Revista Mexicana de Biodiversidad 81: 163- 175, 2010 Is geographical rarity frequent among the cacti of the Chihuahuan Desert? ¿Es la rareza geográfi ca frecuente entre las cactáceas del Desierto Chihuahuense? Héctor M. Hernández*, Carlos Gómez-Hinostrosa and Gibrán Hoffmann Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, Apartado postal 70-233, 04510 Mexico D. F., Mexico. *Correspondent: [email protected] Abstract. With the aim of assessing the extent of geographical rarity of Mexican Cactaceae, we calculated the distribution size (area of occupancy) of 142 species from the Chihuahuan Desert. In addition, using 2 variables (number of localities and range size), we preliminarily assessed their conservation status using the current IUCN Red List criteria. The results showed enormous variation in the areas of occupancy, although from the biogeographic and conservation perspective the most exceptional group comprises the extremely narrow endemics (42 species), whose range is restricted to areas smaller than 10 km2. Our results reinforce the reputation of this plant family as exceptionally rare geographically. We suggest that geographical rarity of Cactaceae in the Chihuahuan Desert is a natural phenomenon; however, we propose that the range of several species has been infl uenced by human activities. Regarding the conservation status of the species, 75 of them are categorized as Least concern. The remaining 67 species (47.2%) fall in 1 of the 3 categories of threat (27 Vulnerable, 11 Endangered, and 29 Critically endangered). These fi gures confi rm the critical conservation status of Mexican Cactaceae. Key words: areas of occupancy, cacti, Chihuahuan Desert, distribution size, Mexico, IUCN Red List. Resumen. Se calculó el tamaño de la distribución (área de ocupación) de 142 especies de cactáceas del Desierto Chihuahuense, con el objeto de evaluar su grado de rareza geográfi ca. Además, mediante el uso de 2 variables (número de localidades y tamaño de distribución), se estimó de manera preliminar su estado de conservación usando los criterios actuales de la Lista Roja de la UICN. Los resultados mostraron gran variación en las áreas de ocupación. Sin embargo, desde una perspectiva biogeográfi ca y de la conservación, el grupo de especies más excepcional corresponde a las endémicas restringidas (42 spp.), cuyas áreas de distribución son menores de 10 km2. Los resultados fortalecen la reputación de las cactáceas de ser una familia de plantas excepcionalmente rara geográfi camente. Se sugiere que la rareza geográfi ca en cactáceas es un fenómeno natural; sin embargo, se propone que la distribución de algunas de las especies ha sido infl uenciada por actividades humanas. En lo que respecta al estado de conservación de las especies, 75 pueden ser consideradas bajo la categoría de Preocupación menor, mientras que las restantes 67 (47.2%) caen en alguna de las categorías de amenaza (Vulnerable = 27 spp., Amenazada = 11 spp., Críticamente amenazada = 29 spp.). Los resultados confi rman el grave estado de conservación de las cactáceas mexicanas. Palabras clave: áreas de ocupación, cactáceas, Desierto Chihuahuense, México, tamaño de distribución, Lista Roja de la UICN. Introduction (CDR) have provided a considerable amount of quantitative and qualitative data suggesting that a signifi cant number Rare species are usually defi ned as those having low of cactus species is particularly rare geographically and local abundance, small range size, and/or high habitat ecologically (Hernández and Godínez, 1994; Hernández specifi city (Rabinowitz, 1981; Gaston, 1994). Rarity and and Bárcenas, 1995, 1996; Bárcenas, 1999; Gómez- conservation are closely linked, because rare species Hinostrosa and Hernández, 2000; Hernández et al., 2001, have a greater likelihood of extinction than common ones 2004, 2008; Hernández and Gómez-Hinostrosa, 2004; (Gaston, 1994). In fact, the degree of rarity of the species Goettsch and Hernández, 2006). Specifi cally, geographical has often been a useful parameter for their assignment to rarity, recognized as the occurrence of particular species in different categories of threat (IUCN, 2001). Investigations relatively small areas, has been conceived as a recurring on the Cactaceae from the Chihuahuan Desert Region fact in Mexican Cactaceae. However, we are just starting to understand the real magnitude of this phenomenon and Recibido: 16 febrero 2009; aceptado: 08 julio 2009 the way it is expressed in the CDR. 164 Hernández et al.- Rarity in Cactaceae Essentially confi ned to the New World, the cactus Species selection. A total of 142 cactus species from family comprises an estimated number of 1 500 species the CDR were selected for this study, of which 129 (Anderson, 2001; Hunt, 2006). Mexico has unequivocally (85.2%) are endemic to the region (Appendix 1). The been identifi ed as the main centre of diversity of Cactaceae species were chosen because they were reasonably well and a focal point for the conservation of members of this defi ned taxonomically and adequately represented by plant family. This country, primarily its dry areas, is herbarium collections. We excluded several species from home of almost 40% of the global diversity. In particular, the analysis, either because they were taxonomically the CDR, the largest desert region in North America, is problematic or because they were underrepresented in the home of the richest assemblage of Cactaceae worldwide. database. This was the case of several extremely wide- Hernández et al. (2004) reported 329 cactus species and 39 ranging species, such as Opuntia lasiacantha, O. rastrera, genera for the region, with 70% of the species and 43% of O. streptacantha, Cylindropuntia imbricata, and C. the genera being strictly endemic. tunicata, as well as several narrowly distributed ones (e.g., In a recent investigation (Hernández and Navarro, Coryphantha wohlschlageri, Mammillaria longifl ora, M. 2007), we calculated the distribution size of some stella-de-tacubaya, M. roseoalba, etc.). Data on the general Chihuahuan Desert cacti, confi rming that some species geographic distribution of all of the studied species may be have extremely limited geographical ranges; however, found in Hernández et al. (2004) and are also summarized we still lack comprehensive information about the extent in the Appendix 1. Taxonomic nomenclature follows Hunt of geographical rarity of Cactaceae in the region. In (2006). this paper we estimate the area of occupancy (AOO) of Defi nitions. We followed the IUCN (2001) defi nition a representative selection of Chihuahuan Desert cacti, of area of occupancy, as the area occupied by a taxon ranging from widely distributed species to extremely within its more general extent of occurrence. The concept narrowly endemic ones. The results allowed us to obtain recognizes the fact that organisms usually are absent in a broad perspective of the nature and magnitude of some areas within its usually broader extent of occurrence, geographical rarity of the Cactaceae in the Chihuahuan where uncolonized or unsuitable habitats exist. We Desert. In addition, using 2 biogeographical variables consider AOO as a synonym of distribution or range size. (number of localities and AOO), we preliminarily assessed Also, we defi ne locality as a graphic point on a map, taken the conservation status of all of the studied species using from a geo-referenced herbarium record, and separated the IUCN Red List current criteria. by its nearest neighbour by at least 797.9 m, the radius of a 2-km2 circle. Points located within this radius were considered part of the same locality. In concordance with Material and methods the UICN (2001) convention, we considered a 2-km2 area as a reasonable measure to quantify individual localities. Study area. The Chihuahuan Desert is a vast area extending Data sources. from central Mexico, in the states of Guanajuato, Querétaro, Geo-referenced specimen data were taken and Hidalgo north to southern Texas, New Mexico, and a from a database of cactus collections from North and small part of Arizona. To the west and east, the region is Central America. The database contained at the time of bordered by the Sierra Madre Occidental and the Sierra this study over 26 300 herbarium records of Cactaceae from Madre Oriental, and it has an approximate extension of 35 herbaria, including our own collections resulting from 533 600 km2 (the area was recalculated by us in this extensive fi eldwork conducted in the CDR over the last 15 paper, but see Hernández and Gómez-Hinostrosa, 2005; years. A total of 6 496 geo-referenced records were taken Hernández, 2006). From the conservation perspective, from the database for this study (Fig. 1). The specimens the CDR stands among the 3 most remarkable deserts included in the database had been collected between 1823 in the world, thanks to its relatively high biodiversity, and 2007, with 35.4% collected between 1950 and 1989, numerous endemic species, and the existence of and 41.9% since 1990 to the present. We estimate that in the CDR the collecting effort is one herbarium specimen of extensive well preserved areas (Dinerstein et al., 1999; 2 2 Mittermeier et al., 2002). Thorough descriptions of Cactaceae per every 61.1 km (total area = 533 600 km ). the climatological, ecological, biogeographical, and Cartographic method by conglomerates. For the vegetational characteristics of the CDR have been calculation of the species geographic ranges, we used provided by numerous authors (e. g., Shreve, 1942; the cartographic method by conglomerates (CMC) Johnston, 1977; Schmidt, 1979; Medellín-Leal, 1982; proposed by Hernández and Navarro (2007). This method Henrickson and Johnston, 1986; Hernández and Gómez- incorporates elements of the areographic and cartographic Hinostrosa, 2005; Hernández, 2006). methods, previously used by different authors to calculate Revista Mexicana de Biodiversidad 81: 163- 175, 2010 165 Figure 1. Point occurrences of the studied species. Each circle represents the occurrence of 1 or several species in a given locality. distribution size. The CMC allows the recognition of particular species. discrete aggregations of 2 or more points (conglomerates) 4.