UNIT 15 FATE OF THE EARTH: THE BALANCE OF NATURE

A. INTRODUCTION

Overview

The establishment of life, early in planet Earth's history, was an event that would totally transform our planet. The atmosphere originally was virtually oxygen-free, and in this environment the basic building blocks of life, the amino acids, appeared. We don't know how the next step, the production of nucleic acids to enable reproduction of cells, was achieved, though interesting hypotheses have been proposed. Once this step was achieved, life and its environment evolved together, and the development of life proceeded to ever higher levels of sophistication. With photosynthesis, life acquired a practical method for obtaining energy from the Sun, and the subsequent rise of oxygen in the atmosphere had profound consequences for both geosphere and biosphere. With the arrival of mankind, a new force for change appeared -- one that acts with great rapidity and force, though by no means always benignly. An examination of the energy budget of the Earth and of three important biogeochemical cycles -- those of carbon, nitrogen, and phosphorus -- illustrates the interconnectedness of the planet-wide systems that comprise the biosphere and geosphere. Before we can predict the consequences of our actions as stewards of Planet Earth, we must have a reasonable understanding of this complex system of interacting cycles that serve to keep the environmental house in order. This system is what is popularly referred to as the "balance of nature." You will see, however, that natural "balances" are not static but are dynamic and have undergone dramatic shifts throughout geologic time.

B. GEOSPHERE AND BIOSPHERE

Of all the thousands of photographs taken by the Apollo astronauts, the one that has most captured the attention of the public, non-scientists and scientists alike, is the remarkable picture taken from a command capsule in orbit around the Moon that serves as the symbol for this book. Above the bleak and cratered terrain of the Moon's horizon rises the blue, white, and brown orb of Planet Earth. When this picture first appeared in magazines and on television it seemed to proclaim, "We have truly cut loose from Earth and traveled in space", showing both the destination and starting point of our travels. But as time went on, the picture seemed to take on new meaning: the connotation of Spaceship Earth. It is hard to deny the finite nature of a world that can be viewed in a single glance.

The concept of that small glittering sphere as home to the entire mass of humanity, in all its forms, customs, tribes, and nations is humbling and sobering, reminding us of the overwhelming need for all humanity to coexist in peace and cooperation as we share this small planet. The sense of finiteness conveyed by the picture also seemed to echo environmental concerns that had grown and matured in preceding decades. These concerns had earlier roots in thinkers like St. Francis, Darwin, Emerson, Thoreau, and Muir, who marveled at the finely tuned balance of natural processes and worried about the impact that intruding mankind would have on such delicate machinery.

But now a new interpretation of this photograph became possible, using a line of reasoning followed by a growing band of scientists with eclectic backgrounds and wide-ranging interests. This was to be a holistic view of the planet, in which mankind, life, oceans, atmosphere, and the rocky tectonic engines of the physical planet itself were not to be regarded as separate and compartmented, but one vast, intricate, interconnected living machine. With its immediate roots in the relatively new science of ecology, this view also stretches out tendrils to gain support and understanding from such diverse sciences as astronomy, climatology, oceanography, geology, geophysics, paleobiology, and even planetology -- in short, from all the fields mentioned so far.

In this view, life is inextricably part of the Earth and part of the balance of nature. To separate studies of the geosphere (the physical world of geology) from those of the biosphere (that part of the world inhabited by life) or vice versa is to break so many connections as to render the smaller view incomplete and often misleading. It is not too hard to accept the influence of the geosphere upon the biosphere -- the adaptation of life to its physical surroundings -- but more difficult is the notion that the biosphere has an important counter-influence upon the geosphere. Earth itself has been shaped by life, and for that reason is unique in so many ways among the planets of our solar system.

Consider the remaking of the atmosphere by life on Earth. Only on this one planet do we find an atmosphere rich in oxygen and nitrogen. Even so distant, separate, and physical a phenomenon as the aurora has been shaped by life. The principal colors that we observe in these high-altitude light shows are due to the presence of oxygen and nitrogen in the atmosphere. The colors we see are a function of the intensity of aurora activity and the proportion of these gases in our atmosphere, of which the presence and proportion of oxygen, is closely linked with the history of life on our planet.

C. THE ORIGINS OF LIFE

Let us take a closer look at the original events that so changed Earth. Recall from Unit 10, Origins, that Earth's first 1,000 million years were extremely turbulent, with a rain of infalling debris left over from the formation of the solar system dominating the geological processes. We don't know exactly when or how life first made its appearance on Earth, but we do know that life had already been around for some time when Earth was only about 700 million years old, 3,800 million years ago.

The earliest fossils discovered so far date from that time and are found in Australian rocks. These are fossils of very primitive single-celled organisms of a group called prokaryotes (recall that these were briefly introduced in Unit 9 as part of the discussion regarding the possibility of life on Mars). Today this group is represented by blue-green algae and bacteria, which reproduce asexually by cell division. As a result, the offspring are genetically similar to the parents. Just how long life had existed before these oldest fossils is not known. It has been suggested that life developed on earth only 200 million years after the end of the most intense period of bombardment.

By the time these microscopic messengers became sealed in their rocky time capsules, the final stages of volcanic flooding of the maria were drawing to a close on the Moon. On Earth, the landmasses were barren; the sea was to be the mother of all life. Yet the earth would be transformed by life, within it's oceans, air, soil, frozen methane ice beneath the ocean bottoms, on it's land surfaces, even within pores of rock hundreds of meters below the surface, life would invade and thrive. Indeed, the blue-green algae themselves would completely change the atmospheric composition of the Planet in a way unique in our Solar System. Indeed the result of life on Planet Earth by any measure is staggering, consider a single portion of the earth system, the atmosphere.

Scientists believe that when collected and considered together observations from global sites, ranging in age between 3.0 billion years and 0.6 billion years, of preserved soils, oxidized sedimentary strata (red beds), uranium ores and uranium in black shales (both sensitive recorders of oxidation), and banded iron formations, show that the partial pressure of Oxygen (pO₂) was £ 1% of the present atmospheric level about 2.2 billion years ago and rose to ³ 15% pO₂ about 1.9 billion years ago. This rapid change in pO₂, presented in the following section, was purely a consequence of living organisms.

Recent advances in molecular phylogeny have provided further evidence of the process of evolution at molecular scales and suggest a very distant common ancestor to all present forms of life. How did this remarkable phenomenon come about? What was the origin of life on Earth? There is hardly any other question that holds such a fascination for our species. Every religion and every primitive folklore has its own particular answer. Science can only look at the available evidence and construct logical models that predict the eventual appearance of tiny fossils in the rocks of Australia.

However, in science there is a remarkable excitement and reverence in these investigations which is well expressed by Charles Darwin in the final paragraph of his The Origin of Species (1859):

• It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

1.  The First Living Cells

One way to go about answering this question is to reason back from what we know today about the chemical basis of life. The unit of life is the cell and at some point cellular organisms must have assembled from the components available on the early Earth. If we could somehow transport ourselves nearly 4,000 million years back in time, what would the first living cells be like? In order to be called alive, they would certainly have the primary characteristics of the living state, which include growth, reproduction, and the ability to evolve. Let's examine these properties individually.

Growth in cells today occurs through a process by which energy and nutrients are extracted from the environment, the energy being used to cause the nutrients to be assembled into new cell components. The assembly depends on metabolism, particularly enzyme-catalyzed reactions, and is directed by a kind of molecular blueprint containing the information needed for the assembly process. The information itself is copied during cell reproduction by replicating the molecules in which it is stored. In even the simplest modern cell, vast amounts of information are present, and this is used to direct the synthesis of thousands of enzymes and other proteins involved in metabolism and structure.

In the earliest life forms, probably nothing more was present than several small, slowly replicating molecules, together with a few equally small catalytic molecules that happened to be able to enhance the rate of replication. Most important, a mechanism must also have been present which permitted the replicating molecules to direct the synthesis of the catalytic molecules. Only when this loop between information and catalytic activity is closed can evolution occur, because only then can random changes in the replicating molecule occasionally produce a favorable change in the catalyst that in turn increases the ability of the system to extract nutrients and energy from the environment. Finally, for an actual cell to exist, by definition there would need to be a boundary membrane of some sort that could encapsulate the system and provide a microenvironment conducive to its overall function.

Even at this simplified level, the first cells seem fairly complicated. Is it conceivable that such a system could have assembled by chance on the early Earth? First we can ask what sorts of molecules might have been available. The major molecular components of living cells today include nucleic acids like DNA and RNA for the replicating molecules that transmit information, and proteins called enzymes for the catalytic molecules. The genetic code carried in DNA directs the synthesis of proteins from amino acids , while lipid bilayers2 form the boundary that separates the cell interior from the external environment. Could any of these general classes of molecules have been present 4,000 million years ago?

The answer to this question is a cautious yes, and this is where our first step toward a plausible scheme for the origin of life begins. First we can ask whether any of the simpler molecules of the life process might have been available. The early solar system involved not only geological processes, but also chemical processes, with vast clouds of gas and dust being acted upon by violent energies, including ultraviolet light and heat from the sun, electricity (lightning), and heat from volcanic activity. In 1953, Stanley Miller conducted a historic experiment which attempted to simulate these conditions in order to observe the chemical reactions that might occur. At that time, the composition of the early atmosphere was believed to be what chemists call "reducing." That is, no free oxygen was present, but instead the mixture was dominated by hydrogen, the most abundant element in the solar system, with smaller quantities of the reduced forms of carbon (methane), nitrogen, (ammonia), and oxygen (water). By way of contrast, today's atmosphere, containing 20% oxygen and 80% nitrogen, would be considered oxidizing.

In the experiment performed by Miller, a mixture of methane, ammonia, water and hydrogen was subjected to an electrical discharge to simulate lightning, and the composition was analyzed over a period of time. The results were truly astonishing. The mixture turned reddish-brown after only a day or so, and a week later, when analyzed, was found to contain numerous organic substances including several amino acids, the building blocks of proteins. This clearly suggested that there was nothing special about amino acids. Instead it now seemed plausible that they could have been readily available as dilute solutions in the lakes and oceans of the early Earth. Similar experiments have been repeated under a variety of conditions, and it is now clear that the main components needed to form the major kinds of biomolecules could have been synthesized by chemical processes occurring in a reducing atmosphere.

Despite the scientists' satisfaction of discovering a chemical basis for early chemical evolution, simulation experiments still leave something to be desired. They only represent our best guesses at the actual conditions 4,000 million years ago, and in fact, we already are reasonably certain that the original guess about a reducing atmosphere was off the mark. Instead, the Earth was probably at volcanic heat early in its history, due to the energy content of the solids that accreted to form the planet. It follows that the early atmosphere would have been rapidly replaced by a "volcanic" atmosphere of carbon dioxide, water, and nitrogen. Such an atmosphere is neither strongly reducing nor strongly oxidizing, but somewhere in between the two extremes. Under these conditions, synthesis of organic compounds by electric discharge becomes much less efficient, presenting something of a dilemma for scientists working in the field. On the other hand, there had to be some source of organic compounds, otherwise processes related to the origin of life would not have had the set of organic compounds required.

Is there any way to test the concepts brought out by the simulation experiments, and to be more confidant that they are meaningful? Probably the best test comes in the form of certain meteorites that crash to Earth every few decades. Most meteorites are made of stony material or metallic alloys of iron and nickel. However, on rare occasions a meteorite falls that contains small amounts -- a few percent -- of carbon. Furthermore, when this carbon content is analyzed, a substantial portion of it turns out to be organic, composed of amino acids, hydrocarbons, lipid-like molecules, and even traces of purines and pyrimidines, building blocks of nucleic acids . All of this organic material was synthesized by abiotic processes, probably in planetesimals (small planets a few kilometers in diameter) that never got large enough to become true planets. Because similar chemical processes must have occurred on the early Earth, this discovery represents a remarkable confirmation that the simulation experiments are on the right track. It has even been proposed that comets and small meteorites may have delivered substantial amounts of organic compounds to the Earth's surface, thereby bypassing the problems associated with terrestrial synthesis.

We now come to the most important question of all, and yet the one we know the least about: assuming that the simpler organic compounds of the life process were present on the prebiotic Earth, how could they gather into the first cells? An important concept related to this question is that many biologically relevant molecules have an extraordinary ability to fit together into larger aggregates called supramolecular structures. This process of self-assembly is still not understood in detail, but some important examples include the manner in which purine and pyrimidine bases form hydrogen bonds to stabilize the double helical structure of DNA . Another example is the self-assembly of lipid molecules into the lipid bilayer structure of membranes. Self-assembly is fundamental to the architecture of all modern cells, and certainly must have been involved in the earliest forms of life.

The question can now be stated more clearly: what kinds of self-assembling molecules could have provided the first replicating information carriers, the first catalysts and the first membrane boundaries? We can begin be asking what sort of replicating molecules might have been present? The only one we know of is nucleic acid, which has the ability to make copies of itself with the help of enzymes called polymerases. Recently, an experiment has been performed which in its way is as important as the original finding that amino acids were produced under prebiotic conditions. In the experiment, chemically active forms of nucleotides were mixed with a small amount of nucleic acid which could act as a template. When the mixture was analyzed a week later, it was found to contain polymers of the nucleotides, with chains up to 40 nucleotides long. This is the first indication that polymerization and replication can occur in the absence of enzymatic catalysis, as it must have in the earliest chemical systems on the evolutionary pathway to the first true cells.

What about catalysts? A catalyst is anything that speeds up a chemical reaction without actually taking part in it and becoming changed itself. Biological catalysts today are all proteins which have active sites with catalytic activity. Could amino acids have assembled into protein-like compounds on the early Earth? This turns out to be relatively simple. When mixtures of amino acids are heated to 60°C or higher, after a few weeks numerous chemical linkages have formed and polymeric compounds can be isolated. Some of these polymers have modest catalytic ability, suggesting that enzyme-like catalysts may have been available to assemble into the first forms of life.

Even more intriguing is the recent discovery that RNA itself can have catalytic properties. Such RNA molecules, now referred to as ribozymes, offer an entirely new possibility for the origin of life: perhaps an "RNA World" came first. That is, it may not be necessary to propose that the first life used a complex system of DNA, RNA and protein. Instead, RNA may have served all these functions, combining in one molecule the catalytic and information-carrying properties required by the first cells. This is now a focus of intense research interest, with new discoveries being reported almost weekly. For example, it has recently been found that RNA ribozymes can catalyze their own polymerization reactions. In other work, ribozymes were shown to "evolve" under laboratory conditions, changing from one kind of catalyst into another. These results have convinced most of the scientists active in this field that the concept of an RNA World is very useful in suggesting new experiments, and the most plausible of the various scenarios proposed for how the earliest cells connected catalytic functions to genetic information in a macromolecular system.

The last question is related to the boundary membrane. Again, this has turned out to be relatively straightforward because of the self-assembly properties of lipid-like molecules, which form small sac-like structures called vesicles when exposed to water. Such vesicles are routinely used in research as models of the boundary membrane that surrounds all living cells. Is there a way that membranome boundary structures could be produced on the early Earth? It has been found that fatty acids, glycerol and phosphate, when dried under the same conditions described above for the formation of protein-like polymers, are able to produce simple phospholipids. These in turn readily form membranome vesicles in water, and the vesicles can encapsulate other molecules even as large as DNA. Recently it has been discovered that some of the organic components of carbonaceous meteorites are able to form membrane-bounded vesicles, again showing that we are not limited to simulation experiments.

We can now return to our original question. How might the first cell have been assembled? Imagine an environment such as a tide pool which undergoes daily cycles of drying and heating by the Sun, followed by rehydration as the tide comes in. Under these conditions, the dilute solutions of organic molecules described above would be concentrated to thin films on the surfaces of rocks and sand particles, and the heating would drive polymerization reactions so that larger molecules would be continually forming. Lipid-like molecules present would tend to encapsulate the polymerized products, and on rehydration membranome sacs containing the polymers would float away to land somewhere else for another drying cycle. Now imagine this process of natural experimentation occurring along tens of thousands of miles of coastline, and over millions of years of time. It seems almost inevitable that just once, a simple replicating molecule would find a way to interact with a simple catalyst in a membrane-bounded vesicle, and life would begin.

2.  Did Clay Provide the Framework for Life?

While the basic building blocks of a cell can be constructed by random events such as in the spark experiment, it seems unlikely to some scientists that a fully-functional cell could result from random processes. As one scientist puts it, if you take a pile of aluminum, copper, and insulation, grind it all up and throw it down a flight of stairs, you don't expect a jetliner to fly away at the bottom.

Recent observations, however, have suggested a surprising possibility. Ordinary clay is made up in part of very small mineral grains that are constructed of flat layers like sheets, piled one atop another. These layers are only lightly bonded together, resulting in the slipperiness characteristic of clay. Like cards in a deck, clay mineral layers can pack a lot of surface area into a small volume. In addition, the surfaces of each layer are chemically active and can bond organic molecules to them. It has been suggested that clay surfaces might act as a kind of template, governing the structure of bound molecules and aiding in their assembly into more complex and regular forms.

It is even possible that the clay itself is able to replicate its own structure, including variations that might exist within that structure. Water causes clay layers to expand and separate slightly. New layers can then form in the spaces opened up between the old layers, and the new layers are imprinted with the structure of the old layers. In this way, distinctive clay layers have the potential to "breed true" in generation after generation.

The combination of large surface area, possible template activity, and a crude kind of inorganic self-replicating ability might have introduced the necessary degree of order and reproducibility to have allowed a primitive type of natural selection to begin its task. As an example of natural selection in such a system, certain chance clay-organic combinations may have proved especially successful at self-replication and so they would have become more numerous.

Finally, if a molecule like a nucleic acid were to result from large numbers of repetitions of this process, the newly-formed organism would be able to replicate itself in a wholly organic manner and would no longer require the clay, but could cast it aside like a crutch that is no longer needed. It is a highly speculative but captivating idea, that clay might have provided the catalyst that enabled the quantum leap from random organic molecules to the first living, reproducing cell.

3.  The Next Steps

However that momentous first assembly was accomplished, our only evidence is in those 3,500 million year old single-celled fossils. Already, these ancestors of modern blue-green algae were fairly complex and may have been capable of photosynthesis, the conversion of light into chemically useful energy. An important byproduct of this process is the conversion of carbon dioxide and water into hexose carbohydrates and water, with the release of spare oxygen. Before the ancient algae must have come more primitive forms yet, tiny chemical factories that scavenged the abundant organic molecules that had been produced by lightning and other nonbiological processes. Next came single-celled organisms that could manufacture their own food from readily available inorganic compounds such as carbon dioxide, hydrogen sulfide, and ammonia.

Some structures built by blue-green algae (called cyanobacteria) today bear striking resemblance to early fossils found in western Australia. They form dome-shaped masses about the size of cabbages, and are similarly layered. The structures, commonly made of carbonate rock, are called stromatolites. Today they form mostly in intertidal zones, alternately being exposed to air and covered by water as the tides come and go, but also extend into the subtidal. The first Precambrian fossil stromatolites very much resemble the structures formed by their present-day counterparts. Colonies of microscopic blue-green algae trap and bind sediment and can bring about the precipitation of mineral material, which in time builds up the layered structures characteristic of stromatolites.

With the development of photosynthesis, the release of oxygen by chlorophyll began a slow but profound change in Earth's atmosphere. The first half of Earth history, until about 2,000 million years ago, was characterized by anaerobic conditions (deficient in molecular oxygen). However, the rock record contains abundant rocks, called banded iron formations, that are rich in oxidized iron minerals. How can this apparent enigma be explained? Under anaerobic conditions in the ancient oceans, the reduced variety of iron, ferrous iron (Fe²⁺), is soluble in water. If oxygen is introduced, the ferrous iron will oxidize, combining with oxygen to form the ferric (Fe³⁺) oxide mineral hematite (Fe₂O₃), which will precipitate out of the water and be deposited. Stromatolites are constructed by blue-green algae and can produce oxygen. The oxygen produced by the stromatolitic blue-green algae and phytoplankton reacted with the ferrous iron dissolved in the seas, resulting in the accumulation of beds rich in iron oxide. Indeed, most of our iron ore comes from these ancient banded iron formations.

The seas had many hundreds of millions of years in which to build up considerable quantities of ferrous iron and this had to be swept from the oceans before oxygen could begin to accumulate in the atmosphere. With the buildup of atmospheric oxygen, ozone (O₃) could be created by the action of sunlight in the upper atmosphere, establishing a filter for harmful ultraviolet radiation. With the establishment of the ozone layer, the level of ultraviolet radiation from the Sun at the Earth's surface was reduced to the point where organisms could live in very shallow water environments and later could even live full-time out of the water.

The change from a reducing to an oxidizing atmosphere has been described as the greatest air pollution event in the history of Earth. For it almost certainly brought disaster to the anaerobic prokaryotes that had flourished until then. Oxygen is a poison to all life. We and other aerobic organisms have evolved special enzymes to neutralize the toxic effects of oxygen yet permit the oxygen to function in metabolism. The evolution of oxygen-mediating enzymes must have co-occurred with the first release of oxygen from blue-green algae.

Once oxygen appeared, the dominant anaerobic biota were presented with three possible paths: 1) retreat to permanently oxygen-deficient places such as muds; 2) evolve oxygen-coping abilities; or 3) become extinct. No doubt all three occurred. The remaking of planet Earth by its own life forms had begun.

The next major step was the development of eukaryotes. Unlike the prokaryotes, these organisms are far more organized and complex. Their genetic material is organized into a well-defined nucleus that is surrounded by a nuclear wall membrane within the cell. In addition, eukaryotes contain small specialized units called organelles. Among these are plastids such as chloroplasts, which carry out photosynthesis, and mitochondria, which combine carbohydrates and fatty acids with oxygen to release energy. Working together, the chloroplasts and mitochondria act as a means of gathering and utilizing solar energy, storing it in the form of carbohydrates and drawing on these reserves when there is no sunlight.

The first eukaryotes may have formed as a symbiotic association of specialized prokaryotic individuals, each providing a different function and gradually evolving into organelles within a single eukaryotic cell. This new complexity and degree of organization opened the way to additional developments: the appearance of multicelled organisms and, most importantly, sexual reproduction. The latter opened up tremendous variability within a species because offspring now derived their genetic makeup from a mixture of genes from both parents. The processes of evolution could now advance at a far more rapid pace.

Unlike the early prokaryotes, which were adapted to living in an anaerobic environment, eukaryotes were aerobic, adapted to deal with an environment rich in oxygen. By the time that they appeared, oxygen must have begun to accumulate in the atmosphere and oceans of Earth. The first eukaryotic fossils appear in rocks about 1,700 million years old.

With the remaking of the atmosphere, life placed its stamp on the third planet, the only planet with liquid water. Eventually, a species would evolve that would begin anew the process of alteration of the natural environment. Homo sapiens had arrived, and even from space, the dark side of Earth could be seen to be spangled with the lights of his cities.

D. THE ENERGY BUDGET OF THE BIOSPHERE

Embedded in the environmental ethic is the concept that nature is governed by many interacting parts that, prior to the influence of mankind, had evolved into a state of balance and internal harmony. This concept has become familiar in the term the balance of nature. To many, it seems that the introduction of man's influences have hopelessly upset that balance. Let us look at this situation in a little more detail.

Taken literally, the idea of a balance in nature can be misleading, especially if it is considered to be a static balance. Rather, what we are really talking about is a complex, internally balanced set of interactions that essentially produce the distribution of species and the physical and chemical environment. As the species have changed with evolution, so have the interactions, the balances, and the environment as well. To change any of the latter is certainly going to change the species distributions; some more, some less. Some balances are very delicate; others are not. As one of the species inhabiting the biosphere, we affect those balances. For the remainder of this course, we shall use this rather more complex and dynamic meaning for "the balance of nature."

From an environmental point of view, the principal issue of science is to identify those points at which our interventions most seriously change the distributions presently found in nature, and those which are not so likely to produce significant change. Once potential changes and their likely causes have been identified, then society as a whole can make intelligent value judgments as to whether those changes are desirable or undesirable.

Before we can properly assess the impact of mankind on the biosphere and on the biota that inhabit it, we should gain a better understanding of the processes that maintain ecological conditions. In the limited time and space remaining to this course, we cannot cover all the appropriate principles of ecology, but we should examine at least those that are most important to interactions with the geosphere, which has been the focal point of our interests so far. Then, perhaps, we will be in a better position to determine which among mankind's activities are most hazardous in the long run to us and to all the other inhabitants of our planet.

You have already seen the prodigious increase in our use of energy, much of it in the form of fossil fuels drawn from the Earth. The biota of Earth is also a consumer of energy on a grand scale, though nearly all of the energy consumed in nature is solar. Only about half of incoming solar radiation makes it through the atmosphere to the ground or water surface; about 30% is reflected back into space; and the rest is absorbed by the atmosphere. Much of what strikes the surface is immediately converted into heat, but a small portion -- less than one percent -- is utilized in the process of photosynthesis to produce complex carbohydrates from water, carbon dioxide, and nutrients. This process adds to the biomass -- the total mass of living matter.

The process is not efficient for a number of reasons. For instance, the annual net photosynthesis in the higher plants requires that a significant portion (probably 10 to 20%) of the net heat available at Earth's surface must be used to drive transpiration (the transfer of water in vapor form from the plant to the atmosphere). This is more than ten times greater than the biochemical energy stored in the carbohydrates.

The process of photosynthesis is a highly complex process that may be simplified as a simple chemical equation:

Formula 1

This formula does no more than state that six molecules of carbon dioxide along with twelve molecules of water are changed in the process of photosynthesis into one molecule of the carbohydrate sugar (C₆H₁₂O₆), six molecules of oxygen, and six molecules of water. The process requires energy from sunlight and the intervention of chlorophyll, an organic substance that is itself constructed of carbohydrates and nutrient minerals. You may check for yourself to see that there are the same number of carbon, hydrogen, and nitrogen atoms present on either side of the equation. The individual atoms are conserved; it is only their arrangement into molecules that is changed.

Sugar is one substance in which energy may be stored by organisms for use when needed. The equation may be reversed, oxidizing sugar to produce carbon dioxide and water, which are released into the atmosphere in the process of respiration. The energy formerly needed to produce the sugar is now released. This, of course, is just the process that occurs within your body when you eat a candy bar and then "work it off" by jogging or by some other physical activity. First, you store the energy chemically in the form of sugar; then you "burn up" (oxidize) the sugar to provide needed energy.

Photosynthetic plants are called producers, in that they produce carbohydrates and form the base of the food chain. Figure 15-1 shows how energy and nutrients move through the biosphere starting with the producers.

• Figure 15-1  Energy and Nutrient Transport in the Biosphere

  

Herbivores are animals that eat plants, and carnivores are animals that eat other animals. Some carnivores eat only (or primarily) herbivores, while some eat other carnivores. There are, of course, omnivorous animals like ourselves who will eat almost anything. Collectively, we are all called consumers. In the process of consuming other organisms, we who are farther along the food chain gain energy and nutrients from earlier members of the chain.

In the end, though, all energy is derived from the photosynthetic producers. At each step of the way, heat is lost via body heat, heat produced by friction in movement, and so on. In fact, most of the energy that is being passed along the chain is lost to heat. This results in smaller and smaller populations that can be supported at each stage of the food chain.

Producers are most abundant, but only about 10% of their stored energy is passed along to herbivores. Only about 10% of the stored energy of herbivores is passed on to primary carnivores, and the secondary carnivores get only 10% of that. This is why there are huge numbers of edible plants, lots of rabbits, and relatively few foxes. It also explains why there is a great advantage to being omnivorous, a condition that allows us to break the restrictions of being at the end of the food chain by picking and choosing from whatever part of the chain we wish. Many of the more highly developed mammals share this trait with us. Our own specialty, however, is technology. We are able to intervene in the natural cycles, adding energy wherever it suits our purposes in hopes of achieving a simplification of the cycle and a resulting greater efficiency of energy use.

In addition to this somewhat linear food chain, there are also the decomposers (see Figure 15-1). These are the agents of decay and decomposition, the bacteria and the fungi. They take the wastes and the remains from all the other stages and complete the process of oxidation, returning water and carbon dioxide to the atmosphere and nutrients to the soil. In effect, they close the cycle, reclaiming these substances for use again by the producers. You may recall encountering this concept earlier, in Unit 6, Dynamics of the Oceans. There it was used to explain the high level of nutrients found in ocean bottom water due to a steady rain of decomposing organic material from the surface layers.

Energy, however, is not fully recycled. It passes in only one direction along the food chain and is eventually lost to heat. This heat contributes to raising the temperature of the environment and is finally lost to space as infrared radiation, along with heat from sunlight that never participated in the activity of life (which accounts for greater than 99% of the solar energy flux).

Where does modern, industrialized mankind fit into this scheme of things? First, as omnivores, we have an enormous variety of food choices available to us, giving us considerable adaptability to changing conditions. The diet of a Chinese farmer is vastly different from that of the average American, and yet both manage to survive. Second, through agriculture and animal husbandry we can manipulate the food chain itself. Cattle, which many of us like to eat, are encouraged to multiply, while mice, which most of us do not like to eat and which tend to compete with us for food, are discouraged. Third, and most important, we are able to go outside the food chain entirely and manipulate energy directly. In this way we can intervene in the natural energy cycle and manipulate populations, as in the case of domesticated cattle, and the environment itself, as in the burning of fossil fuels.

Our heavy dependence on fossil fuels is merely a way of drawing energy from long-vanished ecosystems, but our manipulation of direct solar energy, geothermal, tidal, and nuclear energy is a radical departure for life on Earth. Because the machinery by which we accomplish this is relatively compact, the principal effects are often localized in nature. A large power plant that discharges waste heat into a river can raise the temperature of river water for kilometers downstream to beyond the tolerance limits for some species of fish, resulting in massive kills. To avoid this, heat is more often vented in huge cooling towers that use the evaporation of water to carry waste heat off into the surrounding atmosphere. Even this, though, may result in slight, vestigial, local weather modification, leading to increased precipitation and the creation of "heat islands". The effect has been called thermal pollution, and is one of the most difficult kinds of pollution to avoid, since all our activities result in the production of waste heat.

How does our energy production compare to that of nature? On an annual basis, mankind's total world energy consumption is roughly 300 times the energy released in all the earthquakes that occur on Earth and about three times the total energy dissipated by the tides. It is about one-fifth as great as the radioactive heat generated within the entire planet. Because most geothermal heat escapes quietly by conduction through Earth's crust, this means that our rate of energy production is probably at least comparable to that of all the volcanoes on Earth. Clearly, our activities are not to be dismissed as inconsequential.

On the other hand, the net radiation energy from the Sun at Earth's surface is some 6,000 times our own heat production. If photosynthesis utilizes about 0.2% of the solar energy flux, then the energy stored in biomass on a worldwide basis is only about 12 times our total energy production. While this may seem small on a worldwide basis, much of our energy consumption is carried out in a few fairly restricted geographical regions, mostly in the northern hemisphere. This should (and does) give rise to noticeable effects on a local scale. Overall the global atmospheric heat engine, another inefficient mechanism, works at about 1.0*10⁺¹⁴ to 1.0*10⁺¹⁵ watts.

A 1977 report from the National Academy of Sciences3 concluded that direct energy release from mankind's activities would probably cause no more than a 0.5°C (0.9°F) increase in global temperature over the next century or so, but it also warned that much more severe effects might be seen due to increasing carbon dioxide levels in the atmosphere, acting via the Greenhouse Effect.

To what extent can the balances of nature deal with our increased production of carbon dioxide? In the next section we shall look at how these balances work for carbon and two elements important to agriculture: nitrogen and phosphorus.

E. BIOGEOCHEMICAL CYCLES

Many elements essential to life are cycled and recycled through both the geosphere and the biosphere. As a result, their concentrations in the atmosphere, oceans, and soil are dependent on the balances that are struck within the cycles that involve them. Though we shall treat each cycle separately, you should not assume that they operate independently from one another. In each of them, you will see common components, particularly the photosynthetic producers, the carbohydrate consumers (herbivores, carnivores, omnivores), and the decomposers. Changes in population and makeup of any of these components cause changes in all of the biogeochemical cycles and generate interactions between them.

1.  The Carbon Cycle

Carbon is the fundamental building block of life. The reason is that carbon is one of the few common elements whose chemistry is sufficiently complex to permit the construction of the tremendous variety of compounds required in life processes -- the organic molecules. Something as complex as the genetic materials DNA and RNA are only possible because of the large number of ways in which carbon can form chemical bonds with other elements.

The carbon cycle is diagrammed in a simplified way in Figure 15-2. Each box represents a reservoir or storage component for carbon; in its cycling through the process, carbon may reside in any of these reservoirs for a greater or lesser length of time before continuing its journey through the cycle. The arrows connecting the boxes represent the paths that carbon may take in moving from one reservoir to another.

Consider the atmospheric portion of the cycle, in the upper left part of the diagram. Photosynthetic producers (plants) draw carbon from the atmosphere in the process of converting carbon dioxide and water to carbohydrates. Plants not only breathe out (respire) oxygen, but parts of their tissues also respire carbon dioxide, just as we do. These processes provide the plant with energy for growing and maintaining its life support systems, and go on at all times. During the sunlit day, more carbon dioxide is consumed than is released in respiration, but at night photosynthesis ceases and the plant respires pure carbon dioxide, returning a portion of its carbon to the atmosphere.

The consumers (herbivores, carnivores, omnivores) eat plants and each other and pass carbohydrates along the food chain. Each of them respires carbon dioxide, returning a portion of the carbon to the atmosphere. Waste products contain carbon, and when the organisms die, their bodies are attacked by decomposers who oxidize the carbohydrates and return the carbon to the atmosphere as carbon dioxide. A forest fire might consume the entire local food chain at some point, oxidizing producers, consumers, and even decomposers, and returning at one stroke most of the carbon to the atmosphere in the process of combustion.

Not all terrestrial carbon is recycled back into the atmosphere, however. As you may recall from Unit 12, Energy Resources, coal is formed from decaying vegetation that is prevented from oxidizing in an anaerobic environment such as a swamp. The carbon in coal is effectively withdrawn from the cycle for very long periods of time and remains trapped within sedimentary rocks until it is exposed by erosion or by mining.

• Figure 15-2  The Carbon Cycle

  

The right side of the diagram is concerned with the oceanic part of the carbon cycle. Producers (phytoplankton), consumers (zooplankton, fish, crabs, etc.), and decomposers operate much as their terrestrial counterparts, except that outright combustion does not occur. Again, carbon is withdrawn from the cycle in the process of oil and gas production. In the sea, however, a far more important withdrawal mechanism exists: the production of calcium carbonate, the principal constituent of limestone. Calcium carbonate has the chemical formula CaCO₃ and is the principal geological repository for carbon. Other carbonate rocks, like dolomite which is composed of calcium magnesium carbonate (Ca,MgCO₃), also serve in this capacity. Freshwater limestones also form in lakes, but marine carbonates are far more abundant.

• TABLE 15-2 Amount of Carbon Residing in Different Reservoirs (data from U.S. Department of Energy).

  Reservoir		Quantities in Gigatons (109 metric tons)
  Atmosphere		710
  Continents
  	Biomass	590
  	Litter	60
  	Soil	1,670
  	Fossil Fuels	5,000
  	Carbonate Sediments	20,000,000
  Oceans
  	Biomass	4
  	Oceans
  	Surface Layers	680
  	Intermediate Waters	8,200
  	Deep Waters	26,000
  	Sediments	4,900

Table 15-2 lists the approximate sizes of the different reservoirs in the carbon cycle. Note that the combined terrestrial and oceanic biomass reservoir is comparable to that of the atmosphere, followed in size by soil carbon, deep ocean sediments, seawater, and, dominating the total, the carbonate sediments. Here, then, is where the vast bulk of the carbon dioxide that may at one time have dominated Earth's atmosphere has gone, placed in the Earth's crust by biological action.

Some aquatic plants are so efficient in converting dissolved carbon dioxide to calcium carbonate that they can precipitate it at the rate of two percent of their own weight for every ten hours of exposure to sunlight. Perhaps it is appropriate that many of our finest buildings -- the great cathedrals among them -- are often constructed from limestone, one of our most useful rocks. These stones contain the vast stores of carbon that have been collected from the atmosphere over the eons and trapped safely in a benign solid form.

Notice also that the atmosphere and the oceans are able to exchange carbon dioxide between them. This is accomplished at the sea-air interface, where carbon dioxide is readily dissolved in seawater. Because the oceans hold in seawater 50 times the carbon dioxide found in the atmosphere, they provide not only a large reservoir but also a mechanism for regulating the carbon content of the atmosphere. On short time scales, however, only the surface layer of the ocean exchanges carbon dioxide freely with the atmosphere, and this constitutes a reservoir comparable to that of the atmosphere.

Recall from Unit 8, Climates of Earth, that the carbon dioxide content of the atmosphere has increased by some eight percent between 1958 and 1982. This effect is due to the burning of fossil fuel, but the total amount of carbon dioxide released to the atmosphere is actually twice the amount of the observed increase. The oceans (and perhaps increased biomass production as well) have apparently absorbed half of the total excess carbon dioxide released in that time.

2.  The Nitrogen Cycle

Nitrogen is the most abundant gas in the atmosphere, accounting for 78% of the total. It is also biologically critical to organisms because of its role in the structure of proteins and nucleic acids (RNA and DNA). Like carbon, nitrogen supports a complex chemistry that allows it to participate in many and diverse organic molecules.

With its great abundance, you might expect that a nitrogen shortage would be difficult to produce in the environment. In fact, molecular nitrogen in the atmosphere cannot be used by most organisms, but must first be "fixed" or converted into the highly soluble forms of ammonia (NH₃), or the nitrate (NO^3-) or nitrite (NO^2-) ions.

Few inorganic processes can muster sufficient energy to accomplish the task. Meteors, cosmic rays, and lightning fix some nitrogen, but the amount is negligible. The primary natural agents for this task are nitrogen-fixing bacteria in the soil, many of which exist in symbiotic relationship with plants. These plants -- alfalfa, clover, peas, beans, and others -- have long been used to "build up" soil that has become depleted in nitrogen. In addition, non-leguminous trees fix nitrogen. A good example is the alder genus, so widespread in the wetlands. When depletion occurs, leaves turn yellow and plant growth and productivity are stunted. Blue-green algae are also able to fix nitrogen.

Other bacteria in the soil derive their energy by converting fixed nitrogen into nitric oxide (NO), nitrous oxide (N₂O), and molecular nitrogen (N₂), which are then returned to the atmosphere. Providing the principal return path to the atmosphere, these denitrifying bacteria serve to keep the overall nitrogen cycle in balance.

Table 15-3: Estimated Sources and Sinks of Nitrous Oxide

From Climate Change, The IPCC Scientific Assessment, 1991.

Plants take up fixed nitrogen through their roots and pass it on in the form of proteins and amino acids to the rest of the food chain. The nitrogen is returned to the soil in the form of wastes and the products of decay. Note that the atmosphere is not directly involved at this stage. So far as your body is concerned, nitrogen in the atmosphere is simply an inert gas that carries vital oxygen to your lungs. The reason is that dinitrogen (N₂) is inert, while odd nitrogen (N) is chemically active.

• Figure 15-3  The Nitrogen Cycle

  

Soil erosion carries fixed nitrogen into streams, lakes, and the oceans, often leaving behind barren and infertile soils. In addition, intensive farming practices do not allow fields to lie fallow long enough to regain their depleted fixed nitrogen supplies by natural processes. For this reason, large quantities of industrially fixed nitrogen are added to the soil in the form of chemical fertilizers. It is produced by passing atmospheric nitrogen and hydrogen over a catalyst at high temperatures (about 500°C or 900°F).

It is interesting to note that we must take such extreme and energy-consuming measures, while nitrogen-fixing bacteria can do the same thing very efficiently at low temperatures by processes that are not yet understood. They are slower to act, however, and modern farming cannot always wait for natural replenishment of the soil to occur. Critics of modern farming practices have sardonically labeled them a means by which oil and gas are converted into food. Indeed, when the energy requirements of fertilizer production, plowing, tilling, and harvesting are added up, the energy yield of the crop may exceed them by little. Some highly processed foods actually yield less energy than that consumed in their production.

There is some question as to whether the denitrifying bacteria can keep up with the added influx of fixed nitrogen from chemical fertilizers. Increased nutrient levels in stream runoff and in the lakes and bays that they feed have led in some cases to the condition of eutrophication -- an oversupply of nutrients in water that can cause algal blooms -- rapid growth of algae and bacteria that can use up all the oxygen in the water and choke out other life. In addition, excess nitrous oxide and ammonia may be released into the atmosphere, perhaps contributing to the dissociation of ozone in the stratosphere, a problem to which we shall return in the next unit.

Volcanic gases contain nitrogen that is added to the atmospheric supply, and incorporation of fixed nitrogen into sedimentary rocks serves to remove it from the biosphere for periods of geological time. The overall nitrogen supply within the biosphere is maintained at high levels because of the critical need for nitrogen in complex organic molecules. And so, along with oxygen, virtually the entire composition of our atmosphere has been influenced by the presence of life on Earth.

3.  The Phosphorus Cycle

Though phosphorus is found in fairly small quantities in organic matter, it is nonetheless essential to life. It is necessary to the manufacture of proteins and is found in nucleic acids. It is found to be concentrated in the hard parts of our bodies, in our teeth and bones. Occurring as the phosphate ion (PO^4-), it is leached from phosphate-containing rocks by water and deposited in soil, where it is taken up by the roots of producers and introduced into the usual food chain. No chemical compound of phosphate is gaseous, however, and so the atmosphere does not come into play in this cycle. The return path to the soil is via waste products and decomposition (see Figure 15-4).

Phosphates leached from soil and rocks find their way into streams and eventually to the continental shelves where they are deposited in sediments as the mineral phosphorite. Eventually, these sedimentary rocks may be uplifted and eroded to provide new sources of phosphate rocks.

It is often found that the lack of phosphorus as a nutrient limits the growth of the primary producers in an ecosystem. As a result, phosphates are mined extensively for incorporation in chemical fertilizers along with nitrogen. Excessive use of fertilizer can result in phosphate overload in streams and rivers, a condition made worse by the extensive use of phosphate in detergents. As in the case with excess nitrogen, excess phosphate can lead to eutrophication, explosive growth of algae and bacteria, and depletion of oxygen. The result is most visible as a massive fish kill, but in the process, most of the river's life can be destroyed. Far downstream it may recover as the water becomes oxygenated once again.

• Figure 15-4  The Phosphorus Cycle

F. THE GAIA HYPOTHESIS

Our discussion so far of the balances of nature as exemplified in Earth's energy budget and in the biogeochemical cycles has shown that the biota interact closely with the geosphere, and this over Earth history has brought about a state of affairs quite different from what would be expected if biota were not present. What is remarkable, though, is that in most cases the changes wrought are ones that have made Earth into a more secure environment for present-day life.

So far we have treated these changes as the natural consequences of the physical environment and of the evolution of life, without providing any real insight into how such a situation would come about. That living organisms can flourish on Earth is not merely due to chance, according to an English scientist, Sir James Lovelock, and Lynn Margulis of Boston University. In their view, life is not simply a passenger on the planet, passively adapting itself to the existing physical environment. Rather, it actively manipulates Earth to make the environment hospitable for itself.

According to them, the biota of Planet Earth may be viewed as a single giant organism, with elaborate mechanisms for regulating itself. Lovelock summarizes the hypothesis as follows:

• ... the biosphere is a self-regulating entity with the capacity to keep our planet healthy by controlling the chemical and physical environment.5

Lovelock calls this idea the Gaia Hypothesis, after the Greek goddess of Earth, and traces its roots back to ancient conceptions of a protective and nurturing Mother Earth.

What follows is a synopsis of the Gaia Hypothesis, and of Lovelock's arguments in support of it. It should be emphasized that many scientists are highly skeptical of it, and that it is by no means the only plausible explanation for the present Earth environment. In the next section, Dr. Stephen H. Schneider. ;of Stanford University will present an alternative view. These two somewhat different views will provide you with an opportunity to explore the role of controversy in the development of scientific knowledge, for all competing hypotheses must be challenged and tested to determine which is the best rational explanation for all available observations. At the present time, there is insufficient evidence to make a definitive choice between these two fascinating hypotheses.

Let us consider some of the evidence marshaled by Lovelock. You have already noted the remarkable transformation of Earth's atmosphere by life. Lovelock emphasizes the magnitude of these changes in the following table, in which the present atmosphere of Earth is contrasted with those of Mars and Venus, and also with a model Earth atmosphere that he believes might have resulted if the biosphere had never become active.

The striking differences in the composition of Earth's atmosphere from those of Mars and Venus are not what would be expected if an atmosphere were allowed to come into equilibrium on a purely physical or chemical basis, Lovelock argues. Many of the properties of Earth's atmosphere happen to fall within quite narrow ranges of tolerance for life.

• TABLE 15-3

  Gas	Venus	Earth (without life)	Mars	Earth (as it is)
  Carbon dioxide	98%	98%	95%	0.03%
  Nitrogen	1.9%	1.9%	2.7%	78%
  Oxygen	trace	trace	0.13%	21%
  Argon	0.1%	0.1%	2%	1%
  Surface temperature (°C)	477	290	-53	15
  Total pressure (bars)	90	60	0.0064	1.0

Take Earth's surface temperature, for example. The range of surface and water temperatures in the tropical regions of Earth are precisely optimum for the existence of present-day life, yet the temperature of the planet under conditions of a carbon-dioxide-dominated atmosphere surely would be much higher. Fossil evidence indicates that the prevailing temperatures in the tropics have been stable enough to support life throughout the past 3,500 million years, even though solar output has increased by as much as 30% during this time, according to fairly reliable models for solar evolution. This has given rise to what is referred to as the Early Faint Sun Paradox. Lovelock maintains that an active controlling mechanism could have manipulated the workings of Earth's atmosphere in such a way as to keep the temperature nearly constant throughout the whole of this time. He points to the inherently unstable conditions that can be produced by positive feedback mechanisms in climatic systems and suggests that a controlling mechanism would have been necessary to prevent temperature extremes from occurring that might have destroyed all life on Earth.

The oxygen level in our atmosphere is clearly a result of biological activity, but what is not so obvious is that the percentage of oxygen is precisely at optimum levels for use by life. Increasing oxygen levels provide greater energy conversion efficiency, but too much oxygen could bring about worldwide holocaust. The present level stands at 21%, but recent experiments have shown that the probability of a forest fire being started by a lightning stroke increases by 70% for each 1% rise in oxygen concentration above the present level. Ignition is strongly dependent upon the moisture content of combustible material, and so naturally-set fires are much more prevalent after prolonged drought. If the oxygen content should exceed 25%, however, ignition becomes highly probable even in the damp vegetation of a rain forest.

How is the oxygen concentration regulated? Lovelock thinks that methane released into the atmosphere by the operation of the carbon cycle provides the necessary mechanism since it is readily oxidized, converting molecular oxygen into water vapor. On the other hand, nitrous oxide produced in the nitrogen cycle eventually decomposes in the atmosphere into molecular nitrogen and oxygen. This source of oxygen may provide the opposite controlling mechanism by which Gaia keeps the oxygen level optimized.

In Lovelock's view, even the acidity of the atmosphere and oceans and the degree of saltiness of the oceans are under biological control. We live in an environment that is nearly neutral -- neither acidic nor alkaline. This is in fact the optimum condition for most life forms, though some have adapted to local extremes that may be found in certain environments. Most natural atmospheric processes tend toward the acidic, and the environments of Venus and Mars are in fact highly acidic. Ammonia produced in the nitrogen cycle, thinks Lovelock, serves to keep Earth's acidity in check.

In Unit 5, Physical and Chemical Makeup of the Oceans, you learned that the saltiness of the ocean has been maintained at a nearly constant level throughout geological time in spite of continual production of new salts from river input and from hydrothermal activity at the spreading ocean ridges. This level, too, is poised in the optimum range for present-day life, at 3.4%. Salt levels higher than 6% would disrupt cell walls, a fatal situation. It is not at all clear just what biological regulation mechanism might be active in this case, however.

If the Gaia Hypothesis has any validity, then it becomes critically important that we understand the mechanisms by which Gaia exerts her control. Lovelock likens them to those of a cybernetic entity, in which sensors are linked through a feedback mechanism to controls.

A modern home heating and cooling system provides an example of such a system. A thermostat is linked to furnace and air-conditioning controls in such a way that if the temperature falls below the desired range, the furnace is turned up or the air-conditioner is turned down, whichever is appropriate. If the temperature rises, the furnace is turned down or the air-conditioner is turned up. When appropriate, control may be switched from furnace to air-conditioner or vice versa. Each individual part of the system has a simple and well-defined function, but taken together, the system functions as a highly efficient regulator of the house environment, protecting it from external changes.

Such a feedback mechanism provides an important element of resilience to disruption. For instance, if you leave the house on a cold winter's night and accidentally leave the front door wide open, your family sleeping inside will not freeze to death because the house's environmental controls will spring into action, turning up the furnace to compensate for the sudden heat loss. A price will be paid for this accident when the heating bill arrives, but the inhabitants will have survived.

Many organisms possess the ability for self-regulation in order to withstand stresses produced by a changing environment. This ability, called homeostasis, is familiar in the human body's ability to regulate its temperature to within one degree Celsius of an optimum level, even though the temperature of the surrounding environment may vary over a very wide range.

With the help of an ingenious computer model, Lovelock has shown how life might regulate the temperature of a planet. His simplified Daisy World model has just three kinds of creatures: black daisies that grow better when it is cool, white daisies that grow better when it is warm, and cows -- who eat the daisies.

Suppose that Daisy World is initially quite cold. Black daisies will grow faster and the planet's surface will darken. The darker surface absorbs more sunlight and the planet begins to warm. The cows notice the black daisies and begin eating them. Meanwhile, the rising temperature allows the white daisies to grow faster. Soon, there are more white daisies than black. The planet's surface lightens again, and it begins to cool. The cows turn their attention to the by now more numerous white daisies...

Whatever the starting conditions, warm or cool, the result of the computer simulation is always the same: the Daisy World model settles down to a temperature that is at least tolerable to both kinds of daisies and, presumably, to the cows as well.

Lovelock sees in this resilience to environmental change strong echoes in the actual biosphere. For decades, environmentalists have warned of the dangers of ocean pollution through oil spills and climatic change due to thermal and atmospheric pollution. Yet in almost every case, the actual changes observed have proved to be less than those predicted on the basis of the direct consequences of the effect itself. Oil spills can have severe local consequences, but the rebound of even local ecosystems has been more rapid than previously expected. Presently, the annual build-up of global atmospheric carbon dioxide is about the half of the fossil fuel consumption. The ocean's precise amount of pick up is uncertain. The Global Change project JGOFS (Joint Global Ocean Flux Study) aims at empirical proof of the ocean's role. The present data may be interpreted to suggest that half of the carbon dioxide released into the atmosphere from fossil fuel combustion has already been removed by the oceans.

Quick recovery from environmental disaster is clearly a trait that contributes high survival value, and the self-regulating mechanisms of Gaia may well have evolved in response to such Darwinian pressures. Lovelock cites the changeover from an anaerobic to an oxidizing atmosphere as an environmental disaster of the first rank for the biota that existed in the early history of life on Earth. In this case, however, Gaia's response was not to restore the status quo but to generate new forms of life that could take advantage of this new world in which energy was more readily available. Here is one of the weakest parts of the Gaia Hypothesis. The rise of oxygen in Earth's atmosphere was clearly a stimulus for immense change, not for stability. An effective self-regulating mechanism, one would think, would have prevented it from happening. Perhaps in this case, Gaia lost control of the situation and proved powerless to prevent the onset of an environmental disaster of her own making.

From the foregoing argument, the Gaia Hypothesis might be construed as going against the grain of the environmental movement that is presently active in many countries. To a certain extent it does in that it admits of greater resilience in the response of the biosphere to environmental changes caused by mankind than previously acknowledged. If, however, Gaia truly exists, then the hypothesis indicates those features of the environment that must be preserved at all cost. These are the essential workings of the biogeochemical cycles and the huge biomass contained in the photosynthetic producers that flourish in tropical rain forests and in shallow seas on the continental shelves.

Lovelock sees an important threat to the integrity of Gaia's operation in uncontrolled human population growth. A larger human population requires more and more energy consumption, competing with the rest of Gaia for available supplies. There is also competition for space, and large-scale deforestation of tropical regions could act to cripple substantial parts of the self-regulating mechanisms of the biosphere. He also sees danger in large-scale ocean farming on the continental shelves for the same reason.

Gaia, which is the biosphere with its biogeochemical cycles and other self-regulating mechanisms, is not regarded by Lovelock as a sentient being. In speaking of Gaia as though she were an embodiment of Mother Nature, he says:

• This is meant no more seriously than is the appellation "she" when given to a ship by those who sail in her, as a recognition that even pieces of wood and metal when specifically designed and assembled may achieve a composite identity with its own characteristic signature, as distinct from being the mere sum of its parts.

Even so, Lovelock suggests that Gaia has evolved a form of intelligence in the collective intellect of humankind. We are certainly a part of the biosphere, and what we choose to do as a species is clearly of a magnitude that can bring about vast changes in Gaia's ability to function. What seems to be imperative is that we recognize the consequences of our actions. But consequences cannot be foreseen reliably without a thorough understanding of the extremely complex interactions between biosphere and geosphere, and throughout the entire web of life. Gaining that understanding, it would seem, constitutes a vital agenda for the coming decades.

G. GAIA OR COEVOLUTION OF CLIMATE AND LIFE?

It's hard for us to imagine a more original and profound concept than the Gaia Hypothesis, for which its popularizers Sir James Lovelock and Lynn Margulis deserve significant recognition. The realization that climate and life mutually influence each other is profound, and provides an important counterpoint to the parochial view of the world as physical environment dominating life; this had been the predominant paradigm in the physical sciences for many decades. Nevertheless, the fact that climate and life "grew up together" and mutually influenced each other -- the concept of coevolution7 -- is not the same thing as to say that life somehow self-optimizes its own environment. It is the latter idea, the most radical of Lovelock and Margulis, on which I feel some elaboration -- and caution -- is needed. Moreover, there is also competition at all levels of organization from cells to communities.

Lovelock wrote that the "climate and chemical properties of the Earth, now and throughout history, seem to have been optimal for life." But let's examine that a bit more. What does it mean? For example, it is pointed out that production of oxygen, a largely biological phenomenon, helped make the world fit for modern life. But at the same time, it made the world unfit for other forms of anaerobic life that had preceded it. How is this profound change an optimization? From the point of view of oxygen-loving life, it is certainly welcome! From the point of view of anaerobic life, it was a disaster that relegated these forms to small niches on Earth where oxygen is barely present.

The early physical environment largely carved out ecological niches which early life forms had. And, life altered the physical environmental constraints on itself by changing the composition of the atmosphere. This changed the competitive balance of species, and forced evolutionary change -- indeed, coevolutionary change between organic and inorganic parts of the environment. Change, yes; but optimization, I don't quite see how. Only if one defines life in terms of the best current adaptations to the current environment might life be optimizing itself. What about the losers? No doubt they won't see the current environment as having been optimized.

One of our principal difficulties with the whole idea of self-regulation of life is what "life" is. Is "life" to be optimized by maintaining for the longest period of time the stability (i.e., the survival) of extant species? Is "life" the maintenance of the maximum biomass? Is life the maintenance of the maximum diversity of species? All of these seem to us to be legitimate definitions of life, yet to "optimize" each one is inconsistent.

Consider a specific example. At the end of the Pleistocene, when the last Ice Age receded, the carbon dioxide content of the atmosphere was perhaps a third less than it is today. This implies that the weakened Greenhouse Effect would have made Ice Ages even colder then they otherwise would have been. A plausible explanation for the decrease in CO₂ during the height of the Ice Age has to do with the biochemistry of the oceans and the planktonic response to altered nutrient runoff associated with exposed continental shelves when sea levels were lower. In other words, life was altering the chemical composition of the atmosphere, the climate, and its own environment. But it seems hard to imagine how making an Ice Age even colder could be "optimization of life."

We now have considerable evidence that the biomass on Earth was some ten percent smaller during the Ice Age than now. It is reputed that Einstein once said when asked by a lay person how many experiments it would take to prove the theory of relativity: "No number of experiments can prove us right, but one can prove us wrong." Indeed, scientific paradigms often fall when some wicked fact force alteration. If carbon dioxide content was diminished during an Ice Age by life, this certainly seems to throw a monkey wrench into any general law that somehow environmental conditions were being altered by life for its self-optimization.

For us, what Lovelock and Margulis have really done is to point out the role of feedback mechanisms between organic and inorganic components of Earth. This is a major contribution and a brilliant insight. But, feedback can be a two-way street, so to speak. Feedback processes are not just interactions which tend to stabilize, but also can be interactions which tend to destabilize -- like the Ice Age-life-CO₂ case.

I believe that life and the environment have coevolved, but I also believe that those interactions have not always been optimum to all forms of life, but simply interactions which lead to mutual change -- some beneficial and some detrimental for some forms of life at some times. That alone is enough for us to sing the praises of those looking beyond the narrow disciplines of biology, climatology, geophysics, and so forth -- people who insist that the organic and inorganic parts of the planet must be viewed as coupled systems. However, it strikes us as speculation at best and environmental brinkmanship at worst to believe that somehow Gaia, through self-regulation, will protect the planet from the negative consequences of all human intervention.

Coevolution is not the same as homeostasis -- that is, self-regulation. To most scientists I know, the idea of planetary-scale homeostasis, which is the principal intellectual thrust behind the Gaia Hypothesis, is more religion than science. As religion I find Gaia deep, beautiful, and fascinating. As science, I find the hypothesis not very well formulated, hard to test empirically, and too full of contradictory examples to show much promise. Brilliant ideas are what drive future understanding, regardless of whether the initial hypotheses emerge intact. Let us then do the interdisciplinary work needed to help understand the coevolutionary path of our physical and biological worlds.

RECOMMENDED READING

For a thorough presentation of evolution refer to Price (1996) or Strickberger (1996). Introductory essays which describe evolution and the geological record are presented in the numerous books of Gould, including, but not limited to, Gould (1996) and (1977). Detailed and highly interesting descriptions of models for the origin of life and the evolutionary process are presented in de Duve (1995), Dawkins (1987) and (1996), Cairns-Smith (1982) and (1971), Bengtson (1994), Benner et al., (1993), Majerus et al., (1996), Miller and Orgel (1974), Stebbins (1982), Vidal (1984), and Stanley (1981). The larger picture of the Gaia model is well described in Schneider and Londer (1984), Schneider and Boston (1991), and Lovelock (1979) and (1988). Models of ecological management are presented by Chiras (1985), Goudie (1992), Kormondy (1984), Miller (1994) and Williams (1993). For additional information describing the fascinating life of Charles Darwin refer to the excellent biography by Bowlby (1990) and review a couple of Darwin's own writings, such as Darwin (1988) and (1859).

• Stefan Bengtson, editor, Early Life on Earth, Nobel Symposium No. 84, Columbia University Press, 1994.
• S. A. Benner, M. A. Cohen, G. H. Gonnet, D. B. Berkowitz, and K. P. Johnsson, in, The RNA World, R. F. Gesteland and J. F. Atkins, editors, Cold Spring Harbor Laboratory Press, 1993.
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