I'll describe a mutualistic relationship in a coral reef ecosystem.
Organisms: Coral and Zooxanthellae
Coral and zooxanthellae have a mutually beneficial relationship. The coral provides the zooxanthellae with a safe and stable environment to live in, while the zooxanthellae produce oxygen and remove waste for the coral through photosynthesis. This relationship allows both the coral and zooxanthellae to thrive in the coral reef ecosystem.
If the zooxanthellae were to go extinct, the coral would no longer receive the benefits of the zooxanthellae's photosynthesis. This would result in a decrease in the coral's health and could potentially lead to its death. Additionally, the loss of the zooxanthellae would disrupt the balance of the coral reef ecosystem, potentially affecting other species in the area.