Secondary production is generally defined as the elaboration of new tissue that is added to the standing crop biomass of animals. Though technically correct, this definition is inadequate since it does not specify the levels of ecological organization used, and the concept is not identical at various levels of organization. The above definition is really valid only for an individual animal.
For a population, secondary production is the increase in biomass of the individuals concerned; in other words, it is the sum of the growth of the individuals minus their loss of weight together with the population’s nationality, mortality, immigration, and emigration. Similarly, at the level of the ecosystem, the secondary production of the ecosystem is not the sum of the productions of the populations since one population may feed upon another, secondary production at this level is the increase in the biomass of the total fauna.
The unit of time interval used for estimating secondary productivity at different levels also varies; for an individual animal, it is the individual’s life span; for the population it is generally one year or one season; and for the heterotrophic component of the ecosystem, it may be one year or several years.
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The relationship between basal metabolism (log O2 in µ1) and body weight (log W, in µ g, mg, g or kg) of most animals has been found to be linear, and if the diversity of individual heterotrophs and their body size or weight are known, the basal metabolic rate can be determined from standard graphs.
It has been demonstrated by McNeil and Lawton (1970) that production and metabolism are related in field populations and also that the homoeotherms (birds and mammals) have, for the same metabolic level, a lower production rate than poikilotherms (fish and invertebrates).
The sum of production and metabolism constitutes the material assimilated by a population. If the percentage of the food actually assimilated from the food consumed is known, we can estimate the input to the population from the sum of production and metabolism.
However, assimilation percentages are highly variable in different species and even in the same animal from time to time. Some vertebrates may assimilate as much as 90 per cent of the food consumed, whereas such detritus-feeders as millipedes only assimilate about 20 per cent. Recent researches have raised the hope that it may be possible to extrapolate the data on the ecological energetic of specific animals to the level of the population within a given habitat and season.
Regrettably, little, if any work on secondary production estimates of various animal populations has been done in India. One serious gap in our knowledge relates to the series of important functions that heterotrophs perform which, though associated with their feeding activities, seldom figure in energy and material flow calculations. These activities include pollination, seed dispersal, and predation of seeds and fruits, and are particularly prominent in tropical climates. The quantitative significance of these important biological activities to ecosystems is virtually unknown.
Several research projects to estimate faunal and microbial production in terrestrial, freshwater and marine ecosystems were undertaken under the IBP. But several reasonable estimates of the total secondary productivity have already emerged and the IBP projects have at least generated an understanding and awareness of the limits to the rates at which materials can be processed along a food-web.
One most interesting inference forthcoming from the secondary production studies has been that most of it takes place below the ground, with as high as 99 per cent of secondary production being often due to the activity of decomposers occurring in soils or leaf litters. Thus it seems that in contrast to birds and bees, the earth-worms and other organisms found underground are far more important in respect of secondary production.
Some information on the complex interrelations between primary and secondary productivity is available, especially for lakes. Certain parts of primary production, e.g., respiratory losses, are nonavailable to consumers. Aquatic algae often release soluble organic carbon and nitrogen compounds as extracellular products into the lake water and not all of these may be taken up by herbivores or zooplankton, etc., since a substantial part is thought to be utilized by bacteria. But these bacteria may be ingested by the same animals which graze on the algae that released the organic compounds.
In terms of energy transfer, it has been estimated that the best utilization of the energy of the primary producers by herbivores rarely exceeds about one-fourth of the plant production, with the usual order being much less, e.g., one-tenth (see Lund, 1967).
The rate of primary and possibly also secondary production per unit area of surface in an oligotrophic lake can and does sometime equal that in a eutrophic lake (Lund, 1967).
Some planktonic rotifers and crustaceans tend to ingest the smaller algae, flagellates and diatoms more easily than larger forms such as colonial diatoms and filmentous or colonial cyanophytes. The zooplankton can, however, digest the dead remains of such larger forms.
When one considers the pattern of world distribution of primary production, one finds that both deserts and deep oceans are much less productive (primary production less than 0.5 grams dry matter/m2/day) as compared to (/) continental shelf waters, and (ii) grasslands, deep lakes, mountain forests and some agriculture (each with 0.5-3.0 gm dry matter/m2/day).
The estimated primary production values for moist forest, shallow lakes, moist grasslands and most agriculture are 3-10, whereas those for some estuaries, springs, coral reefs, terrestrial communities on alluvial plains and intensive year-round agriculture (e.g., sugar cane cultivation) are the highest, viz., 10-25 gm dry matter/m2/day.
These data suggest that:
(a) basic primary productivity may not necessarily be a function of the kind of producer organism or the kind of habitat medium, but is determined by local supply of raw material, sunlight energy, and the ability of biotic communities to utilize and regenerate materials;
(b) a major portion of the earth’s surface is open ocean or desert land and these have low productivity because of lack of nutrients in the former and lack of moisture in the latter. Augmentation of arid lands with water or of sea with nutrients can increase their productivity considerably.