5.10, 5.11 - Productivity and trophic transfers

Productivity and standing biomass

Productivity measures how quickly organisms produce new organic material, so its time interval matters as much as its amount.

Plants use photosynthesis to fix carbon dioxide into organic compounds. These compounds can be used in respiration or become material such as carbohydrates, lipids and proteins in plant biomass. Biomass is the mass of living biological material in a defined population or area. Dry mass is useful for comparisons because differences in water content would otherwise alter the mass without representing a corresponding change in stored chemical energy.

Gross primary productivity (GPP) is the total rate at which producers fix chemical energy into organic compounds by photosynthesis. Plants also respire: some of these compounds are broken down, with energy transferred through ATP to processes such as active transport and biosynthesis and eventually dissipated as heat. Plant respiration, $R$, in a productivity budget is the rate of this respiratory use of organic material or its chemical energy, expressed in the same units as productivity. It is not the total respiration of plants, animals and decomposers combined.

Net primary productivity (NPP) is the rate of production remaining after the plants' own respiratory use has been subtracted. It represents new plant organic material available for growth and reproduction and, subsequently, to consumers or decomposers. Plants respire in both light and darkness; daytime photosynthesis does not switch respiration off.

Energy productivity might be expressed as $\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$; biomass productivity might be $\mathrm{g}$ dry mass $\mathrm{m}^{-2}\,\mathrm{year}^{-1}$. The negative powers mean “per square metre per year”. An energy value cannot be subtracted from a mass value without an appropriate conversion.

Standing biomass is the biomass present at one moment. It has no “per year” term. A grassland could produce substantial new biomass while grazing removes it at almost the same rate, leaving little change in standing biomass. Therefore, subtracting a final standing biomass from an initial one measures the change in stock; it does not by itself measure all the new biomass produced. A snapshot biomass pyramid likewise cannot, by itself, establish annual productivity or transfer efficiency.

Calculating and rearranging productivity

A productivity calculation begins by identifying which quantity is gross, which is net and whose respiration is included.

The following original teaching budget describes one grassland over one year. All values are chemical-energy production or use per square metre; they are illustrative values, not field measurements.

Estimate before calculating: round GPP to about 25,000 and respiration to about 10,000. NPP should be about $15{,}000\,\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$, smaller than GPP. Now substitute the supplied values:

The exact result agrees with the estimate. The complete plant budget checks because $9{,}650+15{,}100=24{,}750$. This is chemical energy fixed by photosynthesis; incoming sunlight that was never fixed is outside this GPP budget.

Finding a different unknown

Rearrange the symbols before substituting numbers. Adding $R$ to both sides gives

For a separate habitat with NPP $14{,}200$ and plant respiration $9{,}800$, both in $\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$, the estimate is $14{,}000+10{,}000\approx24{,}000$. The calculation gives $\mathrm{GPP}=14{,}200+9{,}800=24{,}000\,\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$.

To find respiration instead, rearrange the same relationship:

Do not subtract animal respiration when a question asks for plant NPP. That would answer a different budget question.

Solving when the unknown appears twice

Suppose a third habitat has NPP equal to 64% of GPP and plant respiration of $9{,}000\,\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$. Let $G$ represent its GPP. Because $\mathrm{NPP}=0.64G$, substitute that expression into the relationship and solve algebraically:

Before exact arithmetic, 36% is a little more than one third; a respiration rate of 9,000 therefore suggests GPP a little less than 27,000. Substitution gives

Then $\mathrm{NPP}=0.64\times25{,}000=16{,}000\,\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$. Check by a second route: $25{,}000-9{,}000=16{,}000$. Dividing respiration by 0.64 would confuse the fraction remaining with the fraction respired.

Consumer energy budgets

Only part of the organic material produced at one trophic level becomes new biomass at the next level.

A trophic level is a feeding position in a food chain: producers form the first level, primary consumers eat producers, and secondary consumers eat primary consumers. A consumer obtains chemical energy by eating organic material. Three quantities describe successive stages:

The grassland below continues the first plant budget: GPP 24,750 minus plant respiration 9,650 gives NPP 15,100. Every entry has units $\mathrm{kJ}\,\mathrm{m}^{-2}\,\mathrm{year}^{-1}$. The stated budget includes all relevant routes for these herbivores, with no other food source or net withdrawal from pre-existing body energy reserves.

The 9,100 not ingested may remain as plant material or later enter a detrital food chain. It has bypassed the particular herbivore transfer, rather than all immediately becoming heat. The 1,800 in faeces was swallowed but not assimilated. Egestion removes undigested material; excretion removes metabolic waste produced within the body. They are different routes. The small excretion term is supplied explicitly here so that the budget is complete, rather than silently treating all assimilated energy as respiration or growth.

Respiration supports maintenance, movement, active transport and other cellular processes. Its energy ultimately dissipates as heat instead of remaining stored in new consumer biomass. The 700 in new herbivore biomass is the production that can provide food for the next consumer level; it is neither the 6,000 swallowed nor the 4,200 absorbed.