Importance–value curves and diversity indices applied to a species-rich heathland in Western Australia

RANKING of species in a community from most to least important (abundant) to yield an ‘importance-value’ ( I – V ) curve is a device often used by ecologists to elucidate features of that community. Increasingly in the literature 1–5 , an I – V curve which obeys the lognormal function is being accepted as describing the ultimate in plant species diversity. South-western Australia is renowned for its apparent floristic richness and high degree of endemism. An ecological study of sand heath, considered by botanists the major repository of this supposed ‘incredible richness’ 6,7 , has revealed a type of I – V curve not previously noted for vegetation. The values for six of seven indices of species diversity examined were as high as, if not higher than, those for the most diverse communities previously recorded, including the richest of tropical rainforests 2,4,8–10 . All earlier indices of equitability (evenness of species' importance) are based on models of limited applicability and a new index, based on the actual slope of the I – V curve, is proposed here.