The world distribution of primary production is shown schematically in figure 3 -7. Values represent the average gross production rate per square meter of area to be expected in an annual cycle. As previously indicated, as much as 90 percent of gross production may be available to heterotrophy, but usually only about 50 percent is actually is actually utilized. It should be remembered that man or any other single species cannot assimilate all of the net production. For example, cornstalks and wheat stubble and roots would be unclouded in the total production of these crops, but only the grain is currently consumed by man.

potential biological fertility

      As may be seen from figure 3 -7, there are about three orders of magnitude in potential biological fertility of the world: (1) large parts of the open oceans and land deserts ranging around 1000 kcal/m²/ year or less; these are the solar – powered ecosystems that are nutrient or water limited. (2) potential biological fertility many grasslands, coastal seas, shallow lakes, and ordinary agriculture range between 1000 and 10,000; these are the energy subsidized solar – powered system. (3) potential biological fertility certain shallow water system such as estuaries, coral reefs, and mineral springs together with moist forests, intensive agriculture (such as year – round culture of sugar cane or cropping on irrigated deserts), and natural communities on alluvial plains may range from10,000 to 20,000. Production rates higher than 20,000 have been reported for experimental crops, polluted waters, and limited natural communities. A probable upper limit of 40,000 – 50,000 has already been noted.

tentative

       Two tentative generalizations may be made from the data at hand. First, basic primary productivity is not necessarily a function of the kind of producer organism or the kind of medium (whether air, fresh water, or salt water), but is controlled by local supply of raw material, sun energy, supplemental energy, and the ability of local communities as a whole (and including man) to utilize and regenerate materials for continuous reuse. Terrestrial systems are not inherently different from aquatic situations if light, water, and nutrient conditions are similar. However, large bodies of water are at a disadvantage because a large portion of light energy may be absorbed by the water before it reaches the site of maximum mineral supply in the deep water. Secondly, a very large portion of the earth’s surface is open ocean or arid and semiarid land and thus in the low production category, because of lack nutrients in the former and lacky of water in the latter. Many deserts can be irrigated successfully, and it is theoretically possible and perhaps feasible in the future to bring up ‘’lost’’ nutrients from the bottom of the sea and thus greatly increase production at, of course, the expenditure of some form of energy. Such an ‘’upwelling’’ occurs naturally in some coastal areas, and these have a productivity many times that of the average ocean. A famous example of the effect of upwelling on productivity is found along the coast of peru. Currents are such that nutrient – rich bottom waters are constantly being brought to the surface so that phytoplankton does not suffer the usual nutrient limitations of the sea. The area supports very large populations of fish and fish – eating birds; so much guano is produced by the birds as they nest along shore man is able to harvest it for fertilizer on a continuous – yied basis. Ryther (1969) has called the Peruvian upwelling area the world’s most productive natural fishery. Some 10⁷ metric tons of anchovies are harvested annually from 60 . 10³ km² which comes to about 300 kcal/ m^2, a very secondary production. Because the fleets of all fishing nations fish this area, even this bonanza is now in danger of being overfished.

distribution of primary production

The world distribution of primary production is displayed in more detail in table 3 – 1, which also includes an estimate of the global area occupied by major ecosystems, and also an estimate of total gross production of the biosphere. The word ‘’estimate’’ should be emphasized since there is yet no accurate inventory of productivity on a global basis, although the beginning of such an inventory is underway as part of an ‘’international biological program’’ which is being funded by governmental agencies of many of the nations of the world. For the most recent survey see the symposium entitled ‘’ the primary production of the biosphere,’’ edited by whittaker and likens (1973).

When the first estimates of global productivity were made in the 1940s it was assumed that the productivity of the ocean was greater than that of the land because it was larger. Then it was discovered that much of the ocean was ‘’desert,’’ so the consen us now is that the land areas contribute more than half of the total. The estimate of 10¹⁸ kcal/ year for global productivity is less than 1 percent of the solar energy entering the biosphere, as we have already noted. But this does not mean that it will be easy to increase world productivity or divert a larger share to food for man.