Paul R. Ehrlich & Anne H. Ehrlich, The Population Explosion, 1990.


Aspects of How Earth Works

A lack of basic background on the history and functioning of our planet is a major barrier to many people's understanding the seriousness of the population/resource/environment crisis. It's a pity, for the history and functioning of our planet and our population are fascinating subjects, as well as being crucial knowledge if we are to increase our individual and collective chances of survival. Here we expand on some background topics that are important to understanding the influence of human population growth in shaping our future.

Earth, the only known life-bearing planet, is a marvelously intricate self-sustaining system. During the four billion or so years that life has existed here, it has evolved into millions of distinctly different kinds of organisms. Over the ages, these evolving life forms have reshaped the planet itself in a variety of ways to make it more hospitable. For instance, the very composition of the atmosphere -- the oxygen-rich air that animals breathe -- is a result of the activities of organisms, primarily green plants, over eons of Earth's history. Nonhuman [253] organisms are still active in maintaining a favorable balance of atmospheric gases.


The sun provides virtually all the energy that powers the Earth system -- both living and nonliving components.1 Green plants and some microorganisms (which from here on we will often simply call "green plants"),2 both in the oceans and on land, are able to capture the energy in sunlight in a complicated process known as photosynthesis and convert it into chemical energy. In this process, they extract carbon dioxide from the atmosphere and combine it with water to produce carbohydrates (sugars, starches, and cellulose). Some of the sun's energy is captured in the bonds that hold these large chemical molecules together. The "waste" product of photosynthesis is oxygen; if it were not for green plants, Earth's atmosphere would have no oxygen.

The green plants use some of the energy "fixed" in this fashion in the chemical bonds of carbohydrates for their own life processes -- maintenance, growth, and reproduction. The energy stored in green plants is the foundation of life for all other organisms. All animals and decomposer organisms gain their energy by feeding on green plants, directly or indirectly. And they use the plants' waste oxygen to extract energy efficiently from their food and drive their own life processes.3

Green plants, in addition to providing the energy to support the rest of the living world, also make proteins and other relatively complex organic chemicals (such as vitamins), using not only carbon dioxide and water but a variety of other minerals obtained from soil or (by aquatic plants) water. These elements, needed by most forms of life and made available by plants, are known as nutrients.

Some nutrients are quite common in the Earth system (carbon, nitrogen); others exist in smaller amounts (sulfur, iron, calcium) or in tiny trace amounts (copper, selenium). All are required by most life forms, including ourselves. Because they are essential for life, the global community of organisms cycles these materials in complex paths through the living and [254] nonliving parts of the system. The green plants -- producers -- first provide energy and nutrients to animals that feed on these plants, and these animals in turn pass them on to animals that eat the plant-eaters, and so on. All of these are consumers, and this series of steps from producers to plant-eaters and those that devour them is known as a food chain. At each step in a food chain, decomposer organisms feed on wastes and dead bodies of both plants and animals, digesting them and returning the elemental nutrients in them to the soil or the body of water, where they will again be available to green plants.4

Biotic Communities

This picture highlights the interdependence that indeed exists among life forms. Groups of plants, consumers, and decomposers in any one place are known as communities. Often, the plants, animals, and microorganisms that coexist in a community have evolved together over a long period of time.

Natural communities are relatively rarely plagued by "pests." Most ecosystems are composed of thousands of different species of plants, animals, and microorganisms; and the existence of several potential predators and parasites for each organism, whether herbivore (plant-eater) or carnivore (animal-eater), usually prevents the buildup of any population to pest levels. Despite the rarity of pest outbreaks in ecosystems, producers run the risk of being consumed completely, which, of course, would be the end of the community.

Green plants can't run away, but they are by no means defenseless. A plant species can hide among other plants by having a widely scattered population, which defeats animals and plant diseases that specialize on it. This creates some problems for the plants' reproduction, but various devices, such as relying on the wind, water, or animals (birds, insects, mammals, etc.) to transfer pollen and disperse seeds, have been developed to overcome this disadvantage. Camouflage and mechanical defenses, such as thorns and tough skin, also discourage consumers. And many plants, having coevolved with their consumers for ages, have developed a great variety of chemical defenses -- poisons and noxious odors and flavors -- to protect themselves. [255]

Animals too have evolved numerous ways of defending themselves against predators, some of which resemble the plants' strategies. They hide, rely on camouflage, use poisons and noxious substances (sometimes obtained directly from plants), wear armor (clams, beetles, turtles, and armadillos), live in large groups (herds of antelope, schools of fishes), fight, and escape by flying, swimming, or running away.

At the same time, predators have evolved ways to overcome the defenses of their prey. Herbivores have ways of finding their preferred foods and of circumventing physical and chemical defenses. Plant-eating insects, with their short generation times, are especially adept at rapidly evolving resistance to the poisons employed by plants. Predators on animals too have evolved ways to overcome the defenses of their prey: by detecting hidden quarry, pursuing it, or cutting vulnerable individuals out of the herd. Plants and herbivores, like predators and prey, are often involved in "coevolutionary races," in which the winner gains access to rich resources and the loser faces extinction.

Coevolution has produced any number of intimate relationships among organisms besides that of predator and prey. Many species compete for the same or similar resources, such as food, water, or living space. But competition for food may be reduced by specializing on slightly different prey organisms, or by feeding at different times or in different places.

Another kind of coevolved relationship is mutualism -- in which two very different organisms may coexist in mutually beneficial ways. Examples include the algae and fungi that coexist as complex structures called lichens; the algae obtain energy through photosynthesis, and the fungi supply protective covering and storage capacity and extract nutrients from rock, soil, or air. Somewhat less intimately, a honeybee obtains nectar in exchange for transferring a plant's pollen. Another very important case is certain kinds of bacteria that live in nodules on the roots of legumes (members of the pea family), capturing atmospheric nitrogen and converting it to a form usable by plants in exchange for sugar provided by the legume. This particular mutualistic relationship thus plays an important part in one of the most critical nutrient cycles operated by ecosystems. [256]

Many coevolutionary interactions occur in the most complex ecosystems on Earth, humid tropical forests, which contain by far the greatest diversity of organisms. Perhaps 40 percent or more of Earth's complement of species are found in those regions, although they occupy only a small fraction -- less than 7 percent -- of the world's land surface.6


The organisms that comprise a community and the physical environment with which they interact -- including soil, water, atmosphere, terrain, etc. -- are collectively known as an "ecosystem."

While both nutrients and energy move through the food webs (interwoven food chains) of an ecosystem, they follow paths that differ in one important respect. Nutrients are continually cycled through an ecosystem and are mostly retained within it (although major parts of the cycles may occur in the nonliving portions of the system). They make up the great "biogeochemical cycles" of carbon, nitrogen, sulfur, potassium, and so on.7 When an animal dies, for example, the materials of its body are broken down by decomposers and the nutrients are released to soil, water, or air, from which they again become available to plants. Energy, by contrast, moves only one way through food chains. Usually, most of the energy is lost at every step, because each organism uses some of it to operate its own life processes, and when energy is used, some of it is always degraded to less available forms.8 As a rule of thumb, only about one tenth of the energy present at each level in a food chain is available to the consumer at the next step upward.

This loss of usable energy has crucial implications for the way ecosystems are organized. A diagram of energy availability is shaped like a pyramid, with the greatest amount flowing into the green plants, much less being available for the plant-eaters (herbivores), and rapidly diminishing quantities going into each level of meat-eaters (carnivores). Likewise, the weight of living material at each level in an ecosystem is usually pyramidal.9 The green plants at the base have the greatest mass, and succeeding levels of consumers are proportionately smaller, with [257] the predators at the tops of food chains generally having the least living weight (biomass). There is a greater weight of grass than of antelopes, a greater weight of antelopes than of lions, and a greater weight of lions than of ticks that live on the lions. The small amount of energy available at the upper ends of food chains limits the population sizes of animals in those positions. Human beings get an important part of the protein for their diets by feeding high on food chains. One likely consequence of the explosive growth of the human population is that Homo sapiens, on average, will be forced to find more of its food lower down -- that is, become more vegetarian.

A Tamed Planet

The taming of Earth, a planet originally very hostile to life, was a long and arduous process. For most of the nearly four billion years Earth has existed, life was confined to the oceans. In those oceans, the first photosynthesizing organisms were bacteria. Very gradually, over billions of years, the photosynthetic activities of oceanic organisms built up the concentration of free oxygen -- that waste product of photosynthesis -- in the oceans and the atmosphere.

The slow buildup of oxygen in the atmosphere made it possible for life to survive on land in two ways. One important change was the availability of oxygen in the air for respiration -- the main process by which energy in carbohydrates is released and used -- by both plants and animals. Respiration is the slow "burning" process by which oxygen is used to release the energy stored in chemical bonds; it is, in essence, the opposite of photosynthesis, resulting in an uptake of oxygen and a release of carbon dioxide.

The other way was less obvious, but was essential for life to flourish on land. Some of the oxygen reacted under the influence of ultraviolet radiation to produce an unusual form, ozone, which has three oxygen atoms joined together instead of the two found in a normal oxygen molecule. Ozone formed a diffuse layer high in the upper atmosphere, the stratosphere, where it absorbs the sun's radiation in the near-ultraviolet part of the spectrum.10 Ultraviolet radiation of those wavelengths is oroadly damaging to most life forms, and until it was screened [258] out by the ozone shield in the atmosphere, organisms could survive only under water, which also filters out ultraviolet.

In the photosynthetic process, carbon was taken (as carbon dioxide) from the oceans and the atmosphere and incorporated into the tissues of organisms.11 Much later, some of this carbon was isolated from the natural carbon cycles for eons as undecomposed dead plants and microbes were buried and preserved by geological processes. This had two important consequences: the accumulation of oxygen in the atmosphere was accelerated, and the preserved carbon-rich organic material was slowly transformed into fossil fuels.

Once established on land, plants, animals, and microorganisms evolved into a diversity of new forms and in turn substantially modified their new environments. The process was anything but smooth, however. Continents move around the planet's surface on gigantic "plates" in a process called plate-tectonics.12 The drifting continents crept from one climatic region to another -- from the tropics to the poles -- subjecting their living passengers, over time, to enormous changes in climatic regimes as well as to physical changes in Earth's surface and varying sea levels.13 The result of all these slow environmental changes was a further diversification of organisms as they evolved in response to changed conditions. The distribution of mineral deposits beneath Earth's surface today also is the result of billions of years of this tectonic movement and associated geological phenomena, such as volcanic activity and mountain-building.

Ancient Extinctions

In addition to the gradual environmental changes induced by the extremely slow geological phenomena such as shifts of the continents, relatively rapid and severe environmental changes apparently occurred on several occasions, with catastrophic consequences for the life forms existing at those times. The best-known and most recent such event took place some 65 million years ago, when the dinosaurs disappeared from Earth along with a wide array of other kinds of organisms.14 It is not clear from the fossil record whether this massive extinction event took place over a few hundred [259] thousand years in response to a widespread change in climate or other environmental factors, or whether it occurred much more suddenly as a result of some externally caused cataclysm.

Possible causes for such a catastrophe include Earth's being sideswiped by a comet with a very erratic orbit or struck by an unusually large meteorite, either of which could have hurled billions of tons of dust into the atmosphere, blocking out most of the sunlight for several months. The darkness and lower temperatures would have killed or severely damaged most green plants and decimated the animals dependent on them both on land and in the sea. The fireball resulting from the impact would have produced enough nitrogen oxides to destroy the ozone layer completely, subjecting Earth's surface to a deadly flux of ultraviolet-B radiation.15 A large fraction of the life forms existing then, even major groups of plants and animals, could have perished from such conditions.

The consequent gross impoverishment of flora and fauna would have led to profound changes in communities and ecosystems, and the recovery of life to its former luxuriance took a very long time. Climates apparently changed substantially around that time, favoring new directions for evolution. The absence of many kinds of organisms also opened new opportunities for those that had survived. Thus the disappearance of the previously dominant dinosaurs is thought to have opened the way for diversification of the previously obscure mammals, a diversification that eventually produced humanity.

Whether these wrenching changes in Earth's flora and fauna occurred abruptly or over many thousands of years, their long-term result is our own existence and the assemblage of species that now share the planet with us. Indeed, this episode in the evolution of our planet not only led to the appearance of human beings but put the finishing touches on our hospitable environment. Many of the kinds of organisms and communities most familiar to us, and on which we have based our civilization, also appeared during the past several million years. Thus gradually, over its long history, Earth has been dramatically changed by the evolution of hfe itself, making it increasingly habitable (from the human point of view). [260]

Vegetation and Climate

But, unbeknownst to most people, the process continues. Natural ecosystems are still actively engaged in maintaining the planet's habitability -- making it possible for over 5 billion people to survive and a billion or two to thrive. Other organisms are functioning parts of those natural ecosystems; to the degree that we exterminate them, we imperil the capacity of Earth to support us and our descendants.

The community of life on Earth generated and still maintains the composition of the atmosphere. Green plants capture and make available the energy that sustains all other life forms. And organisms of all kinds participate in the great biogeochemical cycles. These movements of materials through the living and nonliving parts of the biosphere constantly renew supplies of essential nutrients, replenish soils, and dispose of wastes. But several other life-supporting functions are also carried out continuously by members of living communities.

The climate of any given place on Earth is governed principally by the amount of sunlight received at the surface, which in turn is determined by latitude and season, and the circulation patterns of the atmosphere and the oceans, which are affected by the locations and shapes of landmasses. But climate, particularly over land, is also heavily influenced and moderated by the kind of vegetation present. This determines its surface reflectivity and thus its surface temperature. The cycling of water from land surfaces to the atmosphere and back to the surface in precipitation is regulated to a large extent by forests and other plant communities.

Similarly, the fate of water that reaches the land surface is very largely determined by the presence and type of plant community there. Forests are most efficient at capturing, retaining, and recycling water. Tropical rain forests, such as cover the Amazon basin of South America, capture and recycle prodigious quantities of water. Less than half of the rainfall runs off into the rivers -- and the mammoth size of the Amazon and its tributaries can give a small idea of how much water does fall there. Most of the moisture is reevaporated from the trees, forming the almost constant cloud cover characteristic of the region. [261]

Forests and dense grasslands can keep even heavy rainfalls from running off rapidly; instead the water sinks deep into the soil, and some of it recharges underground stores of fresh water (aquifers). Soils in such areas typically remain moist for long periods between rains, and water is released slowly in springs and steadily flowing streams. Sparse vegetation, by contrast, such as one sees in semiarid regions, is much less efficient at retaining moisture, which tends to run off in floods, often carrying soil with it (especially from steep slopes). Between rains, such areas are frequently subject to drought. These conditions are most extreme in areas where land is bare, such as the driest deserts or as a result of human intervention, as in clear-cut forest areas, overgrazed ranges, or fallow cropland.

Of course, we have just touched on a few aspects of the functioning of the global ecosystem here. More detailed information can be found in references cited in the Notes.


1. The major exception is the energy of radioactive decay which drives tectonic movements.

2. Other photosynthetic organisms include several kinds of bacteria, algae, and some algae-containing mutualistic associations such as lichens and corals.

3. Plants also oxidize carbohydrates to extract the energy from their chemical bonds and use it to grow, repair themselves, and reproduce. One can think of the oxidations that go on in the cells of organisms as the equivalent of slow chemical "fires" -- since fires transform energy by rapid oxidation.

4. Much of the material on ecology in this appendix is expanded upon at a nontechnical level in P. R. Ehrlich, The Machinery of Nature (Simon and Schuster, New York, 1986). A more technical treatment can be found in P. R. Ehrlich and J. Roughgarden, The Science of Ecology (Macmillan, New York, 1987).

5. Coevolution is simply the reciprocal evolutionary interaction of two kinds of organisms that are ecologically intimate, such as herbivores influencing the evolution of plants and vice versa (see Ehrlich, Machinery of Nature, chap. 4). For more technical information see P. R. Ehrlich and P. H. Raven, "Butterflies and Plants: A Study in Coevolution," Evolution, vol. 8, pp. 506-8 (1964), and P. R. Ehrlich, "Coevolution and the Biology of Communities," in K. L. Chambers, ed., Proc. 29th Ann. Biol. Colloq. 1968 (Oregon State Univ. Press, Corvallis, 1970). More recent reviews are: D. J. Futuyma and M. Slatkin, Coevolution (Sinauer, Sunderland, Mass., 1983), and K. C. Spencer, ed., Chemical Mediation of Coevolution (Academic Press, New York, 1988).

6. The reasons there are so many more species in the tropics than in other regions are poorly understood. For a discussion of current conjectures, see P. R. Ehrlich and J. Roughgarden, The Science of Ecology, pp. 400-402.

7. For details on biogeochemical cycles see P. Ehrlich, A. Ehrlich, and J. Holdren, Ecoscience: Population, Resources, Environment (Freeman, San Francisco, 1977).

8. This loss of useful energy is described by the second law of thermodynamics. For more details see Ehrlich, Ehrlich, and Holdren, Ecoscience.

9. We say "usually" because if the producers in a food chain are small organisms such as oceanic phytoplankton, they can live and die so fast that they will have a smaller biomass at any given time than longer-lived organisms higher on the food chain. High turnover, in this case, substitutes for high biomass; and the pyramid of biomass is turned upside down. More energy is still flowing into the lower level, however, and the energy pyramid is right side up.

10. Ozone high in the atmosphere serves as a critical shield for life; near the surface (where it is produced by human activities) it is also a pollutant that is damaging to plants.

11. Carbon, of course, was incorporated into organisms by other processes before photosynthesis evolved.

12. At least once, probably several times, the continents were melded into one huge supercontinent, then slowly broken up into the six major land-masses existing today.

13. For a summary of plate tectonics, see Ehrlich and Roughgarden, The Science of Ecology.

14. For more on extinctions and the dinosaurs, see Ehrlich and Roughgarden, op. cit.

15. As shown by a recent computer simulation by Starley Thompson and Paul Crutzen (S. H. Schneider, personal communication, July 1989).