Among the easiest cacti to grow and flower here in the UK are the various species of Rebutia. This group of small, mostly globular cacti originates from the Andes of Bolivia and Argentina along with many other species. The exact number of species in the genus is rather debateable as they are quite variable and there are numerous local forms.
True cacti are only native to the Americas, with the sole exception of the epiphytic Rhipsalis baccifera which has managed to spread – possibly naturally – to Africa, Madagascar and Sri Lanka. People have translocated many species for agricultural or ornamental purposes to many other parts of the world where they have often become invasive weeds. In the Old World native cactus-like plants are mostly euphorbias, which are completely unrelated.
The species in flower in my conservatory, Rebutia fabrisii (syn R.fiebrigii) originates from Argentina in the province of Jujuy, at elevations over 1400m. The area of occurrence is only around 100 km2, but the habitat is not troubled by human habitat modification so it is currently listed as Least Concern by the IUCN.
The climate of the area where the species originates is characterised by high summer temperatures accompanied by heavy rainfall from thunderstorms, and cold dry winters with occasional snow and frost. In cultivation they need a well-drained compost with watering only during the growing season and kept dry during the winter. Low temperatures probably encourage flowering in the spring. As with all cacti, overwatering outside the growing season is a major killer.
Many cacti are pollinated by various species of bee, and for Rebutia species the main pollinators are probably local bumblebees. Unfortunately, many of the bumblebees of South America are threatened as a result of diseases spread from feral populations of introduced European species of bumblebee imported for greenhouse pollination of crops. While people are most familiar with the pollination services from honey bees, some groups of plants, especially the squashes and cultivated Solanaceae (tomatoes/ eggplants/ peppers etc.) require “buzz pollination”. The anthers of the flower only release pollen when they are vibrated at the correct frequency by a pollinating insect, and bumblebees are commercially produced as colonies to be introduced into the glasshouses as required. While this saves the labour costs of hand pollination, the only alternative, there is insufficient disease control and most of these colonies carry various parasitic mites and diseases. When fertile queens or workers escape through open vents or doors the result is disease-bearing insects visiting flowers used by local bees and competing with them for food. In the US non-native species are no longer used (too late) but at present there is no similar legislation in place in South American countries as far as I am aware, and the native species are already being devastated.
(image my own)