© 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi 25 Diversity and Distributions, (Diversity Distrib.) (2005) 11, 25–32 BIODIVERSITY RESEARCH ABSTRACT Dioecious clades have been observed to have lower species richness than their non- dioecious sister groups indicating that dioecious species experience higher extinc- tion rates and (or) lower speciation rates. To determine whether current threats to biodiversity may exacerbate this pattern, we examined the threat to exclusively dioecious families of angiosperms among the 13,013 species of threatened plants included in the IUCN Red List of Threatened Species. When examined phylogenetically, dioecious families had proportionally more species listed than their sister groups. We then examined whether ecological traits correlated with dioecy, namely tropical distribution, woody growth form, and fleshy fruits, are associated with having higher proportions of threatened species. Ignoring breeding system, woody growth form was the only trait that was associated with a greater than expected proportion of threatened species per family. Red-Listed dioecious families were more likely to have a woody growth form than non-dioecious families. Woody growth habit is likely contributing to the higher incidence of dioecious species being at risk of extinction but is not solely responsible for the pattern because higher risk within dioecious groups was also apparent in a comparison of exclusively woody sister-group pairs. Our results indicate that dioecious plants may warrant special attention in conservation practices. Keywords Angiosperms, biodiversity, dioecy, evolutionary history, extinction, sister-group comparisons, species richness. INTRODUCTION The world is facing an extinction crisis (Koopowitz & Kaye, 1990; McKinney, 1999; von Euler, 2001). Without question, some species will be more at risk than others (Bennett & Owens, 1997; Russell et al., 1998; Purvis et al., 2000; Cardillo & Bromham, 2001). Thus, it is imperative that we begin to assess which species are more at risk than others and the most likely causes of extinc- tion in order to formulate plans for preserving biodiversity and evolutionary history (Nee & May, 1997; Heard & Mooers, 2000). The IUCN Red List of Threatened Species™ (IUCN, 2003; here- after referred to as ‘Red List’), the most recent worldwide list of endangered and threatened species available, allows us to per- form comparative tests to infer whether species with certain distributions, life histories, or breeding systems are more at risk than others. This is a potentially fruitful avenue for research (see Cardillo & Bromham, 2001), but one where plants have been particularly neglected. Considering how the loss of evolutionary history is greatly magnified when extinction is concentrated on certain branches of a phylogeny over others (Heard & Mooers, 2000), performing these studies is of great importance. Based on empirical studies of local flora (Gillespie, 1999; Somanathan & Borges, 2000) and phylogenetic analyses of species richness (Heilbuth, 2000), various authors have hypothesized that angio- sperms with a dioecious breeding system may be more prone to extinction. Here we examine how breeding systems, especially dioecy, may affect the risk of extinction in angiosperm families on a global scale. Dioecy, the breeding system whereby there are separate male and female individuals, is thought to evolve frequently within angiosperms due to short-term benefits accrued in terms of reduced inbreeding depression and/or more efficient allocation of resources (reviewed by Charlesworth, 1999). However, despite the potential benefits of separating sexual functions, dioecy is found in only a very small proportion of flowering plants (c. 6%; Renner & Ricklefs, 1995), partially because dioecious clades have lower species richness than their non-dioecious sister groups (Heilbuth, 2000; Vamosi & Vamosi, 2004). It is unknown, at present, whether the cause of this pattern is due to higher extinc- tion rates or lower speciation rates or both. Several hypotheses have been proposed as to why dioecious plants may be at a severe disadvantage compared to non-dioecious plants, including Department of Biological Sciences, University of Calgary, 2500 University Drive N.W., Calgary, AB T2N 1 N4, Canada. E-mail: jvamosi@ucalgary.ca *Correspondence: Jana C. Vamosi, Department of Biological Sciences, University of Calgary, 2500 University Drive N.W., Calgary, AB T2N 1N4, Canada. Tel.: (403) 220 1136; Fax: (403) 289 9311; E-mail: jvamosi@ucalgary.ca Blackwell Publishing, Ltd. Present day risk of extinction may exacerbate the lower species richness of dioecious clades Jana C. Vamosi* and Steven M. Vamosi