Final answer:
A hawk nesting on a saguaro cactus represents a commensal relationship where the hawk benefits, and the cactus is unaffected. The saguaro's adaptations to store water enable its survival in the desert while providing a nesting site for the hawk.
Step-by-step explanation:
A hawk building its nest on an arm of a saguaro cactus is an example of a commensal relationship in biology. In this type of relationship, one species, the hawk, benefits by having a place to build its nest and raise its young, while the other species, the saguaro cactus, neither benefits nor is harmed by the hawk's presence. The saguaro cactus is well adapted to its environment; it can soak up to 750 liters of water in a single rainstorm, thanks to its widespread root system and barrel-shaped stem that stores water. Its adaptations, which include having no leaves and being covered with thorns, help it preserve water and protect it from thirsty animals.
The commensal relationship here allows the hawk to take advantage of the saguaro's height to stay above potential ground predators and closer to the sky for hunting. The cactus stands unaffected, continuing its life cycle in the harsh conditions of the Sonora desert. This interaction between species illustrates the complexity and diversity of ecological relationships within ecosystems.