Ecosystem Energetics

The diverse forms of life in all ecosystems are powered by a single energy source, sunlight, which enters the biosphere through a process called photosynthesis. Organisms such as plants and algae capture a small fraction of sunlight's energy, storing it as chemical energy in sugar or other complex organic molecules. The energy then passes through an ecosystem via different feeding (trophic) levels. Each time the energy is passed on, a portion of it is lost as heat. Thus, energy needs constant replenishment from the original source, the Sun.

How does energy flow through communities? It flows through ecosystem energetics. Of the energy that reaches Earth, much is either reflected or absorbed as heat by the atmosphere and Earth's surface, leaving only about 1 percent to power all life. Of this 1 percent, green plants capture 3 percent or less. All life on this planet is therefore supported by less than 0.03 percent of the energy reaching Earth from the Sun. Photosynthetic organisms are called auto-trophs, or producers, because they produce food for themselves. Directly or indirectly, they produce food for nearly all other forms of life as well. Organisms that cannot photosynthesize are called heterotrophs, or consumers, because they must acquire energy prepackaged in the molecules of the bodies of other organisms.

There are three basic principles that govern ecosystem energetics. First, the amount of life an ecosystem can support is defined by the energy captured by the producers. This energy, made available to consumers over a given period, is called net primary productivity. It is usually measured in units of energy stored per unit area in a given period, or measured as biomass, the dry weight of the total organic material added to the ecosystem per unit area in a given time span. The net primary productivity is influenced by a variety of environmental factors, including the amount of sunlight, the availability of water, the amount of nutrients available to producers, and the temperature. Among all environmental variables, the most limiting variable is the one that determines net primary productivity, for instance, water in the desert or light in the deep ocean.

Second, within the community, energy is passed from one feeding level to another. Energy flow moves from producers to primary consumers to secondary and tertiary consumers. Primary consumers are normally herbivores that directly consume producers. Secondary and tertiary consumers are typically carnivores that eat meat of other consumers. Certain consumers may occupy more than one feeding level. As energy is passed through feeding levels via food chains or food webs, the transfer is never efficient. This is the third basic principle of ecosystem energetics. Each time the energy is transferred to the next feeding level, the bulk of it is lost as heat. On average, only 10 percent of the energy is transferred from one feeding level to the next. In other words, the higher the feeding level an organism occupies, the less energy is available to it. This so-called 10 percent law or energy pyramid puts a cap on how much life a particular ecosystem can sustain.

—Ming Y. Zheng fishes, are tertiary consumers at the fourth trophic level. A vast array of zooplanktons, invertebrates such as sponges, the poisonous blue-ringed octopus, and so on, also live in coral reef ecosystems to make extremely complex marine food webs. For example, the Great Barrier Reef in Australia is home to more than two hundred species of coral, and a single reef may harbor three thousand species of fish, invertebrates, and algae.

Similar to terrestrial ecosystems, aquatic ecosystems are also prone to human disturbance. Of all aquatic or marine ecosystems, coral reefs are probably most sensitive to certain types of disturbance, especially silt caused by soil eroding from nearby land. As silt clouds the water, light is diminished and photosynthesis reduced, hampering the growth of the corals. Furthermore, as mud accumulates, reefs may eventually become buried and the entire magnificent community of diverse organisms destroyed. Another hazard is sewage and runoff from agriculture. The dramatic rise in fertilizer in near-shore water causes eutrophication, by which excessive growth of algae blocks sunlight from the corals, deprives corals of nutrients, and suffocates corals and other organisms. A third threat to coral reefs, overfishing, is also strictly a result of human interference. It is estimated that in over eighty countries, an array of species, including mollusks, turtles, fish, crustaceans, and even corals, are being harvested much faster than they can replace themselves. Collectively, these human activities had destroyed over 30 percent of coral reefs worldwide by year 2000. Assuming no effective measure is taken to preserve or restore coral reef ecosystems, another 50 percent of reefs will disappear by year 2030. The message is clear: Once humans disturb an ecosystem, through damaging one or more species in an intricately networked food web, balance and sustainability within the whole system is affected. The price for such disruption is high and far-reaching.

—Ming Y. Zheng See also: Biodiversity; Biogeography; Carnivores; Chaparral; Digestion; Digestive tract; Ecosystems; Forests, coniferous; Forests, deciduous; Grasslands and prairies; Habitats and biomes; Herbivores; Ingestion; Lakes and rivers; Marine biology; Mountains; Nutrient requirements; Omnivores; Predation; Rain forests; Savannas; Symbiosis; Tidepools and beaches; Tundra.

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