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I am a biology student who has just completed drawing a food web for class. As I was making it, I learned that a food web is the sum of all feeding interactions. My teacher associated food webs with communities, but I do not understand what the communities of a food web are.

How are communities related to food webs?

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  • $\begingroup$ Tim can you add some information about your drawing? Also, where have you looked for this information already? $\endgroup$ – Vance L Albaugh Feb 23 '17 at 3:02
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The Details

Ecological communities are groups of multiple (2+) species of either plants, animals and/or other organisms that interact in a unique habitat. More accurately (from Wikipedia and corroborated by Begon et al. (2006)), we can define an ecological community as an:

assemblage or association of populations of two or more different species occupying the same geographical area and in a particular time.

These organisms could be interacting in a number of ways:

http://eebweb.arizona.edu/animal_behavior/lycaenids/grid1.gif

As you can see, one or both species can be effected either positively, negatively or neutrally (not at all) in these various types of interspecific interactions.

I should note that a community doesn't have to contain all organisms within a certain habitat.

  • Sometimes we might limit our discussion of a community to a single guild or single trophic level. Other times we might use the terminology simply to describe two interacting species (either within or between guilds/trophic levels).

  • Really, "communities" are more conceptual "units" used to describe two or more interacting species. More accurately, it's an interaction between populations of 2+ species. In this way, community ecology is an extension of population ecology.

Food webs (i.e., a system of interlocking and interdependent food chains) are essentially summaries of the feeding relationships within an ecological community (Molles, 2010). In other words, they are focused strictly on trophic interactions. These trophic interactions are primarily concentrated within the (+,-) interactions between species. This means that the interactions that dictate a food web are going to consist mainly of consumption (predation/herbivory).

  • From University of Michigan (my emphasis):

    Food webs are graphical depictions of the interconnections among species based on energy flow. Energy enters this biological web of life at the bottom of the [web], through the photosynthetic fixation of carbon by green plants.

    Energy moves from lower to higher trophic (feeding) levels by consumption.

Example

An example of a community that does not qualify as a food web:

  • A community of plants includes just the plants that are competing in the same space and as a result make up (i.e., are classified as) some sort of vegetation type. These species are competing at the same trophic level and do not consume one another. This is classic ecological competition.

  • If we examine the trophic interaction between each of these plants with each of their sources of energy, each of their consumers, each of their consumers' consumers, etc. then we are talking about a food web.

Final Thoughts

So the term "community" is used generally to describe interacting species of organisms in a given habitat, while "food webs" describe a more specific aspect of a community -- the trophic interactions of the members of that community.

  • Of course, it could be the case that all species in a given community are represented in a food web, but this is not necessarily the case.

  • It could also be the case that not all species represented in a food web be part of the same community...

One could also consider food webs to be broader than a community because they can describe the trophic interactions of species/populations across communities. From this perspective, food webs "lie at the interface of community and ecosystem ecology" (Begon et al. 2006).


Citations:

  • Begon, M., C.R. Townsend & J.L. Harper. 2006. Ecology: From individuals to ecosystems. Fourth edition. Blackwell Publishing Ltd. Malden, MA. pp 469 - 601.

  • Molles, M.C. 2010. Ecology: Concepts and Applications. Fifth edition. McGraw-Hill. New York, NY. p. 544.

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    $\begingroup$ I give up :-) +1 $\endgroup$ – AliceD Feb 24 '17 at 16:08
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A food web is a representation of who eats whom.


A qualitative food web provides for every pair of vertices just the information whether or not one feeds on the other.

This can be represented e.g. by some directed graph with a set of vertices $V$ and a set of arrows $A$ between the vertices. An arrow has the form $$a: v_1\rightarrow v_2$$ for some vertices $v_1$ and $v_2 \in V$ meaning $v_1$ feeds on $v_2$.


A quantitative food web provides for every pair of vertices a quantity (e.g. in form of a postive real number) defining how much one feeds on the other.

This can be represented e.g. by some weighted directed graph with a set of vertices $V$, a set of arrows $A$ between the vertices and additionally a set of weights $\mathbb{R}^{\geq0}$ providing one weight for each arrow. An arrow has the form $$a: v_1\rightarrow_x v_2$$ for some vertices $v_1$ and $v_2 \in V$ and a positive real number $x\in\mathbb{R}^{\geq0}$ meaning $v_1$ feeds on $v_2$ to an extend of $x$.

The specification of the weights $x$ can vary (bio mass, energy, ...).


The arrows in a graph representation of a food web represent a feeding relationship - the exact specification can be versatile: qualitative, quantitative (diverse units).

The vertices in a graph representation of a food web can be species (i.e. grouping according to genetics), traits (i.e. grouping according to certain properties or combinations of properties).


Community. In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area and in a particular time.

See Wikipedia: Community

Thereby a species based food web (i.e. a representation of the feeding relationship between species) that is e.g. represented as a graph with a set of vertices $V$ and arrows $A$ thereby implicitely also represents a community by forgetting the representation of the feeding relations $A$ and regarding just the set of the included species $V$.

A trait based food web can also be regarded as a community in the above sense. E.g. assume the food web is represented as a graph with a set of vertices $V$ (not being species) and arrows $A$. Assume the traits are about individuals (respectively species). Then translating $V$ in the case of species based traits to the set of involved species:

$$S(V):= \{s|s \text{ is a species for which some trait }v\in V \text{ is true}\}$$

or in the case of individual based traits:

$$S(V):= \{s|s \text{ is the species of some individual for which some trait }v\in V \text{ is true}\}$$

yields again a community.


Summarized. You should always be able to yield a community from a food web by forgetting the feeding relationship and taking all involved species.

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