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Given a scientific passage and an answer, generate a question for the given answer.

Passage: How many species actually exist in the world? No one knows for sure because only a small percentage of them have already been discovered. Estimates range from 5 to 30 million total species currently in existence. Many of them live on coral reefs and in tropical rainforests (see Figure below ). These two ecosystems have some of the greatest biodiversity on the planet. Answer: biodiversity
Output:
Coral reefs and tropical rainforests are ecosystems known for what, contributing greatly to the species in existence on earth?