Ecologists cannot count every single species directly, so they use several clever methods to estimate the total number of species on Earth. Here’s how they do it:

1. Sampling and Extrapolation:

• Scientists study a small, well-known area (like a rainforest plot or coral reef section).
• They count all species there carefully.
• Then, they scale up (extrapolate) their findings to larger areas based on habitat types and conditions.

Example:
If 100 species are found in 1 hectare of rainforest, and there are 100 million hectares of similar rainforest, they estimate from there.

2. Ratios Based on Well-known Groups:

• Some groups like birds and mammals are well studied.
• Ecologists use the known ratios of birds/mammals to less studied groups (like insects, fungi) to guess the unknown diversity.

Example:
If there are 10 bird species for every 100 insect species in a studied area, they use that ratio for other areas too.

3. Using Discovery Curves:

• As more species are discovered over time, the rate of discovery slows down.
• By plotting species discovery over time, ecologists can predict how many more species remain undiscovered.

4. DNA and Molecular Techniques:

• Scientists now use DNA barcoding to detect hidden (“cryptic”) species.
• This reveals species that look identical but are genetically different, helping correct underestimates.

Current Estimates:
• Scientists believe there are about 8.7 million species on Earth (±1.3 million).
• Only around 1.5–2 million species have been formally described so far.