In this article we will discuss about the process of energy partitioning in food chains and food webs.
Introduction to Food Chains and Food Webs:
The transfer of food energy from its source in autotrophs (plants) through a series of organisms that consume and are consumed is termed the food chain. At each transfer, a proportion (often as high as 80 or 90 percent) of the potential energy is lost as heat. Therefore, the shorter the food chain—or the nearer the organism to the producer trophic level—the greater the energy available to that population. However, whereas the quantity of energy declines with each transfer, the quality or concentration of the energy that is transferred increases.
Food chains are of two basic types:
(1) The grazing food chain, which, starting from a green plant base, goes to grazing herbivores (organisms eating living plant cells or tissues) and on to carnivores (animal eaters); and
(2) The detritus food chain, which goes from nonliving organic matter to microorganisms and then to detritus-feeding organisms (detritivores) and their predators.
Food chains are not isolated sequences they are interconnected. The interlocking pattern is often spoken of as the food web (Fig. 3-17). In complex natural communities, organisms whose nourishment is obtained from the Sun through the same number of steps are said to belong to the same trophic level.
Thus, green plants occupy the first level (the producer trophic level), plant eaters (herbivores) occupy the second level (the primary consumer trophic level), primary carnivores occupy the third level (the secondary consumer trophic level), and secondary carnivores occupy the fourth level (the tertiary consumer trophic level). This trophic classification is one of function and not one of species as such. A given species population may occupy one or more trophic levels according to the source of the energy actually assimilated.
Transfer of Energy in Food Chains and Food Webs:
Food chains are vaguely familiar to everyone, as we eat the big fish that ate the little fish that ate the zooplankton that ate the phytoplankton that fixed the energy of the Sun; or we may eat the cow that ate the grass that fixed the light energy of the Sun; or we may use a much shorter food chain by directly eating the grain that fixed the energy of the Sun. In the last case, human beings function as primary consumers at the second trophic level.
In the grass-cow-human food chain, we function at the third trophic level, as secondary consumers. In general, humans tend to be both primary and secondary consumers as our diet most often comprises a mixture of plant and animal foods. Animals that consume both plant and animal matter are frequently referred to as omnivores. Accordingly, the energy flow is divided between two or more trophic levels in proportion to the percentage of plant and animal food eaten.
Potential energy is lost at each food transfer. Only a small portion (typically less than 1 percent) of the available solar energy is fixed by the plant in the first place. Consequently, the number of consumers (such as people) that can be supported by a given output of primary production very much depends on the length of the food chain; each link in our traditional agricultural food chain decreases the available energy by about one order of magnitude (about tenfold).
Therefore, fewer people can be supported on a given amount of primary production when large amounts of meat are part of the diet. However, as emphasized in the statement, as the quantity goes down with each transfer, the energy concentration (quality), goes up, a sort of “bad news-good news” story.
Where the nutritional quality of the energy source is high, transfer efficiencies can be much higher than 20 percent. However, because both plants and animals produce a lot of hard-to-digest organic matter (cellulose, lignin, and chitin) together with chemical inhibitors that discourage would-be consumers, typical transfers between whole trophic levels average 20 percent or less.
Table 3-8 provides approximations of the proportion of assimilated energy at each trophic level that is shunted into either production or respiration; the utilization efficiencies for each trophic level, illustrating the percentage of available energy from the preceding trophic level that is consumed (used) at that trophic level; and the assimilation efficiencies (I — NA) for each trophic level. Naturally, these percentages vary depending on food quality, homeothermy versus poikilothermy, and the stage of each species’ life history.
The approximations, however, do provide estimates regarding the diversity of species that function at each trophic level. The percentages expressed in Table 3-8 can be used to illustrate energy flow through the producer, primary consumer, and secondary consumer trophic levels, as depicted in Figure 3-15.
In Figure 3-18, the grazing and detritus food chains are shown as separate flows in a Y-shaped, or two-channel, energy flow diagram. This model is more realistic than the single-channel model because (1) it conforms to the basic stratified structure of ecosystems; (2) the direct consumption of living plants and the consumption of dead organic matter are usually separated in both time and space; and (3) the macro- consumers (phagotrophic animals) and the micro-consumers (saprotrophic bacteria and fungi) differ greatly in size-metabolism relations and in the techniques required for studying them.
The proportion of net production energy that a flow down the two pathways varies in different kinds of ecosystems and often varies seasonally or annually in the same ecosystem. In some shallow waters and in heavily grazed pastures or grasslands, 50 percent or more of the net production may pass down the grazing pathway. In contrast, marshes, oceans, forests, and indeed most natural ecosystems operate as detrital systems, in that 90 percent or more of the autotrophic production is not consumed by heterotrophs until the leaves, stems, and other plant parts die and are processed into particulate and dissolved organic matter in water, sediments, and soils.
In all ecosystems, the grazing and detritus food chains are interconnected, so shifts in flow can occur quickly in response to forcing function inputs from outside the system. Not all food eaten by grazers is actually assimilated; some (such as undigested material in feces) is diverted to the detritus pathway. The impact of the grazer on the community depends on the rate of removal of living plant material, not just on the amount of energy in the food that is assimilated. The direct removal of more than 30 to 50 percent of the annual plant growth by terrestrial grazing animals or by mowing makes the ecosystem less able to resist future stress.
The many mechanisms in nature that control or reduce grazing or herbivory by native species are as impressive as humanity’s past ability to control domestic grazing animals is unimpressive. Overgrazing has contributed to the decline of past civilizations. The choice of words is important here. Overgrazing, by definition, is detrimental, but what constitutes overgrazing in different kinds of ecosystems is only now being defined in terms of energetics, long-range economics, and ecosystem sustainability.
Under-grazing can also be detrimental. In the complete absence of direct consumption of living plants, detritus would accumulate faster than microorganisms could decompose it, thereby delaying mineral recycling and, perhaps, making the system vulnerable to fires.
Energy flows originating from nonliving organic materials involve several distinct food chain pathways, as shown in Figure 3-19. What was labeled the detritus pathway in Figure 3-18 is subdivided into three flows in Figure 3-19. One flow, often the dominant one, originates with particulate organic matter (POM); the other two pathways start with dissolved organic matter (DOM). Symbiotic fungi called mycorrhizae, aphids and other parasites, and pathogens extract photosynthate (DOM) directly from the plant’s vascular system or tissues, whereas the great majority of saprotrophic microorganisms (decomposers) consume the DOM most frequently in the form of exudates from cells, roots, and leaves.
Two distinct subsystem food chains are largely restricted to terrestrial or shallow-water ecosystems, as shown in Figure 3-19, the granivorous food chain, originating from seeds, high-quality energy sources that are major food items for animals and humans; and the nectar food chain, originating from the nectars of flowering plants that depend on insects and other animals for pollination.
Finally, Figure 3-20 shows yet another way to picture food chains, this one especially applicable to aquatic environments.
Three examples should suffice to illustrate the major features of food chains, food webs, and trophic levels. First, in the far north, in the region known as the Tundra, only relatively few kinds of organisms have become successfully adapted to low temperatures. Food chains and food webs are thus relatively simple. The pioneer British ecologist Charles Elton realized this early and, during the 1920s and 1930s, studied the ecology of Arctic lands. He was one of the first to clarify the principles and concepts relating to food chains.
The plants on the Tundra—reindeer lichens (Cladonia; often called “reindeer moss”), grasses, sedges, and dwarf willows— provide food for the caribou of the North American Tundra and for its ecological counterpart, the reindeer of the Old World Tundra. These animals, in turn, are preyed on by wolves and humans. Tundra plants are also eaten by lemmings and by the ptarmigan (Lagopus lagopus) or Arctic grouse (Lagopus mutus).
Throughout the long winter and during the brief summer, the Arctic white fox (Alopex lagopus) and snowy owl (Nyctea scandiaca) and other raptors prey on the lemmings, voles, and ptarmigans. Any radical change in the numbers of rodents or the density of Cladonia affects the trophic levels, because the alternative choices of food are few. This is one reason why the numbers of some groups of Arctic organisms fluctuate greatly, from superabundance to near extinction. The same has often happened to human civilizations that depended on a single or on relatively few local food sources, as for example in the Irish potato famine.
Second, a farm pond managed for sport fishing, thousands of which have been built over the years, provides an excellent example of food chains under fairly simplified conditions. Because a fishpond is supposed to provide the maximum number of fish of a particular species and a particular size, management procedures are designed to channel as much of the available energy as possible into the final product by restricting the producers to one group, the floating algae or phytoplankton. Other green plants, such as rooted aquatics and filamentous algae, are discouraged.
Figure 3-21 shows a compartment model of a sport-fishing pond in which transfers at each link in the food chain are quantified in terms of kilocalories per square meter per year. In this model, only the successive inputs of ingested energy are shown; the losses during respiration and assimilation are not shown. The phytoplankton is fed on by the zoo planktonic crustaceans in the water column, and the planktonic detritus is taken in by certain benthic invertebrates, notably bloodworms (chironomids), which are the preferred food of sunfishes; these sunfishes in turn are fed on by bass.
The balance between the last two groups in the food chain (sunfish and bass) is very important to the harvesting of fish by humans. A pond with sunfish as the only fish could actually produce a greater total biomass of fish than one with bass and sunfish, but most of the sunfish would remain small because of the high reproduction rate and the competition for available food. Fishing by hook and line would soon be poor. As the sportsperson wants large fish, the final predator (tertiary consumer) is necessary for a good sport-fishing pond.
Fishponds are good places to demonstrate how secondary productivity is related to (1) the length of the food chain; (2) the primary productivity; and (3) the nature and extent of energy imports from outside the pond system. Large lakes and the sea yield fewer fish per hectare, or per square meter, than do small, fertilized, and intensely managed ponds, not only because primary productivity is lower and food chains are longer, but also because only a part of the consumer population (the marketable species) is harvested in large bodies of water.
Likewise, yields are several times greater when herbivores, such as carp, are stocked than when carnivores, such as bass, are harvested; the latter, of course, require a longer food chain. High yields are obtained by adding food from outside the ecosystem (by adding subsidies such as plant or animal products that represent energy fixed somewhere else). Actually, such subsidized yields should not be expressed by area unless one adjusts the area to include the from which the supplemental food was obtained.
As might be expected, fish culture depends on the human population density. Where people are crowded and hungry, ponds are managed for their yields of herbivores or detritus consumers; yields of 1000 to 1500 pounds per acre (450-675 kg/ha) are easily obtainable without supplemental feeding. Where people are neither crowded nor hungry, game fish are desired.
As these fish are usually carnivores at the end of a long food chain, yields are much lower— 100 to 500 pounds per acre (45-225 kg per 0.4 ha). Finally, the 300 kcal • m-2 -year-1 fish yield from the most fertile natural waters or ponds managed for short food chains approaches the 10-percent conversion of net primary production to primary consumer production.
A third example is a detritus food chain based on mangrove leaves, which was described by W. E. Odum and E. J. Heald (1972, 1975). In southern Florida, leaves of the red mangrove (Rhizophora mangle) fall into the brackish waters at an annual rate of 9 metric tons per hectare (about 2.5 g or 11 kcal per m2 per day) in areas occupied by stands of mangrove trees. Because only 5 percent of the leaf material is removed by grazing insects before leaf abscission, most of the annual net primary production is widely dispersed by tidal and seasonal currents over many hectares of bays and estuaries.
As shown in Figure 3-22, a key group of small animals, often called meiofauna (“diminutive animals”), comprising only a few species but many individuals, ingest large quantities of the vascular plant detritus along with the associated microorganisms and smaller quantities of algae. The meiofauna in estuaries generally comprises small crabs, shrimp, nematodes, polychaete worms, small bivalves and snails, and, in less salty waters, insect larvae.
The particles ingested by these detritus consumers range from sizable leaf fragments to tiny clay particles on which organic matter has been adsorbed. These particles pass through the guts of many individual organisms and species in succession (a process of coprophagy) resulting in the extraction and re- absorption of organic matter until the substrate has been exhausted.
The model in Figure 3-15 can serve as a model for all ecosystems such as a forest, grassland, or an estuary. The flow patterns would be expected to be the same; only the species would be different. Detrital systems enhance nutrient regeneration and recycling because plant, microbial, and animal components are tightly coupled, so that nutrients are as rapidly reabsorbed as they are released.
The quality of the food resource is as important as the quantity of energy flow involved in the different food chains. In Figure 3-19, the grazing and detritus pathways are subdivided to show six pathways that differ greatly in resource quality. From the viewpoint of the consumer, plant products differ greatly in resource quality depending on the amount of readily available carbohydrates, lipids, and proteins, and on the amount of recalcitrant materials, such as lignocellulose, that reduce edibility.
The presence or absence of stimulants and repressors, and the physical structure of plant parts (size, shape, surface texture, and hardness) determine the rate and timing of the transfer of energy from autotrophs into the food web. Seeds, for example, have a much higher resource quality than vegetative leaves and stems. Exuded or extracted DOM (dissolved organic matter) is more nutritious than detrital POM (particulate organic matter).
Nectar and exuded photosynthate provide very high-quality food for adapted heterotrophs, and these plant outputs are very rapidly consumed when available. For example, nectarivores in entire families of insects (such as bees and butterflies), birds (such as hummingbirds and sunbirds), and even certain bats are extremely active around flowers when nectar is being secreted.
Many species have coevolved with flowering plants for mutual benefit. Colwell (1973) documented a case in which nine species of animals in widely different taxonomic groups (insects, mites, and birds) are closely associated with flowers of four kinds of tropical plants, thus creating a closely knit sub-community.
Coleman et al. (1998) has estimated that the metabolic “hot spots” of rhizospheres and active decomposition sites in the soil may occupy less than 10 percent of the total soil volume. A similar “halo” of bacteria surrounds living algal cells in the marine environment in what Bell and Mitchell (1972) have termed the phycosphere. These bacteria do not make contact with or penetrate the cell membrane and are clearly living off of the exuded organic matter.
The ratios between energy flows at different points along the food chain are of considerable ecological interest. Such ratios, when expressed as percentages, are often termed ecological efficiencies. Table 3-9 lists some of these ratios and defines them in terms of the energy flow diagram. The ratios have meaning in reference to component populations as well as to whole trophic levels.
Because the several types of efficiencies are often confused, defining exactly what relationship is meant is important; the energy flow diagrams (Figs. 3-14 and 3-15) help clarify this definition. We encourage students of ecology to read Raymond L. Lindeman’s classic paper “The Trophic-Dynamic Aspect of Ecology” for an understanding of ecological efficiencies.
Most important, efficiency ratios are meaningful in comparisons only when the numerator and denominator of each ratio are expressed in the same units of measurement. Otherwise, statements about efficiency can be misleading. For example, poultry farmers may speak of a 40 percent efficiency in the conversion of chicken feed to chickens (the Pt/It ratio in Table 3-9). But this is actually a ratio of “wet” or live chicken (worth about 2 kcal/g) to dry feed (worth about 4 kcal/g). The true ecological growth efficiency (in kcal/kcal) in this case is more like 20 percent. Thus, wherever possible, ecological efficiencies should be expressed in the same “energy currency” (such as calories to calories).
The production efficiencies between secondary trophic levels typically approximate 10 to 20 percent. Because the proportion of assimilated energy that must go to respiration is at least 10 times higher in warm-blooded animals (homeotherms), which maintain a high body temperature at all times, than in cold-blooded animals (poikilotherms), production efficiency (P/A) must be lower in warm-blooded species.
Accordingly, the efficiency of transfer between trophic levels should be higher in an invertebrate than in a mammalian food chain. For example, the energy transfer from moose to wolf on Isle Royale is about 1 percent, compared with a 10 percent transfer in a Daphnia-Hydra food chain. Herbivores tend to have higher P/A but lower A/I efficiencies than do carnivores.
To many people, the low primary efficiencies characteristic of intact natural systems are puzzling in view of the relatively high efficiencies apparently obtained in human-designed and other mechanical systems.
This has led many to consider ways of increasing nature’s efficiency. Actually, the primary efficiencies of long-term, large- scale ecosystems are not directly comparable to those of short-term mechanical systems. For one thing, much fuel is used for repair and maintenance of mechanical systems, and depreciation and repair typically are not included in calculating the fuel efficiencies of engines.
In other words, much energy (human or otherwise) other than the fuel consumed in its operation is required to build a machine and keep it running, repaired, and replaced. Mechanical engines and biological systems cannot fairly be compared unless all energy costs and subsidies are considered, because biological systems are self-repairing and self-perpetuating.
Moreover, under certain conditions, more rapid growth per unit time probably has greater survival value than maximum efficiency of energy use. By a simple analogy, it might be better to reach a destination quickly at 65 mph than to achieve maximum efficiency in fuel consumption by driving slowly.
Engineers should understand that any increase in the efficiency of a biological system will be obtained at the expense of maintenance; thus, a gain from increasing the efficiency will be lost in increased cost—not to mention the danger of increased disorder that may result from stressing the system. Such a point of diminishing returns may have been reached in industrialized agriculture, because the energy output-energy input ratio has declined as the yield per unit area has increased.