A significant proportion of the global diversity of flowering plants has evolved in recent geological time, probably through adaptive radiation into new niches. However, rapid evolution is at odds with recent research which has suggested that plant ecological traits, including the b-(or habitat) niche, evolve only slowly. We have quantified traits that determine within-habitat a diversity (a niches) in two communities in which species segregate on hydrological gradients. Molecular phylogenetic analysis of these data shows practically no evidence of a correlation between the ecological and evolutionary distances separating species, indicating that hydrological a niches are evolutionarily labile. We propose that contrasting patterns of evolutionary conservatism for a-and b-niches is a general phenomenon necessitated by the hierarchical filtering of species during community assembly. This determines that species must have similar b niches in order to occupy the same habitat, but different a niches in order to coexist.

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The need to estimate the magnitude of undescribed species richness arises from the limited resources available to the description and conservation of biodiversity, the continuous loss of diversity that we are currently experiencing, and the sheer scale of the task of accurate measurement. Several estimation methods have previously been described and discussed in some detail, but the reliability of these methods is difficult to assess. In this study, we use two independent methods to predict the global species richness of the subfamilies of the parasitic wasp family Braconidae. The first is to extrapolate from the decreasing rate of species descriptions to the point at which this rate reaches zero. The second method uses the geographical distribution of species in two well-studied taxa (butterflies and mammals) to extrapolate from our knowledge of braconid diversity in the Palaearctic. For the subfamilies which currently contain at least 50 species, there is a significant correlation between the proportions of undescribed species predicted by each method. Each method predicts an average increase of between 100% and 200% for the Braconidae as a whole. Applying the figures we obtain to the class Insecta yields an estimate of 2.05-3.4 million global insect species.

The need to estimate the magnitude of undescribed species richness arises from the limited resources available to the description and conservation of biodiversity, the continuous loss of diversity that we are currently experiencing, and the sheer scale of the task of accurate measurement. Several estimation methods have previously been described and discussed in some detail, but the reliability of these methods is difficult to assess. In this study, we use two independent methods to predict the global species richness of the subfamilies of the parasitic wasp family Braconidae. The first is to extrapolate from the decreasing rate of species descriptions to the point at which this rate reaches zero. The second method uses the geographical distribution of species in two well-studied taxa (butterflies and mammals) to extrapolate from our knowledge of braconid diversity in the Palaearctic. For the subfamilies which currently contain at least 50 species, there is a significant correlation between the proportions of undescribed species predicted by each method. Each method predicts an average increase of between 100% and 200% for the Braconidae as a whole. Applying the figures we obtain to the class Insecta yields an estimate of 2.05-3.4 million global insect species. 0 2001 The Linnean Society of London ADDITIONAL KEY WORDS: biodiversity -extrapolation -species description -indicator group.~~