Global Patterns of Mangrove Extinction Risk: Implications for Ecosystem Services and Biodiversity Loss

Mangroves are unique plant species found in tropical and subtropical estuarine and nearshore marine regions worldwide. Mangrove species have several physiological adaptations to saline, water-saturated soils, including viviparous or cryptoviviparous seeds that disperse by water, and salt-exclusion or salt-excretion capabilities to cope with high salt concentrations in nearshore saturated soils and sediments. Many species also have specialized aerial roots, or pneumatophores, that enable oxygenation of roots in water-logged soils. Species restricted to tropical intertidal habitat have been defined as "true mangrove" species, while those not exclusive to this habitat are sometimes referred to as "mangrove associates" (Lugo & Snedaker, 1974). Others include as mangroves any tree, shrub, palm, or ground fern exceeding 0.5 min height and which normally grows in the intertidal zone of tropical coastal or estuarine environments (Duke, 1992). In view of the global variety of mangrove types and their :floristics, there are approximately 70 species of mangroves, which are quite taxonomically diverse, as they represent 17 families (Table 2.1). The Mangrove Reference Database and Herbarium provides a larger overview of all known species, subspecies and hybrids (Masso i Aleman et al., 2010).

Compared to other forest types, mangrove forests generally lack an understory and usually exhibit distinct zones of species (Figure 2.r) based on variation in elevation, salinity, and wave action (Duke et al., 1998). Many species are more common in environmental conditions characterized by the low or high intertidal zone, or in the downstream or upstream estuarine zones. For example, species found primarily in the upstream estuarine or high intertidal region often have specific freshwater-dominated habitat preference, while other species that have higher inundation tolerance can be found in the downstream estuarine or low intertidal zone and along beaches. However, zonation or correlation with environmental gradients is not always apparent and can be easily disrupted by anthropogenic disturbance (Ellison et al., 2000).

All mangroves disperse their propagules by water, and many mangrove species produce seedlings through vivipary. For many of the mangrove species that reproduce through vivipary or cryptovivipary, an embryo remains on the parent tree for months sometimes before it detaches, germinates, and grows roots (Tomlinson, 1986; Hogarth, 2007). The hypocotyls of some species are more adapted to long-distance dispersal and exhibit a period of postponed dormancy before rooting. A few mangrove species do not reproduce through this process, but rather disperse more conventionally via floating fruits and seeds (Tomlinson, 1986; Hogarth, 2007).