ECOSYSTEMS

An ecosystem is the most complex level of organization in nature. It is made up of a community and its abiotic (nonliving or physical) environment, including climate, soil, water, air, nutrients, and energy. Ecologists who try to link together the many different physical and biological activities in an environment are called systems ecologists. Their studies often focus on the flow of energy and the cycling of materials through an ecosystem. They generally use powerful computers to help understand the data obtained from field research and to predict future development. Energy flow. Ecologists categorize the elements that make up or affect an ecosystem into six main parts, based on the flow of energy and nutrients through the system:

(1) the sun, (2) abiotic substances, (3) primary producers, (4) primary consumers, (5) secondary consumers, and (6) decomposers. A simplified ecosystem is illustrated in this article.

The sun provides the energy that nearly all primary producers need to make food. Primary producers consist mainly of green plants, such as grass and trees, which make food by the process of photosynthesis. Plants also need abiotic substances, such as phosphorus and water, to grow. Primary consumers include mice, rabbits, grasshoppers, and other plant-eating animals. Foxes, skunks, and other secondary consumers-or predators-eat animals. Decomposers, such as bacteria and fungi, break down dead plants and animals into simple nutrients. The nutrients go back into the soil and are used again by plants.

The series of stages energy goes through in the form of food is called a food chain. In one simple food chain, grass is the primary producer. A primary consumer, such as a rabbit, eats the grass. The rabbit, in turn, may be eaten by a secondary consumer, such as a fox or a hawk. Decomposing bacteria break down the uneaten remains of dead grass, rabbits, foxes, and hawks, as well as animal body wastes. One of the food chains on Isle Royale has trees as primary producers, moose as primary consumers, and wolves as secondary consumers.

Most ecosystems have a variety of producers, consumers, and decomposers, which form an overlapping network of food chains called a food web. Food webs seem especially complex in many tropical and oceanic ecosystems.

Some species eat many things, but others have very specific food requirements. Such primary consumers as koalas and pandas eat chiefly one type of plant. Koalas eat primarily eucalyptus and pandas eat primarily bamboo. If these plants died off, so would the animals.

Energy moves through an ecosystem in a series of transformations. First, primary producers change the light energy of the sun into chemical energy that is stored in plant protoplasm (cell material). Next, primary consumers eat the plants, changing the energy to a different kind of chemical energy that is stored in body cells. This energy changes again when the secondary consumer eats the primary consumer.

Most organisms have a low ecological efficiency. This means they are able to convert only a small fraction of the energy available to them into stored chemical energy. For example, green plants can change only about 0.1 to 1 percent of the solar energy that reaches them into plant protoplasm. Most of the energy captured by the plants is burned up during plant growth and escapes into the environment as heat. Similarly, herbivores (plant-eating animals) and carnivores (meat-eating animals) convert into their own body cells only about 10 to 20 percent of the energy produced by their food.

Because so much energy escapes as heat at each step of the food chain, all ecosystems develop a pyramid of energy. Plants (primary producers) form the base of this pyramid. Herbivores (primary consumers) make up the next step, and carnivores (secondary consumers) form the top. The pyramid reflects the fact that more energy passes through the plants than through the herbivores, and more through the herbivores than through the carnivores. In many land ecosystems, the pyramid of energy results in a pyramid of biomass. This means that the total biomass (weight) of the plants is greater than the total weight of the herbivores, which in turn exceeds the total weight of the carnivores. In the oceans, however, the biomass of plants and animals is about the same. Small plants grow so rapidly in the oceans that they can support proportionately more animals than can the plants on land.

Ecologists have collected information on a pyramid of biomass on Isle Royale. They studied the relationship in the pyramid among plants, moose, and wolves. In one study, ecologists found that it takes 762 pounds (346 kilograms) of plant food to support 59 pounds (27 kilograms) of moose. This is the amount of moose needed to support 1 pound (0.45 kilogram) of wolf.

Cycling of materials. All living things are composed of certain chemical elements and compounds. Chief among these are water, carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. All of these materials cycle through ecosystems again and again.

The cycling of phosphorus provides an example of this process. All organisms require phosphorus. Plants take up phosphorus compounds from the soil, and animals get phosphorus from the plants or other animals they eat. Decomposers return phosphorus to the soil after plants and animals die.

In natural, undisturbed ecosystems, the amount of phosphorus remains fairly constant. But when an ecosystem is disturbed, especially by human activity, the phosphorus often "leaks out." This reduces the ability of the ecosystem to support plants. One way people alter the phosphorus cycle is by replacing forests with farmland. Without the protection of the forests, phosphorus is eroded with the soil and swept away into rivers and lakes. There, it often causes undesirable excess growth of algae. Eventually, the phosphorus becomes locked in sediments at the bottom of lakes or the sea. Because of this loss of phosphorus, farmers must use costly fertilizers to put the element back into the soil.

Changes in ecosystems occur daily, seasonally, and, as in the case of ecological succession, over periods of many years. Sometimes changes take place severely and abruptly, as when a fire sweeps through a forest or a hurricane batters a seashore. But most of the day-to-day changes, especially in the nutrient cycles, are so subtle that ecosystems tend to appear stable. This apparent stability among plants and animals and their environment has been called the "balance of nature." In the past, this concept of balanced, largely unchanging ecosystems was thought to be especially descriptive of climax communities. But these earlier views were based on short-term studies. Now that ecologists have had an opportunity to study ecosystems over longer periods, they have had to alter some of their ideas.

Conclusions based on population studies from Isle Royale point out some of this change in thinking. For a long time, Isle Royale had neither moose nor wolf. Then, the first moose swam to the island in about 1900. By 1930, ecologists estimated that the moose population had reached about 3,000. There was evidence that the moose were eating many of the plants on the island. In 1933, the moose began to die of starvation. Ecologists had predicted this decline because they understood the food relationship between the moose and plants.

The moose population increased again between 1948 and 1950. However, about this time, wolves made their way to the island. As they killed moose for food, the wolf population grew. Eventually, an apparently stable balance of about 600 moose and 20 wolves became established. Ecologists pointed to Isle Royale as an example of the way in which predators can control prey and thus contribute to the development of stability in ecosystems.

But beginning in the mid-1960's, the moose and wolf populations began to fluctuate. The apparently stable system, in which predators controlled their prey, turned out to be more complex. During the 1950's, when it looked as if wolves were controlling the moose, the winters were characterized by an unusual pattern of deep snows followed by rain and then a hard freeze. This resulted in snow with a hard crust. Wolves could run easily on the surface of this snow, but the heavier-bodied moose broke through the crust. Thus the moose could not easily escape from wolves, nor could they effectively use their sharp, powerful hooves for defense. Under these conditions, the wolves could easily kill moose.

Around 1965, winters on Isle Royale returned to normal, and the wolves caught fewer moose. By the early 1980's, the moose population had again become extremely large, even though the wolf population had also grown. Then the wolf population began to decline, despite the abundance of moose. By the late 1980's, ecologists feared that wolves might disappear from Isle Royale. All of these population changes forced ecologists to reevaluate their thinking about how predators and prey control one another's populations. Ecologists recognized that although wolves and moose certainly can influence the size of each other's populations, these animal groups can completely determine one another's population size only under unusual circumstances.

In the 1990's, Isle Royale's moose population declined again. Ecological studies indicated that changes in the availability of food plants and nutrients were important factors in this decline. For example, the moose would eat the leaves of aspen trees, but not the unpleasant-tasting needles from spruce and fir trees. And since spruce and fir needles also did not taste good to the island's decomposers, the needles piled up on the forest floor, trapping nitrogen and other nutrients from entering the soil. Thus the quality of the soil declined, and the growth of trees was stunted. This has meant less food for the moose and a decline in moose population.