Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

Which of the following relationships exemplifies commensalism?

1. A bird building a nest in a tree

2. A lion hunting a zebra

3. A parasitic worm in a human

4. Two flowers competing for sunlight

The correct answer is: A bird building a nest in a tree

The relationship exemplified by a bird building a nest in a tree accurately represents commensalism because it describes a situation in which one organism benefits while the other is neither helped nor harmed. In this case, the bird gains a safe place to nest and raise its young, providing it with shelter and protection from predators. The tree, on the other hand, is largely unaffected by the presence of the bird’s nest. In other scenarios such as a lion hunting a zebra, there is clear harm to one party, demonstrating predation rather than commensalism. Similarly, a parasitic worm residing in a human's body showcases a relationship where one organism benefits at the expense of the other, characterizing parasitism instead. Lastly, the competition of two flowers for sunlight illustrates interspecies competition, where both organisms are adversely affected in their struggle for a limited resource. This further distinguishes it from the non-harmful interaction seen in commensalism.