Practice question

Using Simpson’s dominance definition, D = Σpᵢ², diversity is greatest when D is smallest. The index represents the probability that two randomly sampled individuals belong to the same species. As richness and evenness increase, that probability declines because individuals are spread among many taxa. A value of zero is the theoretical lower limit and therefore represents maximum diversity among the listed values. In a real finite community containing individuals, D ordinarily remains above zero; it approaches zero as the effective number of equally abundant species becomes very large. A value of one indicates complete dominance by a single species. This direction reverses for the complementary Gini–Simpson index, 1 − D, where values near one indicate high diversity, and for the reciprocal 1/D, where larger numbers indicate more effective species. The formula must therefore accompany the name “Simpson’s index.” The keyed zero assumes the dominance convention consistently used in these rows. Interpreting the scale probabilistically prevents memorization errors: low same-species draw probability means high diversity, whereas high same-species draw probability means low diversity.