Since the beginning of the Anthropocene, sea levels have been rising inexorably, with varying degrees of intensity depending on the region concerned. In the Mediterranean, it has already risen by 16 cm in 100 years. Although this increase may seem modest, it is already having an impact on one of the Mediterranean’s most emblematic habitats: Lithophyllum byssoides sidewalks. This endemic habitat is unique in that it forms only at the interface between sea and land.
Its vertical distribution is limited to around 20 cm.

It cannot grow much higher due to insufficient humidification, nor can it spread too far downwards due to excessive humidification.

–> Consequently, its growth occurs mainly horizontally, seaward, by coalescence with living individuals settling on dead ones.

This process leads to the formation of a sidewalk, but only if the sea level is stable for several centuries. Today’s sidewalks were formed during the Little Ice Age, from the 13th to the mid-19th century. The most spectacular sidewalks can be up to 2 metres wide and several dozen or even hundreds of metres long. Such is the case of Cala Litizia in Scandola, Corsica, or the sidewalk of the Calanques des Contrebandiers in Marseille.

The study of the impact of sea-level rise on marine ecosystems is still underdeveloped in the Mediterranean. In a large-scale study carried out in Provence and Corsica, both in areas impacted by human activity and in marine protected areas, we demonstrated that most sidewalks were strongly impacted by rising sea levels. In fact, the vitality of sidewalks has deteriorated significantly over the years. They are experiencing a reduction in the coverage of living L. byssoides individuals, invasion by infralittoral algae and bioerosion, leading to the formation of holes that weaken them, eventually breaking off and sinking into the sea (Figure 1).