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In one section of the GED study book I'm looking through to prepare myself for exams, they define parasitism, mutualism, and commensalism, and give examples.

One of the examples they give for mutualism is ants living in an acacia tree. The ants get a home, and they defend the tree.

One of the examples they give for commensalism is a spider building a web on a plant.

Is this actually an example of commensalism? This seems more like mutualism to me. The spider benefits from being able to put its web somewhere, and the plant benefits from protection from bugs (as the spider's web will catch them).

Is this relationship an example of commensalism or mutualism?

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You're thinking too broadly and rigidly about these concepts -- both commensalism and mutualism are types of interspecific interactions.

Commensalism is when one species in a given interaction is benefited while the other is neither benefited nor harmed. In a mutualistic interaction, both species benefit from their interaction with each other.

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Like any interaction between two organisms, these interactions can vary between individuals. Although some species pairs will consistently undergo the same type of interaction (e.g., shark and prey -- predation), some groups of species may vary in their paired interactions. This variation may result from two different scenarios:

  1. Variation in interaction type between different species within a taxonomic group

    • E.g., one type of spider catches insects that don't harm the plant -- commensalism -- while another spider species catches insects that would harm the plant -- mutualism. In other words, not all spider-plant interactions are the same, and the interaction type may be dependent on the species involved.
  2. Variation even from the same individual in different circumstances.

    • E.g., A spider catches an insect that doesn't eat the plant it's growing on (commensalism) but at another time it catches an herbivorous insect that does eat the plant the spider has built its web on (a mutualistic behavior). In this scenario, the interaction type between the same two species (even the same individuals) can vary based on circumstances.

Put differently, commensalism, mutualism, etc. are not behavioral traits, but interactions, and so are dependent on not only the species involved but the circumstances as well.

Note: To further complicate things, each given interaction can be qualified in various ways. For example, from Wiley ELS:

Positive interactions can be trophic and nontrophic, can act directly or indirectly (mediated by a third species) and can be symmetric (species have equal effects) or asymmetric (species have unequal effects).

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For the scenario you laid out, the short answer is yes.

Like @theforestecologist pointed out, interactions can be complex and contingent.

Although, in the example you give, it can be easily tested: grow plants with spiders and without, and see if there is a difference in plant fitness. Fitness can be measured directly by looking at, for instance, the number and quality of seeds the plants in each group produces. But fitness can also be measured indirectly by looking at the amount of herbivory on the plants (e.g., scars), the number or area of leaves before and after the spider treatment, photosynthetic rates, etc.

If a fitness benefit can be demonstrated with both species involved, then it is considered a mutualism.

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    $\begingroup$ Great experimental/application point! +1. Very clear that this fitness experiment could be used to begin getting at the predominant interaction type. Thanks $\endgroup$ – theforestecologist Apr 12 at 19:49

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