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Mth 221 - Food Webs

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Food Webs

Chance Kellar


Chastity Qualkenbush

                                                June 4, 2017

        Food webs are used to show organisms and how they provide sustenance to each other within an ecological community. For any organism that is consumed by another organism within the community, a line is drawn between the two with an arrow in the direction of the organism that is consumed. In this way, the definition of a food web of an ecological community is “a directed graph with a vertex for each species in the community and a directed edge from the vertex representing species A to the vertex representing species B if and only if A consumes B” (McGuigan n.d., p. 227).

Figure 1: The Food Web of a Pond 

[pic 1]

        Competition is an important element to any ecological community as the level of competition that on organism faces will impact the population of that species. “Two species compete if and only if they have a common prey” (McGuigan n.d.). Survival for organisms is dependent on certain conditions being met - animals need to be able to breathe oxygen, and to obtain water and nutrients in order to function (Cohen 1978).

        Food webs are a very useful means by which to demonstrate competition within an ecological community as it is a pictorial representation that makes it very clear which organisms experience the most competition. This is important as when certain conditions are altered, the food web shows which organisms are affected and the likely degree to which they are affected. For example, in the case of a food web that shows the ecological community of a pond (figure 1), if there are strong winds, then there will be less filamentous algae, which will impact tadpoles more than pond snails because pond snails can also obtain their nutrients from macrophytes, which are less affected by strong winds, whereas tadpoles are highly dependent on filamentous algae for their nutrients ("Predation And Food Webs", n.d.). 

        In a complex ecological community, it can be helpful to use a competition graph based on the food web as this more clearly demonstrates which organisms compete with each other, since a competition graph has vertices representing the organisms and edges between organism A and organism B if and only if organisms A and B compete with each other. See figure 2 for an example of a competition graph representing part of the food web in figure 1.

Figure 2: Competition graph

[pic 2]

        Clearly, in the real world, ecological communities are not impacted by one factor changing and the rest remaining constant, but by multiple factors changing at the same time. The impact of these many factors on each organism can be demonstrated on a graph within n-dimensional Euclidean space where n is the number of factors (ie. an axis for each factor). This n-dimensional Euclidean space with axes for each of the factors supporting an organism is called en ecological phase space. The sub space defined for each species, which shows the combination of factors in which the organism survives is called the ecological niche set of that organism.

        Certain factors will always have more of an impact on a particular organism than others and, in order for the ecological phase space approach to be more useful, it is better to limit the factors to those that have higher impacts as a smaller number of dimensions is always going to be easier for us to visualize. In this way, food webs are useful as they demonstrate the reliance on certain factors that organisms have. For example, in figure 1, the stickleback is heavily reliant on worms and water fleas as these are its only food source, whereas the hawker dragonfly would not be especially impacted by a drop in the number of worms and water fleas because it also feeds on tadpoles, and mayflies. As a result, the number of worms and the number of water fleas could be factors included in the ecological phase space of a stickleback, but not necessarily in the ecological phase space of a hawker dragonfly.



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