4.3.3: Symbiotic Interactions

Unit 4.3.3 - Symbiotic Interactions

• Please read and watch the following Learning Resources
• Reading the material for understanding, and taking notes during videos, will take approximately 30 minutes.
• Optional Activities are embedded.
• Bolded terms are located at the end of the unit in the Glossary. There is also a Unit Summary at the end of the Unit. 
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Learning Objectives

• Describe the types of symbiotic relationships between species

Introduction

Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time that impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction. In this discussion, the broader definition will be used.

Video

In this 10-minute video, learn how evolutionary biologists understand symbiosis, all its different forms, and why it matters.
Question after watching: what other examples of commensalism, mutualism, and parasitism can you think of?

Commensalism

A commensal relationship occurs when one species benefits from a close prolonged interaction, while the other neither benefits nor is harmed. 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. Many potential commensal relationships are difficult to identify because it is difficult to prove that one partner does not derive some benefit from the presence of the other.

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)

Mutualism

Mutualism occurs when two species benefit from their interaction. For example, termites have a mutualistic relationship with protists that live in the insects' gut (Figure \(\PageIndex{2}\)a). The termite benefits from the ability of the protists to digest cellulose. The termite itself cannot do this: without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa benefit by having a protective environment and a constant supply of food from the wood-chewing actions of the termite. In turn, the protists benefit from the enzymes provided by their bacterial endosymbionts, while the bacteria benefit from a doubly protective environment and a constant source of nutrients from two hosts.

Lichen are a mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria (Figure \(\PageIndex{2}\)b). 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. The algae of lichens can live independently given the right environment, but many of the fungal partners are unable to live on their own.

Figure \(\PageIndex{2}\): (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 in close association. (credit a: modification of work by Scott Bauer, USDA; credit b: modification of work by Cory Zanker)

Parasitism

A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this relationship, the parasite benefits, but the organism being fed upon, the host, is harmed (Figure \(\PageIndex{3}\)). The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host, especially not quickly, because this would allow no time for the organism to complete its reproductive cycle by spreading to another host.

Figure \(\PageIndex{3}\): A fish parasite, the isopod Cymothoa exigua, replacing the tongue of a Lithognathus

Like predation, parasitism is a type of consumer-resource interaction, but unlike predators, parasites (with the exception of parasitoids) are typically much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialized and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the malaria-causing Plasmodium species, and fleas.

Parasites reduce host fitness by general or specialized pathology, from parasitic castration to modification of host behavior. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their transmission from one definitive (primary) host to another. Although parasitism is often unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic.

The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed (Figure \(\PageIndex{4}\)). The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is bringing into its gut by eating, and may grow to be over 15m long by adding segments. The parasite moves from species to species in a cycle, making two hosts necessary to complete its life cycle.

Figure \(\PageIndex{4}\): This diagram shows the life cycle of a pork tapeworm (Taenia solium), a human worm parasite. (credit: modification of work by CDC)

Optional Activity \(\PageIndex{1}\)

A symbiotic relationship where both of the coexisting species benefit from the interaction is called ________.

1. commensalism
2. parasitism
3. mutualism
4. communism

Answer

c. mutualism