The Sierra Morena of Seville borders on three provinces: Badajoz to the north, Huelva to the west and Córdoba to the east; it is flanked to the west and east respectively by two natural parks: Sierra de Aracena and Picos de Aroche (province of Huelva) and Hornachuelos (province of Córdoba). The southern boundary is formed by the border with the fertile plain of the Guadalquivir River. Much of the Sierra Morena is classified as a natural park and together these three bordering protected areas make up the Dehesas de Sierra Morena Biosphere Reserve. Designated as a Special Protection Area for Birds (SPAB) in 1989, it is part of the European ecological network Natura 2000.
It is an integral part of the Andalusian Sierra Morena and, as such, it preserves the natural values of the Mediterranean mid-mountain, characterised by the presence of holm oak and cork oak dehesas, leafy riverbanks and occasional traditional crops, such as olive groves and vineyards. This predominantly wooded area is a magnet for forest birds, but also provides for the presence of birds from open habitats, or those that prefer ecotone areas. The large number of watercourses and reservoirs encourages biodiversity. Mediterranean vegetation, with areas of holm oak and cork oak is interspersed with others where trees have been replaced by rockrose, heather, strawberry tree and mock privet scrub. In addition to some areas of olive groves and ancient holm oaks, the kermes oak that gives its name to the area is the most prominent plant species and is accompanied by white rockrose and a wide variety of aromatic plants.
The predominant vegetation is wooded pasture with holm oaks and cork oaks along with Portuguese oaks in the shadier areas. Another outstanding feature is a small juniper grove in the Barranco de Viar. These patterns change in the lower areas, where fan palms and wild olive trees appear; but it is in the higher areas with greater rainfall where the flora is more diverse.
The rivers and streams are lined by impressive alder gallery forests, a tangle of brambles and flora more typical of the green north of the peninsula, creating magical environments with wild cherry and hazelnut trees. The mountainsides are also covered with heather and a few patches of Pyrenean oak and chestnut trees (Cerro Negrillo). It is here where the endemic species grow, such as the Narcissus fernandesii, a rare fern (Asplenium billotii), the silene mariana and the most precious species, only present here and in the province of Leon, Gyrocaryum oppositifolium. The fauna is determined by the river courses and the slope, with a predominance of birds. These are either rock-dwelling - eagles, griffon and black vultures and black storks (as migrants), Egyptian vultures and black kites; forest-dwelling: booted eagles, buzzards, and goshawks; or river-dwelling: wagtails and kingfishers. There is an interesting fish population, as highlighted by the presence of a native trout. Amphibians include salamanders and pygmy newts. There are also bats.
However, the rarest species is a small crustacean that lives only in a cave in Cazalla, the Hexabathynella sevillaensis.
Wolves patrol the area, along with wild boars, small mammals such as rabbits and mongooses, and numerous birds. It is an exceptionally good area for the observation of large birds such as the black stork, Egyptian vulture, griffon vulture, black vulture, imperial Iberian eagle, golden eagle and Bonelli's eagle.