What is species richness and what is its relation to biodiversity?

Species richness is defined as the number of species within a community and is related to biodiversity depending on how biodiversity is defined.

Biodiversity can be defined and measured in multiple ways. The Convention on Biological Diversity defines it as, "... the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems". You can read more about how they define biodiversity here.

If referring to genetic diversity, a greater number of species within a community would likely have a higher genetic diversity than a community with a smaller number of species.

If you are referring to species diversity, this is composed of species richness along with species evenness (relative abundance of each species) and often how genetically dissimilar species are. (Read about the how species diversity is different from species richness here).

Finally, if referring to ecosystem diversity, or the variety of ecosystems within a specific location, the relationship with species richness is not so straightforward. Some ecosystems are particularly rich whereas others are typically not. You could have an area with low ecosystem diversity, say a single ecosystem, but if that ecosystem could still have high species richness compared to an area with multiple alpine ecosystems but low species richness within each ecosystem.

Species richness refers to the number of different species present in a particular area or ecosystem. It is a measure of biodiversity, which encompasses the variety of life forms within a given habitat or on Earth as a whole. The higher the species richness, the greater the biodiversity of an ecosystem.