Saltmarshes are highly productive ecosystems rich in biodiversity, consisting of a variety of plant species that provide important habitats for a wealth of fauna. In addition, they contribute to livelihoods locally and globally through the associated ecosystem services they provide. In particular, they are recognised as important carbon sinks that protect coastal areas from natural disasters, such as erosion and storm surges. Tidal saltmarsh communities (hereafter saltmarshes) comprise the upper, vegetated portion of intertidal mudflats, lying approximately between mean high water neap tides and mean high water spring tides. Saltmarshes occur worldwide, particularly in middle to high latitudes, and are usually restricted to comparatively sheltered locations. The vegetation of a saltmarsh consists of halophytic (salt-tolerant) herbs, grasses and low shrubs adapted to regular or occasional immersion by the tides. Within the saltmarsh community, there are often clear patterns of zonation, typically linked to inundation and salinity. Saltmarshes can be highly dynamic - prone to erosion, but also rapidly colonizing new sediments. Their distribution is not only influenced by, but also influences local physical processes and geomorphology including topography and creek patterns (Adam 1990). The species composition and type of saltmarsh varies from site to site and regionally (Fischer et al. 2000).

Saltmarshes are unique ecosystems, situated at the interface between terrestrial and marine ecosystems, and often further connecting saline and freshwater ecosystems. They provide critical habitat for numerous aquatic and terrestrial organisms, which use them as a refuge from predators and as a location to feed, shelter and spawn (Deegan et al. 2000). Furthermore, they act as high tide refuges for birds feeding on adjacent mudflats, as breeding sites for waders, gulls and terns and as a source of food for passerine and migratory birds (Greenberg et al. 2014). In addition, these wetlands provide valuable ecosystem services (Gedan et al. 2009) that are of significant commercial and recreational value. For example, saltmarshes increase the abundance of fish invertebrate stocks of importance ofr both commercial and recreational fishing; high diversity of birds found within marshes make them a popular location for tourists and bird watchers; they protect shorelines from erosion by buffering wave action and they protect water quality by filtering runoff and metabolising excess nutrients (Barbier et al. 2011).

Despite their importance, saltmarsh areas are declining around the world, having lost between 25% and 50% of their global historical coverage (Crooks et al. 2011, Duarte et al. 2008). Vast areas have been lost through conversion to agriculture, urban and industrial land, while remaining saltmarshes face many other current and potential threats (Gedan et al. 2009): land claim for industry, port facilities, transport infrastructure and waste disposal is still comparatively common; invasive species such as Spartina which threatens mudflat fauna and birds but also saltmarsh vegetation itself; many saltmarshes are being 'squeezed' between an eroding seaward edge and fixed flood defence walls and grazing has a marked effect on the structure and composition of saltmarsh vegetation by reducing the height of the vegetation and the diversity of plant and invertebrate species A range of other human activities can be locally damaging, including waste tipping, pollution and turf cutting (Neubauer 2009).

Due to the continued pressures placed upon saltmarshes, it is essential to establish a mapped baseline in order to assess change and enhance management efforts. Recent best estimates of the global area of saltmarsh range from 2.2 to 40 Mha Pendleton et al. 2012). One reason for the relatively large range in estimated area is that whist saltmarshes have been mapped locally, and even nationally in many places, there has never been a systematic attempt to map the distribution and extent of saltmarshes globally. This has prevented any accurate accounting of overall declines in this critical habitat, or indeed of geographic patterns in such declines. This is in stark contrast to other coastal systems such as coral reefs (Spalding et al. 2001), seagrasses (Green and Short 2003), and mangrove forests Giri et al. 2010, Spalding et al. 2010, Spalding et al. 1997).

Synthesis of existing saltmarsh distribution data into a global map began in 2007, but was never formally published. Using the groundwork established previously, the same general approach of constructing a mosaic map from multiple sources was resumed in 2013, and is presented here as the first detailed global map of saltmarshes.

The authors gratefully acknowledge financial support from the GEF Blue Forest project (CM, LVW), UNEP-WCMC's Proteus Partnership (CSM, LVW), and the “Building the European Biodiversity Observation Network” (EU BON) project (www.eubon.eu), a 7th Framework Programme funded by the European Union under grant agreement No. 308454 (CSM, LVW). The authors would also like to acknowledge Kamila Jarnia for her assistance with compiling the data layer.

The authors would also like to extend their gratitude to the many data providers who kindly contributed their data to this effort; without their support, this dataset would not exist:

The European Environment Agency, Denmark; The Ramsar Convention Secretariat; UK Environment Agency; Rijkswaterstaat Dienst Zeeland, Netherlands; National Parks and Wildlife Service, Ireland; Humedale Conservation and Inventory Service, Subdirectorate General of Natural Environment, General Directorate of Quality and Environmental Assessment and Natural Environment, Ministry of Agriculture, Food and Environment (Servicio de Conservación e Inventariación de Humedale. Subdirección General de Medio Natural, Dirección General de Calidad y Evaluación Ambiental y Medio Natural, Ministerio de Agricultura, Alimentación y Medio Ambiente), Spain; Department of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, and the Coastal Services Center, USA; Waterway and Shipping Office Bremerhaven (Wasser- und Schifffahrtsamt Bremerhaven), Germany; Federal Institute of Hydrology, Germany; Land Office for Agriculture, Environment, and Rural Areas (Landesamt für Landwirtschaft, Umwelt und ländliche Räume), Schleswig-Holstein, Germany; Florida Fish and Wildlife Conservation Commission - Fish and Wildlife Research Institute, Center for Spatial Analysis, USA; Cawthron Institute (and various regional and city councils), New Zealand; National Office of the Environment, Madagascar; Swedish Environmental Protection Agency (Naturvårdsverket), Sweden; Abu Dhabi Environment Agency, United Arab Emirates; US Fish and Wildlife Service, USA; Geoscience Australia; Prince Edward Island Department Of Agriculture and Forestry, Canada; Nova Scotia Department of Natural Resources, Wildlife Division, Canada; New Brunswick Department of Natural Resources, Fish and Wildlife Branch, Canada; Ministry of Agriculture (Ministerio de Agricultura), Lima, Peru; Consorzio Venezia Nuova, Venice, Italy; Division of Water Resources, Hydraulic and Maritime Engineering, Athens, Greece; European Topic Centre on Biological Diversity; National Museum of Natural History (Museum National d'Histoire Naturelle), France; Estonian Environmental Information Centre; National Commission of Natural Protected Areas (Comisión Nacional de Áreas Naturales Protegidas), Mexico; The Nature Conservancy; Johannes Burmeister (Manfred-Hermsen-Stiftung); Elier Tabilo Valdivieso (Centro Neotropical de Entrenamiento en Humedales); BirdLife International, UK; Institute of Nature Conservation, Polish Academy of Sciences, Poland; Komarov Botanical Institute Russian Academy of Sciences; Gerasimov YN (2003; Pacific Institute of Geography, Russian Academy of Sciences); Institute of Remote Sensing and Digital Earth, China; Environmental Hydraulic Institute (IH Cantabria), University of Cantabria, Spain; Otars Opermanis, Latvia; Institute of Marine and Coastal Research, National University of Mar del Plata (Instituto de Investigaciones Marinas y Costeras, Universidad Nacional de Mar del Plata), Argentina; Ping Zuo (Nanjing University); Tomkovich PS et al. (2002); Spilling et al. (1995); Miniev (1991); Syroechkovski (1995); Tolvanen et al. (1996); Syroechkovski et al. (1998); Sandring et al. (2001); João M. Neto (IMAR-Univ. Coimbra); Isabel Caçador (CO-FFCUL); Pilar García (INDUROT); Botany Department, Nelson Mandela Metropolitan University, South Africa; Neil Saintilan (NSW Department of Energy, Climate Change and Water, Australia); Olga A. Mochalova (Institute of the Biological Problems of the North); The British Columbia Marine Conservation Analysis, Canada.