Species diversity refers to the number of different species in an area and how evenly they are distributed. It is important for understanding and protecting biodiversity. Researchers have found ways to estimate species richness accurately, even with some errors. They have also developed measures that combine species richness and abundance, taking into account factors like rarity and ecological function. These measures help compare diversity across different areas. Overall, there are between 5 million to 50 million species on Earth, and diversity varies globally.