| Summary: | Background and Aims: Mammillaria is one of the most representative genera of Cactaceae. In the Bajío region, Mexico, approximately 70 species and nine subspecies are distributed, of which seven species and one subspecies correspond to M. ser. Leucocephalae. During recent explorations, individuals were found whose morphology does not match with the species described in the updated taxonomic treatments. The objective of this work is to characterize and describe a new species, based on morphological and environmental evidence.
Methods: Field work was carried out in June 2022. Seven morphological characters were analyzed using Canonical Discriminant Analysis (ACD, by its Spanish acronym). Additionally, Pearson residuals were calculated for two environmental variables and significance was estimated with a chi-squared goodness of fit test. The area of occupancy and extent of occurrence were calculated to assign the conservation category according to the IUCN Red List criteria and categories.
Key results: Mammillaria ariasii is described and illustrated, similar to M. hahniana, from which it differs by the number of radial spines, the length of the lower central spine and the color of the internal tepals of the perianth. The presence of extracellular crystals confirms its inclusion in M. ser. Leucocephalae. Mammillaria ariasii is characterized by living in haplic kastanozem soils and volcanic reliefs in an approximate area of occupancy of 28 km2 and an extent of occurrence of 345 km2. An identification key is provided in which it is compared to other members of Mammillaria ser. Leucocephalae.
Conclusions: Mammillaria ariasii is a new species endemic to the Bajío. According to our results, it should be considered an Endangered species (EN). Although it is locally abundant, erosion and livestock farming must be taken into account as potential risk factors.
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