The scope of the Complexo Ons – O Grove SAC (ES1140004), located within the Atlantic biogeographic region, includes the Ons Archipelago, the Umia-O Grove intertidal complex, the A Lanzada dune system, a coastal section of the peninsula of O Grove and the southern part of the Illa de Arousa and its associated islets; This entire system is located between the Ría de Pontevedra and the Ría de Arousa
(Pontevedra, Galicia, Spain).

Thus, in the north of the Complexo Ons – O Grove space (ES1140004) and inside the Ría de Arousa is Punta Carreirón located in the south of the Illa de Arousa and several nearby islets of a rocky and sandy nature (Rúa, Guidoiro Areoso and Guidoiro Pedregoso), the relief of this territory being very smooth, with granite ridges, strongly eroded (Perez Alberti, 1993).

To the south of this territory, and still within the Ría de Arousa, the SAC Complexo Ons – O Grove (ES1140004) houses the O Grove cove and the Umia river estuary that make up a very protected, shallow bay with extensive intertidal surfaces, which is separated from the open sea by a tombolo formed by sandbanks and dunes, which is the isthmus. from A Lanzada.

Close to this sector, but on the open coast, is the O Grove peninsula, which is home to a coastal stretch with alternating small sandbanks and a rocky coast of medium and low height, in which the dune system of the beach stands out. of Mexilloeira and a small coastal lagoon, Lagoa Bodeira.

Finally, the Ons Archipelago occupies the south of the SAC Complexo Ons – O Grove (ES1140004), already located in the Ría de Pontevedra. This Archipelago is part of a sunken mountain range a few million years ago, which reaches gentle heights, the highest being 128 m, where the lighthouse is located. All the faces of Ons are cliffs, except for the east face, which is less abrupt and on it appear sedimentary formations that make up beaches, which gives it a geomorphological originality (Perez Alberti, 1993).

A large part of the territory of the SAC Complexo Ons – O Grove (ES1140004) corresponds to the marine surface (64%), depending on the degree of exposure to waves and the type of dominant substrate, a series of zones can be differentiated, clearly differentiating the external coasts and from the interior of the estuary, the second with greater protection against hydrodynamics than the first. Highlighting several enclaves with species of interest, such as the frequent horny corals Lophogorgia sp., or the bryozoan Pentapora foliacea, the high specific diversity in vertical walls, overhangs and the entrance of crevices and furnae, or the field of anemones Anemonia viridis, between the two islands of the Ons Archipelago, with an abundance of the large mollusk Aplysia punctata, or the associated species Paracentrotus lividus and Inachus dorsettensis, as well as Necora puber (Conaima, 2006). Separate mention within the marine ecosystems deserves the maërl beds present in this territory, with the coralline algae Lithothamnion corallioides and Phymatolithon calcareum, of interest for conservation (Peña & Bárbara, 2006).

Among the terrestrial ecosystems, Ons Island stands out for its majority coverage of coastal heaths defined by a coastal mosaic of Ulex spp. that stand out for their abundance and for having in their specific composition the recently described endemism Cytisus insularis (Ortiz et al., 2001), with the dune and middle marsh systems being dominant in the rest of the territories. Also noteworthy are the important rocky outcrops present in its coastal cliffs, of undoubted landscape value shaped throughout history by marine influence where cavities called "furnas" or sea caves are found.

Among the species of flora and fauna present in this territory, it is worth noting the occasional presence of two priority species of sea turtles Caretta caretta* and Chelonia mydas*, on the other hand and referring to the flora of this SAC, a species included in the Annex II of CD 92/43/EEC, Rumex rupestris. It also has a total of 17 species in Annex II of CD 92/43/EEC, 26 in Annex IV, and 37 belonging to Annex I of CD 2009/147/EEC, as well as a good number of wintering birds. that visit this space, highlighting the colonies of Pluvialis squatarola, Limosa lapponica, Platalea leucorodia and Haematopus ostralegus. Appearing as unique species are those linked to cliffs, fundamentally the rock dove (Columba livia), the royal swift (Tachymarptis melba) and the red-billed chough (Pyrrhocorax pyrrhocorax). There are also abundant populations of the Yellow-legged Gull (Larus cachinnans) and the Shag (Phalacrocorax aristotelis), finding one of the largest Galician colonies in this area.

The Complexo Ons – O Grove SAC (ES1140004) is home to a highly diversified herpete community, with species of interest such as Discoglossus galganoi, Chioglossa lusitanica or Lacerta schreiberi in Annex II and IV of the Habitat Directive, in addition to the Iberian newt (Lissotriton boscai) , or the Iberian skink (Chalcides bedriagai) considered in danger of extinction according to the Galician Catalog of Endangered Species for its insular populations, this saurian is the rarest and most difficult to locate of all the insular herpetofauna, among other species (Galan , 2003).

Likewise, in the waters of the SAC Complexo Ons – O Grove (ES1140004) the presence of various aquatic mammals has been detected, among which 2 species of Annex II of CD 92/43/EEC stand out, the harbor porpoise (Phocoena phocoena) and the bottlenose dolphin (Tursiops truncatus) as well as the presence of the Annex II land mammals Lutra lutra, Myotis myotis, Rhinolophus ferrumequinum and Rhinolophus hipposideros.

The high conservation values ​​of the Ons archipelago make it a remarkable space at the Spanish level, presenting a high interest for the interpretation of natural heritage by society. This has led to its inclusion, among other figures, in the Parque Nacional Marítimo-Terrestre das Illas Atlánticas de Galicia (Law 15/2002), which receives around 450,000 visitors throughout the year, receiving the Ons archipelago approximately 25% of visits. The declaration of the National Park makes it possible to regulate visits in an orderly and sustainable manner (which are carried out by disembarking at the points set up for this purpose), through the corresponding Governing Plan for Use and Management (PRUG) approved by Decree 177/ 2018, avoiding significant effects on the state of conservation of natural components.