Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

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What defines commensalism in ecological relationships?

  1. Both organisms benefit equally

  2. One organism benefits while the other is harmed

  3. One organism benefits and the other is unaffected

  4. Neither organism benefits from the interaction

The correct answer is: One organism benefits and the other is unaffected

In ecological relationships, commensalism is characterized by one organism benefiting from the interaction while the other organism remains unaffected. This means that the benefitting organism gains some advantage, such as food, shelter, or transportation, without causing any harm or providing any benefit to the other organism involved. For example, epiphytic plants that grow on trees demonstrate commensalism. They receive access to sunlight and moisture without harming the tree. The tree remains unaffected by the presence of the epiphyte, illustrating the essence of commensalism. Understanding this relationship helps clarify the nature of interactions within ecosystems, which can be complex and varied. Other types of ecological relationships can involve mutual benefit or harm, but commensalism distinctly identifies that there is no negative impact on the unaffected organism.

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