Key Ecological Concepts: Productivity, Homeostasis, Energy Flow, Nitrogen Cycle

Posted on Jun 21, 2025 in Biology

Understanding Ecological Productivity

Productivity refers to the amount of organic matter, or food, prepared by a plant. When measured at any unit of time, it is known as the rate of productivity of that ecosystem.

Productivity is categorized into the following types:

  • Primary Productivity
  • Secondary Productivity
  • Net Productivity

Primary Productivity

Primary productivity is the production capability of a plant. It is always associated with autotrophs or photosynthetic organisms, primarily green plants. Some microorganisms can also carry out this process and are known as photosynthetic microorganisms.

Thus, primary productivity is defined as the rate of fixation of solar energy from the sun through the photosynthetic activity of these organisms.

Primary Productivity has two main types:

  1. Gross Primary Productivity (GPP)
  2. Net Primary Productivity (NPP)

Gross Primary Productivity (GPP)

Gross Primary Productivity (GPP) is the total rate of photosynthesis or food production by a photosynthetic organism. It depends on the chlorophyll content of a plant. Therefore, 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

Net Primary Productivity (NPP)

Living organisms require energy for every activity they perform. Plants, for instance, need energy for respiration. The energy fixed during photosynthesis is partially used for this purpose. As a result, the remaining energy after loss due to respiration is known as the Net Primary Productivity (NPP). In simple terms:

NPP = GPP – Respiration Energy

Secondary Productivity

In contrast to primary productivity, Secondary Productivity is related to heterotrophs. It represents the energy stored at the consumer level. Ecologist Odum (1971) preferred to use the term assimilation rather than production at this level. Secondary production is not a fixed level of energy utilization or production but rather moves from one consumer level to another through the food chain.

Net Productivity

Net Productivity refers to the storage of energy by consumers. This is the energy that remains in the body of the consumer after utilization in respiration or any other work performed by the consumer. Thus, it can be measured as biomass. Net productivity can be expressed as the production of carbon in mg/meter2/day. From this, one can calculate how much energy per biomass the consumer gains in a year.

Homeostasis in Ecosystems

Homeostasis is defined as “the ability to maintain a constant internal environment in response to environmental changes.” This is a unique principle in biology. Similarly, natural ecosystems are also capable of maintaining their internal regulations, often referred to as self-regulation 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 a state of equilibrium. This implies that within an ecosystem, there always exists a balance between production, consumption, and decomposition, and all living organisms within a particular ecosystem at a particular time follow this kind of equilibrium.

Energy Flow in Ecosystems

According to the first law of thermodynamics, energy cannot be created nor destroyed; it can only be transferred from one form to another. In an ecosystem, energy is fixed when autotrophs (producers) prepare their food material in the presence of sunlight. Heterotrophs obtain their food and energy from these autotrophs (plants).

Energy is essential for all living organisms to perform their work or metabolism. Energy utilization in an ecosystem occurs in two primary ways:

  1. The quantity of solar energy plants receive from the sun for photosynthesis.
  2. The quantity of energy flow that occurs from plants to consumers.

This behavior of energy transaction within an ecosystem is known as energy flow.

Energy flow occurs in two primary models within the ecosystem:

  • Single Channel Energy Flow Model
  • Y-Shaped Energy Flow Model

Single Channel Energy Flow Model

This type of energy flow operates according to the food chain of the ecosystem. For example, in a grassland ecosystem, grasses are the producers. They fix carbon dioxide from the atmosphere and produce carbohydrates as their gross productivity. This represents a one-way direction of energy flow. It clearly indicates that if the food chain is longer, the energy reaching the top carnivores will be less; conversely, if the food chain is shorter, more energy will be available to the topmost trophic level organisms.

Y-Shaped Energy Flow Model

In nature, besides the single channel model, another way of energy transfer is found. The food web provides a more realistic and complex picture of energy flow with more intricate combinations. The Y-model specifically explains the connection between grazing and detritus food chains, illustrating a more comprehensive energy transfer pathway.

The Nitrogen Cycle

The Nitrogen Cycle is a complex biogeochemical cycle that occurs in nature through various steps. Approximately 78% of the atmospheric air is Nitrogen gas (N2). This atmospheric nitrogen enters the biotic world, gets assimilated, and then returns to the atmosphere.

The following key steps are involved in the completion of the Nitrogen Cycle:

  1. Nitrogen Fixation (Nitrogen enters living organisms)
  2. Ammonification
  3. Nitrification
  4. Denitrification (Nitrogen gas back to the atmosphere)

Nitrogen Fixation: Nitrogen Enters Living Organisms

Pure nitrogen gas (N2) cannot be directly used by green plants. Only Nitrate (NO3–) and Ammonium (NH4+) forms of nitrogen can be utilized by them. Thus, atmospheric nitrogen gas must first be fixed into usable forms like ammonia (NH3) or ammonium (NH4+) in nature, primarily by nitrogen-fixing bacteria.

Ammonification

Ammonification is an important step in the nitrogen cycle. It is the process of producing ammonia (NH3) or ammonium (NH4+) compounds from the decomposition of organic matter by bacteria and fungi. Upon the death and decay of plants and animals, complex organic compounds are released into the soil, where microorganisms decompose them into simpler compounds, releasing energy.

Examples of ammonifying bacteria include various decomposers.

Nitrification

When ammonium (NH4+) gets converted into nitrates (NO3–), it is called Nitrification. This process typically occurs in two steps:

  1. Ammonium (NH4+) is converted to nitrites (NO2–) by bacteria like Nitrosomonas and Nitrosococcus.
  2. Nitrites (NO2–) are then converted to nitrates (NO3–) by bacteria like Nitrobacter.

Nitrates can be directly absorbed by plants and incorporated into proteins, nucleic acids, and other nitrogenous organic compounds. Some nitrates may be stored in the humus of the soil, immobilized by bacteria, or reach water bodies with runoff.

Denitrification: Nitrogen Gas Back to the Atmosphere

Through the process of Denitrification, nitrogen gas (N2) returns to the atmosphere. Certain bacteria, known as denitrifying bacteria (e.g., Pseudomonas), convert nitrates (NO3–) back to nitrites (NO2–), and then ultimately to gaseous nitrogen (N2) or nitrous oxide (N2O), which are released into the atmosphere, completing the cycle.