40 Mutualism involving Animals

Key Idea: Mutualistic relationships benefit both species involved. Often the benefit (to at least one party) is food. Mutualism is a symbiotic relationship between two different species in which both interacting species benefit from the association. It can be contrasted with exploitation or parasitism in which one animal benefits while the other does not. Some mutualistic relationships are obligate, meaning that one (or both) species can not survive without its mutualistic partner. Other mutualistic relationships may not have the same level of dependency, and the relationship is not necessary for the survival of either species. These associations are termed facultative mutualism.

Types of mutualistic relationships Resource-resource relationships: One resource is traded for another (usually food or a nutrient).

Many reef building corals rely on a mutualism with algae in their tissues. The corals obtain some of their energy from the algae. The algae obtain a habitat and utilise the coral's nitrogenous waste and carbon dioxide. Termites, which feed on wood, rely on a community of microbes in their gut to break down the cellulose in wood and produce the fatty acids the termites use for energy. The obligatory relationship (opposite) provides food for both microbes and termites.

Service-resource relationships: A service is performed in exchange for a resource, e.g. food for protection.

Service-service relationships: Both organisms provide a service to each other.

Some species of ants "farm" aphids by protecting the aphids from predation by ladybirds. In return the ants harvest the honeydew produced by the aphids. Many large grazing mammals rely on oxpeckers to remove parasite infestations. The grazers provide food to the birds and the birds provide an anti-parasite service.

True service-service mutualisms are very rare and there is usually some sort of resource component present.

Clownfish protect their home sea anemone by chasing away predators, e.g. butterfly fish. In return, the anemone protects the clownfish from its predators. However, the anemone's symbiotic algae also benefit from the nitrogen excreted by the clownfish. Acacia ants nest within the thorns of the bullhorn acacia. In exchange for shelter, the ants protect acacias from attack by herbivores. There is a resource component though because the ants feed on the lipid rich bodies at the tips of the acacia leaflets.

Pollen grains

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White-tailed deer

In an obligate mutualism, neither species can survive without the other. There is a mutualistic relationship between many herbivores and the microbes in their gut, which enables cellulose to be digested. In ruminants, the rumen microflora break down the cellulose in forage and the ruminant obtains energy from the volatile fatty acids released by the microbial activity. The microbes benefit by having a stable growth environment and a food supply. In a facultative mutualism, both species benefit from interacting with each other but can survive without the interaction. In many cases, a species may interact mutualistically with many similar species. For example, bees pollinate many different types of flower. The flower can use any type of bee as a pollinator and the bee can visit any type of flower to gather nectar.

1. (a) Define the term mutualism:

(b) Distinguish between facultative and obligate mutualism:

2. The image shown right shows the Riftia tube worm, which lives near hydrothermal vents deep in the ocean. The worms have no digestive tract and rely on symbiotic bacteria for nutrition. In return, the bacteria are provided with the safe stable internal environment of the worm in which to live.

(a) Is the relationship between the worm and the bacteria an obligate or facultative relationship?

(b) Explain your answer:

3. Describe three broad classes of benefits that seem to be most common in mutualism:

(a)

(b)

(c)

4. Service-service mutualisms, such as the clownfish-anemone relationship, appear to be very rare. What might suggest that there is a resource component to these relationships?

Honeybee pollinating a purple crocus