Productivity in Ecosystem – Primary and Secondary Productivity

Productivity in Ecosystem: Did you know the length of the day on Earth is increasing? Don’t be surprised; it’s a fact. When Earth was formed nearly 4.5 billion years ago, its day used to be roughly six hours long. During the course of 4.5 billion years, the spinning rate of the Earth was slowed down due to the gravitational pull of the moon and continues to do so at the rate of 1.7 milisecond/year.

Now the question is, if the length of a day becomes 6 hours today, how will that affect our planet, our ecosystem, and its productivity? To understand that, we need to understand the functioning and productivity of our ecosystem. Read on to know productivity in ecosystem, primary and secondary productivity, ecological productivity and how it is affected by sunlight, duration, and other factors.

What is Productivity?

Productivity in ecosystem can be defined as the rate at which the biomass increases per unit area. It can be measured in units of biomass per unit volume per unit area, i.e., gm^–2 yr^–1 or (kcal m^–2) yr^–1.  Biomass can be defined as dry matter or dry mass. Productivity often depends on the ecosystem. Each ecosystem has a different number of producers, consumers, and decomposers, which affects productivity greatly. The ecological productivity can usually be characterized as a pyramid structure that depicts the food web. Moreover, primary and secondary productivity is as important as ecological productivity.

Productivity can be studied at two different levels:

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Fig: Types of Productivity

Primary Productivity in Ecosystem

1. It is carried out by autotrophs or producers.
2. Primary productivity can be defined as the amount of solar energy converted to chemical energy by an ecosystem’s producers for a given area during a certain time period.
3. Producers produce biomass by performing photosynthesis.
4. Some primary producers can be chemosynthetic as well, i.e., they produce biomass by chemosynthesis. E.g., Purple sulfur bacteria.
5. Primary productivity can be divided into two types as below:

Fig: Types of Primary Productivity

Gross primary productivity (GPP): The total solar energy used to produce biomass by the process of photosynthesis is referred to as gross primary productivity (GPP). It is the rate of production of organic matter. Some amount of GPP is utilized by the plant itself, and a very little amount is transferred to the consumers. Most of the GPP is lost due to plants’ internal metabolism or cellular respiration or growth and repair mechanism. The lost  GPP is referred to as respiratory loss (R).

Net primary productivity (NPP): Net primary productivity is the amount of biomass available for the consumers or heterotrophs after respiratory loss. This biomass is stored in the plants. Net primary production forms the base of ecological food chains and is heavily manipulated by humans in the production of food, fiber, wood, and increasingly biofuels. 

This can be described as:

NPP = GPP – R

Fig: Depiction of GPP, NPP, and R

The tropical forest has the highest primary productivity in terrestrial regions, while the desert shows the least primary productivity. According to an estimate, approximately 170 billion tons of net primary productivity occurs per year in the entire biosphere, while the ocean is responsible for only 55 billion tons of biomass.

The primary productivity of a natural ecosystem largely depends upon the amount of solar energy; there is a positive correlation between primary productivity and solar radiation. Since solar radiation received at the Earth’s surface decreases as we move from the equator towards the poles, primary productivity also decreases remarkably towards the poles. According to E.P Odum, there are three levels of productivity on a world scale:

1. Shallow water areas, moist forests (tropical and temperate), alluvial plains, and regions of intensive farming represent the regions of high ecological productivity.
2. Arctic snow-covered wastelands, deserts, and deep ocean areas represent the regions of low ecological productivity.
3. Grasslands, shallow lakes, and farmlands represent regions of intermediate ecological productivity.

Secondary Productivity in Ecosystem

Secondary productivity can be defined as the rate of biomass production by consumers or herbivores. The organism that cannot produce its own food and can obtain energy from biomass produced due to primary productivity. Now, the thing to be noticed here is unlike primary productivity, at the level of secondary productivity, different trophic levels exist like herbivores, carnivores, omnivores, etc.

Each trophic level consumes biomass but assimilates only 10% of it into energy, and the rest of the undigested biomass is released in the form of fecal waste and left to be used by other trophic levels or decomposers. The assimilated energy is used for various metabolic processes, growth, and reproduction, etc. Entire secondary productivity represents energy flow through trophic levels. Secondary productivity is basically the amount of energy stored in the tissues of consumers of each trophic level.

Fig: Secondary Productivity and Energy Flow

In terrestrial ecosystems, productivity is highest in Swamps, marshes, tropical rain forests (most productive), while lowest in the desert. In contrast, in aquatic ecosystems, productivity is highest in the Estuaries, lowest in the open ocean.

Fig: Productivity and Energy Flow Through the Different Trophic Level

Ecological Productivity in Ecosystem

Ecological productivity is defined as the subsequent use of energy by herbivores, detritivores, carnivores, and the primary fixation of solar energy by green plants. Green plant productivity is referred to as primary productivity by ecologists. Gross primary productivity refers to the entire quantity of energy fixed by plants, whereas net primary productivity is lower since it accounts for energy losses associated with plant respiration. The biomass of vegetation increases over time if the net primary productivity of green plants in an ecosystem is positive.

Factors Affecting Components of Ecosystem

The components of ecosystem is affected by the following factors:
1. Solar radiation: Increased solar radiation increases productivity
2. Temperature: Productivity is highest near the equator terrestrial region
3. Moisture: Leaf water potential, soil moisture, fluctuation of precipitation, and transpiration depend upon the moisture content of the atmosphere.
4. Mineral nutrition: Higher uptake of minerals from the soil increase productivity and nitrogen metabolism while salinity and heavy metals decrease productivity.
5. Biotic activities: Above-ground herbivores,  below-ground herbivores, predators, and parasites decrease primary productivity.
6. Impact of human populations:  Increased population causes reduction of plants and reduces productivity.
7. In aquatic systems, deeper parts have scarce light and nutritional availability, which limits productivity.

Productivity in Ecosystem: Summary

Productivity in ecosystem can be defined as the rate of biomass production in the form of the dry weight of biomass. Productivity can be categorized into two types- primary productivity and secondary productivity. Green plants do primary productivity by the process of photosynthesis. The unit of productivity is gm^–2 yr^–1 or (kcal m^–2) yr^–1. The rate of production of organic matter during photosynthesis is referred to as gross primary productivity. Most of the gross primary produce is lost in the form of respiratory loss. Net primary productivity is the available biomass for the consumption of heterotrophs (herbivores and decomposers). Gross primary productivity minus respiratory loss gives net primary productivity. Secondary productivity is the rate of biomass production by consumers or herbivores.

FAQs on Productivity in Ecosystem

Some of the frequently asked questions on productivity in ecosystem are mentioned below:

Q.1: Why is productivity important to an ecosystem?
Ans: Ecosystems have characteristic amounts of biomass at each trophic level. It is important to maintain balance at each trophic level. Productivity is important for the following reasons:
(a) To initiate the flow of energy from the sun to organic form
(b) To bring energy to other living organisms like consumers and decomposers
(c) To maintain the balance of energy.

Q.2: What are the major productivity functions of the ecosystem?
Ans: There are four major productive functions of an ecosystem:
1. Productivity
2. Energy flow
3. Decomposition
4. Nutrient cycling

Q.3: Which ecosystem has the highest and which ecosystem has the lowest productivity?
Ans: Tropical rainforest has the highest productivity while desert has the lowest productivity.

Q.4: How is ecosystem productivity measured?
Ans: Ecosystem productivity is measured in terms of biomass per unit volume per unit area, and its unit is gm^–2 yr^–1 or (kcal m^–2) yr^–1.

Q.5: What are the factors affecting the productivity of an ecosystem?
Ans: Factors affecting the productivity of an ecosystem are:
(a) Type of ecosystem
(b) Amount of producer and consumer at each trophic level
(c) Human indulgence in a particular ecosystem.

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