Food chains and food webs

All activities of life are powered, whether directly or indirectly, by a single source of energy: sunlight. The energy enters the biosphere through primary producers, which trap and convert solar energy into chemical energy, first in the form of sugars and, ultimately, in other complex organic molecules. Energy in its chemical form is then passed along from one type of organism to another through a complex feeding relationship. Two fundamental laws underlie this function: Energy moves through ecosystems in a one-way street, while nutrients cycle and recycle. Each time the energy is used, some of it is lost as heat. Energy needs constant replenishment from an outside source, the sun. In contrast to energy, nutrients constantly cycle and recycle in a circular flow. Nutrients enter the system from soil or water or atmosphere through primary producers (plants), and pass along to herbivores, then to carnivores. The wastes and dead bodies or body parts degraded by detritus feeders return nutrients to the ecosystem.

Feeding Relationships Within a Community

Feeding relationships in a community are often defined or described through food chains and food webs. A linear feeding relationship spanning all trophic levels is called a food chain, whereas many interconnecting food chains in a community make up the food web. Obviously, different ecosystems have drastically different food chains. To illustrate who feeds on whom in a community, it is better first to examine some basic laws and structures that govern a community in general. Energy and nutrients are two common elements that sustain all communities and ecosystems. Energy enters communities through the process of photosynthesis, by which plants and other photosynthetic organisms trap a small portion of sunlight and convert it into sugars. Photosynthetic organisms, from the mighty sequoia to the zucchini and tomato plants in a garden to single-celled diatoms in the ocean, are called autotrophs or producers, because they produce food for themselves. They also produce food for nearly all other organisms, called heterotrophs or consumers.

The amount of life a community can support is determined by how much energy the producers within it can capture. Within the community, energy flows from producers, occupying the first trophic level, through several levels of consumers. The consumers that feed directly and exclusively on producers are herbivores, ranging from caterpillars to buffaloes to wheat aphids. These herbivores, also called primary consumers, form the second trophic level. Carnivores, such as the spider, eagle, fox, and birds that eat caterpillars, are meat eaters, feeding primarily on primary consumers. Carnivores are secondary consumers that form the third or higher trophic level. Some consumers, such as the black bear that eats both blueberries and salmon, occupy more than one trophic level.

A food web, however, represents many interconnecting food chains in a community, describing the actual, complex feeding relationships within a given community. A food web also reflects the feeding nature of organisms that occupy more than one trophic level. Animals such as raccoons, bears, rats, and a variety of birds are omnivores, eating at different consumer levels at different times. In addition to producers and consumers, a functional ecosystem also consists of detritus feeders and decomposers that release nutrients for reuse. The extremely diverse network of detritus feeders is made up of earthworms, mites, protists, centipedes, nematodes, worms, some crustaceans and insects, and even a few vertebrates such as vultures. Except for vultures, these organisms thrive in the garden, and compost by extracting energy stored within dead organic matter, in turn releasing it in a further decomposed state. The excretory products released serve as food for other detritus feeders and decomposers, which are primarily fungi or bacteria. Fungi and bacteria digest food outside their bodies by releasing digestive enzymes into the environment. They then absorb the nutrients they need and leave the remaining nutrients for recycling. Without detritus feeders and decomposers, nutrients would soon be locked into organic matters, and the ecosystem will cease to be functional.

Energy Transfer and the Nutrient Cycle

One important principle that governs the flow of energy through the ecosystem is that the energy transfer from one trophic level to the next is never efficient. The net transfer of energy between two trophic levels is roughly 10 percent. During the transfer, 90 percent of the energy is lost as heat or in other forms. Of one thousand calories stored in the producer (plants), the caterpillars that consume all plant tissues will obtain one hundred calories, a bird that eats caterpillars will extract ten calories, and when a hawk catches the bird, that portion of the energy is reduced to a mere one calorie. This inefficient energy transfer between trophic levels is called the 10 percent law, or an energy pyramid. This 10 percent law has profound impacts within an ecosystem. Plants have the most energy available to them; the most abundant animals will be those directly feeding on plants, and carnivores will always be relatively rare, especially those of large size that occupy a higher trophic level.

However, when toxic substances pass through trophic levels, the exact opposite is true. While energy diminishes in the process of flowing from lower to higher trophic levels, toxic substances progressively increase in concentration along the food chain. This phenomenon, called biological magnification, was discovered through the study of the use of the pesticide Dichloro-diphenyl-trichloroethane (DDT). Tests of water samples following the use of this pesticide showed a trace amount of DDT. Tissue analyses of predatory birds in the same aquatic ecosystem, however, revealed a DDT concentration a million times greater than that in the water. Fish caught from the same waters also contained much higher DDT levels than the water, but substantially lower levels than that of the birds that consumed those fish. DDT has since been confirmed as the cause for population declines of several predatory birds, especially fish eaters such as bald eagles, brown pelicans, and cormorants. Understanding biological magnification is crucial to the prevention of widespread loss of wildlife.

Unlike energy, no mechanism or source exists to allow a constant replenishment of nutrients. The same pool of nutrients has been supporting life from the beginning. Nutrients are elements and small molecules that form all the building blocks of life. Macronutrients are those acquired by organisms in large quantities, including water, carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, and calcium. Micronutrients, including zinc, molybdenum, iron, selenium, and iodine, are acquired in trace quantities. Nutrients cycle from producer, to consumer, to detritus feeder and decomposer, and eventually back to producer. The major reservoirs of nutrients are in the nonliving environment, such as soil, rock, water, and atmosphere. In an undisrupted ecosystem, nutrients have cycled and recycled in a sustainable manner for thousands of years. However, human intervention, either through industry or agriculture, has created enormous problems for sustainable nutrient cycling. One example is the nitrogen cycle. Although plants may obtain nitrogen from soil, the primary source of nitrogen is the atmosphere. Nitrogen gas (mainly N₂) makes up over 70 percent of the air. N₂ may be extracted by some plants or converted by lightning into usable forms that drop as rainfall. Through industrial production and use of nitrogen fertilizers in agriculture, humans have overcharged the nitrogen cycle, causing acid rains and surface water and groundwater contamination, which pose a serious threat to natural ecosystems.

Food Chains and Food Webs in a Terrestrial Ecosystem

Since different ecosystems have drastically different food chains and food webs, they can be better illustrated using examples of different communities. The first example is a land community, to provide an overview of terrestrial food chains and food webs. Plants such as maple trees and squash are the producers that occupy the first trophic level. Aphids, caterpillars, grasshoppers, and other animals that forage directly on plants or plant tissues are primary consumers that occupy the second trophic level. Birds, spiders, and other insects that feed on primary consumers are secondary consumers occupying the third trophic level. Large birds, such as eagles, owls, and hawks, that eat secondary consumers are tertiary consumers making up the fourth trophic level. This food chain can go on to even higher trophic levels.

However, natural communities rarely contain well-defined groups of primary, secondary, and tertiary consumers in a linear pattern. In reality, a food web showing many interconnecting food chains in a community describes much more accurately the actual feeding relationships within a given land community. This is in part due to the omnivorous, or “eating all” nature of some animals. These animals include but are not limited to bears, rats, and raccoons. They act as primary, secondary, and even tertiary consumers at different times. Many carnivores will eat either herbivores or other carnivores, thus acting as secondary or tertiary consumers, respectively. An owl, for instance, is a secondary consumer when it eats a mouse, which feeds on plants, but a tertiary consumer when it eats a shrew, which feeds on insects. Once a shrew eats carnivorous insects, it is by itself a tertiary consumer, making the owl that feeds on the shrew a quaternary consumer that occupies the fifth trophic level. Since organisms are interlocked in such complex yet organized networks, disruption at any particular point of the food web (damage to one group of organisms) might have far-reaching effects on a whole community or ecosystem.

Food Chains and Food Webs in a Marine Ecosystem

Coralreefs will be used as an example for marine ecosystems. Reefs are created by concerted efforts of producers—algae—and consumers—corals. In warm tropical waters, with just the right combination of bottom depth, wave action, and nutrients, specialized algae and corals build reefs from their own calcium carbonate skeletons. The reef-building corals grow best at depths of less than forty meters, where light can penetrate and allow their algal partners to photosynthesize. Algae and corals are involved in a mutualistic relationship, where algae benefit from the high nitrogen, phosphorus, and carbon dioxide levels in the coral tissues. In return, algae provide food for the coral and help produce calcium carbonate, which forms the coral skeleton.

Coral reefs provide an anchoring place for many other algae, a home for bottom-dwelling animals, and shelter and food for the most diverse collection of invertebrates and fish in the oceans. In essence, algae is the producer that occupies the first trophic level. Corals that feed on algae are primary consumers sitting at the second trophic level. Many fish (such as blue tang) that feed on corals are secondary consumers occupying the third trophic level. Larger fish, such as sharks that eat small 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 2000. Assuming no effective measure is taken to preserve or restore coral reef ecosystems, another 50 percent of reefs will disappear by 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 are affected. The price for such disruption is high and far-reaching.

Principal Terms

Biological Magnification: The increasing accumulation of a toxic substance in progressively higher feeding levels

Consumer: An organism that eats other organisms

Decomposers: Microbes such as fungi and bacteria that digest food outside their bodies by secreting digestive enzymes into the environment

Detritus Feeders: An array of small and often unnoticed animals and protists that live off the refuse of other living beings

Energy Pyramid: A graphical representation of the energy contained in succeeding trophic levels, with maximum energy at the base (producers) and steadily diminishing amounts at higher levels

Nutrient Cycle: A description of the pathways of a specific nutrient (such as carbon, nitrogen, or water) through the living and nonliving portions of an ecosystem

Producers: Organisms that produce food for themselves as well as for nearly all other forms of life, including plants, plantlike protists, and cyanobacteria

Trophic Level: The categories of organisms in a community, and the position of an organism in a food chain, defined by the organism’s source of energy

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