The correct answer is Only (c) and (d). Explanation & Key Takeaways: Alpha, Beta, and Gamma Diversity • Diversity is of three types which include alpha-beta and gamma diversity. • The counting of the number of taxa that is present in a particular area, community, or ecosystem falls under Alpha diversity. • The species between two adjacent ecosystems which is obtained by the comparison of the number of species of the ecosystem which is unique to each other falls under beta diversity. • An example of Alpha, Beta, and Gamma diversity is explained through a mountain landscape. • Under a mountain landscape, there are many patches of forests and grasslands and the species under each grassland or forest patch under the slope will denote the Alpha diversity. • The beta diversity is represented between the two patches and their communities consisting of the species. • The diversity of species along the mountain slope of the entire range will denote the gamma diversity of the mountain slope. • Whittaker (1972) described three terms for measuring biodiversity over spatial scales: alpha, beta, and gamma diversity. Hence, Statements c and d are correct. • Alpha diversity • It describes the species diversity within a community at a small scale or local scale, generally the size of one ecosystem. • When we casually speak of diversity in an area, more often than not it refers to alpha diversity. • Beta diversity • It describes the species diversity between two communities or ecosystems. • It is at a larger scale and looks to compare the species diversity between two separate entities that are often divided by a clear geographical barrier like a river or a mountain ridge. • Gamma diversity • It is studied at a very large scale—a biome—where species diversity is compared between many ecosystems. • It could range over areas like the entire slope of a mountain, or the entire littoral zone of a seashore. Related Concepts: Species richness • It is the number of species within a defined region. Hence, Statement a is not correct. • The species richness of a region is obtained through sampling or via a census. Simpson's Diversity Index • It is a measure of diversity which takes into account the number of species present, as well as the relative abundance of each species. Hence, Statement b is not correct. • As species richness and evenness increase, diversity increases.