Food Chains

A food chain, or trophic chain, is a group of organisms in an ecosystem connected consecutively with each other by the "consumer-food" principle. The food chain concept relates to the position of a group of organisms in an ecosystem but not to their taxa, since a species may occupy different trophic levels at different parts of its life cycle, or eat from more than one trophic level. Food chains begin with primary producers or autotrophs that obtain their energy directly from the sun (plants, algae, and cyanobacteria) and move through one or more levels of consumers or secondary producers (heterotrophs). The simplest food chain in the Arctic begins, for example, with lichens, grasses, and undershrub consumed by reindeer, or caribou, which in turn become the prey of the wolf. Marine food chains (see Food Webs, Marine) generally have a higher number of trophic levels.

Food chains result in the transformation of energy and materials in ecosystems, and a major part (c.80-90%) of the potential energy (stored as carbohydrates) is lost at each transition to a higher trophic level. For example, the formation of 1 kJ of energy of a predator biomass requires the consumption of about 100 kJ from the primary producer, in a three-level food chain. This low efficiency of food conversion to biomass is due to the energy required for respiration, motion, and keeping warm, and incomplete consumption or indigestibility of the food mass (much of the food mass may be poorly digestible chemical substances, such as bones, cellulose, and chitin, or may contain chemical inhibitors). The efficiency is also represented by the number of steps in the food chain. Organisms acquiring their energy from the sun through a similar number of steps belong to the same trophic level. The organisms of the lower trophic levels generally consume a higher relative mass of food than those from the higher levels: for example, herbivores require relatively more food in proportion to their body mass than predators. The number of trophic levels in food chains is usually three or four, and even in the impoverished Arctic communities the food chains are usually no longer than in the tropics. Several hypotheses have been proposed to explain this, including hypotheses based on the energy flow, selective feeding, and ecosystem dynamics.

There are two main types of food chains: those where consumers feed on living organisms, and detritus chains where consumers are decomposers or eat dead organisms. The first type is represented by autotrophic organisms as the base, followed by phytophagous animals (herbivorous mammals, phytophagous zooplankton, etc.), then first-order predators (e.g., some fishes and salamander larvae consuming zooplankton), then second-order predators (e.g., predatory fishes and birds). In the detritus chains, a major part of the plant production is not consumed by phytophagous animals, but is decomposed by saprotrophic organisms. Such food chains are most widespread in forests, but have a lesser role in terrestrial ecosystems of the Arctic. However, they play a more important role at considerable depths of the ocean and in small eutrophic water bodies on land.

The two types of food chain are not isolated from one another. For example, the annual primary production in Arctic seas is shared between the zooplankton (floating) and the detritus (benthos, or sea-bottom) chains. The switch between these energy flows may take place as a response to environmental change, ensuring the productivity of populations of fish that consume different groups of invertebrates.

Primary production in the Arctic is low, which causes low levels of the production at higher trophic levels. Both parameters increase significantly from the polar deserts and semideserts to the Low Arctic. The intensity of energy transfer through food chains in the Arctic varies more significantly than in the more southern regions due to sharp seasonality in behavior and migrations of many dominant species of animals. The level of influence of herbivorous animals on the community depends not only on their energetic peculiarities but also on the exemption of plants from the community. Marine zooplankton usually consumes more phyto-plankton than it may assimilate, and the excess enters the detritus chain. The food chains undergo significant seasonal changes, both in aquatic and terrestrial ecosystems. For example, there is a clear annual cycle in the diet of cod feeding in the Barents Sea. In February-April, after the winter period of scarce feeding, the cod move eastward and forage intensively on pelagic fishes such as herring. The shoals of the spawned groups come somewhat later and also forage on fish. In summer, the cod forage on higher crustaceans, Euphasiacea and Hyperiidae in the central regions, and are concentrated in autumn in the eastern parts of the sea, where they use the secondary food resource, crustaceans and molluscs. The beginning of the westward migration coincides with the transition to fish feeding, and some groups may cease feeding for a certain time. Food chains in the Arctic seas are also significantly dependent on the seasonal development of the plankton, which comprise a significant basis of the chain. For example, the spatial distribution of herring in the Barents Sea and the directions of its horizontal movements depend, in large part, on the composition and distribution of this food resource. The southwestern part of the Barents Sea represents the main foraging area of the herring, whereas at the end of winter, when plankton production is decreased, the fish recede back into the Norwegian Sea. The seasonality in fish and marine mammalian migrations influences the plans of fishing and the size of catches.

In terrestrial ecosystems, small herbivores, and especially rodents from the genus Lemmus (Lemming), may comprise more than 90% of all terrestrial vertebrates. Populations of these rodents undergo significant cyclic increases and decreases, which leads to a corresponding change in their pressure on the herbaceous cover, and especially overgrazing of monocotyledons during years of peaks of population. The fluctuations in population numbers of vertebrate predators in tundra, for example, the Arctic fox (Alopex lagopus), follow the fluctuations in their main prey, the rodents. This is caused by the scarcity of alternative food sources (resulting from low overall species diversity) in severe conditions of the Arctic. The existence of some predatory birds in tundra in winter is probably possible only due to high abundance of lemmings in winter. The smallest mammalian predators of the Arctic, the short-tailed weasels (Mustela rixosa and M. erminea), also depend primarily on lemmings, but often eat bird eggs and nestlings. Thus, the fluctuation of the primary food source of predatory Arctic animals causes significant corresponding changes in higher trophic levels.

Large herbivorous mammals also play an important role in food chains in the Arctic. This especially concerns the wild reindeer or caribou. They influence most seriously areas with a weak plant cover, and a significant destruction of plant cover occurs in some places. Muskox (Ovibos moschatus) is the largest High Arctic terrestrial herbivore. As with reindeer, it displays significant feeding migrations. In winter, it grazes mainly on lowlands, where plants are more available, and occurs in more marginal lands in summer. The muskox tends to be a more intensive grazer than the sympatric caribou.

Herbivorous animals in the Arctic terrestrial ecosystems depend in large part, on the plant resource abundance and availability. Lichens constitute a very important component in the food of reindeers. Other herbivorous animals consume mainly green parts of higher plants. Stems, seeds, and underground organs, having lower energetic value, are consumed in smaller amounts. Berries play an insignificant role in the feeding of tundra animals, because such plants are more widely distributed southward, in the southern tundras, forest tundras, and forests. The abundance and availability of plants are lower in the tundra ecosystems as a result of low primary productivity in high latitudes. In addition, the rate of recovery of the plant cover there is lower than southward. In this situation, overgrazing represents a serious threat, and the population number, density, spatial distribution, and behavior of wild herbivorous animals there are adapted to peculiarities of the Arctic vegetation. These feedback connections between components of trophic chains were formed in the course of evolution of organisms and ecosystems to ensure the long-term coexistence of predator and prey natural populations. Artificial interference in this system leads to its disturbance, which may result in a significant destruction of ecosystems and impoverishment of natural communities (as in the conditions of unsustainable cattle breeding in the North).

Although the number of bird species is not high in Arctic ecosystems and seasonality is one of the main traits of the Arctic bird fauna, the number of trophic levels occupied by them is similar to that in the temperate zone. Birds play the most important role in the coastal zone, where the nesting aggregations may cover some areas almost totally. Their predatory pressure on fishes is more significant than the pressure on components of terrestrial ecosystems. The diversity of trophic niches is highest in semiaquatic birds, and many of them reach high trophic levels. Some marine birds, for example, large gulls, display cleptoparasitism, where one bird often forages by attacking another individual with a fish and taking its food. Such individuals may occupy highest trophic levels. Like in predatory mammals, predatory birds also depend on fluctuating prey, for example, the abundance of owls follows that in lemmings. Herbivorous birds may influence significantly the plant cover in tundra. For example, different species of migrating geese, forming dense aggregations, destroy the herbaceous cover to such an extent that moss tundras are formed on the places of cotton grass—moss tundras with subsequent development of other types of tundras. Thus, the trophic activity of animals is a factor of community successions in the Arctic. Carrying of seeds by birds may represent another factor of change in plant associations in the Arctic. As a rule, such transfer occurs within a community, but in some cases the seeds may cross considerable distances, and new species may appear in some habitats.

Species foraging on insects and other invertebrates may be seasonally abundant in Arctic regions, but their active periods are short and they have to migrate southward or hibernate due to impossibility of feeding in cold time. Their feeding ecology undergoes some changes resulting from their adaptation to Arctic environment. For example, the Siberian newt (Salamandrella keyserlingii) and some brown frogs (Rana amurensis and R. arvalis) consume more aquatic prey in tundra than in the temperate forest zone, because of their tendency to stay near small water bodies. Like in other geographic zones, the newt at larval and adult phases of its life cycle is the top predator in such fishless habitats.

Among higher vertebrates of the Arctic, there are almost no animals that store their food. Thus, the majority of animal species in the Arctic are polyphages, for example, the forms with relatively broad food spectra. Food specialization, for example, adaptations to feeding on a specific type of food, is rare due to the relatively low prey species diversity. Under conditions of sharp fluctuations of prey abundance and availability both within and between years, predators should be able to switch quickly to alternative prey types for survival. The absence of food specialization corresponds, neverthe less, to more or less developed selective feeding. The adaptive significance of selective feeding is the enhancement of net energy intake under conditions of short vegetative period and scarcity of food.

Food chains result in the transfer, and sometimes concentration, of certain contaminants such as heavy metals, radionuclides, and pesticides. The rate of their concentration depends on the particular organisms, their habitats, type of contaminant, and many other factors. In many cases, it is not easy to identify the disturbance at early stages of pollution, and the situation becomes clear only after significant changes have taken place. The pollutants may concentrate with increasing trophic level, and long chains, such as marine food chains, are the most vulnerable. The low rate of renewal of components of Arctic ecosystems makes them highly sensitive to pollution, and the possibility of concentration of toxic substances in food chains should be considered in the revision of protective measures for Arctic ecosystems, for example, in plans for oil transportation, extraction of minerals, and burial of radioactive substances at sea.

Finally, it should be noted that many species consume organisms belonging to different trophic levels, and it is more realistic to consider the real situation in nature as a trophic network (food web) instead of a linear chain. Although the number of trophic levels is similar in the Arctic and in regions with higher species diversity, the complexity of the food network depends on the community diversity, which increases southward from the Arctic.

Sergius L. Kuzmin

See also Food Webs, Marine; Primary Production; Secondary Production; Trophic Levels

Further Reading

Giller, P.S., Community Structure and the Niche, London and

New York: Chapman and Hall, 1984 Murray, J.L., "Ecological Characteristics of the Arctic." In AMAP Assessment Report: Arctic Pollution Issues, Oslo, Norway: AMAP Secretariat, 1998 Steele, J.H. (editor), Marine Food Chains, London: Oliver and

Boyd Publ., 1970 Zenkevich, L.A., Biologiya Morei SSSR, Moscow: USSR Academy of Sciences, 1963; as The Biology of the Seas of the USSR, New York: Wiley, 1963

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