Elements of Physical Biology

These living tow nets (i.e., gizzard shad) do not get caught on snags and roots, the string does not break, and the algal collection is very representative of the body of water from which the fish were taken. It is only necessary to catch the young fish and examine their stomachic and intestinal content to secure a proportionate sample of the plankton. Tiffany examined specimens from streams and ponds in Illinois, and also from Ohio. He points out that the gizzard shad fulfills an important réle as an intermediary link in the food chain: algae, shad, game-fishes, man.

Thus, the gizzard shad is making useful for man the energy stored in plant forms which occupy no land areas, which do not interfere with the ordinary disposition or utilization of bodies of water (except the occasional contamination of water for drinking purposes by some algae), which involve no labor of cultivation on the part of man, and which are of no value for direct human consumption. The world’s population in the last hundred years has increased about 150 per cent. Along with this increase has had to come a corresponding increase in the world’s food supply. One of the ways in which this necessity has been met is the securing of new acres of soil in which to grow crops. It is easily seen, however, that there is a limit to new acreage. In the future, therefore, we may have to turn more of our attention to the cultivation of the waters for food supplies. We may have to develop an industry of aquiculture as we have developed an industry of agriculture. The time is rapidly approaching when fish will be more highly prized as food and more extensively used than now. As that time comes, the cultivation of algae will be a first step toward greater fish production. A second step may be the introduction of fish like the gizzard shad into fish ponds and lakes to make more readily available the phytoplankton for fish food.

In a summary survey of the Food Resources of the Sea,? G. W. Martin makes similar observations. He remarks: ‘So far as the actual cultivation of the sea’s resources, as distinguished from their mere exploitation, is concerned, we have made only the feeblest beginnings.”” Somewhat in the same vein is W. A. Herdman’s comment: ‘‘Aquiculture is as susceptible to scientific treatment as agriculture; and the fisherman who has been in the past too much the hunter, if not the devastating raider, must become in the future the settled farmer of the sea, if the harvest is to be less precarious.” Viewing the matter from a slightly different angle W. F. Wells of the New York Conservation Commission draws attention to the fact that in our modern great cities, with the widespread adoption of the water system of sewage disposal, valuable fertilizer material is lost from its natural place in the fields. By enriching

the vegetation in the water and furnishing abundant life thereto, it may again be restored to the people as fish and shell fish in the place of beef and mutton. And the exchange is not such a bad bargain. For it has been found that in carp ponds, for example, the production of market ware was 95 pounds per acre, as contrasted with 73 pounds of beef per acre of farmland.’ Perhaps one of the most telling illustrations of the economic importance of pisciculture is presented to us in the Alaska purchase: Within fifty years after the acquisition of our Northern province, it had yielded, in its salmon fisheries alone, seven and a half times its purchase price.

The quantitative study of ocean life (on which must be based an intelligent system of marine aquiculture) may be said to date from 1880, when Henson introduced the use of the dragnet. To this has since been added the use of bottom samplers or grabs; also an ingenious, if less trustworthy method of making a census of the marine population, which consists in catching a number of live fish, marking them, throwing them back in the water, and then noting the percentage of marked fish in the fishermen’s catch during the period that follows. In this way it has been estimated, for example, that the North Sea contains about fifteen hundred million plaice, a figure about equal to the earth’s human population. |

The field of utility of the dragnet and most of the other methods described is evidently limited to the larger denizens, such as are effectively held in the meshes of a net. Even a fine silk net will fail to hold a very numerous constituent of the population of the sea, a constituent that is highly important not only on account of its great extent, but also because of the réle it plays in the food traffic of marine life. Lohmann showed (1911) how these fine organisms (the nannoplankton) can be collected by the use of a centrifuge.t Allen has developed a special dilution culture method of count for the nannoplankton organisms, which is modelled after the pattern of bacterial count technique. He showed sea water to contain 464,000 organisms per liter (exclusive of bacteria). Al-

3 It is true that beef is much superior in food value, pound for pound, but it is also much more costly to produce. It should also be noted that the yield from carp ponds is very high as compared with that of marine fisheries. E. J. Allen (Food from the Sea, 1917, quoted by W. F. Thompson, Scientific Monthly, 1922, p. 546) estimates the yield in the North Sea at 15 pounds per acre. lowing for systematic errors Allen thinks a population of one million organisms per liter (one per cubic millimeter) to be a conservative estimate. This is not excessive crowding, in view of the size of these organisms, as illustrated by the diagram, figure 36, reproduced from G. W. Martin’s article. Before the extent and significance of this nannoplankton was realized, the amount of food required by the animals of the sea seemed so much in excess of the amounts

Fig. 36. Densiry or MicroorGANISMS (EXCLUSIVE OF BacTEeRIA) IN SEA About one organism, measuring 6 microns in diameter, is found per cubic millimeter of water. The relative dimensions are here shown. After G. W. Martin. revealed by the earlier methods of collection, as to give rise to the suggestion (Piitter 1907-1909) that the nutrition of marine animals was on an entirely different plane from that of land animals, and that a large number of them, especially the smaller ones, absorbed dissolved organic matter directly from the water, without the mediation of plants. Piitter’s arguments have not been generally accepted, and more recent studies have invalidated many of them. Nevertheless, it is possible that something of this sort is more general than we realize. Mitchell (1917) reported an experiment strongly indicating that an oyster can utilize dextrose dissolved in sea water.

The most elaborate attempts to calculate the production of the sea have been those of the Danish biologist Petersen and his associates. As a result of their studies, these workers have come to the conclusion that the plankton plays a very small part in the nutrition of the animals of the sea, and LVOGALLYY AHL NI GAIT IVWINY 40 SNIVHD-GOOWY LNVLYOdMW] LSOFY AHL 40 AWOG “LE “Oly . sna N UOpUoT pazpszsn}}] ay} fo Aeajsnoo hg eS . Os —— G2 o/ 2f2 Spodosalsog7 ahiv7 YSIJIUS YSIS //ouss "“SuoaIasnsy/Kiopopasy

that the fundamental food of all marine forms in northern waters at any rate is the ‘‘dust fine detritus’ of the sea bottom, derived primarily from the eel grass, Zostera. These investigators have studied in particular the conditions in the Kattegat, a rather shallow arm of the sea between Denmark and Sweden, about 150 miles in extreme breadth and 90 miles in extreme width. Their principal conclusions are exhibited in the diagram figure 37, and are as follows:

It is assumed that about half of the total amount of Zostera annually produced in this area is washed elsewhere by the currents. The balance, estimated at 24,000,000 tons, serves as the basis for the animal life of the area. The useless animals, that is, those that are of no value to man and do not serve as food for fish, feeding directly on the Zostera, amount to about 5,000,000 tons. Useful animals, mainly those capable of serving as food for fish, are estimated at 1,000,000 tons. These are not all utilized by food fish, however. Starfish account for perhaps 200,000 tons; 500,000 tons are eaten by the larger gastropods and crustaceans, of which only a part are consumed by fish; while plaice and other flatfish consume about 50,000 tons producing 5000 tons of human food annually. Cod are much less economical, since they get their food at third hand, so to say, and each ton of the 6000 tons produced annually represents about one hundred times as much of the original synthesized organic food.’ On the other hand, the cod help to keep down the predatory gastropods and crustaceans (see figs. 88 and 39). The. herring is the most important food fish feeding on the plankton, (mainly on copepods) and it in turn is eaten by the cod. Perhaps the most striking feature brought out by these figures is the comparatively trifling amount of human food finally produced from such a large amount of organic material.

Summary of Methods. A tabular summary of some of the principal methods by which data have been secured regarding relative and absolute frequency of organisms and species is given in table 10. Food Chains. It has already been remarked, in dealing with the general kinetics of the type of systems here under consideration, that each component of the system appears as a link in a chain or a network of chains, receiving contributions from components (sources) above, and discharging material into other components (sinks) below. Food chains, such as spoken of by Tiffany in the passage quoted on page 172, are a particular example of such chains of components. The study of food chains is one of the important tasks of the economic biologist. For we cannot afford to restrict our attention to the immediate source from which we draw our

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supplies of food for the human population. The sources of these sources also demand attention. The problem forces itself upon our notice primarily (in the present state of society) in connection with agriculture. The fields cannot continue indefinitely to yield undiminished annual crops if the materials drawn from them are not in some way replenished. One important constituent needs no human intervention: carbon dioxide, owing to its gaseous form, automatically seeps in by diffusion as fast as it is absorbed by the

: s 2” Mery, ; < ae Fe hoe. ; ceolata Aphrodite . Sue ee Aphreeite Se ee green plants. The same is true in some degree of free nitrogen, though the capacity of plants to assimilate this element is strictly limited. Water, also, is, in most agricultural areas, provided by the automatic meteorological processes of evaporation and condensation in rainfall. But as to certain other essential materials, notably combined nitrogen, phosphorus, potash and sulphur, the inherently immobile® constituents of the fertile soil, for these auto-

matic replacement does not occur in sufficient measure to satisfy the agricultural needs of the densely populated countries of this age. It becomes necessary for man to feed his food. Early man and primitive man, may reap where he has not sown. But long ago our tribe turned from the life of a nomad and hunter to tilling the soil and to animal husbandry. Thus was established a system of symbiosis with the links next above us in the food chain: the harvest ripening on the plain; and the cattle, grazing upon the pasture in summer, fed from the crib in winter. But early agriculture was essentially of the nature of a mining industry. It drew from the soil as from a bottomless well, without thought of a possible exhaustion of the source, or of any feasible replenishment to diminishing resources. Except that, by a semi-automatic, semi-empirical process, natural fertilizer was allowed to restore to the soil at least a part of its strength to bring forth a crop. One more step forward and man graduated from mining farmer into manufacturing farmer. The field became a factory fed with raw materials in the form of saltpeter, potash salts and phosphate fertilizer, imported, if need be, from afar; and producing its output of agricultural flora and fauna. Lastly, in our own generation, we have learned to divert into the life stream the sluggish element, so essential to life, so illnamed by the French—azote; to make ourselves independent of the saltpeter beds, to assure our future against a nitre famine by opening the inexhaustible mine of the atmosphere. It isa singular thing that this element, so accessible, so abundant, in which we are literally bathed within and without, every instant of our life, should so long have remained foreign to our industrial economy. Strange circumstances, yet not without close parallel.

For even now we are powerless to avail ourselves effectively of the golden flood of energy that daily pours upon us without limit from above—while we turn earthward to dig laboriously for the plainly exhaustible supply of coal to supplement our limited bodily energies. Food Chains in Aquatic Species. The principle that long food chains are essentially wasteful finds particular application also in the practical problem of the economic and rational utilization of marine organisms for human sustenance. As Professor Martin observes, the most economical course would be to utilize marine vegetation directly as food for man and his domestic animals. The

use of algae as food for the table can hardly be expected to become an item of any consequence. Its use as cattle fodder presents better prospects. But our chief reliance will no doubt continue to be on the assimilation of marine plants by fish. To quote again Professor Martin: Since man prefers to harvest the plant life of the sea indirectly, those animals which feed directly on the plants are able to increase with less waste and at a more rapid rate, considered in total populations, than those which feed on other animals. Most of our food fish, for example, feed on smaller fish; these in turn feed upon small crustaceans and the latter eat the microscopic plants and detritus, so that in many instances the fish we eat are removed three or four steps, perhaps more, from the original food source. This is more significant than may seem apparent at first glance, since it involves an enormous waste. Before any organism can grow, the energy needed merely to live must be supplied, and by the time a crustacean is eaten by a minnow, or a minnow by a food fish, it will, on the average, have consumed a quantity of food several times its own weight. These facts are well brought out in the statistics of Petersen, and the diagram figure 37 based thereon. The edible shellfish, however—oysters, clams, mussels and the like—feed for the most part directly on the marine plants and this is one reason why the extension of the shell fisheries represents so much promise.

Another advantage of this branch of aquiculture is also to be noted, namely, that most shellfish, like land crops, stay where they are planted. Even the scallop, which can swim about after a fashion, is restricted in its movement, and could readily be controlled. Oyster culture is already a great and important industry, but it has not nearly approached its possibilities. Clam culture is still in an embryonic state, and scallop culture has yet merely been suggested. When some of the problems confronting the establishment of these industries have been solved, we may hope to have acquired additional information concerning the ecology of the sea, which will help us in our approach to the more difficult problems of the future.

Primary, Secondary and Tertiary Foods. It is often convenient to classify the foods for human consumption according to their relative position in that portion of the food chain which is under human control. Commonly the meats of domestic animals are 7 For some further bibliographic indications regarding the food consumed by fishes see A. S. Pearce, Ecology, vol. 1924, p. 258. See also W. A. Herdman, Founders of Oceanography, 1923; J. Johnstone, Introduction to Ocean-

classed as secondary foods, since the production of these meats takes plaice in two steps, first the growing of the fodder (for the most part materials not comestible by man), and second the transformation of a part of this fodder, in the animal economy, into food adapted for human consumption. On the other hand the fisherman’s haul is for us primary food, growing feral, without human intervention. This classification must not be pressed. Where fields are supplied with fertilizer it might well be maintained that the crops of wheat, potatoes, etc., commonly classed as primary, are secondary foods, while butchers’ meats are tertiary. Even the catch of fish, and huntsman’s quarry may not be strictly primary, in so far as game laws and regulations regarding the pollution of lakes and rivers represent some degree of symbiotic intervention on the part of man. Fine points apart, the distinction between the primary foods (crops and fish) and the secondary foods (butchers’ meats) is economically most significant, for the consumption of fodder by farm animals is an item not merely comparable with human food consumption, but exceeding this latter manyfold. The fact, of course is, that farm animals are far from being economical and efficient converters of raw materials into food for human consumption. They represent a luxury, a humoring of the tastes of men at the expense of their purses. With the present density of population we can afford the luxury. Presumably the future will see retrenchments, with pastures and cornfields converted to wheat. Still tastes are not accidental things. Allowing for vagaries and exceptions, the things we like are, on the whole, good for us and for the species. Whether man can maintain his present status with a materially abridged meat ration is perhaps an open question. Should the answer be in the negative, the conclusion would seem to be forced upon us that an overcrowding of the earth would react unfavorably upon the vigor of the race; quality would be sacrificed to quantity. How this might affect the ultimate fate of our species is a subject for speculation. The pessimist might take a cue from palaeontology, recalling that the extinction of a species seems to follow, not infrequently, close upon its period of greatest development.

The optimist, on the other hand, might perhaps extend the suggestion that when overcrowding does come, the ones to survive most surely, if not most abundantly, will be those whose superior qualities will enable them, in spite of intensified competition, to draw to themselves a sufficiency of the more desirable, though perhaps not absolutely essential articles of consumption. Natural selection would thus operate by the preferential survival of an aristocracy, while a submerged tenth would furnish a drain for the discharge of the unfit. Certainly, in the interests of the species, it were better that the inferior constituents be purged from the system than that they should drag down the general level to mediocrity and perhaps below the line of viability. But these are speculative reflections.

Cycles. Food chains, were we able to trace them through their entire course, would undoubtedly be found to form closed cycles or a network of cycles. This is indeed a practical necessity for the continued performance of the processes or organic nature, processes that have gone on essentially unchanged in their general character, however modified in detail, for many millions of years. A few of the simpler food chains we may be able to follow with something approaching completeness through their cycle. For the most part, however, the system of interlocking cycles in nature is complex beyond all reasonable hope of detailed analysis in its entirety.

If we are satisfied to omit innumerable details, we can trace, for each of the most important chemical elements’ concerned, the broad outline of its cycle in nature. The elements and simple compounds principally concerned are ae A OTL R EES tae? a ore LG Fs kw Lo AS cheers CO, ORV POM a sick oa esltasieine xilolie'e Bk Re. Rita mts ats eRe ee O, Nitrogen.....................-...-free N, NHs, nitrites and nitrates DO it ne nee a Re Re Tere et icra cur Girogageii ia hen avree cies ie erase as) Sn H,0 TESS Oh UES ca perpen eleeN eee eee een eae oe (phosphates, etc.)

Brief consideration will presently be given to each of these cycles in turn. First, however, it will be well to review some of the essential facts regarding the occurrence of the chemical elements, generally, in nature. For the drama of life is like a puppet show in which stage, scenery, actors and all are made of the same stuff. The players, indeed, “have their exits and their entrances,” but the exit is by way of translation into the substance of the stage; and each entrance is a transformation scene. So stage and players are bound together in the close partnership of an intimate comedy; and

if we would catch the spirit of the piece, our attention must not all be absorbed in the characters alone, but must be extended also to the scene, of which they are born, on which they play their part, and with which, in a little while, they merge again.*® 8 Since the words above were written I have run across the following singularly apposite passage in John Morley’s Introduction to Wordsworth Collected Poetic Works: ‘‘Wordsworth’s claim, his special gift, his lasting contribution, lies in the extraordinary strenuousness, sincerity and insight with which he first idealizes and glorifies the vast universe around us, and then makes of it, not a theatre on which men play their parts, but an animate presence, intermingling with our works, pouring its companionable spirit about us, and ‘breathing grandeur upon the very humblest face of human life.’”’

When the elements have been mingled in the fashion of a man, and come to the light of day, or in the fashion of the race of wild beasts or plants or birds, then men say that these come into being; and when they are separated, they call that in common parlance, death . . . . let not the error prevail over the mind that there is any other source of all the perishable creatures that appear in countless numbers.—Empedocles. Our stage is a tripartite world: The heavens above, the waters of the sea, and the solid ground beneath our feet; the atmosphere,

the hydrosphere and the lithosphere. The total mass of the earth is about 6.5 X 1024 tons. But it is only the outer crust that interests us here, for the deeper layers have little or no part in terrestrial life. If we arbitrarily take a layer ten miles thick for the crust, the distribution of the material among the three main divisions is, according to F. W. Clarke, about as shown in table 11. The Atmosphere. There are two ways of confininga gas. The one most familiar in the laboratory and in products of human workmanship generally, is to enclose the gas in a suitable envelope, such as a glass vessel, or the cylinder of an engine; to put something around the body of gas to be confined. The other way, nature’s way on a large scale, is just the opposite, and consists in putting something into the gas, or putting the gas around something. It is so the earth holds her atmosphere by gravitational attraction. Her hold is not impartial. She draws closest to her the densest constituents, and

gives longer leash to the lighter. The atmosphere, in consequence, is not a homogeneous body, but varies in composition with altitude. At the same time, owing to its elasticity, the air in its lower strata is compressed by the weight of the overlying atmosphere, so that 99 per cent of the whole is contained within a shell 30 km. (183 miles) thick. The remaining 1 per cent extends out into space without any Fra. 40. Cross Section or THE ATMOSPHERE, SHOWING Some FHATURES

assignable limit, but at any rate to a height of some 300 km. (185 miles), as evidenced by the aurora. A graphic representation of the broad divisions of the atmosphere is shown in figure 40, adapted from Wegener and Humphreys.! A more detailed and exact statement of He identified certain green lines in lid nitrogen is rendered Fig. 41. ComposiTIon oF THE ATMOSPHERE AT DIFFERENT LEVELS lately been put forward by L. Végard. the auroral spectrum with lines observed when so phosphorescent by x-rays. He concludes that th