Marine biologists investigated the biodiversity of marine gastropods across three distinct coastal habitats: a rocky intertidal shore, a sheltered mudflat, and a salt marsh. During their sampling, they recorded 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 coastal habitat. Explain why measuring the index of diversity provides a more useful measure of biodiversity than measuring species richness alone.