Callenbach himself tells us the purpose of this book from the very first page:

This little book provides a compact introduction to the fundamental concepts of ecology, the science that studies the marvelously complex interrelationships of life forms on planet Earth. These concepts are the foundation of the environmental movement, which aims to reduce or remedy damage caused to the natural order by humans and gives inspiration for changes in practically every aspect of peprsonal and family life, business management, and community or government policies.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 1)

The book itself is organized as a dictionary containing only the most basic and fundamental concepts of ecology, always discussed from the point of view of someone who obviously shares the principles of the enviromentalist movement. In other words, the author never intended —it is clear to the reader— to discuss the issues from an objective or balanced point of view (i.e., allowing for equal time to the differing views). On the contrary, Callenbach intended this book to be used as a pocket book for anyone interested in viewing the world from an ecological viewpoint.

Still, even people who have an interest in sciences in general and life sciences in particular will no doubt benefit from some of his insights. For example, when discussing the concepto of bacteria, and right after pointing out that they inhabit our planet in trillions upon trillions and how they can be considered the original gene splicers, Callenbach explains:

As a result, all thw world's bacteria essentially have some access to a single gene pool and hence to many ways of living in an ecologically changing world. We might consider them one nearly four-billion-year-old global super-organism, ancient and durable. By comparison, the brief story of humanlike creatures on Earth encompasses about three million years —a mere blip on the planet's time chart. Humans as we see each other today are known to go back only 40,000 years, or at the most, as some recent archaeological findings suggest, 90,000. Earth belongs, then, not to the thundering herds of grazers, the roaring carnivores, the soaring birds of prey, or even to clever, cunning humans, but to the subvisible bacteria in their uncountable numbers, who have been here since life began.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 14-15)

It is one of those facts that puts things into perspective. The big has plenty of them.

A good example of the author's militant approach to the issues covered in the book is the entry on biodiversity:

An ecosystem has greater diversity when it contains more species. We call it impoverished when the number of species is diminished. This happens when a grassland with dozens of types of grasses and flowers, hundreds of insects and birds and small mammals, and innumerable microbes is turned onto a city or suburb whose paved and built-on land can support mainly rats, pigeons, English sparrows, Bermuda grass, a few kinds of trees, and cockroaches —hardy species that find ways to coexist with large numbers of humans.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 15)

Mind you, not that he does not have a point, but plenty of people might be automatically turned off by such overly pro-environmental stance. On the other hand, why would anyone who does not care about the topic would even approach the book? I suppose what I am trying to emphasize is that you should make no mistake, this is not a scientific approach to the topic of ecology, but rather a concise description of the main concepts to help you understand the field of ecology from an environmentalist point of view. And it does an excellent job at that.

The entry on carrying capacity contains a clear warning to modern-day society that, unfortunately, is often ignored:

We're sometimes tempted to believe that, because we are a clever species, we can use technology to escape carrying capacity limitations and perhaps reach a human population of ten billion or more. But the huge human population on Earth now is almost certainly temporary; such expansion of our numbers has been possible solely because we have tapped a limited store of fossil fuels and used their energy to produce food, shelter, and goods. For every calorie of energy we get from our food, industrial agriculture puts between four and twenty calories of petroleum energy into fertilizer, equipment fuel, pesticides and herbicides, processing, and shipping. We are, in effect, eating oil.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 24)

It is an argument that we have heard quite often in the last decade or so. It looks as if we are reaching the summit (perhaps the end?) of the anthropocene era on this planet, marked by a cancerous expansion throughout the whole place. In spite of all the warnings, it looks as if we are not convinced yet that our lifestyle is wasteful, unsustainable and, in the middle-run (not even the long-run!) may lead to our own extinction. The human hubris and attachment to old habits is such that it certainly looks as if we will not try to react until a major catastrophe is already here and, by then, it may already be too late. The fact is that nature moves at a very slow (but constant) pace, in the long-run, while our mind only seems to be able to comprehend the short-term. That is the challenge.

Part of the problem, of course, is the way we organize ourselves, a topic briefly touched upon by Callenbach in the entry for the term community:

In mordern human communities we organize our mutual interdependence through massive cities, corporations, and nations. Some of these institutions foster behaviors rarely found in nonhuman communities. Only a few other species, such as certain ants and our close relatives the chimpanzees, engage in large and serious enough skirmishes to say that they go to war. Individual organisms compete for food and mates and establish dominance hierarchies, but our chronic human divisions between groups of rich and poor, the powerful and the oppressed, are our specialty. It appears, then, that human communities have much to learn from the natural world about living in some degree of harmony.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 26-27)

The key difference, obviously, is the fact that we view the Earth and its resources as tools at our disposal, while animal communities see themselves as integrated in a larger ecosystem. That is the key mental concept that we need to change. That is the philosophical shift we need to promote if we are to survive ourselves. And, by saying this I do not necessarily imply that a human life is worth as much as the life of an insect, as some critics would have it. No. However, we do have to learn to see our own life as a part of a larger story. In that sense, the time has come to put into question the old humanism we inherited from the Renaissance, as well as the human-centered approach to religion that makes us gaze at our own navels in constante amazement. I always find it quite interesting that it is precisely those people who criticize the environmentalist movement of "moral relativism" and attempting to equate the human life with that of a gorilla that apparently have no problem whatsoever to justify the murder of millions of fellow humans in a war to defend a flag or a particular god. Where is the moral relativism, then?

But, together with all these philosophical, social or political considerations, the book also has some scientific content:

Of life's cycles, these are the most fundamental:

• The water cycle provides the supportive surroundings in which life can exist. The bodies of all living beings are largely water.
• The nitrogen cycle provides an element needed for cells to build their proteins and genes.
• The carbon cycle provides another essential material for cells and helps to regulate atmospheric temperature.
• The sulfur cycle also helps to regulate global temperatures, in addition to providing an essential element in all living cells.
• The phosphorus cycle provides material for cell membranes, genes, teeth, and bones.

The acronym CHNOPS [pronounce the C as an S] is a handy way to remember that all organisms contain fixed proportions of the elements carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur —a powerful indication of the close evolutionary connections between all forms of life.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 29-30)

And we make it to the concept that glues it all together (i.e., ecology):

Today, almost everyone has at least a vague idea of what "ecology" means —that it has something to do with human impacts on nature. Actually, the science of ecology studies all interactions among living beings and their environment, whether we humans are involved or not. Air and even some rocks that function as part of life's cycles are included too. Ecology is a study of patterns, networks, balances, and cycles rather than the straightforward causes and effects studied in physics and chemistry. The goal of ecology is to understand the functioning of whole lving systems, not simply to break them down into component parts for analysis. When ecologists look at an earthworm, they aim to understand its functions within its evosystem —the surrounding atmosphere, soil, plants, dead material, competing species and predators, decomposition organisms, moisture, and other factors.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 34)

The entry for the term environment contains a very clear example of how this works:

Ecology, the study of relantionships among organisms and between organisms and their surroundings, lets us see the interconnections and processes that really make up "the environment" and gives us a more fundamental reason to protect it. When we eat an apple, we're taking a part of the environment and putting it inside our bodies for a brief period. The apple came from a tree that gathered the richness of sun and soil, and ran and air. After we digest the apple, extracting the nutrients our bodies can use, our wastes become the food of microbes in sewer plants or marshes and thus, along with the apple core if we compost it, microbes and their wastes reenter the cycle of plant growth. When we breathe, we take in oxygen that phosynthetic plants and microbes have produced and breathe out carbon dioxide, essential to these plants and microbes. While we are alive, we make use of material resources from the environment. We may think we throw garbage and trash "away", but they go around again and again. We are the environment, and it is us.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 46-47)

A key component to understand how an ecosystem works (as a matter of fact, I would propose that a key concept to understand pretty much any system, including social and political ones) is that of energy:

Only two sources of energy power the metabolism of living cells: sunlight received by some bacteria, some protists, and most plants, and certain kinds of chemical energy: sugars and other carbon compounds are energy sources for animals, plants, protists, and bacteria. Certain special bacteria draw energy from hydrogen and some other chemicals, but heat, radioactivity, electromagnetism, sound waves, and other kinds of energy cannot power any life form.

When humans tap fossil-fuel energy, we're using a limited supply of solar energy stored by living organisms in the distant geological past. Nuclear power, which harnesses the purely physical process of nuclear fission to provide a small portion of our electricity, theoretically makes it possible to escape dependence on the sun. In the long term, use of nuclear power is unlikely because it is much more expensive than fossil-fuel power, and without subsidies it cannot even compete economically with renewable energy from solar and wind power —and of course it exposes us to the risk of catastrophic accidents and creates radioactive wastes with grave health dangers.

Until the twentieth century, we had an easy definition of energy: capacity to do "work," like running a machine or moving objects around. But the modern physics of energy and subatomic particles has presented us with a world both startling and mystifying. All seemingly solid objects, we now know, are mostly empty space. The incredibly small protons, neutrons, and electrons that make up matter are separated by vast distances: if we imagine an atom's nucleus greatly enlarged, to a millimeter in diameter (less than a sixteenth of an inch), the outer electrons of that atom would be the length of a football field away. Whirling around so fast that they present apparently solid surfaces of leaves, bodies, or rocks, these particles are themselves constituted of subparticles whose masses are only manifestations of energy —a little like musical tones, which vibrate the air and our eardrums. Sometimes subatomic entities have the character of a particle (a little lump), but at other times they act like a wave (of light, for instance).

Despite these strange discoveries, we know how energy behaves, both in general and in living systems:

• First Law of Energy: Energy cannot be created or destroyed, merely changed from one form to another. Burning a log for heat does not create energy; it just liberated energy previously stored by a tree. To move your hand, your muscles use energy stored in your body by eating food.
• Second Law of Energy: When energy is converted from one form to another, some of it ends up in a more dispersed and less useful form —normally some kind of heat. In a car engine, only about 20 percent of the high-quality energy in gasoline is converted into movements of pistons, gears, and wheels. The other 80 percent goes into heat that is distributed, largely by the radiator, into the air. In addition, half of the 20 percent available for powering the car is used up overcoming friction in gears and tires. So a tenth or less of the original fuel energy actually moves the car. Similar losses occur when living beings use energy.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 41-43)

The entry on the concept of progress sums up what we have already leaarned after a couple hundred years of relentless marching towards the edge of the cliff:

In our dreams of "progress", Western peoples tend to imagine that evolution must have headed toward perfection —which some would like to identify with humanity. However, evolution is a process entirely without preplanning. It just happens. No one species is "better" than another; hierarchies of beings are purely a human invention. Instead each species alive today is as successful as any other, simply because it's alive.

All life forms descended from common ancestors and thus are related. It is not metaphorical to speak of our kinship with other animals or plants, or even with subvisible beings. In our fetal development we traverse the same path as other mammals; we share skills and brains with birds and fish; we share with bacteria genes made of DNA. Our life goes back all the way to the original bacteria from which life sprang, our ultimate ancestors.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 50-51)

Yet, as Callenbach warns somewhere else, it is not necessarily these activities done in the name of progress that are always to blame for the worst ecological damage:

You may have been horrified to look down into the utter desolation of a milewide pit mine or see a dry riverbed from which all the water has been diverted for irrigation. But the most universally devastating ecological impacts of human activities come from the seemingly innocent or sometimes even picturesque activities involved in our taking possession of almost all biologically productive land. We convert plains and prairies to farming and livestock raising, eliminating native grazing animals such as bison and antelope. We bring in foreign grasses and then overgraze them. We clear-cut forests and replace them with sterile single-species tree farms in which only a few animals and birds can live. We pave over fertile, low-lying land for city streets, highways, and buildings —destroying the original plants and making the area unfit for habitation by wild animals, birds, or even many insects.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 76-77)

In reality, the key concept in ecology is that of interrelationship and, ultimately, we all depend on organisms that we tend to willingly ignore due to their small size:

All food webs depend on the Earth's only truly productive organisms: photosynthetic (or chemosynthetic) bacteria, together with algae and plants —organisms we call producers. Consumers include most bacteria, most protists, most animals, and a few carnivorous or parasitic plants that feed on the producers or on each other. Decomposers are mainly bacteria and fungi that eat dead organisms and return their nutrient components to the life process. Producers have the greatest biomass (total weight). Decomposers come next, and consumers —which includes humans— are third. We must remember that large, visible, often dramatic animals, whether tigers or humans, only exist because of billions of leaves on plants and blades of grass, and billions of microbes that recycle their droppings along with uneaten dead plants.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 58)

In the end, although it is too easy to forget, all energy that fuels life on Earth comes from the Sun:

All life on Earth ultimately depends on light from the sun for energy and carbon from the air for food. But only specialized living beings —photosynthetic bacteria, algae, and plants— can accept sunlight energy and use carbon to make food in the form of their bodies. These are the sole truly productive organisms on Earth, and all other forms of life in their vastly complex ecological interrelationships are utterly dependent on them. (This is true even of bacteria that live on the pitch-dark ocean floor and a few other types that live by reacting photosynthesis-derived oxygen with compounds of sulfur, nitrogen, iron, and other elements.) If we look back over the whole history of life on the planet, the evolution of photosynthesis by early bacteria was the most important single event. Without photosynthesis, Earth would have remained a dead planet.

(Ernest Callenbach: Ecology: A Pocket Guide, pp. 85-86)

Yet, small as it is, these life forms have been around for very long. They should legitimately be considered the very foundation of life on the planet. We ignore them at our own peril. As a matter of fact, the overuse of antibiotics may bring them to the fore sooner or later:

Bacteria have been transferring genes almost since life began, and they're extremely good at it —which can be bad news for humans. When drug-resistance genes from harmless bacteria transfer into disease-producing bacteria, the result is a resistant strain that our antibiotics cannot kill. Because bacteria pass genes around quickly, such drug resistance can spread rapidly. Most antibiotic drugs discovered in the past fifty years are now markedly less effective.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 104)

The entry on urban ecology is quite interesting too, for it shows not only the negative, but also the positive side of life in large cities, which is definitely not the typical imafge we have of the environmentalist:

Contemporary cities wipe out virtually all native plant and animal life within their occupied areas. Nonetheless, on a per person basis, they actually cause less ecological destruction than country or dispersed suburban living. Stacked apartments waste less heating energy through their walls and ceilings than separate houses of the same floor space; they require much less piping and wiring. City dwellers walk more, use public transportation more, and have fewer cars and drive less. City services like mail and grocery distribution have shorter routes and thus require less time and less vehicle fuel per person served. Thus the ecological footprint of a city person is smaller than that of a country dweller or suburbanite.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 124)

There are also different ways to organize life in the city. We do not have to continue repeating old and tired ways. There is room for improvement:

A modern city does not generate food or materials on its own, but it could. Older cities were full of productive small gardens and some, like Paris, actually exported food. An intensively planted garden only 10 feet square can provide a whole family's vegetables for the summer. Small urban forests can generate sustainably harvested wood. And cities can recycle almnost all the materials they use: metals, stone, wood, cement, asphalt, paper, even cloth fibers. In rebuilding themselves for energy efficiency and for human convenience, cities will be doing just what a natural ecosystem does: as it matures, it devotes more energy to maintenance and repair and less to growth.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 125)

Finally, in the postscript, Callenbach reflects upon the changes that have happened recently in the world of science and research, which have come to show a progressively complex picture where all components depend on each other:

But in the last fifty years, something extremely strange has happened to these formerly straightforward-appearing ways of understanding and controlling the world. With the more sophisticated analysis possible through modern science, we've learned that the world is in reality more a fuzzy network od interconnected energies than a set of separate objects with neat mechanical relationships. There is no such thing as a thing —that is, a separate, disconected, independent thing. Not only in biology but also in physics, the world is now described as made up of complicated overlapping and interacting patterns. The apparently solid objects and beings we see around us are in fact mostly empty space, in which systems of energetic patterns manifest themselves. There is no fixity or permanence. In mathematics, a special field now deals with chaotic phenomena. All is constant change, cycles without end, the birth and rebirth of stars, rocks, trees, humans, and microbes —in short, the ecological phenomena described in this book.

(Ernest Callenbach: Ecology: A Pocket Guide, p. 144)

Altogether, Ecology: A Pocket Guide is an excellent book if you need a quick introduction to the field or if you just need a short volume to refresh the most basic concepts of ecology every now and then. Said that, it definitely is an introductory (and brief) book for the lay person, not a guide for the expert (neither is a volume for the scientist, who should choose instead one of many excellent textbooks out there). In any case, I highly recommend it if you are interested in the topic.

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