Ecosystems: How They Work

1. Nutrient Cycling.When you look at the various inputs and out­puts of producers, consumers, detritus feeders and decomposers, how they fit together should be conspicuous. The waste, or byproducts, of each is the food or essential nutrients for the other. Specifically, the organic material and oxygen produced by green plants are the food and oxygen required by consumers and other heterotrophs. In turn, the carbon dioxide and other wastes generated when heterotrophs break down their food are exactly the nutrients needed by green plants. Herein is the first basic principle of ecosystem sustainability: for sustainability, ecosystems dispose of wastes and replenish nutrients by recycling all elements. This principle is in harmony with the Law of Conservation of Matter. Since atoms are neither created nor destroyed, nor convert­ed one into another, they can be reused indefinitely. This is exactly what natural ecosystems do; they recycle the same atoms over and over again. We can see this even more clearly by focusing on the pathways of three key elements: carbon, phosphorus, and nitrogen. Since these pathways do lead in a circle, they are known as the carbon cycle, the phosphorus cycle, and the nitrogen cycle.

2. The Carbon Cycle.For descriptive purposes, it is convenient to start the carbon cycle with the "reservoir" of carbon dioxide mole­cules present in the air and dissolved in water. Through photosynthe­sis and further metabolism, carbon atoms from carbon dioxide be­come the carbon atoms of all the organic molecules making up the Plant's body. Through food chains, the carbon atoms then move into and become part of the tissues of all the other organisms in the eco­system. However, it is unlikely that a particular carbon atom will be Passed through many organisms on any one cycle because at each

^eP there is a considerable chance that the consumer will break down ^e organic molecule in cell respiration. As this occurs, the carbon

atoms are released back to the environment in molecules of carbon dioxide, thus completing one cycle, but of course ready to start an­other. Likewise, burning organic material returns the carbon atoms locked up in the material to the air in carbon dioxide molecules. No two successive cycles of a particular carbon atom are likely to be the same. Nor are the two cycles likely to be within the same ecosystem because in the atmosphere wind will carry them around the globe.

3. The Phosphorus Cycle.Phosphorus exists in various rock and soil minerals as inorganic phosphate ion (P0₄³~). As rock gradually breaks down, phosphate and other nutrient ions are released. Phos­phate dissolves in water but does not enter air. Plants absorb phos­phate from the soil or water solution, and as it is bonded into organic compounds by the plant it is frequently referred to as organic phos­phate. Through food chains, organic phosphate is transferred from producers to the rest of the ecosystem. As with carbon, at each step there is a high likelihood that the organic phosphate will be broken down in cell respiration, releasing inorganic phosphate in urine or other waste. The phosphate may then be reabsorbed by plants to start another cycle.

4. There is an important difference between the carbon cycle and the phosphorus cycle. No matter where carbon dioxide is released, it will mix and maintain the concentration of carbon dioxide in the air. Mineral nutrients, however, which do hot have a gas phase, are recy­cled only insofar as the wastes that contain them are deposited on the soil from which the nutrients originally came. This is basically what happens in a natural ecosystem. However, humans are prone to upset this cycle. A very serious case of humans interfering with the phos­phorus cycle is seen in the cutting of tropical rainforests. This type of ecosystem is supported by a virtually 100 percent efficient recycling of nutrients. In other words, there are little or no reserves in the soil When the forest is cut and burned, the nutrients that were locked up in the trees are really washed away, and the land is rendered unpro­ductive. Also, in the human system, phosphate from agricultural crop­lands makes its way, in large part, into waterways — either directly by way of runoff from croplands or indirectly by way of discharge of sew­age effluents. Since there is essentially no return of phosphate from water, this addition results in overfertilization of bodies of water.

Meanwhile, phosphorus is replaced on croplands by mining phos­phate rock — a process that will ultimately result in depletion.

5. The Nitrogen Cycle. Itis more complex than the carbon and phosphorus cycles because it has both a gas phase and a mineral phase. The main reservoir of nitrogen is the air, which is about 78 percent nitrogen gas (N₂). Plants cannot utilize nitrogen gas directly from the air; instead the nitrogen must be in a mineral form, such as am­monium ion (NH₄⁺) or nitrate ion (N0₃~). A number of bacteria and also certain blue-green algae, which are actually bacteria, can con­vert nitrogen gas to the ammonium form, a process called biological nitrogen fixation. Most important among these nitrogen-fixing or­ganisms is a bacterium called Rhizobium, which lives in nodules on roots of legumes, members of the pea-bean family of plants. This is another example of symbiosis. The legume provides the bacteria with a place to live and with food (sugar) and gains a source of nitrogen in return. Fixed organic nitrogen is passed from the legumes to other organisms in the ecosystem through food chains.

6. As animals break down proteins and other organic compounds containing nitrogen for energy in cell respiration, the nitrogen is ex­creted, generally in the ammonium ion form. Bacteria in the soil may convert the ammonium ion to the nitrate form, but either form may be reabsorbed by any plants, thus creating an ongoing cycle. Howev­er, another kind of bacterium in the soil gradually changes the nitrate ion back to nitrogen gas. Consequently, nitrogen will not accumu­late in the soil. Some nitrogen gas is also converted to the ammoni­um form by discharges of lightning in the process known as atmo­spheric nitrogen fixation and comes down with rainfall, but this is estimated to be only about 10 percent of the amount of biological nitrogen fixation.

7. All natural ecosystems, then, depend on nitrogen-fixing organ-^lsms; legumes with their symbiotic bacteria are, by far, the most im­portant. The legume family includes a huge diversity of plants, ranging from clovers (common in grasslands) through desert shrubs to many ^trees. Every major terrestrial ecosystem, from tropical rainforest to desert and tundra, has its representative legume species, and legumes ^a""e generally the first plants to recolonize a burned-over area. Without ^nem, all production would be sharply impaired because of lack of avail-

56 Unit Five

Ecosystems: How They Work 57