What is a mutualism?

Another ecological essay along the same lines as What is a community?  Perhaps pertinent for my thesis?  I remain unapologetic, and perhaps someone will find it useful.

The biotic interactions that comprise ecological communities can be divided into loose categories: antagonistic (beneficial for one organism, negative for the other), commensal (one organism benefits, there is no effect on the other), amensal (negative for one and neutral for the other), competitive (negative for both organisms) and mutualistic (beneficial for both organisms). Hence, a mutualistic community is one comprising mutualistic interactions between organisms in a given location.

Mutualistic interactions are ubiquitous in nature. Indeed, life as we know it would not exist without mutualisms and facilitation. For example, eukaryotes are incapable of fixing nitrogen from the air. Certain species of plants, however, are able to fix nitrogen via a mutualistic symbiosis with nitrogen-fixing bacteria that live in specialized root nodules (Kiers et al. 2003).

While it would be a mistake to say that mutualistic interactions are the sole driver for extant community structure, we would also be remiss to ignore them. However, in spite of their ubiquity, they are relatively understudied compared to competitive or antagonistic interactions, and their implications for natural selection and coevolution are often poorly understood.

Between 78% (temperate) and 94% (tropical) of terrestrial angiosperms depend on animal vectors for pollination services (Ollerton et al. 2011). The global estimated proportion of biotically pollinated flowering plants is 87.5% (J. Ollerton, pers. comm.).  Interestingly, mutualistic interactions between animal-pollinated plants and their pollinators are diffuse and generalized (Waser et al. 1996).

Darwin himself was fascinated by this mutualism, and he wrote about it at great length (e.g. Darwin 1862). Indeed, he was able to predict the existence of a moth by observing the morphology of a flower (to the best of my knowledge, this is the only species whose existence has been successfully predicted before it was discovered) (Arditti et al. 2012). Though extremely specific interactions such as that one do exist, the vast majority of interactions are loose. In aggregate, however, this mutualism has arguably led to the impressive diversity of both floral forms and the insects specially adapted for the collection of floral resources (e.g. bees) (Cardinal & Danforth 2013). It is indeed mind-boggling to think what evolutionary process could be responsible for their development.

When studying pollination mutualisms, it is common to focus on the superfamily Apoidea (bees). This superfamily has specific adaptations for collecting pollen, which is the primary protein source for the developing young. Phylogenetically speaking, the bees are predatory wasps that have replaced insect prey with pollen (Cardinal & Danforth 2012). Their specific adaptations include branched body hairs (which pollen clings to) and often specialized pollen baskets or scopa (the location of which depends on the bee and is usually reserved for females). Bees are effective vectors for the dispersal of the gametic phase of the otherwise sessile plants (Free 1970). At the same time, pollen and nectar are resources which have fueled the radiation of the bees (Cardinal & Danforth 2012).

In spite of the clear impact these two groups of interacting organisms have on each other, there are few studies showing clear signals of community-wide coevolution, and what exactly would constitute evidence for this process is unclear. Many hypotheses of how these communities are structured have recently fallen out of favor in the scientific community (e.g. the pollinator syndrome hypothesis, whereby pollinators segregate along clear lines according to floral morphologies) (Ollerton et al. 2009).

In many ways, mobile mutualisms, where one or both interacting organisms are able to move away from each other, such as plants and their pollinators or seed dispersers, are more difficult to study than intimate mutualisms, where one organism lives within the body of the other (coral/symbiodinium, lichen/algae, etc.) (Thompson 2005). The exchange of resources and population level dynamics of the mobile partner are extremely difficult to track, especially when they are rare in a large area. Thus, sampling effort and resolution remain challenges for the standardization and communication of data on these systems.


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5 thoughts on “What is a mutualism?

  1. Enjoyed your post but can I offer a small correction? When you say that “The mutualistic interactions between entomophilous plants (78-94% of terrestrial angiosperms (Ollerton et al. 2011)”, those figures actually refer to pollination by all animals, not just insects. So bird and bat pollination, etc. will also be included as well as “entomophily”.

    I should also say that the range cited (78-94%) is the variation between temperate to tropical communities i.e. the proportion of animal pollinated plants increases as you move towards the tropics. The global estimated proportion of biotically pollinated flowering plants is 87.5%.

    Best wishes,

    Jeff

  2. Pingback: Elephant week: Elephants are scared of bees | standingoutinmyfield

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