Approximately 1 to 5 % of the solar energy falling under plants is converted into organic material. Primary production or primary productivity are terms used to define the amount of organic matter generated by solar energy over a given period of time in a given area. Gross primary productivity is the total produced organic matter, including that used for respiration by the photosynthetic organism. Net primary productivity is a measure of the amount of organic matter produced for heterotrophs in a community in a given time. It is equal to the primary gross productivity minus the amount of energy spent by the photosynthetic organisms ' metabolic activities. As a result of its net production, the net weight of all organisms living in an ecosystem, its biomass, increases.
The rate of production by heterotrophs is called secondary productivity. Because herbivores and carnivores cannot carry out photosynthesis, they do not manufacture biomolecules directly from CO2. Instead, they obtain them by eating plants or other heterotrophs. Herbivores ' secondary productivity is about less than the primary productivity on which it is based. Much of the biomass is not consumed by herbivores but supports the community of decomposers. In addition, some energy is not assimilated by the body of the herbivore but transmitted to the decomposers as feces.
There are 2 or 4 steps in food chains. At each step, a significant amount of energy is lost and very little usable energy remains in the system after being incorporated into the organism bodies at four successive trophic levels.
A plant fixes about 1% of the energy of the sun falling on its green parts. In turn, the successive members of a food chain process about 10% of the energy available in the organisms they feed into their own bodies. Therefore, at the lower trophic levels of an ecosystem, there are generally far more individuals than at higher levels. Similarly, the primary producers ' biomass present in a particular ecosystem is greater than the primary consumers ' biomass, with successive trophic levels having lower and lower biomass and correspondingly far less potential energy. Such relationships appear as pyramids if they are shown diagrammatically.
Certain aquatic ecosystems have inverted pyramids of biomass. For example, in a planktonic ecosystem — overtaken by small living organisms floating in the water— the lowest-level turnover of photosynthetic phytoplankton is very fast, with zooplankton consuming phytoplankton so fast that phytoplankton (the producers at the food chain's base) can never develop a large population size. This is so because even though phytoplankton reproduces very quickly, the community can support a heterotrophic population that is larger in biomass and more numerous than phytoplankton.
The existence of food webs creates the possibility at different trophic levels of interactions between species. Predators will affect not only the species they prey on, but also, indirectly, the plants they eat. In contrast, higher primary productivity will not only provide more food for herbivores but will also indirectly lead to more food for carnivores.
We see a profusion of plant life when we look at the world around us. Why is it? Why don't the populations of herbivores increase to the extent that they consume all available vegetation? The answer is, of course, that predators keep the populations of herbivores under control so that plant populations can thrive. This phenomenon is called a trophic cascade, in which the effect of one trophic level flows down to lower levels. The existence of trophic cascades was confirmed by experimental studies. For instance, sections of a stream were isolated with a mesh in one study in New Zealand that prevented fish from entering. Brown trout have been added in some of the enclosures, while other enclosures have been left without large fish. After 10 days, in the control, the number of invertebrates in the trout enclosures was half that. In turn, in the trout enclosures, the biomass of algae that feeds on invertebrates was five times greater than in the controls.
Factors at the bottom of food webs, on the other hand, can have consequences that ramify to higher trophic levels, leading to so-called bottom-up effects. The basic idea is to be too small to support any predators when an ecosystem's productivity is low. Increased productivity will be completely consumed by herbivores, whose populations will actually increase in size. At some point, populations of herbivores will be large enough to support predators. Consequently, further increases in productivity will not lead to higher populations of herbivores, but rather to higher populations of predators.
There is no inevitability of trophic cascades or bottom-up effects. For instance, if two species of herbivores exist in an ecosystem and compete strongly, and one species is much more vulnerable to predation than the other, top-down effects will not spread to the next lower trophic level. Increased predation will simply decrease the vulnerable species population while increasing its competitor's population, with no potential net change in vegetation at the next lower trophic level.