Primary Production

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain itself. Plants capture this energy and use it to build organic matter, and everything else in the ecosystem ultimately depends on that supply.

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 can be expressed in terms of weight, such as g m−2g\,m^{-2}, or in terms of energy, such as kcal m−2kcal\,m^{-2}. In other words, it tells us how much organic material the producers of an area have laid down.

From Production to Productivity

While production is an amount, productivity is the rate of biomass production — how fast that organic matter is being made. Because it is a rate, it is expressed with a time unit as well, in g m−2 yr−1g\,m^{-2}\,yr^{-1} or kcal m−2 yr−1kcal\,m^{-2}\,yr^{-1}.

Expressing it as a rate is useful because it lets us compare different ecosystems on a common footing. A productive ecosystem and a sluggish one can be placed side by side once both are measured in the same units per year.

Gross and Net Primary Productivity

Primary productivity is divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity is the rate of production of organic matter during photosynthesis — the total amount the plants fix.

However, a considerable amount of this GPP is used up by the plants themselves in respiration (R). What is left after these respiration losses is the net primary productivity:

GPP−R=NPPGPP - R = NPP

Here GPPGPP is the gross primary productivity, RR is the respiration losses, and NPPNPP is the net primary productivity. NPP is important because it is the biomass available for consumption by heterotrophs — the herbivores and the decomposers that feed on the producers.

Secondary Productivity and What Varies

Secondary productivity is defined as the rate of formation of new organic matter by consumers. Just as producers build biomass, consumers build their own tissues from the food they assimilate, and the rate at which they do so is secondary productivity.

Primary productivity is not the same everywhere. It depends on the plant species inhabiting an area, on a variety of environmental factors, on the availability of nutrients, and on the photosynthetic capacity of the plants. For all these reasons it varies from one type of ecosystem to another.

On a global scale, the annual net primary productivity of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Of this, the oceans produce only about 55 billion tons, even though they cover roughly 70 per cent of the surface; the rest is produced on land.

Quick Recap

  • A constant input of solar energy is basic to any ecosystem.
  • Primary production is the biomass or organic matter produced per unit area over time by plants in photosynthesis, expressed as weight (g m−2g\,m^{-2}) or energy (kcal m−2kcal\,m^{-2}).
  • Productivity is the rate of biomass production, expressed as g m−2 yr−1g\,m^{-2}\,yr^{-1} or kcal m−2 yr−1kcal\,m^{-2}\,yr^{-1}, so ecosystems can be compared.
  • GPP is the rate of production of organic matter in photosynthesis; a large part is used in respiration (R), leaving NPP, given by GPP−R=NPPGPP - R = NPP NPP is the biomass available to heterotrophs.
  • Secondary productivity is the rate of formation of new organic matter by consumers.
  • Annual NPP of the biosphere is about 170 billion tons (dry weight); the oceans contribute only about 55 billion tons despite covering about 70 per cent of the surface.

Solved Examples — Section 3

Q1. What is primary production?

Answer: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, expressed as weight (g m−2g\,m^{-2}) or energy (kcal m−2kcal\,m^{-2}).

Q2. How does productivity differ from production?

Answer: Production is an amount of biomass, while productivity is the rate of biomass production, expressed with a time unit such as g m−2 yr−1g\,m^{-2}\,yr^{-1}.

Q3. Write the relationship between GPP, R and NPP.

Answer: GPP−R=NPPGPP - R = NPP where GPP is gross primary productivity, R is respiration losses and NPP is net primary productivity.

Q4. Why is NPP important for the rest of the ecosystem?

Answer: NPP is the biomass available for consumption by heterotrophs — the herbivores and decomposers.

Q5. Define secondary productivity.

Answer: The rate of formation of new organic matter by consumers.

Q6. What is the annual NPP of the whole biosphere, and how much comes from the oceans?

Answer: About 170 billion tons (dry weight) of organic matter; of this the oceans produce only about 55 billion tons, despite covering about 70 per cent of the surface.