A marine biology team investigated the biodiversity of gastropod molluscs across three distinct intertidal zones: a rocky shore, a sandy beach, and an estuary salt marsh. During their sampling, they identified several species, including Littorina littorea\textit{Littorina littorea}Littorina littorea, Littorina saxatilis\textit{Littorina saxatilis}Littorina saxatilis, and Nucella lapillus\textit{Nucella lapillus}Nucella lapillus.
State what is meant by the term 'species richness'.
Explain what the scientific names of Littorina littorea\textit{Littorina littorea}Littorina littorea, Littorina saxatilis\textit{Littorina saxatilis}Littorina saxatilis, and Nucella lapillus\textit{Nucella lapillus}Nucella lapillus suggest about the evolutionary relationships between these three gastropod species.
The biologists calculated the index of diversity for each intertidal zone. Explain why measuring the index of diversity is more useful than measuring species richness alone.