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8.4: Commensalism and Mutualism

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    Interactions in which one or both species benefit and neither is harmed are called facilitation. There are two types of facilitation: commensalism and mutualism. 


    Commensalism is a type of facilitation that occurs when one species benefits from an interaction, while the other neither benefits or is harmed. Many potential commensal relationships are difficult to identify because it is difficult to demonstrate that one partner is unaffected by the presence of the other. Birds nesting in trees provide an example of a commensal relationship (figure \(\PageIndex{1}\)). The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators.

    This Southern Masked-Weaver is building a nest of green plant material in a tree. The lower portion of this yellow bird is visible behind the nest.
    Figure \(\PageIndex{1}\): The Southern Masked-Weaver is starting to make a nest in a tree in Zambezi Valley, Zambia. This is an example of a commensal relationship, in which one species (the bird) benefits, while the other (the tree) neither benefits nor is harmed. (credit: “Hanay”/Wikimedia Commons)

    Another example or a commensal relationship involves the Little Blue Heron and the White Ibis, which are both wading birds. The Little Blue Heron catches more fish in the presence of the White Ibis, but the White Ibis is unaffected. Interestingly, Little Blue Herons attempt to catch fish more often in the presence of the species, but the success rate of their attempts does not change. Nevertheless, more frequent attempts still increases the total number of fish caught. The White Ibis may make fish more visible to Little Blue Herons, causing changes in their behavior (figure \(\PageIndex{2}\)). 

    A Little Blue Heron hovers behind a White Ibis in a shallow body of water.
    Figure \(\PageIndex{2}\): The Little Blue Heron (left) and the White Ibis (right) have a commensal relationship. Image by Russ (CC-BY).


    In a mutualism, both species benefit from their interaction. For example, pollinators, such as bees, butterflies, and hummingbirds, benefit because they eat the collect pollen and/or nectar that they collect from flowers. The plants also benefit because their pollen is dispersed to other plants, allowing them to reproduce. Both the pollinators and the plants benefit, demonstrating a mutualism (figure \(\PageIndex{3}\)). Clownfish and anemones are another example of mutualisms. Clownfish gain protection from living among anemones. In return, clownfish clean the anemones and scare away predators. In addition, their waste provides nutrients (figure \(\PageIndex{4}\)). 

    A hummingbird with a magenta throat, blue head, and gray body hovers with its beak in a tubular red flower.
    Figure \(\PageIndex{3}\): A male Broad-Tailed Hummingbird visits a scarlet gilia flower at the Rocky Mountain Biological Laboratory. The hummingbird gains food (nectar) while aiding the gilia flower with reproduction. Image by David W. Inouye (CC-BY).
    A large orange and white striped clownfish and a smaller one surrounded by white, elongated sea anenomes.
    Figure \(\PageIndex{4}\): These clownfish have a mutual relationship with the sea anemones in which they live. image by Samuel Chow (CC-BY).


    Co-evolution often occurs in species involved in mutualistic relationships. Coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. For example Central American Acacia species have co-evolved with some species of ant that drink the nectar (Figure \(\PageIndex{5}\)). The Acacia have hallow thorns and pores at the bases of their leaves that secrete nectar rich in lipids and proteins. These hollow thorns are the exclusive nest-site of some species of ant that drink the nectar. But the ants are not just taking advantage of the plant — they also defend their acacia plant against herbivores! This system is probably the product of coevolution: the plants would not have evolved hollow thorns or nectar pores unless their evolution had been affected by the ants, and the ants would not have evolved herbivore defense behaviors unless their evolution had been affected by the plants.

    Pseudomyrmex sp. and Acacia cornigera or collinsii mutualism  Merida, Yucatan, Mexico
    Figure \(\PageIndex{5}\): Ant (pseudomyrmex sp.) on an acatia thorn in Yucatan, Mexico. The ants will chew on the thorns so that they can get inside, they will then colonize the whole plant, feed on the "Beltan bodies" (Which are found in the tips of each leaflet and are rich in lipids and proteins) and in change, they will protect the plant from any other organism, including other plants. Also, the ants will even destroy the seeds which are left around the tree, in order to reduce competition. (CC BY 2.0; by Maximilian Paradiz via flickr)

    Many examples are provided by flowering plants and the species that pollinate them. Plants have evolved flowers with traits that promote pollination by particular species. Pollinator species, in turn, have evolved traits that help them obtain pollen or nectar from certain species of flowers. For example, the plant with tube-shaped flowers shown in (Figure \(\PageIndex{3}\)) co-evolved with hummingbirds. The birds evolved long, narrow beaks that allowed them to sip nectar from the tubular blooms.

    Video: Which Came Frist - Flowers or Bees?

    How did co-evolutionary relationships between flowers and animals begin? Watch the video to find out.



    This page is a Modified derivative of:

    This page titled 8.4: Commensalism and Mutualism is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Sara Kappus (Open Educational Resource Initiative at Evergreen Valley College) .

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