Definition. Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis, expressed in weight (g m⁻²) or energy (kcal m⁻²). The rate of that biomass production is primary productivity, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ so that different ecosystems can be compared. It is divided into gross primary productivity, the rate of production of organic matter during photosynthesis, and net primary productivity, which is GPP minus respiration losses and is the available biomass for consumption by heterotrophs.

The factors that affect it.

(i) The plant species inhabiting the area. Species differ in growth form, leaf area, life span and the efficiency of their photosynthetic pathway, so the same site planted with different species yields different productivity.

(ii) Availability of nutrients. Nitrogen, phosphorus and other minerals limit growth wherever they are scarce — which is the main reason the open ocean is unproductive despite abundant light at its surface.

(iii) Photosynthetic capacity of the plants. The maximum rate at which the plant's machinery can fix carbon sets a ceiling that no amount of extra light or nutrient can exceed.

(iv) Environmental factors. Solar radiation, temperature, water availability, day-length and carbon dioxide concentration all govern the rate at which photosynthesis actually proceeds.

The consequence. Because all of these vary geographically, productivity varies greatly in different types of ecosystems — and the whole biosphere's annual net primary productivity comes to approximately 170 billion tons of dry organic matter.

Primary productivity is the rate of biomass production by plants during photosynthesis — the rate at which new organic matter is created from inorganic materials using solar energy. It is divided into GPP and NPP, where NPP = GPP − R.

Secondary productivity is the rate of formation of new organic matter by consumers. It is sometimes described as the rate of assimilation of food energy by consumers.

The fundamental difference. Primary productivity creates organic matter from inorganic materials; secondary productivity only converts organic matter that already exists into consumer tissue. A consumer adds no new energy to the ecosystem — it repackages what the producers fixed.

Why secondary can never exceed net primary. Net primary productivity is the available biomass for the consumption of heterotrophs, so it is the entire supply from which all consumers must draw. And they cannot convert all of it: some plant material is never eaten, much of what is eaten is not assimilated, and a large share of what is assimilated is spent on the consumer's own respiration and lost as heat. Energy flow is unidirectional, with loss at every step — the 10 per cent law of the next part puts a figure on it. Secondary productivity is therefore always a small fraction of NPP.

The consequence for ecosystems. This is precisely why the number of trophic levels in a food chain is limited, and why a pyramid of energy is always upright and can never be inverted.

The arithmetic. GPP − R = NPP. If GPP is equal and NPP differs, then R — respiration loss — must be higher in forest B. A considerable amount of GPP is utilised by plants in respiration, and forest B is utilising more of it.

What could cause the higher respiration.

Temperature. Respiration rises steeply with temperature, and often more steeply than photosynthesis. A hotter forest — or one in a hotter season — burns a larger share of what it fixes.

The amount of non-photosynthetic tissue. Trunks, branches and roots respire but do not photosynthesise. A mature forest of large old trees carries an enormous mass of such tissue to maintain, so a bigger fraction of its GPP goes on maintenance. A young, fast-growing stand of the same GPP keeps far more.

Species composition. Primary productivity depends on the plant species inhabiting the area, and species differ in how efficiently they convert fixed carbon into retained tissue.

Which forest supports more animal life: forest A. Net primary productivity is the available biomass for the consumption of heterotrophs — herbivores and decomposers. GPP is invisible to consumers; only the remainder after the plant's own respiration reaches them. So the forest with the higher NPP feeds more animals, even though both forests are photosynthesising at the same rate.

The general lesson. GPP measures a forest's performance as a photosynthesiser. NPP measures its value to everything else alive.

Because a total confuses performance with size. Productivity is expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ precisely so that the productivity of different ecosystems can be compared. Divide out the area and you are measuring how well each square metre works; leave the area in and a vast poor ecosystem can outscore a small rich one.

The ocean figures make the point in both directions. Of the biosphere's approximately 170 billion tons of annual net primary productivity, the oceans contribute 55 billion tons. As a total, 55 billion tons is enormous — about a third of all production on Earth, and any sensible person would call the oceans highly productive. But the oceans occupy about 70 per cent of the surface, so per square metre they are producing less than a fifth of what the land produces. As a rate per unit area the open ocean is one of the least productive systems there is.

Which figure answers which question. The total tells you how much the oceans matter to the global carbon budget — a great deal. The rate tells you what is happening ecologically in a given patch of sea — very little, because light penetrates only a thin surface layer and nutrients sink out of it.

The general principle. The same reasoning was met in the previous chapter, where population density had to be expressed per unit area, and birth and death rates per capita. In ecology, comparison almost always requires dividing out the size of the thing being compared.

Why productivity rises. Primary productivity depends on the availability of nutrients, among other factors. In most lakes light is adequate in the surface layer but nitrogen and phosphorus are scarce, so nutrients are the limiting factor. Supply the limiting nutrient and the phytoplankton, which have a very high photosynthetic capacity and a very short generation time, multiply rapidly — hence a tripling within weeks rather than years.

What follows: the productivity becomes the problem.

An algal bloom. The surface becomes crowded with phytoplankton, which shade the water below, so the submerged plants of the lower layers — the lower strata of the aquatic ecosystem — receive too little light and die.

A surge of detritus. Phytoplankton are short-lived, so the greatly increased production quickly becomes greatly increased dead organic matter, which is the raw material for decomposition.

Oxygen collapse. Decomposition is largely an oxygen-requiring process. The decomposers, especially abundant at the bottom, consume the dissolved oxygen faster than it can be replaced, and the deeper water becomes anoxic. Fish and other consumers suffocate and die, adding still more detritus.

Inhibited decomposition. Anaerobiosis inhibits decomposition, so organic material now builds up on the bottom instead of being mineralised, and nutrients are no longer cleanly returned for reuse.

The lesson. High productivity is not the same as a healthy ecosystem. A functioning ecosystem needs its four processes — productivity, decomposition, energy flow and nutrient cycling — to stay in proportion to one another; raising one of them alone can wreck the rest.