Structure and habitat
Sclerocactus dawsoniae has a bluish-green stem, usually solitary, 3–10 cm tall and exceptionally up to 14 cm. Its 8–15 ribs bear areoles with 7–9 spines, generally without hooked tips. The flowers are pink to purplish pink, and their stamen filaments are greenish white. The fruit is cylindrical and 9–22 mm long. It contains black seeds. [McGlaughlin and Naibauer 2024]
The species occurs near De Beque, in Mesa and Garfield counties. It occupies rocky alluvial terraces in the Colorado River basin, containing gravel and rounded stones of volcanic origin. It flowers in April and May. Its name commemorates the botanist Carol Dawson. [McGlaughlin and Naibauer 2024]
From the former circumscription to a separate species
In older accounts, these populations belonged to Sclerocactus glaucus in the broad sense. The current name was introduced in 2024, and POWO recognises it as a separate species. The former use of the name S. glaucus here refers to part of a larger historical circumscription of the taxon. Assigning a particular observation to S. dawsoniae requires information on the origin of the plants studied. [McGlaughlin and Naibauer 2023] [2-WWW]
The genetic study published in 2023 already presented this group as a separate lineage, using the name S. dawsoniae before publication of the formal description. The nearest genetically identified populations of the two species were around 10 km apart. Occurrence in separate areas, known as allopatry, helped maintain differences between their gene pools. [McGlaughlin and Naibauer 2023]
Many markers and a consistent separation
In the comparative analysis of 208 plants, the northern group was represented by 75 individuals. The researchers read 20 991 variable DNA positions in 3537 loci, the genome fragments under study. The phylogenetic tree, similarity analysis and a model assigning plants to genetic groups all indicated the same fundamental distinction between S. dawsoniae and S. glaucus. [McGlaughlin and Naibauer 2023]
The population analysis distinguished two main genetic clusters. Differentiation between them was greater than between Grand Valley and Gunnison River, the two population groups of present-day S. glaucus. A small mixed genetic signal was visible in a few individuals. The authors link it to past gene flow, which left a trace despite the clear separation of the main groups today. [McGlaughlin and Naibauer 2023]
Structural differences supported the molecular result. In the material compared, S. dawsoniae was shorter and narrower on average and had fewer spines than S. glaucus. However, the ranges of individual traits overlap. Species identification follows from their consistent combination and geographical origin, explaining why similar plants were previously grouped under one name. [McGlaughlin and Naibauer 2023]
Variation within a small range
Nucleotide diversity was significantly lower in S. dawsoniae than in S. glaucus. This measures the average differences between sequences in a sample. At the same time, the two species had similar heterozygosity and inbreeding coefficients. The result therefore indicates a smaller stock of variants at one level of comparison, with a similar proportion of individuals carrying different copies of the markers analysed. [McGlaughlin and Naibauer 2023]
The authors link lower nucleotide diversity to the smaller range and abundance of the northern lineage. In the earlier estimates they compiled, the great majority of known plants were concentrated in one management area, Roan Creek. Such spatial concentration links the fate of a large part of the species to changes within a small part of the landscape. [McGlaughlin and Naibauer 2023]
The study describes DNA variation and population structure. Responses to drought, growth rate and reproductive success require separate biological measurements. Preserving known population origins allows future observations to link these traits to a specific part of the range and its genetic resources. [McGlaughlin and Naibauer 2023]
Aktualny status federalny
Since 30 June 2025, S. dawsoniae and S. glaucus have been absent from the US federal list of threatened species following the removal of the previously protected entity known as the Colorado hookless cactus. This legal change was announced on 29 May 2025. Information about the proposed delisting in the 2023 genetic study describes an earlier stage of the proceedings. [22-WWW]