Insects:
- World Wildlife Fund (WWF): Estimates that insect life makes up over half of all living things on Earth. Scientists have identified 10 quintillion (10^18) individual insects belonging to 1 million described species. The WWF suggests that there could be anywhere from 5 to 10 million total species.
- Entomology Department at the University of California, Riverside (UCR): Provides a more conservative estimate of around 6 million insect species remaining to be discovered.
Marine Life:
- WWF: Approximately 250,000 marine species have been officially described, but there could be around 2 million total marine species.
- Census of Marine Life: Estimated in the early 2010s that around 700,000 to 1 million marine species remain to be described.
Animals:
- Convention on Biological Diversity (CBD): Estimated in 2008 that of the 1.9 million species identified at that time, animals accounted for about half of them. According to the CBD's estimates, as many as 13 million species total could exist, with the vast majority of them being invertebrates (non-insect animals without a backbone).
Microbes:
- American Academy of Microbiology: Estimated in 2009 that less than 1% of all microbes have been identified and described. Microbes include organisms like bacteria, archaea, fungi, and protists.
It's important to note that these are rough estimates, and the number of unknown species could vary widely. Scientific expeditions, research, and advancements in technology continuously add new species to our understanding of global biodiversity.