Assertion: The species diversity present in a given community or habitat is referred to as alpha diversity.
Reason: Alpha diversity is usually expressed by species richness and species evenness in that community habitat.

Both Assertion (A) and reason (R) are correct and R is the correct explanation of A.

Alpha diversity within community diversity is species diversity in a given community or habitat. It is dependent upon species richness and species evenness/ equitability. There is a lot of competition, adjustments and interrelationships amongst members of the same community. The number of species per unit area is called species richness. Number of individuals of different species represent species evenness or species equitability.