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THE FLOW OF ENERGY THROUGH ECOSYSTEMS

The passage of energy in one direction through an ecosystem is known as energy flow. Energy enters an ecosystem in the form of the radiant energy of sunlight. Some of it is trapped by plants during the process of photosynthesis. Now in chemical form, it is stored in the bonds of organic (carbon-contain­ing) molecules such as glucose. When these mole­cules are broken apart by cell respiration, the en­ergy becomes available to do work such as repairing tissues, producing body heat, or reproducing. As the work is accomplished, the energy escapes the living organism and dissipates into the environ­ment as low-quality heat. Ultimately, this heat en­ergy radiates into space. Thus, once energy has been used by living things, it becomes unavailable for reuse.

Producers, Consumers, and Decomposers

Three categories based on how they get nourish­ment: producers, consumers, and decomposers. Most communities contain representa­tives of all three groups, which interact extensively with one another.

Sunlight is the source of energy that powers almost all life processes on the face of the Earth-Producers, also called autotrophs (Greek auto, "self," and tropho, "nourishment"), manufacture complex organic molecules from simple inorganic substances (carbon dioxide and water), usually using the energy of sunlight to do so. In other words, producers perform the process of photosyn­thesis. By incorporating the chemicals that they manufacture into their own bodies, producers make their bodies or body parts a potential food resource for other organisms. Whereas plants are the most significant producers on land, algae and certain types of bacteria are important producers in aquatic

Grass, algae, and photosynthetic bacteria are all important producers. In abyssal hot spring commu­nities deep in the ocean, non photosynthetic bacte­ria are the producers (see Focus On: Life without the Sun).

Animals are consumers; that is, they use the bodies of other plant and animal organisms as sources of food energy and body-building materials. Consumers are also called heterotrophs (Greek heter, "different," and tropho, "nourishment"). Consumers that eat producers are called primary

Some consumers, called detritus feeders or detritovores, consume the organic matter originat­ing in plant and animal remains. Detririvores are especially abundant in aquatic habitats, where they burrow in the bottom muck and consume the or­ganic matter that collects there. For example, marsh crabs are detritus feeders in the salt marsh community. Earthworms are terrestrial (land-dwell­ing) detritus feeders, as are termites and maggots (the larvae of flies). Detritus feeders work together with decomposers to destroy dead organisms and waste products. An earthworm, for example, actu­ally eats its way through the soil, digesting much of the organic matter contained there. Earthworms also aerate the soil and redistribute its minerals and organic matter with their extensive tunneling.

Decomposers (also called saprophytes) are heterotrophs chat break down organic material and use the decomposition products to supply them­selves with energy. They typically release simple inorganic molecules, such as carbon dioxide and mineral salts that can then be reused by producers. Bacteria and fungi are important examples of de­composers. Dead wood, for example, is invaded first by sugar-metabolizing fungi that consume the wood's simple carbohydrates, such as glucose.

When these carbohydrates are exhausted, fungi, often aided by termites and bacteria, complete the digestion of the wood by breaking down cellulose, a complex carbohydrate that is the main constituent of wood.

Communities such as the Chesapeake Bay salt marsh contain balanced representations of all three ecological categories of organisms—producers, consumers, and decomposers. Producers and de­composers have indispensable roles in ecosystems. Producers provide both food and oxygen for all life. Decomposers are also necessary for the long-term survival of any community because without them, dead organisms and waste products would accumu­late indefinitely. Without decomposers, important elements such as potassium, nitrogen, and phos­phorus would permanently remain in dead organ­isms and therefore be unavailable for use by new generations of living things. Although consumers are an important part of most ecosystems, they are not essential to the long-term survival of producers or decomposers.

The Path of Energy Flow: Who Eats Whom in Ecosystems

Energy flow in an ecosystem occurs in food chains, in which energy from food passes from one organ­ism to the next in a sequence. Producers start the food chain by capturing the respire organic molecules in the remains (the car­casses and body wastes} of all other members of the food chain.

Each level in a food chain is called a trophic level. The first trophic level is formed by producers (photosynthesizes), the sec­ond trophic level by primary consumers (herbi­vores), the third trophic level by secondary con­sumers (carnivores), and so on.

Simple food chains such as the one just de­scribed rarely occur in nature, because few organ­isms eat just one other kind of organism. More typi­cally, the flow of energy and materials through an ecosystem takes place in accordance with a range of choices of food on the part of each organism in­volved. In an ecosystem of average complexity numerous alternative pathways are possible. Thu¹-a food web, a complex of interconnected food chains in an ecosystem, is a more realistic model of the flow of energy and materials through ecosys­tems. (See Focus On: Changes in Antarctic Food Webs, page 50, for an examination of how human:-have affected the complex food web in Antarctic.

The most important thing to remember about energy flow in ecosystems is that it is linear, or one ­way. That is, energy can move along a food chain or food web from one organism to the next as long as it is not used. When energy is used, it becomes unavailable for use by any other organism in the ecosystem.

Ecological Pyramids

An important feature of energy flow is that most of the energy going from one trophic level to another in a food chain or food web dissipates into the envi­ronment. The relative energy values of trophic lev­els are often graphically represented by ecological pyramids. There are three main types of pyramids—pyramid of energy.

A pyramid of numbers shows the number of organisms at each trophic level in a given eco­system, with greater numbers illustrated by wider sections of the pyramid (Figure 3-11). In most pyr­amids of numbers, each successive trophic level is occupied by fewer organisms. Thus, in typical grassland the number of zebras and wildebeests (herbivores) is greater than the number of lions (carnivores). Reverse pyramids of numbers, in which higher trophic levels have more organisms than lower trophic levels, are often observed among decomposers, parasites, tree-dwelling her­bivorous insects, and similar organisms. One tree can provide food fur hundreds of leaf-eating insects, for example.

A pyramid of biomass illustrates the total bio-mass at each successive trophic level. Biomass is a quantitative estimate of the total mass, or amount, of living material. Their units of measure vary: biomass may be represented as total volume, as dry weight, or as live weight. Typically, pyramids of biomass illustrate a progressive reduction of biomass in suc­cessive trophic levels. On the as­sumption that there is, on the average, about a 90 percent reduction of biomass for each trophic level,⁴ 10,000 kg of grass should be able to support 1,000 kg of crickets, which in turn support 100 kg of frogs. By this logic, the biomass of frog eaters (such as herons) could be, at the most, only about 10 kg. From this brief exercise you can see that, although carnivores may eat no vegetation, a great deal of vegetation is still required to support them.

A pyramid of energy illustrates the energy rela­tionships of an ecosystem by indicating the energy content (usually expressed in caloric) of the bio­mass of each trophic level. On the whole, energy pyramids resemble biomass pyramids in shape, but they help to make another conse­quence of the nature of trophic levels clearer: most food chains are short because of the dramatic reduc­tion in energy content that occurs at each trophic

A secondary consumer requires an enormous. Home range—the area needed Co obtain enough food—to encompass all the necessary producers, especially if it is a large animal. Thus, a large soli­tary predator such as a tiger may have a home range exceeding 250 square kilometers (96.5 square miles), whereas a cottontail rabbit, which occupies a lower trophic level, can live confortably on 6 hec­tares (14.8 acres). These area estimates reflect not only diet, but also other factors such as size of the organism, so the best comparison of the home ranges of primary consumers and secondary con­sumers is between animals similar in every way except eating habits. Such a comparison is possible, for example, between the home ranges of two spe­cies of mice. Living in the same community, the seed-eating white-footed mouse requires about 1.5 hectares {3.7 acres), but the carnivorous grasshop­per mouse requires 5 hectares (12.4 acres).

Variation in Productivity of Ecosystems

The gross primary productivity of an ecosystem is the rate at which energy accumulates (as biomass) during photosynthesis—that is, the total amount of photosynthesis in a given period of time. Of course, plants must respire to provide energy for their life processes, and cell respiration acts as a drain on photosynthesis. Energy that remains (as biomass) after cell respiration have occurred is called net pri­mary productivity. In other words, net primary productivity is the amount of biomass found in ex­cess of that broken down by a plant's cell respira­tion. Net primary productivity represents the rate at which organic matter is actually incorporated into plant bodies so as to produce growth net primary productivity = plant growth gross primary productivity — plant respiration total photosynthesis

Only the energy represented by net primary productivity is available for the nutrition of con­sumers, and of this energy only a portion is actually utilized by them. Both gross primary productivity and net primary productivity can be expressed in terms of kilocalories (of energy fixed by photosyn­thesis) per square meter per year, or in terms of dry weight (grams of carbon incorporated into tissue) per square meter per year.

What determines productivity? A number of factors may interact. Some plants are more efficient photosynthesizers than others. Environmental fac­tors are also important. The influx of solar energy,availability of mineral nutrients, availability of water, and other climatic factors are important, as are the degree of maturity of the community, the severity of human modification, and other factors that are difficult to assess. For example, the high productivity of intertidal communities along an ocean shoreline is largely due to wave action. Many intertidal organisms are sedentary detritus filter feeders (such as mussels) that have their food car­ried to them by wave action and so need to expend less of their own energy to obtain food.

Ecosystems differ strikingly in their productiv­ity. Terrestrial communities are generally more pro­ductive than aquatic ones, partly because of the greater availability of light for photosynthesis⁵ and partly because of higher concentrations of available mineral nutrients. However, adverse temperatures and lack of water limit the productivity of certain terrestrial ecosystems. Aquatic ecosystems have an abundance of water, which moderates temperatures, but are usually limited by the scarcity of min­eral nutrients (especially in the open sea) and the low light intensity.

The net primary productivity of an ecosystem tells us little about how much biomass is present at any given time. Despite plant growth, under natu­ral conditions there is about as much grass in a section of prairie this year as there was last year. The reason is that the turnover of plant biomass from its consumption by animals and decomposers is usually about the same as the net primary produc­tivity of the ecosystem. It is this balance that deter­mines the current plant biomass, or standing crop.