Primary and Secondary Productivity

Production and productivity are different measurements

Primary production is the quantity of new organic material synthesised by autotrophs. In photosynthetic ecosystems, green plants, algae and other producers use sunlight to fix inorganic carbon. An amount of biomass might be expressed as g m⁻². Productivity is the rate at which that biomass is produced and therefore must include a time interval, such as g m⁻² yr⁻¹, or an energy-based rate such as kJ m⁻² yr⁻¹.

Standing crop is the mass of living material present at one time. This is a stock, not a rate. A meadow grazed frequently can have a modest standing stock but generate new tissue repeatedly. A mature forest can store enormous woody biomass even when its present annual growth is relatively slow. Comparing ecosystems therefore requires checking whether a measurement refers to stored living mass or newly produced mass.

Gross and net primary productivity

Gross primary productivity (GPP) is the total rate of organic matter production by primary producers before subtracting the organic matter they use in respiration. Producers respire to obtain energy for their own maintenance and metabolic activity. Their respiration (R) consumes part of the gross organic product, leaving net primary productivity (NPP). The relationship is NPP = GPP − R.

Subtracting producer respiration is essential because not all products of photosynthesis remain available for net growth, storage, grazing and detritus. All three terms must be expressed with compatible units and refer to the same habitat area and time interval. NPP is not the amount herbivores necessarily eat: some new tissue remains living, while other matter is shed or becomes detritus without first being grazed.

A worked biomass budget

Suppose a grassland's GPP is 1,200 g m⁻² yr⁻¹ and its producer respiration corresponds to 400 g m⁻² yr⁻¹ of dry organic matter. Net primary productivity is 1,200 − 400 = 800 g m⁻² yr⁻¹. The calculation is subtraction because respiration is already a component of the gross total, not a separate source of new production.

If another habitat has GPP = 1,100 and respiration = 600 g m⁻² yr⁻¹, its NPP is only 500 g m⁻² yr⁻¹. A higher GPP does not guarantee a higher NPP if producer respiration is proportionately large. Neither site can be assigned a larger standing crop from these rates alone because accumulated biomass also depends on mortality, grazing and turnover.

Secondary productivity in consumers

Secondary productivity is the rate at which consumers create new organic tissue from assimilated food. Herbivores do not fix atmospheric carbon photosynthetically; they ingest compounds produced by plants and convert some into their own biomass. Food that is not assimilated passes into waste, and assimilated compounds are also used for movement, respiration and maintenance. Only part becomes growth and reproduction.

Distinguish the phrase secondary productivity from primary producer. A consumer can add new body mass and therefore contribute to secondary production, yet it is not an autotrophic producer. A consumer's standing biomass at an instant still differs from the rate at which new consumer biomass is formed. This distinction later becomes crucial for interpreting ecological energy pyramids.

Why habitats produce biomass at different rates

Primary productivity is influenced by plant species and their photosynthetic capacity, light, moisture, temperature, availability of nitrogen, phosphorus and other nutrients, and the length of a favourable growing season. In dry lands water may be limiting even under strong sunlight. Under a forest canopy insufficient light may be the most immediate constraint. In oceans, light decreases with depth and available nutrients can restrict growth even near an illuminated surface.

The source gives approximate global annual net primary production of 170 billion tonnes of dry organic matter, with 55 billion tonnes attributed to oceans. These are broad textbook estimates rather than immutable modern totals. Oceans cover much of Earth's area but many open-ocean regions are nutrient poor. Coastal and upwelling areas may support much higher rates than the open ocean. There is no single factor that limits every aquatic ecosystem under all conditions.

Quick Quiz