Whittaker (1972) created a system to describe biodiversity over different spatial scales. He called these alpha, beta, and gamma diversity. Alpha diversity refers to the diversity within a particular area or ecosystem, and it is usually expressed by the number of species in that ecosystem. This is equivalent to measuring the species richness of an area. For example, we might be interested in monitoring aquatic biodiversity on the offshore side of a fringing coral reef on Mauritius in the Indian Ocean. The alpha diversity of this reef is measured by the total number of species found in the area that we are monitoring. If we extend our survey along a transect running inshore from the reef, we cover some new ecosystems; the reef becomes shallower and less exposed as we move from the main fringing reef to an inshore lagoon ecosystem. If we extend our survey far enough, we may reach some submerged coastal mangrove forests. These different ecosystems will have different species and communities. We can compare the degree of species change as we move along the transect from one ecosystem to the next—this comparison of so-called between-area diversity is referred to as beta diversity. Gamma diversity is a measure of the overall diversity within a large region, and so in some respects it is an expanded version of alpha diversity. Thus, if we chose to survey all the reefs of Mauritius and measure species diversity over the whole area, that would be gamma diversity. We could expand the survey even further to include the reefs of the neighboring Mascarene Islands of Reunion and Rodrigues; the gamma diversity would then include the species for all those islands.
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