Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

In which type of relationship does one organism benefit while the other is not affected?

1. Predation

2. Competition

3. Commensalism

4. Parasitism

The correct answer is: Commensalism

In commensalism, one organism benefits while the other is neither helped nor harmed. This type of relationship can be seen in various ecological contexts. For example, consider barnacles that attach themselves to whales. The barnacles gain mobility to access food in nutrient-rich waters as the whale swims, while the whale remains largely unaffected by their presence. This distinction is crucial, as it highlights that the benefitting organism derives an advantage (e.g., better access to resources), whereas the other organism does not experience positive or negative consequences. Predation involves one organism benefiting at the expense of the other, while competition refers to organisms vying for the same resources, which typically adversely affects both. Parasitism is a relationship in which one organism benefits at the cost of the other, leading to potential harm or detriment to the host. Therefore, commensalism is uniquely characterized by its lack of effect on the non-benefiting organism.