Wetlands: a wealth of biodiversity

The wetlands are defined as a "surface that is emerged, saturated with water or flooded, either permanently or seasonally." They act as a transition between land and water. A common classification divides them into three main categories. Continental wetlands include marshes, lakes, rivers, floodplains, bogs and swamps. Coastal wetlands include salt marshes, estuaries, mangroves, lagoons and coral reefs. Artificial wetlands are man-made water features, such as fishponds, rice paddies or reservoirs. This broad definition allows all water points to be included in ecological protection, conservation and restoration measures, each with its own specific features and particular importance for the biodiversity, ecology and ecosystem of which it is a part.

On 2 February 1971, the Convention on Wetlands was signed in Ramsar, Iran. Today bringing together 172 signatory states, its mission is "The conservation and wise use of wetlands through local, regional and national actions and international cooperation, as a contribution towards achieving sustainable development throughout the world." It is estimated that around 35% of the world's wetlands have disappeared since the 1970s. This convention gave rise to a list of "Ramsar sites" designating wetlands of international importance and recognising their value to humanity. Originally, the conservation of these areas was aimed at preserving the diversity of aquatic birds that find refuge there. Since then, the Ramsar Convention has broadened its scope to recognise the importance of wetlands for biodiversity as a whole, as well as the ecosystems they provide and their impact on human well-being.

80% of the surface of wetlands disappeared during the last millennium. A study carried out in France on 223 wetland sites concluded, in 2020, that each site studied was under pressure from an average of 14 different types of human activity. On the podium of the anthropogenic activities most harmful to wetlands are, firstly, soil artificialisation due to increasing urbanisation, secondly, the consequences of intensive farming (soil drainage, pollution and pesticide residues), and thirdly, the deliberate or accidental introduction of invasive alien species disrupting ecosystems. The study also revealed that only 14% of the 223 wetland sites studied are not affected by invasive species. All of these factors, among many others, contribute to the disappearance of wetlands and, consequently, to the fragility of ecosystems, which in turn leads to loss of biodiversity.

Wetlands provide many ecological and hydraulic services. They act as veritable sponges, releasing water as well as absorbing it. If properly managed, they can contain water from excessive rainfall, limiting the risk of flooding in urban areas. Conversely, they release water into the water tables in the event of drought. In addition, coastal wetlands help to reduce erosion caused by storms, thereby limiting the rise in water levels. Studies have shown that mangroves in good condition can reduce wave intensity by up to 75%. Some areas also help to reduce the presence of greenhouse gases in the atmosphere by storing CO2. Peat bogs, for example, sequester twice as much carbon as forests. They can store between 25% and 30% of the carbon in terrestrial ecosystems, even though they only cover 3% of the earth's surface.

In addition to ecological services, wetlands are also known for their ecosystem services. They are among the most diverse and productive ecosystems, providing habitats for many threatened and endemic species. Migratory species of fish use them as corridors of transition between different habitats for their seasonal journeys crucial to their survival, and find refuge there along the way. The population of migratory freshwater fish has declined by 93% since the 1970s, partly due to the degradation and fragmentation of these ecosystems. In addition, wetlands make a significant contribution to sustainable development. They provide sources of renewable energy such as hydropower; filter water by trapping pollutants thanks to their unique aquatic flora; and support economic activities such as tourism or fishing.

Wetlands are home to original and specific biodiversity. Although they cover only 6% of the Earth's surface, they are home to 40% of all plant and animal species. Each type of wetland has a unique and varied biodiversity, determined by factors such as climate, location, size, water quality and nutrient availability. For example, the diversity of bird species is closely linked to the surface area of the wetland, while the proliferation of benthic invertebrates is more dependent onthe age of the area. Because of their positioning between water and land, wetlands are rich habitats that are home to a multitude of plant species, micro-organisms and aquatic and terrestrial animals. Many of these species are threatened, underlining the importance of protecting, restoring and managing wetlands in Europe.

Wetland conditions can be difficult for some plants. These face challenges such as variations in oxygenation (hypoxic zones) or the leaching of minerals due to water fluctuations. The plants that thrive here are known as hygrophilous plants. They play a crucial role in the ecological and ecosystem services mentioned above. One of the best-known hygrophilous plants in Europe is the water lily. Its aerobic respiration system enables it to maintain adequate oxygenation, even in stagnant water. Its roots draw nutrients from muddy soils while its leaves float to the surface to carry out photosynthesis. The water lily blooms above the water, attracting insects pollinators that participate in these interdependent ecosystems, promoting the reproduction of hygrophilous plants.

Among the continental wetlands, mountain areas are reservoirs of biodiversity. Europe's mountains are dotted with glacial lakes, high-altitude marshes, wet meadows or alpine peat bogs. The water found there is generally fresh and cool, providing a source of drinking water for the surrounding wildlife. They also offer migratory species important stopovers during the major migration periods in autumn and spring. In the mountains, temperature variations are more marked than at lower altitudes. Local biodiversity therefore follows the rhythm of the seasons and must adapt by hibernating, migrating or surviving under the ice. For example, the Pyrenean red frog, an endangered species endemic to the Pyrenees region, can move under layers of snow. In winter, it can survive the cold by remaining immersed in damp rock cavities, thanks to its skin respiration.

The coastal wetlands, also known as littoral zones, line the maritime coasts and live to the rhythm of the tides. They create a meeting point between continental freshwater and marine saltwater, forming a salinity gradient that allows different species to flourish there according to their needs. There are mangroves, salt marshes, lagoons and estuaries. They are particularly important for the reproduction of biodiversity, which uses them as nurseries. For example, the white stilt is a bird that typically frequents coastal wetlands. Islets of sand with little grass provide it with the ideal place for nesting. The white stilt feeds mainly on larvae, molluscs and small crustaceans, which it can reach with its adapted beak by pecking in shallow water.

The creation of artificial wetlands has become increasingly widespread in recent years in order to remedy the degradation and reduction of natural wetlands caused by human activity. They are often designed to treat wastewater by exploiting their hydrological processes with purifying virtues. They can take the form of ponds, filtering marshes, retention basins, settling ponds or water reservoirs. In addition to their many ecological and hydraulic virtues, these water features attract and shelter biodiversity or act as a transitional corridor for it. Their role in preserving biodiversity depends on appropriate planning and regular maintenance. Drawing on ecological engineering and biological knowledge, the careful management of artificial wetlands helps to combat the standardisation of habitat and the proliferation of invasive species.

According to the Ramsar Convention, it is urgent and crucial to halt and reverse the destruction of wetlands. This involves the restoration and protection of existing areas, but also the creation of new artificial areas that are planned and managed conscientiously. The Île-de-France region has recently committed itself to this cause. The region launched a "1,000 ponds" plan in April 2024, as a solution deemed "effective and inexpensive" for preserving biodiversity and benefiting from the hydraulic and ecosystem services of wetland sites. There are already 30,000 ponds in the Île-de-France region, home to a unique and flourishing biodiversity. A national "wetlands 2022-2026" plan was launched in 2022, including among its objectives the restoration of 50,000 hectares of wetlands by 2026, the extension of the Ramsar network and the doubling of the area of wetlands benefiting from strong protection in France by 2030.