Science Matters (39 page)

Most biologists study life by tackling a small, manageable system—one organ, one cell, or even one molecule. But living systems never occur in isolation. Life requires the complex interaction of many organisms with their surrounding environment. Organisms cooperate and compete, eat or are eaten. Life on Earth, along with its nonliving environment, functions as a unit, obeying all the physical and biological principles described in earlier chapters. You have to study the whole integrated system if you want to understand our planet, and this is where the science of ecology (a word derived from the Greek term for house) enters the picture. Ecologists study ecosystems, so they concern themselves with all the organisms in a given area and their physical environment.

An ecosystem encompasses no fixed size. Almost any chunk of our planet that includes minerals, air, water, plants, animals, and microorganisms that interact will qualify. An ecosystem could be a swamp, a square yard of meadow, a sand dune, a coral reef, or an aquarium. Natural ecosystems seldom have sharp boundaries: forests merge into fields, shallow water grades into deep water.

Within an ecosystem, each organism fits like a gear in a complex machine. Each organism depends on its fellows, but performs necessary functions for them as well. Termites in a forest depend on trees to produce deadwood, for example, and the trees depend on the termites to clear the ground for new seedlings. The special place occupied by an organism in an ecosystem is called its ecological niche.

All living things on our planet exist in a thin layer at the surface, a layer that extends a few miles below the solid surface and a few miles into the air. We call this region the biosphere, and it can be thought of as Earth’s largest ecosystem.

One rule seems to emerge from studies of ecosystems, a rule that follows from the complexity of the web that connects living and nonliving things. It can be stated simply:

More grandiloquently, it is:

No matter how it’s stated, the rule comes down to this: in a complex system it is not always possible to predict what the consequences of any change will be, at least with the present state of knowledge. This means that seemingly small changes in ecosystems can cause large effects, while huge changes might leave the system pretty much as it was.

Having made this point, we should also note that life on Earth has survived many wild swings in environment in the past. Nature itself is constantly changing the global environment, so that change in and of itself is not necessarily a bad thing, and it’s certainly not unnatural. Nevertheless, the fact remains that we cannot presently predict with certainty what the ultimate effect of any given change will be.

The sun provides the primary source of energy for life on Earth. Plants, plankton, and other green life use this radiant energy to convert carbon dioxide and water to energy-rich chemicals—simple carbohydrates—by the process of photosynthesis. Plants harness that chemical energy to produce the more complex
molecules—proteins, lipids, and sugars—from which leaves and stems and flowers are made. Plants and other photosynthetic organisms are self-sustaining life. They are the primary producers of energy-storing molecules used by all living things, and scientists refer to them as the first trophic level in the environment. It is in the first trophic level that the food chain begins, and it is this level that supplies energy to all living things.

Animals, fungi, and most bacteria can’t convert solar energy directly into the molecules they need to sustain themselves, so they seize their energy by eating other life-forms. Plants provide the energy source for the primary consumer level of the food chain, which includes grazing animals, caterpillars, and vegetarians, as well as a host of species from bacteria to termites that eat decayed plant matter. Most animals occupy this second trophic level.

Higher up the food chain are animals that feed on other animals in one form or another. Primary carnivores (like wolves) eat herbivores (like rabbits); secondary carnivores (like killer whales) eat primary carnivores (like fish). Other feeding strategies include organisms (like many bacteria, termites, and vultures) that scavenge dead bodies and waste products, and omnivores (like human beings and raccoons) that get their food from many sources, both plant and animal. All of these animal consumers, however, are ultimately dependent on the photosynthetic producers at the base of the food chain.

The food chain is a very inefficient system. As energy is transferred up from one level to another, much of it is lost at each step. Plants use only a few percent of the energy in the sunlight that falls on them. Grazing animals typically recover only about 10 percent of the energy stored in the grass they eat. The lost 90 percent escapes through animal metabolism as heat or is locked in molecules that are not easily digested.

Energy is obtained by living things through the food chain, which includes several trophic levels. Energy-producing photosynthetic plants provide energy for animal consumers and decomposers
.

Roughly another 90 percent of the available energy in the food chain is lost in the move from herbivores to carnivores. This ever-diminishing flow of energy from lower to higher trophic levels means that each level supports fewer individuals. Vast schools of small fish will feed relatively few big fish. On the African plains there are few lions compared to the large herds of grazing animals. This fact also explains why beef (from the second trophic level) is approximately ten times more expensive than grain, and why there are no lion steaks available in your supermarket.

Each ecosystem is maintained by the energy that flows through it, but no matter how the energy moves through the biosphere, its ultimate fate is always the same. Sooner or later it is converted into heat and radiated back into space, just one more part of the great energy balance that keeps our planet going.

Unlike energy, which is constantly lost and must be constantly replenished in any ecosystem, the atoms and molecules that make up the structure and nutrients of organisms are recycled. Atoms do not disappear, but move from one organism to another and from one chemical form to another, continuously shifting back and forth between living and nonliving parts of the system. We describe the history of atoms in terms of the so-called chemical cycles. Chemical cycles essential for life include the water cycle, as well as cycles for the elements carbon, nitrogen, oxygen, phosphorus, sulfur, and others.

Each part of the cycle followed by any atom or molecule is complex, with many alternative pathways. Consider the movements of just one carbon atom, entering the cycle from the
atmosphere as a molecule of the gas carbon dioxide (CO
₂
). A blade of grass combines that molecule of carbon dioxide with water through photosynthesis to create part of a glucose molecule. Shortly thereafter, the glucose is processed in cellular chemical factories to form part of the cellulose fibers that support each grass blade. The carbon atom has become an integral part of the structure of grass.

A hungry mouse nibbles at the grass, chewing and swallowing the carbon atom, which is added to the mouse’s chemical stockpile. The unfortunate mouse is spotted and eaten by an owl, who adds the atom to its own energy reserve. As the owl burns its carbon-rich fuel by respiration, the carbon atom returns to the atmosphere in another molecule of carbon dioxide.

Carbon might follow other pathways. Some carbon atoms end up in the soil as animal droppings or by death and decay. There, bacteria, worms or other scavengers obtain raw materials directly from the carbon-rich earth. Layers of dead plant matter can pile up, become deeply buried, and transform by the Earth’s temperature and pressure to form fossil fuel deposits such as coal, oil, and natural gas. Snails and beetles convert other carbon atoms into the chemicals that make their hard outer shells. In the ocean, corals and shellfish use a similar process to manufacture durable carbonate reefs and shells, which can accumulate to form thick limestone formations. In the past century humans have altered the natural carbon cycle by burning hundreds of billions of tons of fossil fuel, thus increasing the concentration of carbon dioxide in the atmosphere.

Every other atom essential to life—oxygen, hydrogen, nitrogen, and so on—follows a similar cycle through the biosphere. The details differ from element to element, of course, but the main principle is the same: materials cycle through the biosphere and never leave.

Humans are an integral part of the ecosystem. Like all other living things, we depend ultimately on the energy in sunlight and the photosynthetic reactions in the first level of the food chain. But humans, unlike any other species, have learned to shape and alter their environment in remarkable ways. By developing agriculture, building cities, and, more recently, building manufacturing plants, we have, for better or worse, profoundly altered the biosphere. Many of the most important questions now on the national and international political agendas have to do with this fact. None of these issues is a purely scientific one; many economic and social factors impinge on one another. Nevertheless, each has a strong scientific component, and it is impossible to discuss any of them intelligently without some basic understanding of the underlying science. Below are three of the many environmental problems about which we all will have to make intelligent decisions as citizens: ozone depletion, acid rain and urban air pollution, and the greenhouse effect.

The ozone molecule consists of three (as opposed to the usual two) oxygen atoms. Only about one molecule per million in the atmosphere is ozone, but these molecules play a crucial role in our environment in two ways. Ozone near Earth’s surface (“bad ozone”) is a noxious pollutant, irritating to eyes and lungs. Ozone 50,000 feet up, on the other hand (“good ozone”), absorbs the sun’s harmful ultraviolet radiation and thus provides an effective sunscreen for those of us living on the ground. Without
the ozone layer, humans and other terrestrial life would be constantly bombarded with high-energy radiation and consequently put at higher risk of medical problems such as skin cancer and eye damage.

The ozone layer was put at risk because of the widespread use of a class of chemicals known as chlorofluorocarbons (CFCs for short). For decades CFCs were used extensively as the working fluids in refrigerators and air conditioners, as cleaners during the manufacture of microchips, and in the manufacture of foam products. When they were widely introduced in the 1960s, the molecules’ stability was considered an asset since they wouldn’t break down and add to pollution. But that very stability led to problems, because CFCs last long enough to filter through to the upper atmosphere. There the molecules’ chlorine atoms act as catalysts in a complex set of reactions that convert two molecules of ozone to three molecules of ordinary oxygen, depleting the ozone layer faster than it can be recharged by natural processes—another example of the law of unintended consequences.

In 1984, scientists working in the Antarctic made a startling discovery that focused world attention on the ozone layer. During the months of September and October (the Antarctic spring), the concentrations of ozone above the pole dropped by 50 percent. This celebrated “ozone hole” has reappeared, with varying degrees of intensity, every year since. It appears that the massive ozone depletion associated with the hole is the result of the special conditions in Antarctica—the isolation of the air during the Antarctic winter and the presence of ice clouds that form during the period when the sun doesn’t shine.

The public was justifiably concerned about the progressive destruction of the ozone layer, and a sense of urgency prompted measures to reduce the use of CFCs. Chemical industries were
quick to respond to the evident danger—DuPont, for example, quickly phased out all production of the chemicals, and all industrial nations soon followed suit.

As environmental problems go, ozone depletion has been a relatively simple one to deal with. The solution was obvious, its cost was relatively low, and it required no real change in behavior or lifestyle to reverse the present trend.

Burning always introduces carbon dioxide and water vapor into the atmosphere, but combustion also produces three other significant sources of pollution:

1. Nitrogen oxides
   . Whenever the temperature of the air is raised above about 500°C, nitrogen in the air combines with oxygen to form what are called NO
   _x
   compounds (pronounced
   nox):
   nitrogen monoxide (NO), nitrogen dioxide (NO
   ₂
   ), and others.
   
2. Sulfur compounds
   . Petroleum-and coal-based fossil fuels usually contain small amounts of sulfur, either as a contaminant or as an integral part of their structure. The result is that sulfur dioxide (SO
   ₂
   ) is released into the atmosphere as well.
   
3. Hydrocarbons
   . The long-chain molecules that make up hydro carbons seldom burn perfectly. As a result, a third class of pollutants, bits and pieces of unreacted hydrocarbon molecular chains, enters the atmosphere.