Food chains, Food Webs and the Flow of Energy in Ecosystems Introduction An ecosystem can be defined as a more or less self-contained function unit in ecology consisting of all abiotic and biotic interactions in a specific area. Flow of energy within an ecosystem is a one-way process; Photosynthesis utilizes light (solar) energy to yield chemical energy that is passed on to organisms at significantly reduced amounts at each level of nutrition. This ‘inefficiency’ in energy transfer is the principal constrain in the food chain length. Food chains can be identified as the sequence of organisms through which energy flows.
Moreover, food chains and food webs-of greater complexity-describe the complicated feeding relationships between the members of a community in an ecosystem and a variety of them exists. Food web research is an area of extensive research and interest and early research started by Ch. Elton that first introduced the pyramid of numbers. Following research by others further contributed scientists understanding of food web complexity via their work. Most recent research into food webs and food chains, discussed in this essay, deals with the effect of climate change and pollution in aquatic ecosystems.
History of research into food webs Charles Elton was a pioneer in the concept of food webs, chains and sizes. His organization of species into functional groups; producers, consumers, decomposers described the characteristic pyramidal shape of food webs, with a larger amount of producers (autotrophs) at the bottom and successively smaller amounts of heterotrophs on the following, higher levels (Elton C. S. , 1926). The Eltonian pyramid of numbers is the basis in describing all food webs. It is based on this pyramid that Raymond L. Linderman published his work upon examining it in terms of trophic dynamics.
Linderman (1942) suggested that the Eltonian pyramid resulted from a successive loss of energy going up trophic levels, which was attributed to the thermodynamic inefficiency of energy transformation (ecological efficiency). Robert Paine’s work on intertidal shores, which suggested that maintenance of species diversity and ecological stability were a key result of food web complexity, was the one that raised awareness and interest of numerous theoretical ecologists. One such example is, Stuart Pimm, whose book discusses amongst other, hypotheses of the food chain length (science. jrank. org, 2012).
Food chains and food webs A food chain is a feeding hierarchy in which organisms in an ecosystem are grouped into nutritional levels. Successive trophic levels represent the flow of food energy and the feeding relationships between them. Food chains usually start with photosynthetic producers because, uniquely, producers have the ability of extracting both energy and matter from the abiotic environment (Communities and ecosystems-KCTS Home). All other living organisms acquire their energy and matter by feeding on other organisms, or breaking them down, recycling their minerals and elements in the process.
In nature, three main types of food chains have been observed; the grazing food chain (Fig. 1), the parasitic food chain and the detritus food chain (Fig. 2). Figure [ 1 ]: Primary producer consumed by herbivore which in turn is consumed by carnivore Figure [ 2 ]: The primary consumer (woodlouse) now feeds on detritus (dead leaves) (Images taken from http://fany. savina. net/2010/02/ecology/, 8 February 2012) Figure [ 3 ]: Representation of a food web in a forest with various interconnecting food chains Image taken fron http://www. utorvista. com/content/biology/biology-iv/ecosystem/food-web. php , 10 February 2012 In reality, in one habitat, most animals have various food sources, and food chains can be interlinked at different trophic levels to form a complex interaction between different species (PRINCIPLES OF ENVIRONMENTAL SCIENCE AND ENGINEERING- CE O71. ) at a food point of view; a food web (Fig. 3). The food web increases the chances of survival of most of the organisms in the ecosystem as it sometimes gives them numerous alternatives of food.
Furthermore, a food web provides greater stability to the ecosystem (Raven et. al. , 2010). There are three types of food webs; topological, flow webs and interaction webs. Energy flow in an ecosystem Living organisms are highly organised and need a constant supply of energy to maintain their internal order in order to survive. Energy is required to drive all physical and chemical processes of life; biosynthesis, active transport and movement. Figure 4(telstar. ote. edu, 12 February 2012) is an illustration of a natural energy system.
Figure [ 4 ]: Each of the arrows in the diagrams represents transformation or transmission of energy. Plants and humans are the agents storing/transforming the energy available. | | During the transfer of energy through successive trophic levels in an ecosystem, there is a loss of energy all along the path. No transfer of energy is 100 per cent. The studies of transfer of energy in different food chains in a large number of ecosystems have revealed a uniform pattern of transfer of energy, which is given by 10 per cent law. Energy is mainly lost as heat and/or undigested food (cell walls, bones etc).
A number of studies have been conducted over the years and the four are the chief hypotheses suggested for the factors constraining food chain length. Primarily, Hastings and Conrad (1979) suggested the optimal foraging theory; efficiency of energy transfer is increased by feeding on lower trophic levels thereby resulting in a shorter food chain. Cousins’ (1987) theory said that animal size may influence chain length as predators are usually of smaller size compared to their prey. Primary productivity and/or energy transfer efficiency might account for short food chains (Pimm, 2002).
Finally, shorter food chains enable communities to return to equilibrium more quickly after disruption (Kim Tania, 2006). Current Research into Food Webs Current research into trophic ecology deals with anthropogenic activities entangled with environmental pollution and global warming. Although, predictions upon the effects of future climate change are difficult to be made, it is safe to assume that further warming will most probably change the spatial distribution of primary and secondary production (Richardson and Schoeman, 2004). Studies by Yule et al. 2006) of different food webs in a tropical river reflected the impact of sediment pollution. Samples taken from unpolluted sites of the river had higher linkage density, more elements and greater food web complexity. Polluted sited, on the contrary, gave conflicting results even having some taxa that were previously grazers in unpolluted sites becoming collector-gatherers. Air pollution in aquatic ecosystems has been documented to contribute largely to increased acidity, eutrophication and, mercury bioaccumulation in higher trophic levels.
Lake acidification for instance, will have crucial consequences for the food web, if certain acid-sensitive species are lost, as not only the species themselves will be affected, but other species feeding on it and similarly the various links along the food web ( Garry M. Lovett et al. , 2009). The impact of climate change on biological communities of immense importance such as marine plankton is an area of concern. Plankton plays a crucial role in the activity of marine ecosystems, not only it is a source of food for marine organisms, it also accounts for 50% of global primary production.
Research done in the Northeast Atlantic has shown that a rise in temperature of the sea surface is coupled by by higher and lower phytoplankton abundance in colder and warmer regions respectively. These results along with investigations into the linkages of the plankton food web support dominant bottom-up control within the community as any changes in plankton availability will also affect copeped herbivores and zooplankton carnivores (Richardson and Schoeman, 2004). Conclusion Food chains and food webs are extremely significant in an ecosystem as they allow energy flow through trophic levels.
Energy transfer inefficiency is responsible for the limited food chain length and helps keep an ecological balance as it maintains and regulates the population size of different organisms. Human activities, though threaten to disrupt this balance and the future of major food webs is uncertain. WORD COUNT: 1237 REFERENCES: * Net Industries, 2012. Food chain-Structure of food webs [Online]. Available at: http://science. jrank. org/pages/2797/Food-Chain-Web-History-food-web-research. html [accessed 8 February, 2012] * Cousins, S. 1987. The decline of the trophic level concept.
Trends in Ecology and Evolution. 2, 312-321 * Elton, C. S. , 1927. Animal Ecology. London, UK: Sidwick and Jackson * Hastings, H. M. and Conrad, M. , 1979. Length and Evolutionary stability of food chains. Nature. 282, 838-839 * Linderman, R. L. , 1942. The trophic-dynamic aspect of ecology. Ecology. 23, 399-417 * Lovett, G. M. , Hear, T. H, Evers, D. C. , Findlay, S. E. G, Cosby, B. J. , Dunscomb, J. K. , Driscoll, Weathers, K. C. , 2009. Effects of Air Pollution on Ecosystems and Biological Diversity in the Eastern United States.
Annals of the New York Academy of Sciences. 1162, 99-135 * Pimm, S. L. , 2002. Food Webs. Chicago, USA: The University of Chicago Press. * Richardson, A. J. , Schoeman, D. S. , 2004. Climate Impact on Plankton Ecosystems in the Northeast Atlantic. Science. 305, 1609-1612 * Yule, C. M. , Bovero, L. , Marchant, R. , 2009. Effects of sediment pollution on food webs in a tropical river (Borneo, Indonesia). Marine and Freshwater Research. 61, 204-213. * Communities and Ecosystems-KTCS Home [Online]. Available at: http://legacy. opkinsville. kctcs. edu/sitecore/instructors/Jason-Arnold/VLI/Modul e%204/M4BCommunitiesandecosystems/M4BCommunitiesandecosystems_print. html [accessed 12 February, 2012] * Food Web [Online]. Available at: http://www. tutorvista. com/content/biology/biology-iv/ecosystem/food-web. php [accessed 11 February, 2012] * All Energy Systems [Online]. Available at: http://telstar. ote. cmu. edu/environ/m3/s3/all_ene_sys. htm [accessed 12 February, 2012] * Kim, T. , 2006. What determines food chain length? [Online]. Available at: