Summary of Chapter Concepts

Mutualisms can improve the acquisition of water, nutrients, and places to live. Mutualisms can be categorized as generalists, which interact with many species, or specialists, which interact with few other species. When both species require each other to persist, they are obligate mutualists. When the interaction is beneficial but not critical to the persistence of either species, they are facultative mutualists. Mutualisms for resources include the algae and fungi that compose lichens and the corals and zooxanthellae that build coral reefs. Plants also participate in this type of mutualism by interacting with endomycorrhizal fungi, ectomycorrhizal fungi, and Rhizobium bacteria. In most animals, protists can play an important role in digesting food. Other animals construct habitats that they share with other species in exchange for other benefits.

Mutualisms can aid in defense against enemies. Plants make use of defensive mutualisms in a number of ways, including mutualisms with aggressive insects such as ants, and with endophytic fungi that produce chemicals harmful to herbivores. Animals that interact as mutualists to defend against enemies include cleaner fish that remove parasites from large fish and oxpecker birds that remove ticks from mammals.

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Mutualisms can facilitate pollination and seed dispersal. Pollinators allow many species of plants to be fertilized and some plants have evolved traits that favor a particular type of pollinator. When this happens, the plants and the pollinators can coevolve. Numerous plants also depend on mutualisms to disperse their seeds. In some cases, seeds are dispersed as the result of animals storing them far from the parent plant. In other cases, animals consume the fruit of plants and the seeds are dispersed after passing through their digestive systems.

Mutualisms can change when conditions change. Although mutualisms benefit all species in the interaction, a positive mutualism can switch to a neutral or negative interaction when conditions change. In some cases, species can respond to cheaters in a mutualism by only rewarding individuals that provide benefits in return.

Mutualisms can affect communities. Mutualisms can increase or decrease the abundance of participating species. An absent mutualist can cause another species to be completely eliminated, thereby affecting the distribution of a species. Mutualists can also affect communities either by directly altering the number of species or by initiating a chain of interactions through a community. At the ecosystem level, mutualists can also have effects such as moving nutrients into producers and increasing the total biomass of producers.