Read the passage and then answer the question.
Symbiotic relationships, which occur when one organism is dependent on another organism for survival, are typically categorised into three different types of relationships: parasitism, mutualism and commensalism. In each of these relationships, the organisms involved are benefited or harmed in different ways.
Parasitic relationships are often the most familiar types of symbiotic relationships. A common example is a flea living on a dog. In cases like these, the parasite gains a viable habitat to support its life cycle, while the host is harmed.
In a mutualistic relationship, on the other hand, both organisms in the relationship are better off because of their arrangement; that is, they benefit mutually from their interactions. For example, lichen is composed of two organisms, an alga and a fungus. The fungus acts like a sponge, holding on to water that the alga needs. At the same time, the alga is constantly converting sunlight into energy, so the fungus, which cannot perform this function for itself, benefits from a reliable energy source. Lichen is, in turn, engaged in a commensalistic relationship with the tree that it grows on. In a commensalistic relationship, one organism benefits and the other organism neither benefits nor suffers. The tree provides lichen with a place to grow, but the tree is unaffected.
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