Symbiosis: mutualism and commensalism
All living organisms of an ecosystem interact with each other. There can be thousands or even millions of organisms in an ecosystem and they interact with each other in many different ways. Some organisms depend on other organisms for food, shelter or protection. These interactions will be explored in this chapter.
Symbiosis is a close association or interaction between two or more different organisms where at least one of them benefits. Both organisms may benefit in some interactions but in other interactions one could be unaffected or one could be harmed or even killed. The three main types of symbiosis are mutualism, commensalism and parasitism.
Mutualism is a type of interaction in which both participating organisms benefit. In many cases neither species can survive without the other (sometimes called a symbiotic relationship). Organisms that interact with each other often experience higher success than those that do not and it is now believed that most species are involved in one or more mutualisms.
Mutualism generally involves an exchange of substances or services that organisms would find difficult or impossible to obtain without interacting with another, i.e. transportation, food, shelter or protection. Some examples of mutualism include lichen, pollination and aphids interacting with ants.
Lichen is made up of a fungus and an alga living together. The alga uses light from the sun to make food that it and the fungus need to survive but the fungus also shelters the alga so that it does not get too hot or dry. Pollination is also an example of mutualism. Certain animals such as bees visit flowers to obtain food but also benefit the flower by transporting its pollen. Another example of mutualism is when ants attack the predators and parasites of certain aphids and in exchange they receive access to the aphids' sugary excretions (honeydew).
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Commensalism is an interaction where one species benefits, while the other is neither helped nor harmed. The species that is generally not affected is called the host. The host often provides a home and/or transportation for the other species. Commensalisms vary in strength and duration from intimate, long-lived relationships to brief, weak interactions.
Some examples of commensalism are epiphytes and trees, the remora fish and sharks, and jellyfish and fish. Epiphytes are plants such as orchids and staghorn ferns that grow on the outside of other plants without taking any nourishment from them. The tree is the host and provides the branches that support the epiphytes but the tree is not harmed and receives no benefit.
The remora is a sucker-fish that lives in close association with sharks or other larger fish. The sucker-fish has a sucker on its fin that it uses to attach itself to the shark. The sucker-fish is small and does not injure the shark but it uses the shark as transportation and protection and lives on the scraps formed as the shark eats its prey. The shark benefits slightly from the association as the remora eat the tiny shrimp-like parasites that live on the shark's skin.
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Another example of commensalism involves fish, often juveniles and jellyfish. The fish swim around the jellyfish in order to gain protection from potential predators. The jellyfish is not affected by the relationship because it is not eaten by the fish nor does it eat the fish.
Parasitism is an interaction where one species (the parasite) lives in or on another species for all or most of the host's life. The parasite obtains food, shelter and other requirements from the host. Some parasites harm their hosts but do not usually kill them. Parasitism is an example of a relationship between two species in which one benefits at the expense of the other.
Some examples of parasitism are the cuckoo, mistletoe plant and tapeworm. The cuckoo is a bird that lays its eggs in the nests of other birds. The cuckoo young are raised and fed by adults of the host species, while adult cuckoos fend for themselves. The mistletoe plant is a partial parasite. It lives on other plants and gets its water and other nutrients from the host plant but it still makes it own food through photosynthesis. The tapeworm is also an example of a parasite. Tapeworms live in the gut of animals and receive their nutrients and a home from the host. Tapeworms can cause severe problems such as malnutrition and intestinal blockages for their host.
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