U Gaedke, University of Potsdam, Potsdam, Germany © 2009 Elsevier Inc. All rights reserved.
An understanding of the role of species populations within an ecosystem is possible only when information on multiple species is systematically joined in a way that reflects mutual interactions. A synthesis of this type produces a view of ecosystem processes that are the by-product of many simultaneous interactions among populations.
Feeding relationships have proven to be an effective means of bridging the gap between populations and ecosystems. Studies of feeding at the ecosystem level may be referred to as 'food-web analysis' or 'trophic dynamics.' Early studies of this type involved diagrammatic portrayal of feeding levels (trophic levels) in an ecosystem (Figure 1). Such a diagram begins with photoautotrophs (level 1), which live by using inorganic substances and solar energy to synthesize biomass. Photoautotrophs are consumed by herbivores, which are the first level of consumers. Herbivores in turn are consumed by carnivores of the first level, which are consumed by carnivores of the second level. When the organisms are arranged by trophic level, they can be shown as a vertical stack of boxes that comprise a feeding pyramid (or trophic pyramid), or they can be shown in more detail as a diagram with spatially separated boxes connected by lines indicating the feeding pathways (Figure 2). Because there are numerous kinds of organisms at any given trophic level, a feeding diagram takes the form of a web, which gives rise to the term 'food web.' A diagrammatic analysis of a food web may be relatively simple if it focuses only on the dominant members of each level, or it may be much more detailed if it includes the subdominant members as well.
While a qualitative diagram is informative if it is accurate, a quantification of the amount of energy or materials passing through the food web produces more insight (Figure 2). Because quantification of a food web involves the linkage of compartments by dynamic pathways representing the flow of materials or energy, the quantitative study of food webs often is conducted through the construction of computer models.
Feeding relationships as portrayed in a food web may be quantified by any of several means, according to the interest of the investigator. For example, the amount of biomass passing from one level to another may be quantified. Because biomass has an energy equivalent per unit mass, quantification of energy flow also is possible, and is especially useful in connecting the flow of energy back to the solar source and to the metabolism of individual organisms or groups of organisms. In addition, it is possible to study food webs quantitatively in terms of the passage of an element or even a class of compounds through the food web. Carbon, a surrogate for biomass, is often traced through food webs, as are phosphorus and nitrogen, the two most critical inorganic nutrients in aquatic ecosystems. Also it would be possible to demonstrate or predict the flow of fatty acids or carbohydrates through food webs. Thus, food-web modeling is applicable to a wide range of interests that involve feeding interactions.
Modern food-web analysis grew from the concept of the trophic pyramid (Figure 1), which was used by the British ecologist Charles Elton (1900-1991) and other founders of modern ecology to summarize information on the roles of organisms in ecosystems. Early portrayals of food webs typically were based on the number of organisms or on biomass. For many environments, such a diagram takes a pyramidal shape. The laws of nature do not require the pyramidal shape, however. For example, a higher trophic level may have more biomass than a lower trophic level adjacent to it.
The concept of the trophic pyramid was advanced significantly toward modern food-web analysis by Raymond Lindeman (1915-1942) who, working with Evelyn Hutchinson (1933-1991), introduced the concept of trophic dynamics. According to Lindeman, the linkages between trophic levels, when portrayed as energy flow, provide a view of the ecosystem that is linked to the solar energy source and to the metabolism of individual organisms. Furthermore, an analysis of trophic relationships based on energy leads to the calculation of energy-transfer efficiencies and other related phenomena that are readily associated with an energy-based analysis.
The early concept of trophic levels contained several important flaws, the solution of which has produced a number of innovations. The first and most obvious flaw is that individual kinds of organisms often cannot be assigned to a single trophic level. For example, an omnivorous organism, such as some types of crayfish, may be able to derive nourishment from both plant and animal matter, and therefore cannot be assigned either to the level of herbivore or to the level of carnivore. It is especially common for high-level carnivores to feed at multiple lower levels, including herbivores and lesser carnivores, at the same time.
The modern view is that species must be assigned fractional trophic levels that reflect their diet. For example, an organism feeding 50% at level 2 and 50% at level 3 would be assigned a level of 2.5. Thus, models can take into account proportionate differences in trophic level.
A second problem arises from organisms that use particulate organic matter (POM, detritus) or dissolved organic matter (DOM; also designated dissolved organic carbon, DOC) as food. Modern food-web analysis views such feeding relationships as a 'detrital food chain,' which is an important complement to the more traditionally recognized
Secondary carnivores Primary carnivores J Herbivores
Figure 1 An example of a pyramid of biomass of organisms arranged by trophic level. Such pyramids led to modern trophic diagrams and food-web analysis.
feeding relationships, the 'grazer food chain,' based on photoautotrophs, which pass organic matter to herbivores. The detrital food chain, like the grazer food chain, contains multiple levels (Figure 3).
Inclusion of the detrital food chain leads to a more realistic view of bacteria in ecosystems. Bacteria are often the direct consumers of detritus (which they dissolve prior to consuming) and dissolved organic matter. Even though they create biomass at the base of the detrital food chain, they cannot do so through use of sunlight, as is the case of production of photo-autotrophs, such as algae and aquatic vascular plants at the base of the grazer food chain. Because detritus and dissolved organic matter are abundant in aquatic ecosystems, the realities of the detrital food chain must be incorporated into quantitative models.
The increasingly realistic view of food webs leads away from the traditional pyramid or four-step energy diagram (Figure 1) and more toward diagrams that show various groups of organisms occupying noninteger positions in the grazing food chain, and connected to the detrital food chain, which incorporates bacteria as first-level producers of biomass (Figure 2).
Use of Trophic Guilds to Analyze the Trophic Structure of Food Webs
A trophic guild consists of all organisms within a food web that have similar food sources and predators (e.g., boxes shown in Figure 2). In most cases, trophic guilds defined in this way would consist of multiple species, all of which would occupy similar positions
23 Herbivorous Crustaceans
1 Carnivorous Crustaceans
Figure 2 A modern food-web diagram for the pelagic zone (open water) of Lake Constance, adjacent to Germany, Switzerland, and Austria. Fluxes are given as mg C m~2 d_1. Source: U. Gaedke.
Grazing chain 0.5% [J
Grazing chain 0.5% [J
Figure 3 Trophic pyramids for the grazing chain and the detritus chain of Lake Contance, shown in terms of annual production. Source: U. Gaedke.
Figure 3 Trophic pyramids for the grazing chain and the detritus chain of Lake Contance, shown in terms of annual production. Source: U. Gaedke.
in the trophic diagram or food-web model. Thus, application of the concept of guilds simplifies analysis and modeling of food webs.
One complication of the use of guilds is change in feeding habits for a species during its growth and development. For example, a carnivorous fish species may feed on zooplankton when it is small but feed on other fish when it is large. In such cases, it would be justified to treat the younger stages of a certain species as belonging to a different guild than the older individuals of the same species.
Figure 2 shows the structure of a food-web model of the open waters of Lake Constance, which adjoins Germany, Switzerland, and Austria. The model is based on boxes corresponding to guilds of organisms as well as a pool of DOM and POM. Carbon comes into the food web through this DOM and POM pool or through a photosynthetic guild (in this case, phy-toplankton) and leaves the food web either as sedimentation of POM to the bottom of the lake or release of CO2 gas to the water as a by-product of respiration. Circles within each of the boxes indicate net growth. Arrows passing from one box toward another indicate consumption, and arrows passing out of each box indicate grazing, predation, fecal output, or other losses. The numbers indicate fluxes (biomass per unit time for a given unit area of the lake, in this case expressed as mg C m~2 d).
A quantitative model such as the one shown in Figure 2 is either an average for a considerable period of time or snapshot of an instant in time. The flow of mass from one compartment to another can be expected to change substantially from season to season and even week to week in a lake. Thus, modeling that encompasses changes occurring over any substantial period of time (e.g., a year) can require a great deal of information.
Although the progressive change in dynamics of food webs over time is seldom quantified because of the large amount of data that would be required, averages or snapshot views of food webs can reveal the main fluxes that account for ecosystem metabolism and the efficiency of transfer of mass or energy through the food web.
Trophic Transfer Efficiency
The trophic structure of a food web is affected greatly by loss of energy that occurs between trophic levels. It is not unusual for loss of energy between any two adjacent trophic levels to reach 90% (Figure 3) because in Figure 3 losses are between ca. 70 and 80% throughout. As recognized by Lindeman when he created the concept of trophic dynamics based on energy transfer, the second law of thermodynamics, when applied to trophic transfers, requires that dissipation of energy as heat will be a substantial loss even for the most efficient transfers. For example, the growth efficiency of individual cells seldom exceeds 40%, even when nonmetabolic losses are disregarded. Ecological factors introduce nonmetabolic inefficiencies related to nongrazing mortality, export of organic matter from the system (outflow or sedimentation), and elimination (fecal output).
Nongrazing mortality, which can occur when organisms are exposed to lethal physical or chemical conditions, physiological death related to age, or inadequate nutrition, divert biomass and energy from the transfer process that connects one trophic level to the next in the grazer food chain. Nongrazing mortality passes energy and mass to the detrital food chain. Elimination (fecal output) also is drain on efficiency in that mass or energy passes from the grazer food chain to the detrital food chain. Losses to elimination may be especially high for organisms feeding on organic sources that are difficult to digest. This is particularly the case for herbivores consuming vascular plant tissues, which are rich in complex carbohydrates that are difficult to digest.
All organisms must respire in order to live. Respiration involves the release of chemical energy from organic matter. The released energy is used in part for synthesis of new biomass, and is also used to maintain the integrity of basic metabolic functions that sustain life. Therefore, a considerable amount of organic matter is lost (converted to CO2 + H2O and energy) for energy production purposes without passing to a higher trophic level.
The various modes by which energy and mass can be lost are the basis for quantifying several kinds of ecological efficiencies that are used in food-web analysis. The ratio of biomass synthesized (either growth or production of reproductive biomass such as eggs) to biomass ingested as food is symbolized K1, the gross growth efficiency. K1 rarely exceeds 0.30-0.35 under natural conditions, and often is considerably lower. The net growth efficiency of an organism (K2) is defined as the ratio of new biomass produced to the amount of biomass that is assimilated (i.e., passing through the gut wall into the body of the organism). This efficiency has a maximum value of 50% for small organisms but is lower for large organisms (e.g., fish).
Trophic transfer efficiency is the ratio of biomass production at one trophic level to the biomass production of the next lower level. In plankton food webs, trophic transfer efficiencies may be high (0.15-0.30) (Figure 3) when compared with webs dominated by a transfer from vascular plants to herbivores. Temporal variability of the trophic-transfer efficiency is high, even within a given ecosystem, because of the instability of the numerous factors that can affect the trophic transfer efficiency. For example, the trophic transfer efficiency is strongly reduced when inedible algae (such as cyanobacteria) dominate or when animals with high respiratory needs (such as vertebrates and especially birds and mammals) prevail.
Food Quality and Quantity
Heterotrophs (consumers, including bacteria) live by consumption of biomass or nonliving organic matter that is derived from biomass. Because the chemical composition of protoplasm in biomass (disregarding skeletal material or support structures) across all heterotrophs falls within a relatively narrow range, heterotrophs that feed on biomass are assimilating approximately the same mixture of elements that they will need in order to synthesize their own biomass (skeletal material and support structure typically pass through the gut, unassimilated). Detritivores also benefit from this carryover of elemental mixtures from one kind of organism to another, although detritus is more likely to show some selective loss of elements that would alter the balance typical of living biomass.
Unlike heterotrophs, photoautotrophs assimilate elements separately from water or, if they are rooted vascular plants, from sediments. For example, carbon is derived from H2CO3 and related inorganic carbon forms dissolved in water, and phosphorus is taken up separately as phosphoric acid that is dissolved in water. Because the inorganic substances required to synthesize biomass are taken up separately, large imbalances may develop when some essential components are much more abundant than others. Thus, autotrophs face greater challenges than heterotrophs in assembling the necessary ratios of elements to synthesize biomass, but even heterotrophs can experience imbalances of elements.
The approximate ratios of elements that are characteristic of autotrophic biomass have been extensively studied. Characteristic ratios of carbon to nitrogen and phosphorus are often the greatest focus of analysis because carbon is the feedstock for photosynthesis and phosphorus and nitrogen are the two additional elements that are often in short supply for conversion of photosynthetic products (carbohydrate) to other molecule types that are needed for the synthesis of protoplasm (e.g., amino acids). The importance of C:N:P ratios in aquatic organisms was first brought out by Alfred Redfield (1890-1983), who discovered that healthy oceanic phytoplankton show a characteristic atomic C:N:P ratio of about 106:16:1. Thus, the nutrient status of a phytoplank-ton community can be judged to some degree from the elemental ratios. For example, a phytoplankton community suffering phosphorus deficiency may show a C:P ratio of 500:1 rather than 106:1, as predicted by the Redfield Ratio for well-nourished phy-toplankton. The analysis of elemental ratios for diagnosis of elemental imbalances is termed 'ecological stoichiometry.'
Imbalances in elemental ratios in one trophic level can create imbalances or inefficiencies at the next trophic level. This is particularly true between primary producers and herbivores. For example, phyto-plankton suffering phosphorus scarcity may pass biomass with a high C:P ratio to grazers that consume phytoplankton. Because of an imbalance of elements in the food, the grazers must consume extra food in order to obtain the correct balance for the synthesis of their own biomass. Similarly, an especially low C:P ratio (e.g., 50:1) will provide an oversupply of phosphorus (typically this happens when bacteria are consumed), some of which would be released to the environment without generating any biomass.
One strategy that herbivores may employ in improving the elemental balance of food intake is to consume heterotrophs in addition to autotrophs (omnivory, which is feeding at multiple trophic levels). Thus, consumption of a phosphorus-rich food could be offset by consumption of a carbon-rich food, and the combination would provide more efficient use of ingested mass than a single food type.
Trophic Structure of Food Webs in Open Water and Near Shore
In the open water of a lake at some distance from the shore (i.e., in the pelagic zone), the dominant auto-trophs are phytoplankton, which live as individual cells or small colonies of cells suspended in the water. Some groups of phytoplankton (e.g., filamentous cyanobacteria) are rejected by most grazers because they are difficult to ingest or difficult to digest. Algae of low palatability may become quite abundant when grazing pressure is high in the pelagic zone. Overall, however, phytoplankton often is composed of a high proportion of edible and digestible species. Vascular plants (macrophytes), which typically grow near the shores of a lake (in the littoral zone), offer a less useful food supply because the digestible component of biomass is embedded in complex carbohydrates that maintain the shape of the plant. For this reason, much of the macrophyte biomass that develops during a growing season in the littoral zone of a lake dies and decomposes rather than being eaten while alive. The detrital food chain benefits from the biomass that was left uneaten by the grazer food chain.
Macrophytes may account for most of the auto-troph biomass in a littoral zone, but attached algae colonize macrophytes and all other surfaces that are illuminated in the littoral zone. These attached autotrophs (periphyton) are an important food for herbivores in the littoral zone, even though the macro-phytes themselves often are not.
The trophic structure in the pelagic zone often is dominated by four significant trophic levels. The top predator guild, which consists of multiple fish species, is not exposed to substantial predation pressure because of its size and mobility. The top predator guild therefore builds up biomass until it encounters a limitation caused by scarcity of appropriate food. This type of limitation is called a 'bottom up' limitation because the trophic level below is restricting increase in biomass of the trophic level above. While the top predator guild is controlled from the bottom up, its prey are controlled from the top down. 'Top down' control in this situation involves suppression of population biomass of the trophic level that serves as food for the abundant top predator.
In a four-level food web, the third level down from the top consists of herbivores. When top predators are abundant, the intermediate predators (e.g., fish that eat zooplankton) are suppressed, which relieves grazing pressure on herbivores. Therefore, herbivores, which consist mostly of zooplankton, may become abundant because of weak top-down control. This, in turn, will impose a strong grazing pressure on phytoplankton, which can cause a decline in phy-toplankton biomass, yielding a higher water transparency. Thus, water clarity may be enhanced by stocking large predatory fish. This technique is an example of biomanipulation.
Physical factors may affect the structure of food webs. For example, the absence of any solid attachment points in the pelagic zone requires that all auto-trophs be small. Thus, the food web must be based on grazers capable of harvesting large numbers of small autotrophs. The grazers that can do this work are also small, which creates a niche for small carnivores that eat the small herbivores. This leaves the opportunity for a top carnivore level that feeds on the small carnivores. Thus, predator and prey are related in body size through feeding behavior.
Food webs of the littoral zone in lakes are more similar to those of terrestrial environments or wetlands than to those of pelagic food webs of lakes if their production is dominated by vascular plants. Grazers (herbivores) may then be larger because larger units of food are available. Thus, top carnivores may feed directly on herbivores rather than on intermediate carnivores and body size and trophic level are not as strongly correlated as in pelagic food webs. In this case, the herbivores come under top-down control, and the efficiency of energy transfer from plants to herbivores is reduced because there are not enough herbivores to consume the total plant production. Thus, the length of the food chain in different portions of the ecosystem, or even in different ecosystems, affects the amount of biomass that can be produced by each trophic level.
Transfer Efficiency along the Size Gradient in Pelagic Food Webs
In pelagic food webs, all autotrophs are small and predators exceed the size of their prey. Thus, the flow of matter and energy in pelagic food webs is from small to progressively larger organisms. The entire food web's trophic transfer efficiency along the size gradient of organisms reflects each of the trophic linkages and the number of trophic levels in the web. For example, efficiency of fish production in oligotrophic lakes is lower than the efficiency of fish production in eutrophic lakes when expressed as a proportion of the total amount of photosynthesis.
In unproductive lakes, food is more strongly dispersed, and is gathered by consumers that search for their prey. Consumers that locate their prey by searching usually are closer in size to their prey than consumers that feed without searching (by filtration of water, for example). When consumers are very close in size to their food source, the transfer of energy and materials along the size gradient up to large fish is less efficient because it requires more transfers to reach a given size. Thus, there is a theoretical explanation for the lower transfer efficiency along the size gradient in oligotrophic lakes. This is an example of the use of food-web analysis to explain observations about productivity and efficiency in lakes.
Quantitative studies of food webs greatly magnify the value of information on individual species populations. Analysis of linkages in the food web provide explanations for the efficiency of energy or mass transfer to various points in the food web and the controlling influences that either enhance or suppress production at a given level in the food web. Thus, quantitative food-web studies support the understanding of mechanisms that govern the functioning of aquatic ecosystems.
Detrital food chain - Food-web components that begin with particulate organic matter (detritus) or dissolved organic matter, and pass through bacteria to other consumers.
Ecological stoichiometry - Study of element ratios within biomass in relation to element ratios within food or in the surrounding environment.
Food web - A diagram or model that shows the feeding connections between all major groups of organisms in an ecosystem.
Grazer food chain - Components of a food web that begin with photoautotrophs (algae, aquatic vascular plants) and pass through herbivores to carnivores.
Trophic guild - A group of organisms that share common food sources and common predators.
Trophic level - A group of organisms that obtain their food from sources of equal distance from the original source. Autotrophs equal level 1, herbivores equal level 2, primary carnivores equal level 3, and secondary carnivores equal level 4. Species feeding from more than one level may be assigned a fractional trophic level.
Trophic transfer efficiency - The fraction of total production at a given trophic level that is converted to production at the next trophic level.
See also: Lakes as Ecosystems.
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