An influx of energy is required to maintain order in ecosystems and to sustain their physical and biological processes. For most ecosystems, the Sun is the primary source of energy. Plants and algae convert this solar energy into chemical energy, which then moves from plant to animal to animal in the form of food. These movements of energy form a web of energy transfers called a food web (Figure 2.9). Food webs show who eats whom in an association of coexisting organisms. One of the basic elements in a food web is the trophic level of each organism, which identifies its position in the overall movement of materials or energy through an ecosystem.

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The primary producers, or autotrophs, always occupy the first, or lowest, level in a food web. Primary producers, mostly plants on land and algae in aquatic ecosystems, produce the materials that form the basis of what all organisms need to stay alive. They do this by means of photosynthesis, a biochemical process that uses solar energy to convert water and carbon dioxide into the chemical energy in a simple sugar called glucose (Figure 2.10).

Ecologists refer to the total amount of organic matter produced by the primary producers in an ecosystem over some period of time as gross primary production. However, plants expend energy for all kinds of things besides growth: in respiration; in maintaining the structure of their tissues; and in defending themselves against attack by viruses, bacteria, fungi, insects, and other organisms. The amount of energy left over after subtracting these factors is called net primary production. Net primary production can also be looked at as the amount of food energy available to other organisms that feed on primary producers.

The organisms occupying trophic levels above the producers, which feed on organic matter produced by other organisms, are collectively referred to as consumers, or heterotrophs. Consumers meet their energy needs by extracting the energy from their food through cellular respiration, a process that occurs in the cells of all organisms (Figure 2.10). Consumers that feed on plants are called herbivores or primary consumers, while those feeding on other consumers are called carnivores, predators, or secondary consumers. Some consumers, called omnivores, eat both plants and animals. Finally, a critical group of consumers in any food web are the detritivores, or decomposers. These organisms, which feed on dead and decaying plant and animal material, are particularly important in the cycling of matter in ecosystems.

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The net amount of consumer biomass, or energy, that goes into growth and reproduction, is referred to as secondary production.

As a result of energy losses at each trophic level (thanks to the second law of thermodynamics), a graph of the net production by all the organisms in each trophic level in an ecosystem takes the form of a pyramid, broader at the base, representing net primary production and narrowing progressively at higher trophic levels. Figure 2.11 gives an example of an energy pyramid from the Silver Springs ecosystem in Florida. Because up to 90% of energy is lost between trophic levels, energy makes a one-way trip through an ecosystem.

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The effects of the second law of thermodynamics on food webs and trophic pyramids have some practical consequences. For instance, carnivores will always be less abundant than their prey species and, consequently, more likely to be in danger of extinction. Also, because of lower production at higher trophic levels, producing enough animal protein for everyone on Earth to have a diet comparable to that in the United States would not be sustainable.

Matter, like energy, also flows through ecosystems. According to the principle of the conservation of matter, matter is neither created nor destroyed—it is conserved during chemical reactions, although it may change forms. Consider a forest fire that destroys millions of tons of wood (Figure 2.12). As the forest burns to the ground, the matter that made up the forest changes form—some is converted to carbon dioxide gas, some to smoke, and some to ash. However, if you could measure the process very precisely, you would find that there is no change in the mass of matter before and after the fire. Even in the hottest of forest fires, not a single atom is lost. This shows us that the movement of matter on Earth is cyclic—nothing is created or destroyed but can be recycled indefinitely.

You can demonstrate this fact for yourself by reviewing the chemical equations in Figure 2.10 (see page 40). Count the number of atoms on the two sides of the reactions in Figure 2.10, and you will find that, though the atoms have been rearranged, their numbers are the same.

Examples of substances that get cycled include water, nitrogen, carbon, phosphorus, sulfur, and iron. The cycles of these and other substances are called biogeochemical cycles because they involve biotic, or living components like plants and animals, and abiotic (nonliving) components, including Earth’s water, minerals, and atmosphere. The biological components of biogeochemical cycles include the producers, herbivores, carnivores, detritivores, and decomposer bacteria and fungi represented in Figure 2.9 (see page 40). Decomposition of organic matter releases inorganic substances, such as nitrogen, phosphorus, or carbon, which are returned to soil, atmosphere, or water.

One molecule that has been making headlines for years is carbon dioxide, an important part of the global carbon cycle. Carbon plays a central role in the lives of all organisms as most of the molecules essential to the structure and functioning of living cells—including protein, DNA, and lipids—are built on a framework of carbon. Carbon is also a key player in two processes we’ve already learned about—photosynthesis and respiration; carbon moves around on Earth, from the tissues of living creatures to sedimentary rocks, and from oceans to atmosphere; and carbon trapped in the bodies of once-living creatures is released millions of years later into the atmosphere by the burning of fossil fuels (Figure 2.13).

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Because matter cannot be created or destroyed, biogeochemical cycles like the carbon cycle move essential elements around Earth continuously. For more on biogeochemical cycles, see Chapter 7 (nitrogen cycle), Chapter 8 (phosphorus cycle), and Chapter 13 (sulfur cycle). Another cycle, the hydrologic, or water, cycle plays a key role in the discussions of water-related issues in Chapter 6.