The lowest estimated number is 3 million, with the highest reaching 1 billion.
Currently, a widely accepted number was proposed by Professor Camilo Mora from the University of Hawaii at Manoa:
8.7 million, excluding viruses.
This number was estimated using the application of biological classification hierarchies—essentially the trends in the number of kingdoms, phyla, classes, orders, families, genera, and species as learned in biology textbooks (org/content/113/21/5970).
For example, a phylum of animals = 10 classes = 100 orders = 1000 families = 10000 genera = 100000 species, with similar trends in the number of classification ranks among different groups, and higher classification levels being more stable compared to species.
Therefore, after a series of simulations, a more reliable total species number can be estimated.
