Biological Productivity

an ecological and general biological concept signifying the reproduction of the biomass of plants, microorganisms, and animals in an ecosystem. In the narrower sense, it means the reproduction of animals and plants used by man. Biological productivity is realized in each individual case through the reproduction of species populations of plants and animals that takes place at a certain rate and that can be expressed by a definite quantity—production per year (or some other unit of time) per unit of area (for terrestrial and bottom-dwelling organisms) or per unit of volume (for organisms living in open water or soil). The production of a particular species population can also be related to its abundance or biomass. The biological productivity of different terrestrial and aquatic ecosystems is manifested in many forms. The products produced in natural communities and used by man (for example, wood, fish, and fur) are correspondingly varied. Man is usually concerned with increasing the biological productivity of an ecosystem because this enhances the possibilities for exploiting nature’s biological resources. However, in some cases a high biological productivity can be harmful (for example, excessive development of a particular phytoplankton species in highly productive waters—blue-green algae in fresh water and toxic peridinean species in the sea).

The concept of biological productivity is similar in many respects to that of soil fertility, but it is broader in content and scope because it can be applied to any biogeocenosis or ecosystem. The term “biological productivity” is also applied from time to time to cultivated communities whose productivity is largely the result of social labor. However, both natural terrestrial and natural aquatic ecosystems are under man’s direct or indirect influence. Therefore, as the population grows and mankind’s scientific and technological resources increase, the biological productivity of increasingly diverse ecosystems reflects not only their original natural characteristics but the result of human influences.

Production, but not the biomass of a community or of its components, is a general and adequate measure of biological productivity. The biomass of an individual species or of the population as a whole can be used to estimate production and productivity only when comparing ecosystems of the same or similar structure and species composition, but it is completely unsuitable as a general measure of biological productivity. For example, as a result of the high intensity of photosynthesis of unicellular planktonic algae, approximately the same amount of organic matter per unit of area is synthesized a year in the most productive parts of the ocean as in highly productive forests, although the forests’ biomass is hundreds of thousands of times greater than the phytoplankton biomass.

The production of each population during a particular period of time is the total increase in all the individuals, including the increase in structures separated from the organisms and the increase in individuals removed (eliminated) for one reason or another from the population during the period in question. In the extreme case, if there is no elimination and all the individuals survive to the end of the period under study, production is equal to the increase in the biomass. If, however, the initial (B₁) and final (B₂) biomasses are equal, it means that the increase is compensated by elimination, that is, under this condition production (P) is equal to elimination (E). In the general case,

P = ǀB₂— B₁ǀ + E.

Production thus defined is sometimes called “net production” in contrast to “total production,” which includes not only the increase but also expenditures on energy metabolism. The terms “net production” and “total production” have become established as far as plants are concerned. As applied to animals, “total production” is assimilated food or “assimilation,” while “production” is used in the sense of net production.

The production of autotrophic organisms capable of photosynthesis or chemosynthesis is called primary production, and the organisms themselves are called producers. Green plants—higher plants on land and lower plants in the water—are a leading factor in the creation of primary production. The production of heterotrophic organisms is usually considered part of secondary production, and the organisms themselves are called consumers. All types of secondary production originate from the utilization of the matter and energy of primary production. However, energy can be used only once to do work, unlike matter which returns again and again to the cycle. The complex trophic relations can be expressed schematically in the form of “energy flow” through an ecosystem, that is, a step-by-step process of utilizing the energy of solar radiation and the matter of primary production. The first trophic level of utilization of solar energy consists of photosynthesizing organisms that create primary production; the second level includes the herbivorous animals that consume them; the third is constituted by carnivorous animals; and the fourth embraces second-order predators. Each succeeding trophic level consumes the production of the preceding one. Moreover, part of the energy of the food consumed and assimilated is expended on energy metabolism and is dissipated. Hence the production of each succeeding trophic level is smaller than the production of the preceding one (for example, the yield based on the same primary production of herbivorous animals is always larger than the predators living at their expense). Biomass as well as production frequently decreases from the lower to the higher trophic levels. However, unlike production, the biomass of the following level may also be greater than the biomass of the preceding one (for example, the phytoplankton biomass is smaller than the total biomass of the entire animal population of the ocean living at its expense). Heterotrophic microorganisms are a prominent factor in the mechanism of biological productivity. They utilize the dead organic matter that reaches them from all the trophic levels, partly mineralizing it and partly converting it into the substance of microbial bodies. The latter are an important source of nutrition for many aquatic (benthic and planktonic filter-feeding and detritus-eating) and terrestrial (soil fauna) animals.

Production is divided according to another principle into intermediate and final. Intermediate production includes the production consumed by other members of an ecosystem, and the substance again returns to the cycle occurring within the ecosystem. Final production is that which is removed in some form or other from an ecosystem, that is, it goes beyond the limits of the ecosystem. Final production also includes the types of production used by man that may belong to any trophic level, even the first level occupied by plants.

The mounting demands and growing technical might of mankind are swiftly increasing mankind’s capability of influencing living nature. It is becoming necessary to control ecosystems. All the means of influencing the biological productivity of ecosystems and controlling it are aimed either at boosting useful primary production (different forms of fertilizer, reclamation, regulation of the abundance and composition of consumers of primary production, and so on) or at raising the efficiency with which primary production is used at the succeeding trophic levels in the direction essential for man. This requires a good knowledge of the species composition and structure of ecosystems and of the ecology of individual species. The forms of economic exploitation and control of living nature that are based on a knowledge of the characteristics of local ecosystems and the form of biological productivity characteristic of them are the most promising.