Distribution and habitat
Frailea erythracantha is a small cactus described from Serra do Caverá in the western part of Rio Grande do Sul, Brazil. It grows on basalt outcrops among weathered rock fragments containing humus and organic material. The original description documented two isolated localities. Plants occurred singly or in small groups on rocky elevations and isolated hills [Pontes et al. 2018].
The localities lie within open vegetation on rocky elevations. The authors recorded accompanying plants including Dyckia waechteri, Cereus hildmannianus, Parodia ottonis and Opuntia elata. Basalt rock, weathered fragments in crevices and small accumulations of organic material together describe the local habitat documented in the study [Pontes et al. 2018].
Stem and spine structure
Young stems are globose or hemispherical and become shortly cylindrical with age. In the descriptive material they reached 60 to 70 mm in height and 20 to 40 mm in diameter. The surface bears 20 to 28 slightly tuberculate ribs, and the lower part of the tubercles may show dark purple pigmentation, especially in young plants. Areoles produce 10 to 12 radial spines and four central spines arranged as a cross. The spines are slender, flexible and reddish, sometimes darkening to almost black [Pontes et al. 2018].
In young plants, dark purple markings beneath the areoles stand out against the olive-green or grey-green stem surface. Their colour may become dark green with age. The authors distinguished the species using a combination of stem size, rib number, four central spines, flower dimensions and seed characters. The diagnosis therefore integrates features from several parts of the plant [Pontes et al. 2018].
Flowers, fruits and seeds
The yellow or pale yellow flowers are funnel-shaped. In the original description they measured 45 to 65 mm in length and 35 to 60 mm in diameter. The authors reported flowering from October to January and fruiting from November to February, corresponding to spring and summer in southern Brazil. In cultivation, flowers opened on very hot days at around 13 h and closed approximately four hours after anthesis began [Pontes et al. 2018].
Mature fruits contained 80 to 110 seeds. The seeds are glossy black and approximately hat-shaped, measuring 1.8 to 2.4 mm long and 1.4 to 2.1 mm high. The testa surface has weakly pronounced elevations and relatively sparse papillose projections. Together with spine arrangement and flower structure, these features form the diagnostic combination presented in the species description [Pontes et al. 2018].
Documented threats
In 2018 the authors described two populations containing fewer than 50 individuals each and proposed a Critically Endangered category. They identified livestock grazing and the spread of the invasive grass Eragrostis plana among the pressures on the species. This assessment refers to the localities and population sizes documented during that study. It highlights the importance of conserving the small patches of rocky vegetation that provide the species’ habitat [Pontes et al. 2018].
Conserving this habitat involves retaining the rock outcrops and their surrounding native vegetation. For the two localities described in the paper, reducing grazing pressure and grass invasion was particularly relevant. The small, isolated populations provided a clear reason to prioritise these sites for further field observation [Pontes et al. 2018].