Food Chains and Food Webs

10 Key excerpts on "Food Chains and Food Webs"

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  Insect Ecology

  • Upadhyay, Shadanan(Authors)
  • 2018(Publication Date)
  • Biotech

    (Publisher)

  Chapter 10: Food Chain: Web and Ecological Succession Food Chains and Webs A food chain is the path of food from a given final consumer back to a producer. For instance, a typical food chain in a field ecosystem might be: grass -> grasshopper -> mouse -> snake -> hawk Note that the food chain is the path of food from a given final consumer back to a producer typically list a food chain from producer on the left (or at the bottom) to final consumer on the right (or at the top). The real world, of course, is more complicated than a simple food chain. While many organisms do specialize in their diets (anteaters come to mind as a specialist), other organisms do not. Hawks don’t limit their diets to snakes, snakes eat things other than mice, mice eat grass as well as grasshoppers, and so on. A more realistic depiction of who eats whom is called a food web. It is when we have a picture of a food web in front of us that the definition of food chain makes more sense. We can now see that a food web consists of interlocking food chains, and that the only way to untangle the chains is to trace back along a given food chain to its source. The food webs you see here are grazing food chains since at their base are producers, which the herbivores then graze on. While grazing food chains are important, in nature they are outnumbered by detritus-based food chains. In detritus-based food chains, decomposers are at the base of the food chain, and sustain the carnivores, which feed on them. In terms of the weight (or biomass) of animals in many ecosystems, more of their body mass can be traced back to detritus than to living producers. Pyramids The concept of biomass is important. It is a general principle that the This ebook is exclusively for this university only. Cannot be resold/distributed.

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  From Populations to Ecosystems

  Theoretical Foundations for a New Ecological Synthesis

  • Michel Loreau(Author)
  • 2010(Publication Date)
  • Princeton University Press

    (Publisher)

  CHAPTER 4 Food Webs, Interaction Webs, andEcosystem Functioning

  A food web describes the network of trophic interactions between species, i.e., who eats whom, in an ecosystem. Since trophic interactions are both the vehicle of energy and material transfers and one of the most significant ways in which species interact, they have always lain at the confluence of community and ecosystem ecology. But they have been approached from different perspectives in different traditions. The energetic view articulated by Lindeman (1942) and developed by ecosystem ecology during the following decades views food webs as networks of pathways for the flow of energy in ecosystems, from its capture by autotrophs in the process of photosynthesis to its ultimate dissipation by heterotrophic respiration. A different approach, rooted in community ecology, was initiated by Elton (1927) and developed by May (1973), Pimm (1982), and many others. This approach focuses on the dynamical constraints that arise from species interactions and emphasizes the fact that too much interaction (whether in the form of a large number of species, a large connectance among these species, or a high mean interaction strength) destabilizes complex ecological systems, including food webs. Food webs have also been studied from a topological perspective: the pattern of trophic interactions in a food web is nonrandomly related to species traits, in particular, body size, which led to the development of size-based models of food-web structure such as the cascade and niche models (Cohen et al. 1990; Williams and Martinez 2000). Perhaps the approach that lies most closely to the interface between community and ecosystem ecology is that based on the trophic cascade concept (Carpenter et al. 1985). Hairston et al. (1960) hypothesized that carnivores control herbivores, thereby releasing plants from control by herbivores, in most ecosystems. This simple idea led to a flurry of studies on the community- or ecosystem-level consequences (though mostly the bio-mass of the various trophic levels) of the top-down control exerted by higher trophic levels on lower trophic levels.

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  A New Ecology

  Systems Perspective

  • Sven Erik Jørgensen, Brian D. Fath, Simone Bastianoni, Joao C. Marques, Felix Muller, S. Nors Nielsen, Bernard D. Patten, Enzo Tiezzi, Robert E. Ulanowicz(Authors)
  • 2011(Publication Date)
  • Elsevier Science

    (Publisher)

  Andrewartha and Birch, 1984 ), but the species is more than the loci of an envirogram; it is those interactions, that connectivity, with other species and with the environment, which construct the ecosystem. The diversity, stability, and behavior of this complex is governed by such interactions. Here we introduce the standard food web treatment, discuss some of the weakness, while suggesting improvements, and end with an overview of the general insights gained from understanding ecosystem connectivity as revealed by ENA.

  A food web is a graph representing the interaction of “who eats whom”, where the species are nodes and the arcs are flows of energy or matter. For example, we show a food web diagram typical to what one would find in an introductory biology or ecology textbook (Figure 5.1 ).

  Figure 5.1 Typical ecological food web.

  The energy flow enters the primary producer compartments and is transferred “up” the trophic chain by feeding interactions, grazing and then predation, losing energy (not shown) along each step, where after a few steps it has reached a terminal node called a top predator (also known, in Markov chain theory, as an absorbing state). This picture of “who eats whom” has several deficiencies if one wants to understand the entire connectedness as established by the matter–energy flow pattern of the ecosystem:

  • First, the diagram excludes any representation of decomposers, identified above as a more fundamental element of ecosystems than more familiar trophic groups like herbivores, carnivores, and omnivores. While decomposers have been an integral part of some ecological research (e.g., microbial ecology, eutrophication models, network analysis, etc.), their role in community food web ecology is just now gaining stature. Prejudices and biases often work to shape science; what food-web ecologist, for example, would a priori classify our species (Homo sapiens

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  Ecology

  From Individuals to Ecosystems

  • Michael Begon, Colin R. Townsend(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  Chapter 17 Food Webs

  17.1 Food chains

  In the previous chapter we considered how direct interactions between species can shape communities. Our main focus was on interactions between species occupying the same trophic level (interspecific competition and to a lesser extent facilitation) or between members of adjacent trophic levels. In fact, of course, the influence of a species often ramifies further than this. The effects of a carnivore on its herbivorous prey may also be felt by any plant population upon which the herbivore feeds, by other predators and parasites of the herbivore, by other consumers of the plant, by competitors of the herbivore and of the plant, and by the myriad of species linked even more remotely in the food web. In this chapter, we broaden our scope by looking at food chains (see Figure 16.1 f) and food webs.

  Food chains may be defined as sequences of species running from a basal, autotrophic species to a species that feeds on it, to another species that feeds on the second, and so on up to a top predator (fed on by no other species). This does not imply a belief that communities are organised as linear chains (as opposed to more diffuse webs). Rather, individual chains are identified purely as a means of trying to understand aspects of the structure of food webs – for example the number of links between the base and the apex. We begin by focusing on the interactions themselves – often indirect interactions, linking one species to another, two or more levels below or above it in the food chain.

  The removal of a species (experimentally, managerially or naturally) can be a powerful tool in unravelling the workings of a food web. If a predator species is removed, we expect an increase in the density of its prey. If a competitor species is removed, we expect an increase in the success of species with which it competes. There are plenty of examples of such expected results. Sometimes, however, removing a species may lead to a decrease in competitor abundance, or the removal of a predator may lead to a decrease in prey abundance. Such unexpected effects arise when direct effects are less important than the effects that occur through indirect pathways. Thus, the removal of a species might increase the density of one competitor, which in turn causes another competitor to decline. Or the removal of a predator might increase the abundance of a prey species that is competitively superior to another, leading to a decrease in the density of the latter. In a survey of more than 100 experimental studies of predation, more than 90% demonstrated statistically significant results, and of these about one in three showed such indirect effects (Sih et al

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  Trophic Ecology

  • James E. Garvey, Matt Whiles(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Box 10.3 ), theoreticians have had a hard time coaxing these relevant locales of strong interactions within food webs from their models (Pascual and Dunne 2006). As Polis and others have pointed out, the frontiers of food-web theory include incorporating species richness and spatial scale into models. Further, the adaptability of webs to changes in internal and external forcing factors will need to be explored.

  10.9  CONCLUSIONS

  Food webs are attempts to bridge the gap between population/community aspects of ecosystems with patterns of materials and energy flux. These constructs comprised of nodes and connections in a network framework vary from capturing simple feeding connections to the intensity of energy flow and indirect effects. Regardless of the resolution of the food webs and their complexity, they, like all models, are simplifications of the trophic relationships occurring in ecosystems.

  Food webs have been studied both empirically and theoretically for several decades. Given the sheer complexity of interspecific and intraspecific relationships in nature, generalities seem elusive. However, some general patterns do seem to exist. Most interactions between organisms are weak, meaning that changes in the density or behavior of one population do not exert much of an effect to the other. In some ecosystems, consumers emerge—likely through coevolutionary processes—that affect characteristics of the food web such as the distribution of biomass, relative productivity, and perhaps the species present. These consumers either have disproportionate effects relative to their density and are considered keystones or have strong impacts because of their high density and consumptive demand (i.e., numerical dominants).

  What drives the structure and ultimately the function of ecosystems has generated much debate among ecologists. Perhaps one of the oldest questions in ecology is whether food-web size and stability are related. The answer is complicated but generally yes. The food-chain length within ecosystems, which is the distance from the basal producer to the most distant consumer, is limited on Earth. The amount of energy available to food webs is finite, and inefficiencies in trophic transfer place an upper limit on feeding steps. Growing complexity of consumer life histories with higher trophic levels and limited space available for trophic interactions also likely limit food-web size.

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  Microbial Ecology in Sustainable Agroecosystems

  • Tanya E. Cheeke, David C. Coleman, Diana H. Wall(Authors)
  • 2012(Publication Date)
  • CRC Press

    (Publisher)

  We start with an overview of food web descriptions and mod-els that have been developed to study the interactions among plants and soil organisms in natural and agricultural ecosystems. The models capture many of the structural and functional components that Odum (1969) used to contrast early development and mature ecosystems (see Table 4.1). Next, we present a set of results that link many of these struc-tural and functional components to dynamic properties and stability. Here, we present the results of theoretical exercises that change (read disturb) the observed patterns in food web structure and assess the effects of these changes on stability. We end the chapter by connecting the theoretical results to empirical observations from studies of agricultural ecosystems demonstrating that agricultural management practices affect soil food webs in ways that parallel those used in the theoretical exercises on disturbance and stability. 66 Microbial Ecology in Sustainable Agroecosystems 4.2 Soil food web descriptions and models A food web is a description of the trophic interactions within an ecosystem. By their nature, they are incomplete, focusing on a subset of organisms and resources and averaging over different spatial and temporal scales. Paine (1980) distinguished three types of food web descriptions— connectedness , energy flux, and functional —that share a common underlying structure but that utilize and convey different types of information about the system. These three descriptions provide a convenient organizational and operational framework to study and model agricultural practices and their impacts on ecosystem structure, function, and stability. Within each description, different modeling approaches and formulations are used (see Hunt et al., 1987; Moore and Hunt, 1988; Moore et al., 1993; de Ruiter et al., 1995).

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  Dynamic Food Webs

  Multispecies Assemblages, Ecosystem Development and Environmental Change

  • Peter C de Ruiter, Volkmar Wolters, John C Moore(Authors)
  • 2005(Publication Date)
  • Academic Press

    (Publisher)

  Measurement has not always had a central place in food web studies; counting organisms and classifying species and their trophic relationships has been more important both recently and in the distant history of our subject. We could probably point to three phases in studying food webs. The initial phase of counting and description, which we might call an ‘intelligent natural history’ of food webs, led to the development of a number of core concepts including food webs themselves (Elton, 1927). The middle phase, where measurement was paramount, quantified the energy transfers between species using the bomb calorimeter to establish unit mass energy contents (Teal, 1962). The more recent (post-International Biological Programme) phase is one where we have returned to the art of classification and observation in food webs in order to identify pattern and universal properties (Cohen, 1978). DeAngelis (1992) has classed these phases as contrasting the ‘process-functional’ school with the ‘population-community’ school. In recent years the latter approach has tended to dominate but with some innovative work also continuing in the process-functional tradition (DeAngelis, 1992 ; de Ruiter et al., 1993a). This tradition remains important since if feeding is about anything it is about gaining energy and nutrients for maintenance, movement, growth, and reproduction. Food webs are structures which only exist such that members of the web gain energy from others, or if they are predated, ‘donate’ energy and nutrients to their predators. We suggest that additional measurement techniques can clarify the energetics of trophic transfers. Although categories of stored energy can be identified in ecosystems, conventional calorimetry only provides a first law of thermodynamics description suitable for energy accounting in food webs in which energy is neither created nor destroyed

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  Environmental Biology

  • Allan M. Jones(Author)
  • 2006(Publication Date)
  • Routledge

    (Publisher)

  Basic ecological concepts and processes

  Key concepts

  • The laws of thermodynamics determine the transfer of energy through an ecosystem.
  • Energy flow through an ecosystem is a one-way process while other materials recycle on various timescales.
  • Energy transfer is subject to a variety of factors and there are various measures of efficiency.
  • The concept of trophic (feeding) levels, Food Chains and Food Webs is important for the understanding of energy transfer and loss in any ecosystem.
  • Feeding relationships are fundamental to the organisation and function of biological communities.
  • Every system has three main components: the autotrophic primary producers, the secondary producers (mainly the herbivores and carnivores) and the decomposers.
  • The biosphere interacts with three main physical compartments: the hydrosphere (water), the lithosphere (land) and the atmosphere.
  • Materials have biogeochemical cycles, the main ones being the water cycle, carbon cycle, oxygen cycle and the nutrient (particularly nitrogen, phosphorus and sulphur) cycles.

  Despite the diversity of living forms and their habitats (outlined in Chapter 2 ), there are a number of fundamental concepts and processes that apply to all ecosystems and the interactions of their components. Any ecosystem is a collection of interdependent components (subsystems), each with its own defined boundary. Each ecosystem has two basic requirements for its successful functioning, namely

  • the acquisition, transformation and transfer of energy and
  • the gathering and the recycling of the materials necessary for life.

  By understanding how these systems function, environmental science is better able to understand and predict the impacts of human activities on such systems. This chapter presents an overview of the key processes involved.

  3.1  Energy in ecosystems Energy comes in many forms, such as heat, light, sound and electricity, and all have in common the capacity to do work. All forms of energy fall into one of two categories:

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  CLEP® Natural Sciences Book + Online

  • Laurie Ann Callihan, David Callihan(Authors)
  • 2013(Publication Date)
  • Research & Education Association

    (Publisher)

  Grasshoppers consume plants and are consumed (in this example) by toads, the secondary consumers, which represent the third trophic level. Snakes consume toads, and are in turn consumed by owls—making these the fourth and fifth trophic levels. In this example, bacteria are the decomposers that recycle some of the nutrients from dead owls (and other levels) to be reused by the first trophic level. The pyramid illustrates a food chain; however, in nature it is never actually as simple as shown. Owls consume snakes, but they may also consume toads (a lower level in the pyramid) and fish (from an entirely different pyramid). Thus, within every ecosystem there may be numerous food chains interacting in varying ways to form a food web. Furthermore, all organisms produce waste products that feed decomposers. The food web represents the cycling and recycling of both energy and nutrients within the ecosystem. The productivity of the entire web is dependent upon the amount of photosynthesis carried out by photosynthesizers. Water Cycle The availability of water is crucial to the survival of all living things. Water vapor circulates through the biosphere in a process called the hydrologic cycle. Water is evaporated via solar radiation from the ocean and other bodies of water into clouds. Water is also released into the atmosphere from vegetation (leaves) by transpiration. Some water is also evaporated directly from soil, but most water in the ground flows into underground aquifers, which eventually empty into the oceans. Water above ground flows into waterways, which also eventually flow into the ocean (a process known as runoff). Water vapor is then redistributed over land (and back into oceans as well) via clouds, which release water as precipitation. The water cycle also has a profound effect on Earth’s climate. Clouds reflect the Sun’s radiation away from the Earth, causing cool weather

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  Ecology

  From Individuals to Ecosystems

  • Michael Begon, Colin R. Townsend, John L. Harper(Authors)
  • 2009(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  20.4 Empirical patterns in food webs: the number of trophic levels In the previous section, we examined very general aspects of food web structure – richness, complexity – and related them to the stability of food webs. In this section, we examine some more specific aspects of structure and ask, first, if there are detectable repeated patterns in nature, and second whether we can account for them. We deal first, at greatest length, with the number of trophic levels, and then turn to omnivory and the extent to which food webs are compartmentalized. (a) Active plant tissue P Respiration Transport Heterotrophs H Inactive organic matter D Net primary production Litterfall and translocation Consumption Defecation Decomposition 10 –2 Energy input per unit of standing crop (energy units) Rate of recovery after perturbation (arbitrary units) (b) 10 –1 10 0 10 1 10 2 Tundra Tropical forest Temperate deciduous forest Freshwater spring Salt marsh Pond Figure 20.12 (a) A simple model of a community. The three boxes represent components of the system and arrows represent transfers of energy between the system components. (b) The rate of recovery (index of resilience) after perturbation (as a function of energy input per unit standing crop) for models of six contrasting communities. The pond community was most resilient to perturbation, tundra least so. (After O’Neill, 1976.) 594 CHAPTER 20 A fundamental feature of any food web is the number of trophic links in the pathways that run from basal species to top predators. Variations in the number of links have usually been investigated by examining food chains, defined as sequences of species running from a basal species to a species that feeds on it, to another species that feeds on the second, and so on up to a top predator (fed on by no other species).

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