Species are the fundamental unit of biodiversity. Biologists define a species as a population that interbreeds under natural conditions. However, there are ambiguities in this definition that add to the difficulty of enumerating species, even those that have already been discovered.
Scientists have different opinions about what constitutes a species or a subspecies.
There have been a number of efforts to estimate the number of species that are known and have been described. This is a challenging job because there are no central registries for species, and no one database. Often species that have been described by scientists in one geographical site, are also recorded and described by scientists at another. It takes time and research for biologists to recognize these redundancies. Global Biodiversity Assessment is often cited, which estimates the number of described species at approximately 1.75 million.