospheric air is Nitrogen. It enters the biotic world and got assimilated then again goes back into the atmosphere. The following steps are involved in the completion of the Nitrogen cycle- Nitrogen enters the living organisms: Pure nitrogen gas cannot be used by green plants. Only Nitrate & Ammonium forms of Nitrogen can be utilized by them. Thus, nitrogen gas is first fixed into Nitrous oxide, Nitric oxide and Ammonium in nature.D) Nitrogen Gas back to the Atmosphere: Through the process of denitrification, nitrogen gas goes back to the atmosphere. Some bacteria there can convert Nitrates (NO3) to Nitrites (NO2). They are called denitrifying bacteria e.g Pseudomonas. Ultimately Nitrates, Nitrites & gaseous Nitrogen are then released into the atmosphere.