A COMPARATIVE-STUDY OF THE DISTRIBUTION OF GENUS SIZE IN 20 ANGIOSPERM FLORAS

The biodiversity of floras has until recently been measured solely in terms of their species number or species density, with little regard t o the breadth of phylogenetic diversity represented by the species. Th e latter is partly a function of the size of the flora, and partly of the pattern of distribution of the species into higher taxa. To determ ine whether floras differ in this respect, this study compares the fre quency distribution of genus size in 20 island and regional floras. Ce rtain floras (Cape Region, S.W. Australia, New Zealand, Hawaii) are fo und to have high concentrations of genera containing many species. Oth ers are notably lacking in large genera (Java,Jamaica, Nepal, Niger), though this group tend to be family-rich. In floras with high endemism (Cape, New Zealand, Fiji, Jamaica, Hawaii), the level of endemism is consistently higher in larger genera. Possible reasons for the observe d differences between floras are geographic and temporal isolation, le vel of habitat diversity, climatic history, volcanic, orogenic and tec tonic events. Clusters of large genera may indicate recent speciation, possibly following the last glaciation. Genus size may be an importan t consideration when limited conservation resources have to be targett ed to retain the maximum phylogenetic diversity in a threatened flora. (C) 1997 The Linnean Society oi London.