Arran whitebeams complex fashion, which involved the common whitebeam ("Sorbus aria") giving rise to the tetraploid rock whitebeam ("Sorbus rupicola") which is still found on Holy Isle. This species is able to survive at higher altitudes and therefore occupies a less competitive niche with fewer tree species able to tolerate the harsher conditions. The rock whitebeam interbred with the rowan / mountain ash ("Sorbus aucuparia") to produce the hybrid, a fertile separate species the Scottish whitebeam ("Sorbus arranesis") which grows well in this zone of reduced competitive growth at higher altitudes. The bastard mountain ash ("Sorbus pseudofennica") arose from a further cross
Sorbus aucuparia birds and are thus propagated by being passed intact in their droppings. The fruit are eaten by about 60 bird species and several mammals. They are liked particularly by thrushes and other songbirds, and are also eaten by cloven-hoofed game, red fox, European badger, dormouse, and squirrel. Fruit of "S. aucuparia" are used as a food source by migratory birds in winter, including Bohemian waxwing, spotted nutcracker, and redwing. Cloven-hoofed game also excessively browse foliage and bark. The plant roots can be found in symbiosis with arbuscular mycorrhizal and less commonly with ectomycorrhizal fungi. It is usually later superseded by