The islands have an asymmetric profile with two distinct slopes. The western slope is steeper shore islands where sea erosion results in high cliffs and caves. The eastern side has a much smoother profile protected from the erosive action of wind and open sea, allowing the formation of beaches and dune systems.
Most of the island's vegetation is scrub, forest and cliff.
The forest vegetation was altered by the hand of man with the introduction of exotic species such as pine, eucalyptus and acacia occupying a quarter of the park's surface. In areas abundant native Pyrenean Pyrenean oak and blackthorn scrub.
In the area of ??scrub and gorse abounds include a Mediterranean species such as black cistus, the spurge and wild broom, asparagus and corn bunting.
The beach and dune plant communities maintain their healthy. Species that inhabit them have to endure very harsh conditions such as high dry, loose substrate, strong winds and high concentrations of salts. You can find species such as sea grass, the beach grass, the wallflower of the sea, the sea lily beach or mugwort.
On the cliffs the vegetation may vary depending on the degree of exposure to bird colonies.Other vegetation on the island are of great interest as the vascular flora, the marsh or other plant groups such as fungi and mosses. In the case of lichen have been recorded over 167 different species or subspecies.
The Cies are home to large numbers of seabirds what he has done which is declared as a Special Protection Area for Birds (SPA). The yellow-legged gull is the most abundant of the Park which accounts for 10% of world population turning to the islands as the world's largest colony of this species. This species nests in the area of ??the island protected access.
The shag is another seabird species most abundant in the island. This species develops more expertise in the marine environment that the gull. In Cíes 1,500 pairs breed annually which represents one of the largest concentrations of Europe.
You can also see other species of cormorants: the cormorant, that nests in the Cies.
On the cliffs you can see the peregrine falcon, the Alpine Swift, crows. And the interior of the island the list grows: robin, wren, blackbird, blackcap, chaffinch, linnet, pigeon, dove,.
The most abundant mammals are the rabbit, the hedgehog, otter, shrew. Amphibians are very scarce because no fresh water trickles continuously.
Reptiles are very abundant: the skink Iberian Iberian lizard (larger than continental), the lizard, the solution and a pair of non-venomous snake species..