Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 116


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 115
FRONTIERS OF BIOLOGY oughly summarized elsewhere, no attempt will be made to do so here. But we would direct attention to a growing body of information that stems from analysis of relatively primitive nervous systems. For example, cater- pillars are relatively highly specific in their choice of plant food, yet there are only four taste-receptor cells on each of their maxillary palps. An acceptance response that initiates feeding, based on the chemical structure of the available food, is determined by one unique pattern of impulse discharge coming from the four axons of these cells; all other discharge patterns lead to rejection of the "tasted" material. Not only is this of great theoretical significance for behavior, but it can also be a practical lead to the control of plant pests. An elegant example is found in the adults of certain moths, which are normally preyed upon by insectivorous bats. The ears of these moths can detect the ultrasonic cries made by their predators when they are echo- locating. Each ear of the moth is supplied with only two acoustic sensory cells, but arrival of sound of the wavelength used by the bats automatically triggers evasive behavior on the part of the moth. Through these and a host of similar examples, the elements of behavior may be observed in primitive animals. From such understanding, ultimately, we may be able to build a platform from which to view the behavior of that most complex of all animals, man. ECOLOGY The objective of man's struggle with the environment is not to win but to keep on playing. The biosphere as a whole is certainly not simple, but it has been remarkably reliable up to now. The objective of applied ecology is to keep it so. Ecologists are generally concerned with the interactions among living forms and between them and their environments. Our society has placed upon the ecologists the fearful responsibility of safeguarding the planet for human habitation. Patently, the working ecologist, on a day-to-day basis, does not live with that concern. On any day he may be on a lake or the ocean, examining bottom mud with the use of carbon-dating procedures; following migratory patterns of birds, mammals, or fish by telemetry; com- puting the energy balance of a lake, of a terrarium in the laboratory, or of the southwestern desert; examining the history of a peat bog as recorded in its fossils; or engaging in a computer simulation of the total ecology of a rain forest. His tools are not uniquely his; they are whatever applicable tools science has made available. Clearly, so all-encompassing a set of 115

OCR for page 115
116 THE LIFE SCIENCES concerns cannot be adequately summarized here, and it must suffice to provide only some indication of the problems that give ecologists concern and the manner in which these problems are approached. Some Areas of Ecological Research ENVIRONMENTAL CHALLENGES TO INDIVIDUAL ORGANISMS Although the concern of the ecologist is with populations or species, under- standing must rest on the behavior of individual organisms as well. No one who has seen a 40-pound salmon Ding itself into the air again and again in a vain effort to surmount a waterfall can fail to marvel at the strength of the "instinct" that draws the salmon upriver to the stream in which it was born. How do salmon "remember" their birthplace and find their way back, sometimes from thousands of miles? The answer has economic and political interest because dams athwart the routes of the salmon have cut heavily into their reproduction, and the diminished numbers of salmon have affected the fisheries of many nations. Salmon literally smell their way home. Each home stream, presumably from the plant and mineral oils of its drainage basin, acquires a unique organic fragrance. Young salmon "learn" this fragrance in the early weeks of life and retain recognition of it thereafter. The chemical nature of these odors has not been identified. How the salmon navigates long ocean distances is unclear, but, as it finds the main river, it selects that tributary that carries the home odor and con- tinues upstream. The energy requirements for this extraordinary journey are prodigious. A group of salmon, observed while migrating 600 miles up the Fraser River in British Columbia, took almost three weeks to make the traverse. By the time the female salmon, which do not feed during migration, had reached the spawning grounds, they had expended 69 percent of their body fat and over half of their protein. Energy had been expended throughout this period at 80 percent of the maximum rate possible for these animals! With only a small reserve remaining for emergencies, one can readily understand why only one additional small dam on a river may cause a run of salmon to collapse. The continual pressure for survival led those organisms that had ade- quately adapted to inhabit a remarkable variety of ecological niches. Of these, none is more remarkable than the animals that inhabit the desert, where water is scarce. How they survive under those circumstances is a matter of considerable import. As we have already seen, desert rodents evolved kidneys capable of secretion of extraordinarily concentrated urine.

OCR for page 115
FRONTIERS OF BIOLOGY At the same time, they "learned" to avoid the harshest aspects of life in the desert by remaining in their thermostatic, humid burrows during the day, emerging to feed and explore only at night. The camel and the ostrich each solved this problem, in part, by acquisition of a heavy coat that serves as an extremely effective insulator. Others learned to permit their body tem- peratures simply to approach that of the environment, rising and falling with the days and nights as with the seasons. Because of its novelty, one should take note of the mechanism that permits marine birds to spend virtually the entirety of their lives at sea. The least saline water available to them is that in the flesh of the fish that constitute their diet. But each such encounter also entails ingestion of some seawater as well. Yet the salt concentration of the blood of these birds is essentially identical with that of terrestrial forms. This is accom- plished by a unique adaptation, the presence in their nasal passages of a "salt gland," an organ that, on sensing an increase in the sodium concentra- tion of the plasma, secretes a highly concentrated salt solution of fixed concentration and continues to do so until the plasma sodium level has returned to normal. The mechanism responsible for this function is not yet known. Plant species are equally sensitive to the water supply, and the record of annual rainfall is to be found in the width and density of the annual deposition of wood. When midday temperatures and radiation exceed a critical level, which varies with the species, the plant's conducting system can no longer transfer water from the soil; evaporation from the leaves continues and water tensions are established within the plants. A suction force equal to 16 atmospheres has been measured in the tops of redwood trees under such circumstances. When internal water stress becomes ex- cessive, the stomata of the leaves close, minimizing water loss but also shutting off uptake of carbon dioxide and release of oxygen, and hence halting ohotosvnthesis. Records of the past found in the rings of trees ~ 1 ~ ~ _ . ~ ~ . ~ ~ ~ ,1 ~ ~ ~ ~ 1 _ ~1_ ~ ~ A_ ~ I,: require careful study, combined with associated fossil analyses and exam~- nation of current climatic conditions, to permit predicting the more im- portant biological effects of future weather modifications when this becomes feasible. THE ABUNDANCE OF LIVING THINGS The numbers of animals and plants in a given area are primary concerns of the ecologist. Both absolute numbers, or density, and the ratios among species are primary variables. Both are subject to external and internal controls. External controls are exemplified by the weather, application of pesticides, natural catastrophes, and variation in food supply or in 117

OCR for page 115
118 THE LIFE SCIENCES numbers of predators. Internal controls are exemplified by the changes in urn patterns oy wn~cn the subject populations respond to all aspects of their environment. Both have been studied in detail, in the wild and under laboratory conditions. In natural habitat, both absolute densities and Prepuces ratios are maintained over remarkably long periods; the objec- tive of population ecology is to understand the play of forces that achieves this. These forces vary from species to species, but for all species the product of the probability of death and the probability of birth, summed for all ages over a generation, must come to an average value of 1.0 if constancy is to be maintained. A few examples will suffice. Daphnia, a small freshwater crustacean, can maintain remarkably dense populations on an adequate food supply. If a modest grade of removal (fishing) is applied, the density falls sharply but not linearly. Even with removal rates as high as 30 percent per day, a small but vigorous population persists. Patently, the effect of fishing mainly for the young is different from that of fishing for adults only. This displays the resilience that man exploits; if we somewhat rarefy a population, we increase its rate of growth, so a certain degree of rarefaction actually increases the steady yield of animals harvested. This is a benefit that works both ways; pest controllers are less than enchanted to find that the more rats they kill the more there are. Some forms of predation can completely exclude an organism from an otherwise suitable environment. Native silkworms can be successfully raised on wild cherry trees as long as each is enclosed in a protective net, with yields as high as 80 percent from egg to adult. But if the trees are left unprotected, not one silkworm survives through the larval stage. The breeding response to adversity is illustrated by the size of egg clutches in bird species. For example, the habitat of the European robin extends over a great area, yet these birds maintain a fairly constant density over 35 degrees of latitude. Since the winter death rate of adults is higher in the colder northern latitudes, the population can remain constant only if this is offset by an increase in the birth rate. And, indeed, per degree of latitude this species adds about 0.1 egg to the annual setting. Some forms of external control are not always obvious. Red tides, the sporadic blooms of a microscopic dinoDagellate that is highly toxic to fish, occur occasionally on our eastern coast. The circumstance that leads to such blooms is not predictable or constant. It usually begins with the for- mation of a pool of nutrient-rich brackish water in an estuary that has been temporarily prevented from tidal flushing and becomes enriched with stream-borne nutrients. When flushed to sea, the toxic water mass main- tains its integrity for days before dissipation. The nutrients that touch off such an event are small quantities of primary elements and of a few organic compounds. A few micrograms of one of these per cubic meter of water 1 1 ~ . . .. . .

OCR for page 115
FRONTIERS OF BIOLOGY 1 l9 appears to make the difference between bloom and no bloom, between en- vironmental health and disaster SPECIES INTERACTIONS Much of the biology of each species is devoted to accommodation to other species, including such phenomena as defense against disease and predators, patterns of courtship and mating, body size, life-span, and reproductive potential. The number of interspecies interactions among the several million species is vast; systematic studies of these are certain to reveal profitable ways of utilizing more of the earth's biota for man's benefit. Biological control tests, new drugs, and repellents are obvious potential applications. It is this largely untapped potential and not sentimentalism that makes ecol- ogists protectors of threatened species and of dwindling habitats that harbor unique combinations of species. It would be tragic if potentially easy solu- tions to future major problems were lost through ignorance or indifference. It is repeatedly impressive that seemingly competitive species manage to accommodate in given ecosystems. For example, study of a spruce forest containing five species of wild warblers, at first glance much alike in their habits, revealed their actual ecologies to be surprisingly diverse. Although they all fed on the insects in the spruce trees, each species had a unique combination of behavior patterns based on the proportion of the time it spent hovering, whether it tended to feed on peripheral or central parts of the trees, how frequently it flew from one place to another, and so on. Thus, in effect, by exposure to different items of food, they share the re sources of the forest. Such understanding can be utilized in practice. For example, the Klamath weed was accidentally introduced into the livestock ranges of northern California about 1900. This plant not only replaces valuable foliage, but it is also highly toxic. But in 1947 a beetle was found in Europe that fed upon this plant, and it was introduced into the affected areas. Within a few years, this beetle achieved mass destruction of the Klamath weed popula- tion. However, along highways and under the shade of trees, the beetle population is ineffective in controlling the plant. As a result, beetle and weed populations have achieved an equilibrium with rather low weed infestation. Meanwhile, the range has been repopulated with useful grasses. No episode in the deliberate manipulation of ecological systems is more dramatic than that which began with the introduction of the European wild rabbit into Britain by the Normans at the beginning of the twelfth century. With it came the flea, which is the only known host for the myxoma virus. The virus itself had long been established in the native rabbits of South America, but it is not pathogenic for that species; yet it induces a rapidly

OCR for page 115
120 THE LIFE SCIENCES debilitating disease in European rabbits. This virus was deliberately intro- duced into Prance in 1952 and into England in 1953. By 1955 rabbits were practically extinct over most of Britain. As the rabbits disappeared, plant species that had previously been highly restricted in distribution by the grazing of rabbits now flourished and spread. Areas that had previ- ously supported a covering of low mosses and turf became covered with deep mats of grass and strands of heather. In turn, the entire insect popu- lation changed with it. Thus, the total ecosystem was profoundly affected by removal of only one link in the food chain. ENERGY FLOW IN ECOSYSTEMS Only recently has ecology become sufficiently sophisticated to be concerned with the flow of matter and energy in ecosystems. A growing body of infor- mation reveals the efficiency of the conversion of solar energy into organic matter in ponds, ocean areas, open fields, and cultivated farms. In turn, these provide lessons for man in his future management of the earth. For example, the average efficiency of an Iowa cornfield is only about 2 percent, whereas the conversion of solar energy into organic matter by algae in a fertilized pond may be as high as 20 percent. The use of radioactive carbon dioxide as a tracer has permitted the construction of balance sheets of carbon flow, particularly in small lakes; chemical analysis reveals the total flow of mineral elements as well. Similar techniques have been applied to small patches of forests. The consequences of removal of tree cover are extremely dramatic. For a long period thereafter, the impinging energy is wasted, and the bulk of the available mineral matter is removed by the leaching action of rain- fall. Studies of these effects must be continued both on a small scale and on the larger scale of total drainage basins, forestlands, and deserts. What has already become apparent is that, although 70 percent of the surface of the globe is covered by ocean, the total biological yield of the oceans is approximately of the same order as that of the land area. STABILITY AND DIVERSITY Much ecological concern has been addressed to those factors that make for a stable ecosystem a system whose numbers and balance of species remain relatively constant over prolonged periods. Examinations of such systems have involved description of the patterns of the food web, the distribution and arrangement of different species in space and time, and the grouping of individuals of several species into higher taxonomic units with varying functional roles. Such studies have led to a few generalities.

OCR for page 115
FRONTIERS OF BIOLOGY In general, the greater the number of species at any level in the food chain, the greater the community stability at that level and at lower and higher levels as well. Amplitudes of fluctuation of herbivore populations are determined by the number of species of plants they eat and by the number of predators and parasites that attack them. The density of food plants determines the extent to which herbivore populations fluctuate, and the size of the herbivore population, in turn, determines population fluctuation among predators. The more diverse the species at any taxonomic level, the more stable the system. A system of ten equally abundant species is more diverse and successful than one with a single very common species and nine relatively rare species. Although stability may be hard to measure satisfactorily under any cir- cumstances, it is easier to measure after the fact, when disturbance, the inverse of stability, has occurred. Such events usually occur spontaneously or are produced accidentally by advancing civilization. Relevant studies have recently been performed deliberately. For example, an hourglass- shaped bog lake was deliberately separated into two lakes by an earthen barrier across the constriction. Lime was added to one lake, raising the pH from 5.9 to 7.3. Within a year the transparency of the latter increased remarkably and, after two years, the well-lighted zone had increased from 2.7 to 7 meters in depth. The Daphnia population was found to replace itself in one third the time in the lime-treated lake, and new species of phytoplankton and of rooted aquatic plants began to thrive. Thus the initial disturbance that had led to acidification of the lake could, in fact, be successfully reversed, and a more stable, more diverse, and, for man, more attractive ecosystem was restored. Many illustrations of the practical applications of ecological understand- ing are to be found in Chapter 2. The foregoing discussion has served only to indicate some of the kinds of problems addressed by ecologists. Only now has the science advanced to a point at which ecologists consider that they can usefully and successfully construct mathematical models of large ecosystems e.~., the coniferous forest, the western grasslands, the south western desert. The success of these efforts remains to be ascertained. Meanwhile, applied ecology is man's greatest hope both for protection of the natural environment and for human survival. Ecological understanding is required to guide intelligent use of pesticides and of biological mech anisms for pest control; to indicate when ciear-cutt~ng or slash-and-burn ~nnronc~.h~.~ should he used as forests are put to new use; to give guidance to the appropriate scheduling for plowing, planting, burning, and normal agricultural practice; to predict the consequences of the introduction of new strains of crop plants (already it is clear that, on a single farm, genetic -en ~