Ecosystems
by Professor Robert M. Hazen

One of the great principles of biology is that all individuals are part of ecosystems, which are complex communities of organisms and their physical environment. Ecosystems are amazingly varied. You have arid deserts, with almost no rainfall at all. You have lush tropical forests, with more than 100 inches of rainfall a year. You have sun-drenched ponds. You have hydrothermal vents in the deep-ocean floor. All these varied ecosystems share six characteristics.

First of all, all ecosystems are dependent on both living and non-living parts. The physical and chemical environment defines the non-living portions of an ecosystem. The environment includes the weather and the climate; it includes the nature of the local rocks and the soils—the temperature and the salinity of local bodies of water, for example, and so forth and so on. All of the different species, that interact in an ecosystem form an ecological community. Ecological communities may include a great variety of plants and animals, but they always include a host of microbes, one-celled organisms. They have to include organisms that convert sunlight, or some other form of energy, into food.

The second characteristic of all ecosystems is that they all require energy. Every natural system requires energy. Energy flows through an ecosystem according to the laws of thermodynamics. The flow of energy through an ecosystem, through the organisms, is called the "food web." Every organism has to obtain energy, either directly from its environment or by eating other organisms. The concept of the trophic level defines a hierarchy of energy producers and consumers in every ecosystem. A trophic level includes all the organisms in an ecosystem that get their energy from the same source. You have plants, for example, which get their energy from the Sun by photosynthesis. They constitute the first trophic level; they are the energy producers in most ecosystems. However, we also have those deep-ocean environments, where microbes that use the chemical energy of rocks are the primary producers. In those ecosystems, it's the microbes that are the primary, or the first, trophic level. The second trophic level includes herbivores; that includes all the animals that eat plants. Then come carnivores that eat herbivores; that would be in the third trophic level. You also have bacteria and scavengers in many ecosystems that eat dead organisms, and they form yet another trophic level.

At each trophic level, most of that available energy goes unused. Plants use only a few percent of the Sun's radiant energy that comes down and hits them. Herbivores harvest perhaps only 10 percent of the energy available in plants; and carnivores, similarly, use maybe only 10 percent of the energy of the animals that they eat. This trend is the direct consequence of the second law of thermodynamics. Since animals are like machines, they can't operate at 100 percent efficiency. This leads to an interesting consequence. The ratio of biomass of plants to herbivores to carnivores is typically about 100:10:1. This is why large carnivores are relatively rare: they get much less energy than the herbivores, and the herbivores get less energy than the original plants. If you can imagine a picture of the Serengeti Plain, you have a few lions, but you have vast herds of wildebeests, hundreds of wildebeests for every one lion. Similarly, dinosaurs had the same kind of population controls. You might not get that impression, because of the fame of Tyrannosaurus rex, but indeed there are only about a dozen known specimens of Tyrannosaurus rex in the entire world, and yet there are hundreds upon hundreds of the herbivorous dinosaurs which the Tyrannosaurus rex must have eaten.

The third characteristic of all ecosystems is that matter is constantly recycled; the atoms are used over and over again. This is just like any geochemical cycle. The atoms and molecules aren't destroyed; they're just reused, recycled, which can be understood by looking at the history of a carbon atom. A carbon atom could start as a gas molecule, for example, a molecule of CO². That CO² molecule could be taken up by a plant and used to construct a blade of grass. You might have a cow come along, and it eats that blade of grass, and the carbon atom is used to make some of the milk that the cow produces. You might drink that milk in your breakfast coffee, and that carbon might now be part of your tissues. Indeed, that carbon atom could remain in your body for the rest of your life; but eventually, you're going to die, and that carbon atom is going to return to Earth, and it will continue its endless cycle, to be used over and over again. It's amazing to think that the atoms that are now in your body were once part of dinosaurs; they were once part of trilobites. That's inevitable, given the way atoms are cycled over and over again, in the history of life.

The fourth characteristic of every ecosystem is that every organism in an ecosystem occupies an ecological niche. An ecological niche represents a specific strategy for obtaining energy and atoms from the environment. There are many, many examples, as you can imagine. If you have an insect that lives off the sap of bark in a specific kind of tree, that's an ecological niche. A worm that scavenges in shallow soil has its own ecological niche; so does a plant that thrives under the shade of a larger bush. Every organism has to compete for resources in its ecological niche. In general, therefore, an ecosystem doesn't support two species in identical ecological niches. Ecologists were puzzled once when they found two very similar species of birds living in the exact same tree in a forest; this is an example of violating this idea of ecological niches being occupied by single species. When the ecologists looked more closely, they found that one bird lived in the lower branches and ate one kind of insect, and the other species lived in the high branches and ate a different kind of insect. In fact, these were separate ecological niches, even though they were in the same tree.

The fifth characteristic of ecosystems: populations of different species achieve a balance in a stable ecosystem. This situation is a consequence of the fact that matter and energy are limited resources; they have to be shared by all the individuals in any ecosystem. While species' populations may vary with changes in weather, or food supplies, from year to year, the relative size of populations usually remains quite similar from year to year.

The sixth and last of these characteristics is that a change in environment, or the introduction or loss of a species, can disrupt an ecosystem. This disruption can be gradual, as in a climate change, or it can be quite sudden and quite dramatic.

Environmentalists now realize that ecosystems can be disrupted in ways you'd never predict, leading to the law of unintended consequences. The law states the following: any change in one part of a complex system may affect other parts of the system, in ways that are often unpredictable. A surprising and telling example is provided by the story of the near-extinction of the Peter's Mountain Mallow; that's a small, flowering plant found only in Giles County, Virginia. The number of individual plants, though they were carefully monitored and protected, had steadily declined year after year, until in 1991 there were only 4 individual plants that had survived. Conservationists took heroic measures to preserve the remnants of this once-thriving population; that included protecting them from any sort of forest fire, keeping them away from any ravages. But in desperation, it finally dawned on them that fire might be a part of the natural life cycle of these plants. They started a few controlled fires in the area where Peter's Mountain Mallow had once been known to exist. The next spring it was remarkable; hundreds of new Peter's Mountain Mallow plants sprouted. It turns out that fire was essential to the plants' germination process. By protecting the plants from fire, conservationists had almost caused it to go extinct; there's sort of a morality play there.

There are other stories that are much less happy in terms of their ending. One of these is the story of Lake Victoria, the disaster that has really transformed Lake Victoria in the last 30 years or so. Lake Victoria is the largest freshwater lake in Africa . Millions of people live by its shore; millions of people rely on fishing from the lake as a source of protein. In the 1960s, a large, aggressive predatory fish called the Nile perch was introduced to the lake; this was to make a challenge for sport fishermen. A lot of tourists came to the lake, at that period, and this was a large, aggressive fish that could be counted on to provide the fishermen with something to really sink their teeth into, because all the other species of fish in the lake were much smaller, and much less aggressive. The perch has no natural enemies in Lake Victoria, and it had vast supplies of these smaller fish, some of which were called tilapia, the fish that was the main staple of the diet of the African people who lived on the shores of the lake. The populations of these smaller fish plummeted as the Nile perch came in, and the number of Nile perch, of course, increased accordingly. The smaller fish had provided all sorts of ecological benefits to the lake; they had controlled surface algae, which they fed on. With the small fish gone, the algae spread over parts of the lake's surface. Then the dead algae sunk, and this decaying algae, as it sunk in the deeper parts of the lake, consumed oxygen where other kinds of fish lived, and so deeper-water fish could no longer live in the lake. The tilapia had also controlled populations of snails, and these snails carry parasites which are harmful to humans. The incidence of parasitic disease has increased dramatically since the Nile perch was introduced.

The native fishermen now catch Nile perch, but this leads to another unintended consequence. The large fish have to be dried over a fire, rather than sun-dried. With a small fish, you just lay the fish out in the Sun; it dries them, and then you can use them as long-term protein supply. You have to build large wood fires to dry the large fish out, and preserve them. As a result, the forests have been stripped away from the lake's shoreline, causing extensive soil erosion into the lake, and a further disruption of the ecosystem of Lake Victoria . Thus, we see that the introduction of a single species of fish into a vast lake ecosystem has drastically altered that ecosystem, perhaps for all time—certainly for centuries, if not thousands of years.

Humans, by their actions, do change ecosystems. What concerns many scientists is that we can't predict how the ecosystems are going to change. The law of unintended consequences is constantly coming into play. We know ecosystems can be affected in dramatic ways; we just can't predict how, we can't predict when. One thing is certain: as human population grows, we're going to alter the balance of matter and energy in several different ways. First, we're constantly commanding an ever-larger share of water and nutrients, and these are resources that all living things need. Natural processes gradually renew supplies of groundwater, and of course, the nutrients in the soil are also renewed on a gradual basis. But human activities often consume these resources much, much faster than they can be replaced. In addition, humans are eliminating habitats through deforestation, through farming, through urbanization, and other sorts of endeavors that are common to the growing human population. By limiting the amount of food and energy available for other organisms, these activities can cause large-scale changes in local ecosystems, and they may affect the diversity of animal and plant species as well.

Why should we care about the disappearance of other species? After all, humans don't appear to be in any immediate danger of extinction. There are three compelling reasons that have been proposed for why we should preserve Earth's biodiversity. The first reason is that all species are interdependent on others; they're all part of complex ecosystems. Loss of one species may adversely affect the whole system, with consequences that we just can't predict. They may be completely unexpected, and they may be very tragic. Damaged ecosystems, in turn, may affect rainfall and climate; they can affect soil nutrient development, erosion processes. You can change pollination rate, you can change pest control, water quality—all of these factors may be dependent on the preservation of ecosystems as they now stand. As more species are threatened, the risk to humans of these adverse conditions increases.

The second argument to preserve species focuses on our health and our well-being. Humans have enjoyed untold benefits from all the new foods, the new drugs, the new chemicals that are discovered as part of living things. Every living thing has its own unique set of genes, and therefore, its own unique set of proteins. Each of those proteins has the possibility of serving a useful function, as a chemical that we could apply to our own needs. There are surely countless natural substances, many of them of great economic value that remain to be discovered. Each lost species, therefore, represents a lost opportunity for improving our lot.

Finally, many people argue for species protection on ethical and on aesthetic grounds. The eminent biologists Paul Ehrlich and E. O. Wilson once said "People have an absolute moral responsibility to protect what are our only known living companions in the universe. Human responsibility in this respect is deep, beyond measure. We cannot easily measure our loss, or our guilt, in causing a species to become extinct."

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