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Why is the total number of species on Earth difficult to estimate?
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A nautilus (Nautilus) floating in the water column of the ocean.
Estimating the total number of species on Earth is a complex challenge, primarily because the catalog of global biodiversity remains significantly incomplete. While approximately 1.9 million species have been formally named, scientific estimates for the total number of living species vary widely—ranging from roughly 8.7 million to as many as 1 trillion, depending on whether groups like bacteria are included.12
Several factors contribute to this difficulty:
- Incomplete Discovery: Most species have not yet been described or discovered. At the current rate of description, it is estimated that completing a full catalog would take approximately 1,000 years.2
- Defining Species: There is no single, universally applied definition for a species; biologists utilize more than 20 different concepts, such as the biological, ecological, and genetic species concepts, which can complicate classification.1
- Practical Limitations: Many known species are documented from only a single location or a limited number of individuals, making it difficult to assess their full range or genetic variation.2
- Extinction Rates: Human activity, including climate change and habitat encroachment, is driving rapid population declines, meaning many species may go extinct before they are ever identified.23