18. Which of the following statements best describes the relationships among species richness, abundance, and the components of regional diversity? (A) Alpha diversity is the total species richness of a region, beta diversity measures evenness of abundance within a single site, and gamma diversity is the average across local sites. (B) Species richness is simply the number of species in a community, while abundance refers to the relative number of individuals per species. (C) Beta diversity is always higher than alpha diversity because it includes rare species missed in local sampling, while gamma diversity is the product of alpha and species abundance. (D) Gamma diversity equals alpha diversity because regional scale simply aggregates local communities without adding new species through turnover. (E) Evenness (abundance distribution) has no effect on species richness, and beta diversity is calculated solely from differences in species abundance, not composition.