The term "mangrove" may be used for a variety of tropical and subtropical plant species (in many cases not closely related to each other) that grow in highly saline coastal environments and as a consequence share many adaptations that allow them to survive and reproduce under these conditions (e.g., thick, waxy leaves to minimize water loss in their salty surroundings). Mangroves are trees or shrubs that grow with their roots partly or wholly submerged in sea water. The Red Mangrove (Rhizophora mangle) is particularly extreme in its ability to survive with its roots bathed in salt water. In Florida, Red Mangroves reach about 6 meters, but in the tropics they may grow to four times that height. The dark green leaves are shiny and broad. Reddish prop roots arch from the trunk into the water; old individuals may have aerial roots hanging from branches. Stalked yellow, waxy flowers are produced in groups of 4. The fruit is a leathery brown, conical berry about 2.5 cm long. The Red Mangrove is one of a number of mangrove species that are "viviparous", i.e., the seeds germinate while still attached to the parent. Red Mangrove seedlings (sometimes known as "sea pencils") up to 30 cm long hang from branches during much of the year. (Kaplan 1988)
Mangroves are builders. They help build up new land along the shore and are the anchors for rich and complex communities involving diverse animals, plants, fungi, and microorganisms (although plant species diversity in mangrove habitats is generally much lower than animal diversity). They serve as nurseries for many fish and invertebrates, including many that migrate along the shore or out into the ocean as adults. They filter the water and buffer the effects of hurricanes. Kaplan (1988) provides an excellent and accessible introduction to the ecology of mangroves and mangrove swamps.
Conservation and Management
Mangroves in general, and Red Mangrove in particular, are highly vulnerable to coastal development, pollution, and other human impacts (Kaplan 1988). In many parts of the world, mangroves are collected for firewood.
Collectively, considering Red Mangrove together with other mangrove species, at least 35% of the world’s mangrove forests have been lost in the past few decades as a consequence of human activities. This loss directly affects ecosystem services such as providing habitat for fishes, prawns, and crabs. Additionally, degradation of the remaining mangrove habitats results in loss of ecological functionality, putting millions of coastal people in jeopardy. Among the threats to mangroves are aquaculture and coastal development, altered hydrology, sea level rise, and nutrient overenrichment. (Feller et. al 2010 and references therein)
Takayama et al. (2008) developed microsatellite markers useful for studying population structure of Rhizophora mangle and related species.
In Florida, Red Mangrove (Rhizophora mangle) can be most obviously distinguished from Black Mangrove (Avicennia germinans) and White Mangrove (Laguncularia racemosa) by the fact that these latter two species have numerous (especially Black Mangrove) erect breather roots emerging from the water, as well as by the Red Mangrove's distinctive seedlings attached to the parent. Gray Mangrove (or Buttonwood, Conocarpus erectus) has alternately arranged leaves, in contrast to the opposite leaves of the Red, Black, and White Mangroves. (Petrides 1988)
An excellent resource for identifying the mangroves of Florida can be found at http://www.selby.org/
Red Mangrove (Rhizophora mangle) is a broad-leaved evergreen tropical shrub or tree that may reach 24 meters. It has conspicuous arching prop, or stilt, roots. Older bark is reddish and smooth; younger bark is grayish. Twigs are silverish to shiny dark brown. The stalked, oppositely arranged leaves (5 to 16 cm long and up to 8 cm wide) are simple, entire, and smooth, with a prominent midrib. They are leathery, dark green and shiny above and lighter green (often speckled) below. The somewhat leathery, 4-petaled flowers are pale yellowish or cream colored. They are borne on stalks (about 1 to 4 cm long) in clusters of 2 or more. The fruit is an elongate greenish capsule, about 2.5 cm before germination. The seed germinates while still on the plant, giving the fruit a curved, elongate appearance. (Tiner 1993)
Gilbert et al. (2002) studied the possible role of salt excretion by mangroves as a defense against pathogenic fungi in a mangrove forest in Panama. Although presumably evolved for other reasons, salt excretion by leaves of some mangrove species may serve as an important defense against fungal attack, reducing the vulnerability of typically high-density, monospecific forest stands to severe disease pressure. In their study, Gilbert et al. found that Black Mangrove (Avicennia germinans) suffered much less fungal leaf damage from than did White Mangrove (Laguncularia racemosa) or Red Mangrove (Rhizophora mangle). Black Mangrove leaves also supported the least fungal growth on the leaf surface, the least endophytic colonization, and the lowest fungal diversity, followed by White Mangrove and Red Mangrove.
Host specificity of leaf-colonizing fungi was greater than expected at random. The fungal assemblage found on Black Mangrove appears to be a subset of the fungi that can grow on the leaves of Red and White Mangrove. The authors suggested that the different salt tolerance mechanisms in the three mangrove species may differentially regulate fungal colonization. The mangroves differ in their salt tolerance mechanisms such that Black Mangrove (which excretes salt through leaf glands) has the highest salinity of residual rain water on leaves, White Mangrove (which accumulates salt in the leaves) has the greatest bulk salt concentration, and Rhizophora (which excludes salt at the roots) has little salt associated with leaves. The high salt concentrations associated with leaves of Black and White Mangrove, but not the low salinity of Red Mangrove, were sufficient to inhibit the germination of many fungi associated with mangrove forests. The authors suggest that efficient defenses against pathogens may be especially important in natural communities, such as mangrove forests, where host diversity is low and the density of individual hosts is high – ideal conditions for diseases to have strong effects on plant populations.
Mangrove forests are unusual among tropical forests for their low tree species diversity and associated high population density of
individual species. Mangrove species are unusual in their ability to grow in flooded, saline soils and for the array of mechanisms they have evolved to tolerate high salt concentrations. The work by Gilbert et al. suggests that some mangrove species may also be unusual in their escape from strong disease pressures, even when growing at high densities, through the inhibitory effects of
high foliar (leaf) salt concentration on fungal infection. (Gilbert et al. 2002)
In the United States, Red Mangrove (Rhizophora mangle) is found in mangrove swamps, salt marshes, and fresh marshes (near the coast) of the Florida Everglades (Tiner 1993).
Ecologically, tropical mangrove swamp forests share many similarities with salt marshes to the north (although mangroves are woody and salt marshes are generally dominated by grasses and other herbaceous vegetation). Both mangrove swamps and salt marshes occur at the interface of land and sea, protect the coast from storm damage (especially hurricanes), and serve as important nurseries for fish and invertebrates. Mangrove leaves are an important source of energy for marine food webs: fallen leaves are colonized by bacteria, fungi, and protozoans, which are in turn fed upon by zooplankton, which in turn are consumed by juvenile fish and larval invertebrates. (Kricher 1988)
Unlike most plants, Red Mangroves (like some other mangrove species) produce seeds that germinate while still attached to the parent. The Red Mangrove embryo grows into a seedling that may be 25-30 cm long before dropping off the tree into seawater. These elongated seedlings float for several days until the pointed end absorbs enough water to become too heavy to float and sinks. The waxy fruit end (from which the seedling sprouted) still floats, causing the seedling to bob along in the water with the pointed end pointed downward. While still in this state, which can last as long as a year, a few leaves may sprout from the upper end and roots may sprout from the lower end. If the young plant comes into contact with sediment, it will take root. This may occur even far from land, where ocean currents have piled up sand within a few centimeters of the water's surface.
In a year the plant may grow to a meter tall. Within three years it will produce many prop roots, which look like pendulous branches growing down into the water. If other mangoves have rooted nearby, a little forest may form in just a few years. The maze of prop roots slows the currents and tiny suspended particles sink to the bottom in a self-reinforcing process as muddy sand builds up. Mangrove leaves fall and become trapped among the roots, where they are broken down and decomposed by diverse small invertebrates, fungi, and bacteria. The resulting rich organic detritus mixes with the sand to form a rich, densely packed sediment. This is the first stage of a continuous transformation involving a succession of organisms that continue to modify the habitat in which they live. In Florida and the Caribbean, once sufficient sediment has built up, Black Mangrove (Avicennia germinans) becomes established alongside the Red Mangrove and on somewhat higher ground, several meters back from the water's edge, White Mangrove and Gray Mangrove form a mixed forest. Eventually this process of natural succession transforms what was once saltwater into dry land. (Kaplan 1988)
Ball (1980) provides a detailed historical analysis of the development and dynamics of "induced" mangrove forests that developed in response to salinization (by human-driven changes to local hydrology) of areas formerly supporting freshwater marshes along Biscayne Bay in North Miami, Florida, U.S.A.
Red Mangrove (Rhizophora mangle) flowers year-round, but, at least in the southeastern United States, especially in spring and summer (Tiner 1993).
Red Mangrove (Rhizophora mangle) is found in peninsular Florida, Bermuda, the West Indies, Central and South America, and Africa (Tiner 1993)
Red Mangrove is found from West Africa to the Pacific Coast of tropical America. In Africa, its latidudinal limits are not clear, but it has been recorded as far south as Angola and as far north as Mauritania. In the Americas it has a wide distribution on the Atlantic side, from about 25° N in Florida south to eastern Brazil; on the Pacific side, Red Mangrove occurs from Mexico to northern Chile, where its southern range is limited by cold, dry climate. Populations in New Caledonia, Fiji, Tonga, and Samoa are treated by some researchers as a form of R. mangle, but by others as a closely related but distinct species, R. samoensis. There has been some suggestion that R samoensis co-occurs with R. mangle in Pacific South America. (Tomlinson 1986)
Red Mangrove propagules from Florida were introduced to the southwestern part of Moloka'i (in the Hawaiian Archipelago) in 1902 to stabilize coastal mudflat erosion from pastures and sugarcane fields and for honey production. The mangrove introduction on Moloka'i was very successful, and today it composes the largest stand of mangroves in the Hawaiian Islands. The first confirmed mangrove introduction on O‘ahu occurred in 1922 when several species of Old World mangroves, possibly including Red Mangrove, were planted in He‘eia by the Hawaiian Sugar Planter’s Association. However, there is a report of a small mangrove tree growing near Honolulu as early as 1917, probably a propagule from Moloka‘i. Most of the (at least) six species of mangroves or closely associated species that have been introduced to Hawai‘i over the years have disappeared or are very limited in their distribution (Allen 1998). However, Red Mangrove has been very persistent and has successfully colonized all the main islands except Kaho‘olawe and Ni‘ihau. (Chimner et al. 2006 and references therein). This species has been introduced and become established in other far flung places as well, such as Tahiti (Zomlefer et al. 2006).
Red Mangrove habitats are of great importance to a wide diversity of organisms, including fish (Thayer et al. 1987; Kaplan 1988). Animal diversity in mangrove ecosystems is generally much higher than plant diversity.
Burrowing isopods, such as Phycolimnoria clarkae and Sphaeroma peruvianum, and encrusting barnacles (Balanus spp.) reduce root growth in Red Mangrove; leaves are consumed by land crabs and caterpillars, such as the larvae of the skipper Phocides pigmalion (Perry 1988: Ellison and Farnsworth 1996 and references therein).
The intertidal air-breathing gastropod Melampus coffeus is a critical component in the breakdown of mangrove leaf litter, and it forms an important link between mangrove forest productivity and estuarine food webs. Although a number of other invertebrate species act to accelerate litter breakdown in mangrove and salt-marsh systems (e.g., shredder snails, sesarmid crabs), M. coffeus belongs to a smaller group that can directly assimilate the resources in mangrove leaves. Thus, where M. coffeus is abundant, substantial portions of mangrove leaf material are converted to snail biomass and larvae. Adult snails are preyed upon by white ibis (Eudocimus albus); juvenile snails may be preyed on by Fundulus heteroclitus killifish, which may forage in the leaf litter at high tide; and larvae are exported to the estuary. (Proffitt and Devlin 2005 and references therein)
Land crabs are very important components of mangrove ecosystems. They differentially prey on seeds, propagules, and seedlings along nutrient, chemical, and physical environmental gradients. Such abiotic factors are well known to influence plant species distributions. Lindquist et al. (2009), however, argue that in mangrove ecosystems crab predation is more important than many of these environmental factors in shaping the dynamics and organization of coastal forests. These authors also found that crabs facilitate forest growth and development through such activities as excavation of burrows, creation of soil mounds, aeration of soils, removal of leaf litter into burrows, and creation of carbon-rich soil microhabitats. Crabs influence the distribution, density and size-class structure of tree populations. Given the evident importance of crabs as among the major drivers of tree recruitment (i.e., establishment of a new generation) in tropical coastal forest ecosystems, Lundquist et al. suggest that their conservation should be included in management plans for these forests. (Lundquist et al. 2009)
The dominant members of the crab fauna in mangroves belong to the families Gecarcinidae, Grapsidae, and Ocypodidae. The grapsid crabs are the primary consumers of propagules in the Indo-West Pacific region. In the eastern Pacific, Atlantic, and Caribbean, the gecarcinids (e.g. Cardisoma spp.) and Ocypodids (e.g. Ucides spp.) are more important than the grapsids. (Lindquist et al. 2009 and refrerences therein)
Lundquist et al. (2009) found that crab predation on Red Mangrove was less severe in canopy gaps than in the understory, similar to the pattern found by Sousa et al. (2003a) for predation by the stem-boring scolytid beetle Coccotrypes rhizophorae. In the study by Sousa et al. on the Caribbean coast of Panama, the authors found that the Red Mangrove's water-borne propagules establish
wherever they strand, but long-term sampling revealed that only those that do so in or near lightning-created canopy gaps survive and grow to maturity. These microsites provide better growth conditions than the does surrounding understory and, equally important, provide refuge from predation by C. rhizophorae. Sousa et al. (2003b) found that if an infestation of C. rhizophorae did
not completely girdle a Rhizophora seedling, the seedling could survive, but grew at a reduced rate.
In a study in southwestern Puerto Rico, Wier (2004) found cankers, dead branches and trunks, and as much as 32% mortality consistently associated with the fungus Cytospora rhizophorae. The presence of this fungus, an agent of the cytospora canker disease, correlated with proximity to arboreal nests of the termite Nasutitermes costalis. High incidence of this termite (40%), was detected in injured red mangroves. Wier presents circumstantial evidence that this fungus is carried and disseminated by Nasutitermes costalis, with spores that enter branch and root wounds germinating and forming canker-weakened trees that may die prematurely.
Gilbert and Sousa (2002) studied the host-associations of wood-decaying basidiomycete polypore fungi on three mangrove species (Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa) in a Panamanian mangrove forest. They note that the pattern typically observed for these fungi in diverse tropical forests is that there are a large number of rare species, with the smaller number of common species necessarily being nonspecialists due to the challenge of host rarity. In contrast, the authors found that in the tropical mangrove forest they studied, the polypore assemblage was strongly dominated by a few host-specialized species. Three fungal species, each with a strong preference for a different mangrove host species, comprised 88 percent of all fungi collected (the authors note, however, that these fungi are all reported from other hosts outside of mangrove forests as well). At least for polypore fungi within tropical mangrove forests, where host diversity is low and the abundance of individual host species is high, the restriction against host specialization typically imposed by host rarity in tropical forests may be relaxed, resulting in a polypore community dominated by a few common host-specialist species. (Gilbert and Sousa 2002)
Extracting tannic acid (which stains the water in a mangrove habitat a transparent brown) from Red Mangrove bark for use in tanning was at one time a major industry in the Florida Keys (Kaplan 1988).