Negative relationships between species richness and evenness render common diversity indices inadequate for assessing long-term trends in butterfly diversity
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Species richness and evenness, the two principle components of species diversity, are frequently used to describe variation in species assemblages in space and time. Compound indices, including variations of both the Shannon–Wiener index and Simpson’s index, are assumed to intelligibly integrate species richness and evenness into all-encompassing measures. However, the efficacy of compound indices is disputed by the possibility of inverse relationships between species richness and evenness. Past studies have assessed relationships between various diversity measures across survey locations for a variety of taxa, often finding species richness and evenness to be inversely related. Butterflies are one of the most intensively monitored taxa worldwide, but have been largely neglected in such studies. Long-term butterfly monitoring programs provide a unique opportunity for analyzing how trends in species diversity relate to habitat and environmental conditions. However, analyzing trends in butterfly diversity first requires an assessment of the applicability of common diversity measures to butterfly assemblages. To accomplish this, we quantified relationships between butterfly diversity measures estimated from 10 years of butterfly population data collected in the North Saskatchewan River Valley in Edmonton, Alberta, Canada. Species richness and evenness were inversely related within the butterfly assemblage. We conclude that species evenness may be used in conjunction with richness to deepen our understandings of assemblage organization, but combining these two components within compound indices does not produce measures that consistently align with our intuitive sense of species diversity.
KeywordsButterfly monitoring Species diversity Abundance data Effective number of species Shannon–Wiener index Simpson’s index
For assistance with butterfly surveys over the years, we thank Benny Acorn, Jesse Acorn, Vanessa Block, Karen Brown, Tory Culen, Chris Fisher, Caroline LeCourtois, Christianne McDonald, Sonya Odsen, Felix Sperling, Dena Stockburger, Gagan Gill, Laura Vehring, Sarah Booth, Jennine Pedersen, and Ulrike Shlägel. All butterfly surveys were coordinated and overseen by the third author, John H. Acorn. We also extend thanks to Sonya Odsen and Federico Riva for their insights and assistance with analyses, and to Uldis Silins for his assistance in acquiring historical weather data. Finally, we acknowledge and thank both Janet Sperling and Felix Sperling for their invaluable insights and reviews of the manuscript.