Chapter 12 - Energetics and Nutrient Cycling

Natural ecosystems comprise an innumerable amount of different organisms. These organisms are not separated, they interact and depend on each other. Today’s ecosystems are facing an enormous decline in biodiversity due to human impacts with thus far unknown consequences. One key objective of ecological research is to understand the mechanisms generating and maintaining this incredible amount of diversity. However, comprehensive analyses of natural ecosystems are impeded by their complexity and diversity. Food webs, therefore, provide an excellent tool to analyze the complexity of ecosystems. They depict the system‘s diversity and species interactions in a condensed form. Furthermore, food-web structure can help to predict the interaction strengths between species and the energy pathways through the system. In my thesis, I use food web structure to analyze structural properties which separate food webs from other network types and furthermore I investigate generalities and differences of food-web structure across different ecosystems. 
\nOne of the most important ecosystems is the soil ecosystem, as it provides the base for aboveground productivity. However, detailed soil food webs are scarce. In chapter 2, I assembled the complex food webs of 48 forest soil communities and analyzed if soil food webs differ in their topological parameters from those of other ecosystems. I found that soil food webs are characterized by a higher number of omnivorous and cannibalistic species. Moreover, they comprise more trophic chains and intraguild-predation motifs than food webs from other ecosystems. Finally, soil food webs showed high average and maximum trophic levels. These differences in network structure to other ecosystem types may be a result of ecosystem-specific constraints on hunting and feeding characteristics of the species that emerge as network parameters at the food-web level. Despite these differences, soil food webs showed the same scaling of their properties with connectance and size. In a second analysis of land-use effects, I found significant but only small differences of soil food web structure between different beech and coniferous forest types, which may be explained by generally strong selection effects of the soil that are independent of human land use. This study has unravelled systematic structures of soil food-webs, extending our mechanistic understanding how their environmental characteristics determine patterns at the community level. Additionally, I have shown that the general scaling laws also apply for soil food webs.
\nIn addition to purely topological properties, I analyzed another important aspect of food webs. The distributions of body masses and degrees across species are key determinants of food-web structure and dynamics. In chapter 3, I analyzed body masses of species and their systematic distributions across food-web structure. In particular, allometric degree distributions combine both aspects in the relationship between degrees and body masses. They are of critical importance for the stability of complex ecological networks. I used an entirely novel global body-mass database including food-web structures of four different ecosystem types to analyze body-mass distributions, cumulative degree distributions, and allometric degree distributions regarding differences among ecosystem types. My results demonstrate some general patterns across ecosystems: the body masses are either roughly log-normally (terrestrial and stream ecosystems) or multimodally (lake and marine ecosystems) distributed, and most networks exhibit exponential cumulative degree distributions except stream networks that most often possess uniform degree distributions. Additionally, with increasing species body masses we found significant decreases in vulnerability in 70% of the food webs and significant increases in generality in 80% of the food webs. Overall, these analyses document striking generalities in the body-mass and degree structure across ecosystem types as well as surprising exceptions (uniform degree distributions in stream ecosystems). This suggests general constraints of body masses on the link structure of natural food webs irrespective of ecosystem characteristics. 
\nWhile I revealed general patterns of food-web topology in chapter 2 and 3, I investigated the drivers of these general patterns in chapter 4. Therefore, I analyzed the influence of different external factors on community (beta diversity) and food-web structure. Two main theoretical bodies explain β-diversity, the niche theory and neutral theory. However, neutral theory predicts only distributions for trophically identical species, whereas influences of local niches or neutral effects on food-web structure as a crucial part of the multitrophic structure of ecosystems are not taken into account. In chapter 4, I therefore analyzed the effects of spatial distance and environmental dissimilarity on the species dissimilarity (beta diversity) and food web dissimilarity (structural dissimilarity) of multitrophic forest communities. I showed that the mechanisms proposed by neutral theory can adequately predict the beta diversity of multitrophic species communities. Furthermore, food-web structure was robust and affected neither by spatial distance (random dispersal, neutral theory) nor by environmental filtering (niche theory). I additionally analyzed model food webs (random and niche topology) and compared their dissimilarities to empirical food webs. The highest dissimilarity was reached by random food webs whereas niche model food webs were in between and the lowest distances were expressed by empirical food webs. Further, random food webs displayed the highest mean trophic level (115), while niche model food webs showed lower (5) and empirical food webs the lowest (4) mean trophic level values. Hence, food-web structure appears to be energetically optimized with local species adapted to energetic niches within the food web while species identity within these niches remains random. This suggests that different species could be adapted to the same energetic niches and, while following random drift, still assemble into similar food web structures.
\nAltogether, the results of this thesis demonstrate the practicality of food-web structure in unravelling generalities across different ecosystems. Furthermore, food-web structure explains species distributions across the environment and provides additional important information on the ecosystem. 
\nThe observed generalities indicate constraints on food-web structure. The allometric degree distributions demonsrate such constraints on food-web structure by distributing the links in dependence of the species body masses. Finally, my results from chapter 4 indicate that, additionally to global topological constraints, local communities have to meet certain energetic constraints to explain the similarity found across food webs.

Nonpoint source pollution entering rivers will pollute water quality, degrading the health of aquatic ecosystems. However, owing to the lack of quantitative research on the effects of nonpoint source pollution on the structure of aquatic food webs, there is a lack of quantitative basis for river management. Nonpoint source pollution is not only difficult to control effectively, but also the success rate of water ecological restoration projects is low. With the increasing proportion of nonpoint source pollution in water environmental problems, it is urgent to quantitatively assess and predict the impact of nonpoint source pollution on the structure of food webs. Therefore, this thesis presents a method for quantitatively assessing and predicting the impact of nonpoint source pollution on the structure of food webs through using fuzzy clustering to screen the typical points of the impact of nonpoint source pollution, then using canonical correspondence analysis (CCA) and partial least squares regression analysis to comprehensively filtrate the driving factors affect food web that results in nonpoint source pollution, and then determining the impact of each driving factor on the structure of food webs. Finally, the change trend of food web structure is predicted. The results show that (1) the driving factors that the nonpoint source pollution that affects the food web structure is NH3‐N and chemical oxygen demand (COD). The increase in NH3‐N and COD promotes the growth of phytoplankton, causing the change of the primary productivity of the ecosystem, and ultimately changes the entire food web structure; (2) NH3‐N and COD affect the stability, maturity, connectivity and complexity of the aquatic food web structure. The increase of NH3‐N increases the connectivity and maturity of the food web structure but reduces complexity and stability; the increase of COD increases the connection of the food web structure, while reducing the other three indicators; (3) in some areas with good water quality, aquatic species diversity is high, the relationship of interspecies dietary is complex, food web structure level index is high and the structure of food web is stable. The food web structure in the rainy season will be better than that in the dry season. In some areas with severe water pollution and poor food web structure, the ability of the food web to resist external interference is weak. The food web structure in the rainy season will be worse than that in the dry season owing to rainfall into the river. The methods and conclusions in this treatise can provide a reliable and quantitative scientific basis for river ecosystem management and ecosystem restoration and can improve the success rate of ecological restoration projects.

Some ecological ideas developed gradually and only gained coherence and details after they had become commonplace. The history of two interrelated ideas, food chains and food webs, is an example of a gradual, cumulative history. Here is a brief survey of these concepts from about 1700 to 1970 (Fig. 1). The earliest identified food chains seem to have concerned hyper-parasitism (Egerton 2005, 2006a), which students of insects discovered in the later 1600s. But it was entrepreneurial naturalist Richard Bradley (Egerton 2006b) who generalized the concept (Bradley 1718, part 3:60–61): …Insects which prey upon others are not without some others of lesser Rank to feed upon them likewise, and so to Infinity; for that there are Beings subsisting, which are not commonly visible may be easily demonstrated…in a Microscope. This was turned into verse by Jonathan Swift (Fig. 2) in 1733 (lines 341–344). So, Nat'ralists observe, a Flea Hath smaller Fleas that on him prey, And these have smaller yet to bite 'em, And so proceed ad infinitum. In Bradley's account, food chains illustrated the balance of nature (Egerton 1973:333–335). Swift was a prominent literary figure who had a general interest in science, but his lines on fleas were meant as a swipe at lesser poets. Neither Bradley nor Swift provided an illustration, so we can help them out with this one from Alfred Elliott's 1957 Zoology textbook (Fig. 3), though Elliott borrowed the idea for the central figures of fleas from Robert Hegner's 1938 book, Big Fleas Have Little Fleas, or Who's Who Among the Protozoa. Carl Linnaeus, in an ecologically important essay, "The Economy of Nature" (Linnaeus [Latin] 1749; [English] 1775:114, 1977), briefly itemized the stages of two food chains, one terrestrial, one aquatic (Egerton 2007a). There are likely other naturalists between Linnaeus and Darwin who reported on food chains, but attracted little notice. Darwin may be the first to report a food web, occasioned by the Beagle's stopover at the rather barren island of St. Paul on 16 February 1832. He gave its location as 0° 58′ north latitude and 29° 15' west longitude, and 540 miles from America. He found only two species of birds, the booby (a gannet) and the Noddy Tern. The latter built a simple nest with seaweed. Then follows his food web (Darwin 1839:10): By the side of many of these nests a small flying-fish was placed; which, I suppose, had been brought by the male bird for its partner…quickly a large and active crab (Craspus), which inhabits the crevices of the rock, stole the fish from the side of the nest, as soon as we had disturbed the birds. Not a single plant, not even a lichen, grows on this island; yet it is inhabited by several insects and spiders. The following list completes, I believe, the terrestrial fauna: a species of Feronia and an acarus, which must have come here as parasites on the birds; a small brown moth, belonging to a genus that feeds on feathers; a staphylinus (Quedius) and a woodlouse from beneath the dung; and lastly, numerous spiders, which I suppose prey on these small attendants on, and scavengers of the waterfowl. After reading this account, Rear-Admiral William Symonds told Darwin that he had seen at St. Paul crabs drag young birds from nests and eat them. Darwin added his information to this passage in the second edition (1845) of his book on the voyage of the Beagle (Edwards 1985:34). In The Origin of Species (1859:73–74), Darwin reported the most famous example of a food chain in the scientific literature (Fig. 4). It is in chapter 3 on the "Struggle for existence," and involves humble bees (called "bumble bees" in America) pollinating red clover; though some bees were eaten by field mice, the mice, in turn, were kept in check by domestic cats. Darwin speculated that if it were not for the cats, the mice would decimate the bees, and the clover would go unpollinated, since only humble bees pollinate clover. A later, unknown commentator extended this chain further (Milne and Milne 1966:6) by suggesting that old maids commonly kept cats, that clover-fed cattle were eaten by British seamen who protected the British Empire, and that if it were not for old maids, the British Empire would fall! In other words, Darwin's food chain became a biological version of Englishman George Herbert's well-known admonition (1640): For want of a nail the shoe is lost, For want of a shoe the horse is lost, For want of a horse the rider is lost. In America, this admonition is attributed to Ben Franklin, who borrowed it without acknowledgement for Poor Richard's Almanac (1757). But getting back to Darwin's food chain, in 1947 W. L. McAtee pointed out that Darwin's food chain dynamic lacks full validity, since we now know that honey bees also pollinate red clover and that humble bees often appropriate mouse holes, so humble bees and mice have an ambiguous relationship. In Darwin's defense, he heavily depended on H. W. Newman's 1850–1851 study "On the habits of the Bombinatrices" (Darwin 1975:183). The next discussion of note for our purposes is from a remarkable German zoologist, Karl Semper. In 1877, he gave 12 lectures at the Lowell Institute in Boston, published simultaneously in English and German editions in 1881. The English title is Animal Life as Affected by the Natural Conditions of Existence. This book was the first detailed synthesis of animal ecology. In a discussion of the food of herbivores and carnivores (Semper 1881:51–52), he pointed out that when herbivores transform vegetation into flesh, there is a loss of mass due to oxidation of organic material, and that the same is true when carnivores transform the flesh of their prey into their own flesh. To illustrate this, he arbitrarily assumed a 10 to 1 ratio of food to flesh. One thousand units of plant food could only support 100 units of a herbivore, and those 100 units of herbivore could only support 10 units of a carnivore. Although his book has 106 illustrations, this generalized food chain was not illustrated. However W. E. Pequegnat's diagram (Fig. 5) from Scientific American (1958:86) captures Semper's concept, even to the point of using a 10 to 1 ratio. Semper wrote at a time when there was little quantified thinking in natural history. He had first trained as an engineer and then as a physiologist (Mayr 1975), and that background came to the fore in this discussion. Although his book was widely read, apparently no one carried this line of quantitative thinking any further in the 1880s or 1890s. We are used to seeing food chains or webs diagrammed. The advantages are obvious: they provide a visual panorama of detailed information. The early history of such diagrams is elusive. The bibliography on food chains and webs that Allee, Emerson, Park, Park, and Schmidt compiled (1949:514) can assist in the search. However, they did not discover the earliest ones now known, published in 1880 by Lorenzo Camerano, which are reprinted in an English translation of his article (1994:377–378). Since Camerano's two diagrams do not resemble any known from later zoologists, it seems likely that he did not have much, if any, influence on later diagrams. Joel Cohen (1994:353–355) suggests that Camerano was influenced by diagrams for other purposes in books by Darwin and by Hermann Helmholtz, though Camerano's diagrams do not resemble theirs. Like Semper's, Camerano's food webs are generalized rather than specific. The earliest specific food web I have found (Fig. 6) is on "The boll weevil complex," published in 1912 by Pierce, Cushman, and Hood in a USDA Bulletin. Their motive was to promote bowl weevil eradication—by encouraging its predators and parasites. Theirs may not have been the first specific diagram published, because others appeared about the same time in different biological specializations, where it is unlikely that the members of one specialization were reading the literature of other specializations. The following year, University of Illinois animal ecologist Victor E. Shelford (Fig. 7; photo, Croker 1991) published Animal Communities in Temperate America as Illustrated in the Chicago Region, which contained diagrams of both aquatic (Fig. 8) and land food webs (Fig. 9). There is no reason to suspect that he was influenced by the boll weevil diagram of 1912. Shelford used both of his diagrams to show how the community tends toward equilibrium, although the terrestrial community was more complex than the aquatic community, and consequently its equilibrium was more precarious. Shelford became a leading American animal ecologist (Croker 1991); his book was reprinted in 1937 and 1977. The earliest known food web diagram for a marine community was drawn by Danish fishery biologist Johannes Petersen (Fig. 10) in "A preliminary result of the investigations on the valuation of the sea"1915. He studied the Kattegat region of shallow water between eastern Denmark and Sweden (Fig. 11), an area with maximum length of 150 miles and maximum width of 90 miles. Significantly, he attempted to establish the annual productivity for this region, and his diagram indicates the thousands of tons of each group of organisms, with both a number and a proportioned rectangle (Fig. 12). In the text he stated that the eel-grass (Zostera marina) figure of 24,000,000 tons represents only the amount produced in the summer, and that the annual production is twice that. Presumably, all the other figures are annual production and not just summer production. The tons of plaice and cod are the actual commercial catch of those fish from International Fishery Statistics for 1910, and that was possibly true also for the tons of herring given, though he did not say so. The numbers given for other animals seem to be estimates. Although he indicated on his diagram that herring fed on plankton, he thought plankton was much less important than Zostera as a foundation for this food web. He concluded that the Kattegat had a "very unfavourable proportion between producers and consumers"(Petersen 1915:32). What he meant by this seems to be indicated by the following sentence in which he stated that carp ponds have "even without artificial feeding, given a yield of fish per hectare several times greater than that of the Kattegat." Petersen reproduced the same diagram with minor alterations in his final report, "The sea bottom and its production of fish-food" (1918:23). His colleague, H. Blegvad, also used rectangles in his diagram of "Food of fish and principal animals in Nyborg Fjord" (1916:24) (Fig. 13) but without attempting to represent precise quantities. However, he did give quantitative data in the text of his article, which provided some sense of the quantities of organisms involved at each level. In the same year as Blegvad, the American zoologist Harold Sellers Colton published what Jonathan A. D. Fisher calls (2005:145) "possibly the first intertidal marine food web ever illustrated." It is in Colton's article on a carnivorous snail, Thais lapillus (now Nucella lapillus), and shows both which animals the snail eats and which animals eat the snail (Fig. 14). Colton did not indicate what inspired his diagram, but his brief bibliography does include Shelford's book (1913). Fisher did not find references in the later relevant literature to Colton's two articles on this snail (probably due in part to Colton's leaving marine biology for archeology [Miller 1991]), so we do not know of any influence that his diagram exerted. Charles Elton (Fig. 15) helped make such diagrams commonplace. He went on an Oxford University Arctic expedition in 1921 to Spitsbergen and took along Shelford's book as a possible model for his own study (Elton 1966:33). However, Elton soon realized that the community he studied had a different dynamic than Shelford's aquatic and terrestrial ones. Elton was impressed by the transfer of food from sea to land, which is reflected in his diagram (Fig. 16) published in 1923. Although V. S. Summerhayes is listed as the senior author of their joint study, since he was a botanist, we can assume that Elton developed this diagram, in which plants are not emphasized. Two years later, in 1925, Elton published this much simpler Canadian food web (Fig. 17), which includes information on the lengths of animals. In 1924, English fishery biologist A. C. Hardy published a diagram (Fig. 18) on food consumed by herring at different stages of development. It bears no similarity to any diagrams previously shown, and it seems likely that he either was inspired by some unidentified example from the fisheries literature, or that he independently developed his diagram. Be that as it may, in 1927 Elton published his classic textbook, Animal Ecology, which reprinted and explained these last three diagrams by himself and Hardy. In that book Elton also introduced (1927:55) the terms "food chain" and "food cycle." Widespread use of his book popularized the use of food web diagrams. In both Hardy's diagram and in Elton's for 1925, more information was conveyed than merely which animal ate which food. Hardy's additional information was on the age of herring in relation to food, and Elton's was on the size of the consumer in relation to food. Elton also popularized the idea of a food pyramid (1927:68–70), which concept had been implied by Semper. In 1926 Germany's leading limnologist, August Thienemann (Fig. 19) published this unique food web of lakes (Fig. 20). His 50-page article on nutrient cycles in lakes introduced into limnology the terms "producers," "consumers," (though Petersen [1915], quoted above, had used both terms in marine biology) and "reducers." Thienemann's 1926 paper and two of his other papers influenced an American postdoctoral student, Raymond Lindeman, who produced one of the most influential diagrams in the history of ecology (Fig. 21), though few if any ecologists have published similar diagrams. It appeared in his posthumous paper, "The trophic–dynamic aspect of ecology" (1942). Like Thienemann's diagram, Lindeman's is a generalized food web, but both men had hard specific data backing up their concepts. In that respect theirs were similar to diagrams by Shelford, Elton, and Hardy, which illustrated specific food webs, and unlike Semper's generalized food web, which was an educated guess. In the caption to his diagram Lindeman indicated that it was similar to one he had published the previous year. A comparison of his two diagrams indicates what he learned in his year at Yale University working under Evelyn Hutchinson (Cook 1977). The 1941 diagram is identical to the 1942 diagram except it lacks the symbols for trophic levels along the side. Lindeman (1942:159) used Thienemann's terms "producers" and "consumers," but suggested substituting the term "decomposers" for Thienemann's term, "reducers," to signify that the indicated process was not just chemical, but also biological. In 1943, a year after Lindeman's 1942 diagram appeared in the journal Ecology, Harvard marine ecologist George Clarke published this conventional food web (Fig. 22), but three years later, after he had studied Lindeman's diagram and its explanation, Clarke published his diagram (Fig. 23) in Ecological Monographs, of a marine food web that emphasizes productivity and human removal of material. It also shows Clarke's concern for the rate of production at each trophic level. The Odum brothers, Eugene and Howard Thomas, carried Lindeman's thinking further. The Atomic Energy Committee became interested in radiation ecology (Kwa 1989:48), and Eugene Odum (Fig. 24; photo, Craige 2001) developed a program at the University of Georgia to study food chains at the Savannah River Research Facility to trace radioactive pollution (Craige 2001). By injecting plant stems with radioactive phosphorus-32, he and his colleagues traced it up the food chain to leafhoppers, beetles, and spiders (Kwa 1989:58). About 1957 the programs at Oak Ridge and Savannah River converged, with both programs using radioactive tracers to measure the flow of materials up the food chain (Kwa 1989:66). In the second edition of Eugene Odum's famous textbook, Fundamentals of Ecology (1959:47), there is a 1949 diagram of a food chain (Fig. 25). When I saw it, I assumed that Lindeman's influence had flowed across the Atlantic in just a few years, but when I compared it with British ecologist Erichsen Jones' own diagram, I discovered what Odum meant when he wrote that his diagram was "redrawn" from the one by Jones: Odum added the labels to the left of the diagram as a pedagogical aid. Howard Thomas Odum (Fig. 26; photo from Katherine Ewel) received his graduate training under Hutchinson at Yale. (In 1954 he taught me freshman zoology at Duke.) In 1956, he produced a diagram (Fig. 27) of matter and energy flow, in steady-state flowing-water communities in Florida. At that point, the reader could still understand the diagram without special training. However, H. T. Odum continued developing his thinking along the lines of systems ecology and used symbols from electrical engineering. By 1971 he published esoteric diagrams (Fig. 28) that integrate humans into the biotic community. This was an important step towards founding several applied ecological sciences (Mitsch 1994, Hall 1995, Egerton 2007b). Other ecologists developed food chain and food web concepts in another direction. In 1948, D. E. Howell reported finding DDT in human fat, and by 1949 biologists were reporting that fish feeding on insects killed by DDT were also being killed (Hoffmann and Surber 1949, Langford 1949). Rachel Carson (Fig. 29) publicized the discovery of insecticides traveling up the food chain in ever-increasing concentrations in her best-selling book, Silent Spring (1962:110–111), as did Robert Rudd in his less-read book, Pesticides and the Living Landscape 1964. Carson did not provide diagrams, and the ones Rudd used were quite simple. Here are four (Fig. 30) of the seven diagrams in his book. DDT was the most notorious insecticide, and in 1967 George Woodwell published a diagram (Fig. 31) in Scientific American showing increased concentrations of DDT as it progressed up the food chain. By 1970, Clive Edwards constructed a much more detailed food web (Fig. 32), showing DDT pathway and concentrations from the time of spraying DDT into the air, all the way up the food chain until it became concentrated in predatory birds, mammals, and humans. From simple narratives around 1700, food chain and food web concepts have been developed into progressively more sophisticated vehicles for conveying ecological ideas (Polis et al. 2004, de Ruiter et al. 2005). Lorenzo Camerano's two 1880 diagrams of food webs had no known influence, but after the visual stimulus of diagrams became established in the early 1900s, many ecologists found creative ways to express visually their discoveries concerning food chains and webs. This is a revised version of a talk given at the ESA Annual Meeting in August 2006 in Memphis, Tennessee. For comments preceding the talk, I thank Robert P. McIntosh, Professor Emeritus of Biology, University of Notre Dame (now in Florida). For several references used in the revision, I thank Jonathan A. D. Fisher, Department of Biology, University of Pennsylvania, Philadelphia.

To test the effects of temperature on food web structure and productivity, Mary O'Connor (above, checking temperatures) and colleagues placed five microcosms of food webs (shielded from full sunlight and UV) in eight independent water tables, each filled with a temperature-conditioned water bath.

Food webs show the architecture of trophic relationships, revealing the biodiversity and species interactions in an ecosystem. Understanding which factors modulate the structure of food webs offers us the ability to predict how they will change when influential factors are altered. To date, most of the research about food webs has focused on species interactions whereas the influences of surrounding environments have been overlooked. Here, using network analysis, we identified how the structure of aquatic food webs varied across a range of geophysical conditions within a whole stream system. Within a headwater basin in the Cascade Mountain Range, Oregon, USA, macroinvertebrate and vertebrate composition was investigated at 18 sites. Predator–prey interactions were compiled based on existing literature and dietary analysis. Several structural network metrics were calculated for each food web. We show that the structure of food webs was predictable based on geophysical features at both local (i.e., slope) and broader (i.e., basin size) spatial extents. Increased omnivory, greater connectance, shorter path lengths, and ultimately greater complexity and resilience existed downstream compared to upstream in the stream network. Surprisingly, the variation in food web structure was not associated with geographic proximity. Structural metric values and abundance of omnivory suggest high levels of stability for these food webs. There is a predictable variation in the structure of food webs across the network that is influenced by both longitudinal position within streams and patchy discontinuities in habitat. Hence, findings illustrate that the slightly differing perspectives from the River Continuum Concept, Discontinuity Patch Dynamics, and Process Domains can be integrated and unified using food web networks. Our analyses extend ecologists’ understanding of the stability of food webs and are a vital step toward predicting how webs and communities may respond to both natural disturbances and current global environmental change.

Food webs

1. Environmental drivers and food web structure in the pelagic zone vary from south to north in the Baltic Sea. 2. While nitrogen is generally the limiting nutrient for primary production in the Baltic Sea, phosphorus is the limiting nutrient in the Bothnian Bay. 3. In the Gulf of Bothnia the food web is to a large extent driven by terrestrial allochthonous material, while autochthonous production dominates in the other parts of the Baltic Sea. 4. Changes in bacterioplankton, protist and zooplankton community composition from south to north are mainly driven by salinity. 5. Bacteria are crucial constituents of the pelagic food web (microbial loop) and in oxygen-poor and anoxic bottom waters where they mediate element transformations. 6. Diatoms and dinoflagellates are the major primary producers in the pelagic zone. Summer blooms of diazotrophic (nitrogen-fixing) filamentous cyanobacteria are typical of the Baltic Sea, especially in the Baltic Sea proper and the Gulf of Finland. 7. The mesozooplankton (mainly copepods and cladocerans) channel energy from primary producers and the microbial food web to fish and finally to the top predators in the pelagic system (waterbirds and mammals). 8. Herring and sprat populations are affected by the foraging intensity of their main predator (cod), and therefore the environmental conditions that affect cod may also influence mesozooplankton due to food web effects “cascading down the food web”. 9. Anthropogenic pressures, such as overexploitation of fish stocks, eutrophication, climate change, introduction of non-indigenous species and contamination of top predators by hazardous substances, cause changes in the pelagic food web that may have consequences for the balance and stability of the whole ecosystem.

Trophic interactions are central to ecosystem functioning, but the link between food web structure and ecosystem functioning remains obscure. Regularities (i.e. consistent patterns) in food web structure suggest the possibility of regularities in ecosystem functioning, which might be used to relate structure to function. We introduce a novel, genetic algorithm approach to simulate food webs with maximized throughput (a proxy for ecosystem functioning) and compare the structure of these simulated food webs to real empirical food webs using common metrics of food web structure. We repeat this analysis using robustness to secondary extinctions (a proxy for ecosystem resilience) instead of throughput to determine the relative contributions of ecosystem functioning and ecosystem resilience to food web structure. Simulated food webs that maximized robustness were similar to real food webs when connectance (i.e. levels of interaction across the food web) was high, but this result did not extend to food webs with low connectance. Simulated food webs that maximized throughput or a combination of throughput and robustness were not similar to any real food webs. Simulated maximum-throughput food webs differed markedly from maximum-robustness food webs, which suggests that maximizing different ecological functions can generate distinct food web structures. Based on our results, food web structure would appear to have a stronger relationship with ecosystem resilience than with ecosystem throughput. Our genetic algorithm approach is general and is well suited to large, realistically complex food webs. Genetic algorithms can incorporate constraints on structure and can generate outputs that can be compared directly to empirical data. Our method can be used to explore a range of maximization or minimization hypotheses, providing new perspectives on the links between structure and function in ecological systems.

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 362:69-83 (2008) - DOI: https://doi.org/10.3354/meps07401 Food web of a SW Atlantic shallow coastal lagoon: spatial environmental variability does not impose substantial changes in the trophic structure Laura Rodríguez-Graña1,6,*, Danilo Calliari2,6, Daniel Conde1, Javier Sellanes3,4, Roberto Urrutia5 1Sección Limnología, and 2Sección Oceanología, Facultad de Ciencias, Universidad de la República, Iguá 4225, CP 11400 Montevideo, Uruguay 3Departamento de Biología Marina, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile 4Centro de Investigación Oceanográfica en el Pacífico Sur-Oriental (COPAS), and 5Centro EULA, Universidad de Concepción, Casilla 160-C, Concepción, Chile 6Present address: Department of Marine Ecology, Göteborgs University, Kristineberg 566, 450 34 Fiskebäckskil, Sweden *Emails: laurod@fcien.edu.uy and laura.rodriguez@marecol.gu.se ABSTRACT: We performed a detailed analysis of the food web structure of Laguna de Rocha, a temporally open coastal lagoon in the Southwest Atlantic, characterised by spatial gradients in salinity, nutrient levels and trophic status. Carbon and nitrogen stable isotopes of main producers, invertebrate and vertebrate consumers, and stomach contents of ichthyofauna were analysed seasonally at freshwater (north) and marine-influenced (south) sites to determine whether environmental differences induced changes in food web structure. Contribution of primary and secondary food sources and trophic linkages were assessed with the multisource-partitioning model IsoSource and an index that quantifies the contribution of each organism as food for the ecosystem. Isotopic analyses were performed for 10 primary organic matter sources (OM), 13 invertebrates and 9 fish species. Suspended and sediment OM constituted the most important primary sources, while direct consumption of macrophytes and macroalgae was marginal. The small invertebrates Neomysis americana, Nephtys fluviatilis, Pseudodiaptomus richardii and an amphipod were the most important intermediate consumers at both sites (except P. richardii, which was present only in the north). Top predators were the fish species Micropogonias furnieri, Paralichthys orbygnianus and Hoplias malabaricus, and the maximum estimated trophic level (between 3.4 and 4.8) varied seasonally, but not between sites. Fish stomach content analyses largely confirmed results from the IsoSource mixing model. Overall results indicated that, despite environmental differences between sites, the structure of the biological assemblages and general trophic patterns were similar at both sites. KEY WORDS: Food web structure · Coastal lagoon · Estuaries · Benthic–pelagic coupling · Isotopic mixing models Full text in pdf format Supplementary appendix PreviousNextCite this article as: Rodríguez-Graña L, Calliari D, Conde D, Sellanes J, Urrutia R (2008) Food web of a SW Atlantic shallow coastal lagoon: spatial environmental variability does not impose substantial changes in the trophic structure. Mar Ecol Prog Ser 362:69-83. https://doi.org/10.3354/meps07401Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 362. Online publication date: June 30, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Adaptive foragers and community ecology: linking individuals to communities and ecosystems

Food web of the intertidal rocky shore of the west Portuguese coast – Determined by stable isotope analysis

Mercury biomagnification in benthic, pelagic, and benthopelagic food webs in an Arctic marine ecosystem

Summary Food webs and trophic dynamics of coastal systems have been the focus of intense research throughout the world, as they prove to be critical in understanding ecosystem processes and functions. However, very few studies have undertaken a quantitative comparison of entire food webs from a key consumer perspective across a broad geographical area, limiting relevant comparisons among systems with distinct biotic and abiotic components. We investigate the structure and functioning of food webs in four tidal ecosystems of international importance for migratory shorebirds along the East Atlantic Flyway: Tejo estuary in Portugal, Sidi Moussa in Morocco, Banc d'Arguin in Mauritania and Bijagós archipelago in Guinea‐Bissau. Basal food sources, shorebirds and their prey (benthic invertebrates) were sampled in all areas, and Bayesian stable isotope mixing models and community‐wide metrics were used in a comparative analysis among areas. Significant differences among study areas were found in the structure of food webs, as well as in the relative importance of basal resource pools supporting each food web. Overall, the food web of Banc d'Arguin was characterized by lower trophic diversity and higher functional redundancy than the other sites. This result might be explained by the low number of trophic pathways of organic matter transfer in this seagrass‐dominated system which, as a fossil estuary, lacks inputs from both freshwater and nutrient‐rich offshore oceanic waters. Structure of shorebird communities was consistent with the main organizational patterns found for each food web, highlighting the less diverse character of the community of Banc d'Arguin. At Banc d'Arguin and Bijagós archipelago, which displayed the smallest and largest isotopic niche widths in bird assemblage, respectively, mean niche overlap among species was low, suggesting high interspecific partitioning in resource use. Tropical systems typically offer comparatively lower harvestable prey biomass for shorebirds and might thus strengthen interspecific competition, leading to low niche overlap among species. Our study reveals relevant differences in the structure of food webs and shorebird communities in coastal areas along an avian flyway. While differences in trophic redundancy of food webs point to distinct levels of ecosystem resilience, contrasts in the organization of shorebird communities highlight the plasticity in the foraging behaviour of species inhabiting areas with distinct environmental conditions.

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 11:279-288 (1996) - doi:10.3354/ame011279 Components, structure and fluxes of the microbial food web in a small, stratified lake Massana R, García-Cantizano J, Pedrós-Alió C We studied the planktonic community established in Lake Cisó (Girona, Spain) during summer stratification, with the aim of describing the food web of a system as completely as possible. The lake was sampled 19 times during 1990 and 1991. We first determined which populations contributed significantly to total summer biomass. Then, we determined the trophic role of these populations by several independent approaches, and aggregated the community into functional groups. The binary food web obtained indicated that the structure of the food web in Lake Cisó was similar to that found in other systems. Finally, we quantified the trophic fluxes among populations using a simple algorithm which considers the vertical distribution of organisms and the functional responses of the different predators. The trophic food web obtained revealed 2 interesting properties. First, the compartments with larger biomass were relatively stable during stratification and presented slow growth and low predatory losses. Second, there was a very inefficient transfer of organic matter from the lower levels (bacteria, algae and protozoans) to the higher levels (rotifers and zooplankton) of the food web. Both properties could be explained by the fact that most biomass of the system accumulated in the metalimnion, along opposite gradients of oxygen and sulfide, which determined an environment with reduced competition and predation. We postulate that metalimnetic communities above anaerobic hypolimnia can be regarded as sinks of organic matter off the epilimnion. Lake Cisó · Binary food web · Trophic food web · Prey refuge Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 11, No. 3. Publication date: December 31, 1996 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1996 Inter-Research.

Simple SummaryIn terrestrial natural ecosystems, more complex and diverse networks of plant–insect primary consumers and their predators are often more productive, stable, and resilient. Plant diversity often positively correlates to the diversity of phytophagous insects and their natural enemies generating multitrophic interactions with changing outcomes (bottom-up effects). The use of cover crops can promote natural enemy populations and their temporal synchronization with a target pest, resulting in greater pest control. Therefore, changes in the habitat conditions can alter food webs. In agroecosystems, characteristics of the food trophic webs, as connectance, measured as the proportion of realized links in the network, could be linked to the efficiency of pest control. In this study, we evaluated how the use of oat cover crops affects composition and structure in the aphid–parasitoid–hyperparasitoid food webs of plum orchards with different habitat management contexts: plums with inter-rows of oats as a cover crop (OCC) and plums with inter-rows with spontaneous vegetation (SV). Quantitative food web metrics differed significantly among treatments showing a higher generality, vulnerability, interaction evenness, and linkage density in SV, while OCC presented a higher degree of specialization.By increasing plant diversity in agroecosystems, it has been proposed that one can enhance and stabilize ecosystem functioning by increasing natural enemies’ diversity. Food web structure determines ecosystem functioning as species at different trophic levels are linked in interacting networks. We compared the food web structure and composition of the aphid– parasitoid and aphid-hyperparasitoid networks in two differentially managed plum orchards: plums with inter-rows of oats as a cover crop (OCC) and plums with inter-rows of spontaneous vegetation (SV). We hypothesized that food web composition and structure vary between OCC and SV, with network specialization being higher in OCC and a more complex food web composition in SV treatment. We found a more complex food web composition with a higher species richness in SV compared to OCC. Quantitative food web metrics differed significantly among treatments showing a higher generality, vulnerability, interaction evenness, and linkage density in SV, while OCC presented a higher degree of specialization. Our results suggest that plant diversification can greatly influence the food web structure and composition, with bottom-up effects induced by plant and aphid hosts that might benefit parasitoids and provide a better understanding of the activity, abundance, and interactions between aphids, parasitoids, and hyperparasitoids in plum orchards.