Haseltonia 28: 25–38. 2021 25 INTRODUCTION Te cactus family comprises about 1500 species (Hunt, 2006), although estimations vary according to taxonomic concepts. Cactus diversity is concen- trated in three main regions: 1) northern Mexico, 2) northern Argentina and Chile, and parts of Peru and Bolivia, and 3) eastern Brazil. Tey inhabit mainly arid and semiarid regions; nevertheless, species may inhabit deciduous and humid, lowland or montane forest areas, and even some rainforests. Mexico has a high species diversity and an elevated level of en- demism of cacti, comprising about 700 species and subspecies, almost half of the known species, 70% of which are endemic to the country (Hernández et al., 1993). Cactus diversity in Mexico is higher in semiarid areas of the Chihuahuan Desert Region (CDR), the Sonoran Desert, and the rather tropical Tehuacán-Cuicatlán valley. Te CDR harbours about 324 species and subspecies, and is the largest desert in North America (Hernández & Gómez-Hinostro- sa, 2011a). Despite having high levels of endemism, in the CDR endemic cacti had not been adequately protected in the Mexican system of Natural Protect- ed Areas (Hernández & Gómez-Hinostrosa, 2011b). Te IUCN Cactus and Succulent Plant Special- ist Group (CSSG) has identifed several threats for the conservation of Cactaceae: 1) deforestation and agricultural developments, 2) development and ur- Abstract: Te Chihuahuan Desert Region (CDR) is the largest desert area in North America, and the south- eastern portions contain the richest areas in Cactaceae in the continent. Tis research was advocated to un- derstand the diversity of cacti in northern Zacatecas and southern Coahuila, as well as to explore species rar- ity, environmental relations and conservation alternatives. A geographical square measuring a quarter degree was intensively surveyed through 50, 3-km long transects, distributed in 25, 6-minute sub-squares. Te geo- graphical square was selected because of its altitudinal and environmental heterogeneity. Cacti were identifed and local rareness observed, 764 specimens were deposited at the National Herbarium of Mexico (MEXU). Specimen and locality information were captured in the Database for Cacti of Central and North America. A general evaluation of the cactus diversity (species richness), levels of endemism and rarity were performed. A total of 45 species were found, just below El Huizache, Tolimán and Mier y Noriega, with 7 new records for Zacatecas, 2 for Coahuila and 3 for both Mexican states. We found 26 endemic species from the CDR, fve endemic to the Main and Meridional subregions and 21 endemic to the Main subregion, sensu Hernández. Two species are endemic to the state of Zacatecas and three species are endemic to both states. A new index for rarity (Average rarity, Ar) is presented, obtained as the mean of fve variables: level of endemism, species relative frequencies, local rareness, and climatic and edaphic (soil) specialization. Rabinowitz rarity (1981) was calculated for all species and transformed into numerical values to compare it with the obtained Ar values. Tere are 26 rare species following Rabinowitz and 16 rare species following Ar (≥0.5). Ar values resulted smooth and with a higher explanation power (r²). Just fve species resulted with a Rabinowitz rarity of 7 (transformed R of 1) and Ar (≥0.82). A general complementarity analysis was used to address conservation in the studied area, which was enhanced with other four complementarity analyses, including our Ar. Nine sub- squares were found relevant for cacti protection. Soil types and a complete climatic analysis from original data were used to search for environmental relations through Principal Component Analyses. Te main preferences for climatic variables and anomalies are highlighted. Te Ar index is an adequate tool for measuring species rarity and presents it in a balanced and smooth way, not just fxed Rabinowitz categories, allowing the under- standing that some species might be rare in diferent ways in diferent areas and environmental conditions, something that seems to occur in cacti. Further research is encouraged on Ar for comparisons and even map- ping, as well as for Natural Protected Areas (NPA) design. Keywords: Cactaceae, biodiversity, biogeography, endemism, environmental relations, Natural Protected Areas, rarity, Zacatecas Cacti in the Mazapil region, Chihuahuan Desert, Mexico: diversity, geographical patterns and conservation David Brailovsky Signoret* and Héctor M. Hernández Departamento de Botánica, Instituto de Biología, UNAM, Mexico City, Mexico. email hmhm@ib.unam.mx * Author for correspondence: cactovsky@gmail.com Manuscript received 24 th July 2020 Author Copy