How do ecologists estimate the total number of species present in the world?

The diversity of living organisms present on the Earth is very vast. According to an estimate by researchers, it is about seven millions.
The total number of species present in the world is calculated by ecologists by statistical comparison between a species richness of a well-studied group of insects of temperate and tropical regions. Then, these ratios are extrapolated with other groups of plants and animals to calculate the total species richness present on the Earth.

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