15 Chapter 15: Species Interactions: Symbioses

Anastasia Chouvalova and Lisa Limeri


Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Symbioses can include relationships where one partner benefits and the other is unaffected (commensalism), one partner benefits while the other is harmed (parasitism), or both partners benefit (mutualism).


commensal relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship (Fig 15.1). 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. Another example of a commensal relationship is the pilot fish and the shark. The pilot fish feed on the leftovers of the host’s meals, and the host is not affected in any way.

Figure 15.1: The southern masked-weaver bird 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)

Reading Question #1

Which of the following is the best example of commensalism?

A. A nematode is ingested by a human and attacks the intestinal lining in order to obtain nourishment.
B. An oxpecker (a type of bird) lands on a rhinoceros and eats parasites such as ticks from its body, getting nourishment in the process.
C. Barnacles settle on the body of a large whale, who remains undisturbed.
D. A shrimp constructs a burrow for goby fish and the fish defends the shrimp from higher-level consumers.


A mutualism is a symbiotic relationship where two species benefit from their interaction. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut (Fig 15.2a). The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this, and without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood chewing actions of the termite. Lichens are not an individual organism, but a mutualistic relationship between fungus and photosynthetic algae or bacteria (Fig 15.2b). As these symbionts grow together, the glucose produced by the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae.

Figure 15.2: (a) Termites form a mutualistic relationship with symbiotic protozoa in their guts, which allow both organisms to obtain energy from the cellulose the termite consumes. (b) Lichen is a fungus that has symbiotic photosynthetic algae living inside its cells. (credit a: modification of work by Scott Bauer, USDA; credit b: modification of work by Cory Zanker)

Reading Question #2

How is mutualism different from commensalism or parasitism?

A. Mutualism is where both interacting species benefit, while in commensalism, one species benefits and the other species is unaffected.
B. Mutualism is where both interacting species benefit, while in parasitism, one species benefits and the other species is unaffected.
C. Mutualism is where both interacting species benefit, while in parasitism, one species benefits and the other species is harmed.
D. Mutualism is where both interacting species are harmed, while in parasitism, one species benefits and the other species is harmed.
E. A) and C)

Fungus/Plant Mutualism

One of the most remarkable mutualisms in all of nature is the relationship between vascular plants and the fungus, mycorrhizaeMycorrhiza, which is derived from the Greek words myco meaning fungus and rhizo meaning root, refers to the fungal partner of a mutualistic association between vascular plant roots and their symbiotic fungi. Nearly 90% of all vascular plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus.

There are several basic types of mycorrhizaeEctomycorrhizae (“outside” mycorrhizae) depend on fungi enveloping the roots in a sheath (called a mantle). Hyphae grow from the mantle into the root and envelope the outer layers of the root cells in a network of hyphae (Fig. 15.3). The fungal partner can belong to the Ascomycota, Basidiomycota or Zygomycota. Endomycorrhizae (“inside” mycorrhizae), also called arbuscular mycorrhizae, are produced when the fungi grow inside the root in a branched structure called an arbuscule (from the Latin for “little trees”). The fungal partners of endomycorrhizal associates all belong to the Glomeromycota. The fungal arbuscules penetrate root cells between the cell wall and the plasma membrane and are the site of the metabolic exchanges between the fungus and the host plant (Fig 15.3). Orchids rely on a third type of mycorrhiza. Orchids are epiphytes that typically produce very small airborne seeds without much storage to sustain germination and growth. Their seeds will not germinate without a mycorrhizal partner. After nutrients in the seed are depleted, fungal symbionts support the growth of the orchid by providing necessary carbohydrates and minerals. Some orchids continue to be mycorrhizal throughout their life cycle.

Figure 15.3: Two types of mycorrhizae. (a) Ectomycorrhizae and (b) arbuscular or endomycorrhizae have different mechanisms for interacting with the roots of plants. (credit b: MS Turmel, University of Manitoba, Plant Science Department)

Some plants are able to survive without an mycorrhizal partner, but they do better when they are benefitting from a mutualistic relationship with mycorrhizae. These plants are an example of a facultative mutualism, where the mutualism is beneficial, but partners are capable of surviving without the other. In contrast, other plants are completely unable to survive without mycorrhizae. These plants are an example of an obligate mutualism, a relationship where one or both partners requires the partnership to survive.

Other examples of fungus–plant mutualism include the endophytes: fungi that live inside tissue without damaging the host plant. Endophytes release toxins that repel herbivores, or confer resistance to environmental stress factors, such as infection by microorganisms, drought, or heavy metals in soil.

Evolution Connection: Coevolution of Land Plants and Mycorrhizae

As we have seen, mycorrhizae are the fungal partners of a mutually beneficial symbiotic association that coevolved between roots of vascular plants and fungi. A well-supported theory proposes that fungi were instrumental in the evolution of the root system in plants and contributed to the success of Angiosperms. The bryophytes (mosses and liverworts), which are considered the most ancestral plants and the first to survive and adapt on land, have simple underground rhizoids, rather than a true root system, and therefore cannot survive in dry areas. However, some bryophytes have arbuscular mycorrhizae and some do not.

True roots first appeared in the ancestral vascular plants: Vascular plants that developed a system of thin extensions from their roots would have had a selective advantage over nonvascular plants because they had a greater surface area of contact with the fungal partners than did the rhizoids of mosses and liverworts. The first true roots would have allowed vascular plants to obtain more water and nutrients in the ground.

Fossil records indicate that fungi actually preceded the invasion of ancestral freshwater plants onto dry land. The first association between fungi and photosynthetic organisms on land involved moss-like plants and endophytes. These early associations developed before roots appeared in plants. Slowly, the benefits of the endophyte and rhizoid interactions for both partners led to present-day mycorrhizae: About 90% of today’s vascular plants have associations with fungi in their rhizosphere.

The fungi involved in mycorrhizae display many characteristics of ancestral fungi; they produce simple spores, show little diversification, do not have a sexual reproductive cycle, and cannot live outside of a mycorrhizal association. The plants benefited from the association because mycorrhizae allowed them to move into new habitats and allowed the increased uptake of nutrients, which gave them an enormous selective advantage over plants that did not establish symbiotic relationships.

Reading Question #3

A farmer is experimenting with different methods to get the best yield from their crops. They discover that their soybeans are capable of growing on their own, but when mycorrhizae are introduced, the soybeans grow larger and produce a greater crop yield. The relationship between the soybean and mycorrhizae is best described as:

A. Obligate mutualism
B. Facultative mutualism
C. Commensalism
D. facultative parasitism


Lichens display a range of colors and textures (Fig. 15.4) and can survive in the most unusual and hostile habitats. They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannot penetrate. Lichens can survive extended periods of drought, when they become completely desiccated, and then rapidly become active once water is available again.

Figure 15.4 Lichens have many forms. They may be (a) crust-like, (b) hair-like, or (c) leaf-like. (credit a: modification of work by Jo Naylor; credit b: modification of work by “djpmapleferryman”/Flickr; credit c: modification of work by Cory Zanker)

It is important to note that lichens are not a single organism, but rather another wonderful example of a mutualism, in which a fungus (usually a member of the Ascomycota or Basidiomycota) lives in a physical and physiological relationship with a photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium). Generally, neither the fungus nor the photosynthetic organism can survive alone outside of the symbiotic relationship. Thus, lichen is the result of an obligate mutualism. The body of a lichen, referred to as a thallus, is formed of hyphae wrapped around the photosynthetic partner. The photosynthetic organism provides carbon and energy in the form of carbohydrates. Some cyanobacteria additionally fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium. The fungus also attaches the lichen to its substrate.

The thallus of lichens grows very slowly, expanding its diameter a few millimeters per year. Both the fungus and the alga participate in the formation of dispersal units, called soredia—clusters of algal cells surrounded by mycelia. Soredia are dispersed by wind and water and form new lichens.

Lichens are extremely sensitive to air pollution, especially to abnormal levels of nitrogenous and sulfurous compounds. The U.S. Forest Service and National Park Service can monitor air quality by measuring the relative abundance and health of the lichen population in an area. Lichens fulfill many ecological roles. Caribou and reindeer eat lichens, and they provide cover for small invertebrates that hide in the mycelium. In the production of textiles, weavers used lichens to dye wool for many centuries until the advent of synthetic dyes. The pigments used in litmus paper are also extracted from lichens.

Reading Question #4

Which of the following describes lichens?

A. A mutualistic relationship between a fungus and an algae or bacteria
B. A commensalistic relationship between a fungus and an algae or bacteria
C. A fungus that parasitizes an algae or bacteria
D. A bacteria that parasitizes a fungus

Evolution Connection: Endosymbiosis

Symbiosis is a relationship in which organisms from two separate species depend on each other for their survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. For example, microbes that live in our gut produce vitamin K. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and from drying out, and they receive abundant food from the environment of the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Scientists believe that host cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the autotrophic bacteria becoming chloroplasts.

Fungus/Animal Mutualism

Fungi have evolved mutualisms with numerous arthropods. Arthropods depend on the fungus for protection from predators and pathogens, while the fungus obtains nutrients and a way to disseminate spores into new environments.

One example is the leaf-cutter ants of Central and South America which literally farm fungi. They cut disks of leaves from plants and pile them up in subterranean gardens (Figure 15.5). Fungi are cultivated in these disk gardens, digesting the cellulose in the leaves that the ants cannot break down. Once smaller sugar molecules are produced and consumed by the fungi, the fungi in turn become a meal for the ants. The insects also patrol their garden, preying on competing fungi. Both ants and fungi benefit from this mutualistic association. The fungus receives a steady supply of leaves and freedom from competition, while the ants feed on the fungi they cultivate.

Figure 15.5 Leaf-cutter ant. A leaf-cutter ant transports a leaf that will feed a farmed fungus. (credit: Scott Bauer, USDA-ARS)

Reading Question #5

Typically, in plant-mycorrhizae relationships, the plants provide sugar to the mycorrhizae and the mycorrhizae provide water and nutrients to the plant. However, sometimes the plant may stop sharing sugar while continuing to receive water and nutrients. What is being described in this situation?

A. A mutualism becomes a parasitism
B. A parasitism becomes a mutualism
C. The relationship is a mutualism in both situations
D. A symbiosis ceases being symbiotic.


Adapted from

Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/45-6-community-ecology


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Introductory Biology 2 Copyright © 2023 by Anastasia Chouvalova and Lisa Limeri is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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