Patterns of rarity and taxonomic group size in plants Felipe Domı ´nguez Lozano 1 , Mark W. Schwartz * Department of Environmental Science and Policy, 1 Shields Avenue, University of California, Davis, CA 95616, USA Received 14 June 2004; received in revised form 13 April 2005; accepted 28 April 2005 Available online 5 July 2005 Abstract The objective of this research is to examine the extent to which phylogenetic and geographical drivers of rarity may interact to help us better predict distributions of rare taxa. This information is useful to conservation biologists when considering plans for the effective capture of biological diversity within reserve systems. We use 33 floras, distributed globally among five biomes, to assess pattern of rarity across taxonomic group size for plants. We show that small plant families (1–10 taxa) contain consistently fewer rare taxa than expected, while rare plants are proportionately over-represented in large plant families (>100 taxa). We also examine the distribution of species within families. The degree to which species are inequitably distributed among families varied systemat- ically across floristic biomes. Mediterranean floras, in particular, are characterized by a greater than expected fraction of their spe- cies pool concentrated within large families, suggesting a high propensity for rarity simply by virtue of having a higher proportion of their constituent taxa within groups with a high likelihood of rarity. Finally, an analysis of a suite of floras using a common system of identifying rarity shows that the fraction of species within floras that are rare is predicted both by flora size and by a general metric (the Gini coefficient) of inequality among family sizes. Together, these patterns suggest a high degree of predictability in the distribution of rarity in plants that may reflect underlying natural speciation and extinction processes. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Rarity; Vascular plants; Conservation; Biogeography; Mediterranean climate; Species accumulation; Taxonomy 1. Introduction Predicting rarity and vulnerability in the face of unprecedented rates of species loss (Pimm et al., 1995; Regan et al., 2001) is a challenge for conservation biol- ogists (Purvis et al., 2000b; Gaston and Spicer, 2001; Mace et al., 2003). Conservation strategies that focus on holistic measures often use hotspots of diversity to prioritize ecosystem targets (Schwartz, 1999). It is important to understand the extent to which hotspots of diversity might also capture hotspots of rarity and vulnerability. To this end, understanding the degree to which diversity and rarity are correlated can help con- servationists devise efficient strategies for protection of biodiversity. Incorporating phylogenetic information in order to prioritize species as a consequence of evolutionary uniqueness is a compelling addendum to priorities sim- ply based on species threat levels (Mace et al., 2003). Testing for general patterns in the taxonomic distribu- tion of rarity can help streamline these efforts to con- serve phylogenetic branch length and better understand processes that lead to rarity. Studies across a wide array of taxa have shown phylogenetic patterns in the distribution of rarity. These studies, however, vary in the direction of this signal. In the case of verte- brates, it appears that species-poor lineages are prone to high rates of vulnerability (Purvis et al., 2000a,b). The opposite, species-poor lineages contain inordinately 0006-3207/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2005.04.024 * Corresponding author. Tel.: +530 752 0671; fax: +530 752 3350. E-mail addresses: felipe.dominguez@uam.es (F. Domı ´nguez Loza- no), mwschwartz@ucdavis.edu (M.W. Schwartz). 1 Present address: Comisio ´n de Bota ´nica, Biology Department Facultad de Ciencias, Universidad Auto ´ noma de Madrid E-28049 Madrid, Spain. www.elsevier.com/locate/biocon Biological Conservation 126 (2005) 146–154 BIOLOGICAL CONSERVATION