Final answer:
A keystone species is one that maintains biodiversity and ecological community structure. The long-spine sea urchin is a keystone species because it controls algae growth on coral reefs, allowing for the growth and survival of other species.
Step-by-step explanation:
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and upholding an ecological community's structure. The long-spine sea urchin, Diadema antillarum, is considered a keystone species due to its significant influence on coral reef ecosystems. They play a crucial role in controlling the algae population on coral reefs, which allows for the growth and survival of other species.
When long-spine sea urchins are present, they graze on algae, preventing it from overgrowing and smothering the corals. This grazing behavior creates open spaces for coral larvae to settle and grow, promoting reef development. Without the presence of the long-spine sea urchins, the algae population would escalate, leading to the decline and potential death of coral reefs.
Therefore, the long-spine sea urchin acts as a keystone species by maintaining the balance and health of the coral reef ecosystem through its crucial role in controlling algae growth.