The dune ridges of Alvor and Meia-Praia were formed by the action of sand and wind. It is the dunes that make possible the existence of the salt marsh and the Ria itself, on which a wide variety of species depend, as well as the local fishing and shellfishing communities.

In the most developed part of the dune, you can see semi-shrubby vegetation consisting of beach grass, scrub grass, and rolling thistle. The sandpiper is one of the most beautiful bird species found in the dunes and the lagoon, with its diving flights over the water in an acrobatic style that is mimicked in winter by the tern. The sandpiper puts on its own spectacle, making rapid sprints across the sand to feed between the breaking waves.

In addition to birds, the most common animals in the dunes are small mammals and their predators— the fox and the hedgehog. There are also some reptiles, such as the sand lizard, the toothed-fingered lizard, and the ladder snake.