Mangroves are the only trees that are capable of thriving in salt water and form unique intertidal forests at the edge of land and sea. Mangroves, defined as ‘tree, shrub, palm or ground fern, generally exceeding one half meter in height, and which normally grow above mean sea level in the intertidal zone of marine coastal environments, or estuarine margins’ (Duke, 2006), constitute one of the most threatened ecosystems (Farnsworth& Ellison, 1997; Valiela et al., 2001; Alongi, 2002; Duke et al., 2007). Due to the widespread distributions of most mangrove tree species few are listed on the IUCN Red List of Threatened Species™ (Polidoro et al., 2010), and many others are listed as ‘least (global) concern’. Yet, both mangrove species and entire mangrove ecosystems are locally threatened throughout their distribution range despite their numerous goods and services.
Mangrove ecosystems provide habitats for numerous animals and micro-organisms (Cannicci et al., 2008; Nagelkerken et al., 2008), live in close interaction with the mangrove vegetation (Bouillon et al., 2004; Kristensen et al., 2008). Mangrove forests provide essential functions and services to coastal populations, such as protection of the coastal zone (e.g. Badola & Hussain, 2005; Dahdouh-Guebas et al., 2005b; Olwig et al., 2007; Barbier et al., 2008; Kaplan et al., 2009) and a variety of timber and non-timber forest products (Bandaranayake, 1998, 2002; Walters et al., 2008).
Many mangroves have been degraded over time as evidenced by numerous retrospective research approaches (Dahdouh-Guebas & Koedam, 2008; Ellison, 2008). Next to direct anthropogenic degradation (Farnsworth & Ellison, 1997; Alongi, 2002), also indirect degradation such as cryptic ecological degradation (Dahdouh-Guebas et al., 2005a) threatens the survival of individual mangrove trees and vegetation assemblages. ‘Cryptic ecological degradation’ (sensu Dahdouh-Guebas et al. 2005a) indicates that introgressive mangrove-associated vegetation or minor mangrove species such as Acrostichum aureum L. slowly start to dominate a forest at the expense of typical, functional, valuable and functional true mangrove species (qualitative degradation) but without loss of spatial extent (no change or an increase in area). In addition, climatic change events such as sea-level rise (Gilman et al., 2008) threaten mangrove ecosystems worldwide.
It becomes increasingly more important to understand the early drivers in mangrove dispersal (Di Nitto et al., 2008, Triest, 2008), mangrove establishment (Krauss et al., 2008), adult mangrove growth and development (Komiyama et al., 2008), regenerative constraints (Bosire et al., 2005), and vegetation dynamics (Berger et al., 2008) in order to design mangrove recovery programmes (Kairo et al., 2001; Bosire et al., 2008).
Mangroves are distributed world-wide in all continents with tropical and subtropical coasts and occur in 124 countries and territories. FAO (2007) estimates a total area for mangroves of 15.6 to 19.8 million hectares. The northern extension limits of mangroves are in Japan (31ºN) and Bermuda (32ºN) and the southern extension limits are in South Australia (38.75ºS) and the east coast of South Africa (32.6ºS) (Tomlinson, 1986). In some countries mangroves are not native such as in the Hawaiian Islands, but since the early 1900's, at least six species have been introduced there (Kathiresan & Bingham, 2001). Also in countries with mangroves exotic mangrove species have been introduced; for instance Nypa fruticans (Thunb.) Wurmb., a mangrove species endemic to the Indo-West-Pacific region only, was introduced in West-Africa (Spalding et al., 1997). Inversely, some other species have gone extinct in some countries, such as Bruguiera gymnorrhiza (L.) Lamk. in Yemen or Heritiera littoralis Dryand. in Bangladesh (loc. cit.).
To document and analyse the historic loss and current extent of mangrove ecosystems and mangrove biodiversity around the world, the Mangrove Reference Database and Herbarium has been created.