0.6 Alpha, beta, and gamma diversity

Whittaker (1972) described three terms for measuring biodiversity over spatial scales: alpha,beta, and gamma diversity. Alpha diversity refers to the diversity within a particular area or ecosystem, and is usually expressed by the number ofspecies ( i.e. , species richness ) in that ecosystem. For example, if we are monitoring the effect that British farming practices have onthe diversity of native birds in a particular region of the country, then we might want to compare species diversitywithin different ecosystems , such as an undisturbed deciduous wood, a well-establishedhedgerow bordering a small pasture, and a large arable field. We can walk a transect in each of these threeecosystems and count the number of species we see; this gives us the alpha diversity for each ecosystem; see (this example is based on the hypothetical example given by Meffe et al. , 2002; Table 6.1 ).

If we examine the change in species diversity between these ecosystems then we are measuring the beta diversity . We are counting the total number of species that are unique to each of the ecosystems being compared. Forexample, the beta diversity between the woodland and the hedgerow habitats is 7 (representing the 5 species found in thewoodland but not the hedgerow, plus the 2 species found in the hedgerow but not the woodland). Thus, beta diversity allows usto compare diversity between ecosystems.

Gamma diversity    is a measure of the overall diversity for the different ecosystems within aregion. Hunter (2002: 448) defines gamma diversity as "geographic-scale species diversity". In the example in , the total number of species for the three ecosystems 14, which represent the gamma diversity.