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Campus Ecosystem Model (CEM)

Campus Ecosystem Model (CEM)

Mangrove Forests

 
 

 

Introduction

Mangrove ForestMangrove forests represent more than a collection of individual trees; they are a functioning system. Mangrove is a general term used to denote approximately 50 different salt-tolerant trees and shrubs occupying muddy, salt- and brackish-water shorelines (Gonick & Outwater, 1996). Although mangroves are also tolerant of freshwater, they are restricted to saltier environments because they are easily out-competed in freshwater systems (Nelson, 1994). These tropical species "flourish in average annual temperature ranges of greater than 65 degrees…[with]…few hard freezes" (Alden et al., 1998). In Florida, there are three true mangrove trees and an implied fourth: red mangrove (Rhizophora mangle), black mangrove (Avicennia germinans), white mangrove (Laguncularia racemosa), and buttonwood (Conocarpus erectus), often referred to as the gray mangrove (Myers & Ewel, 1990). The buttonwood is a pseudo-mangrove because it is often found in association with white mangroves.

Physical Characteristics

Mangrove ForestFlorida mangroves can be distinguished from one another based upon their physical characteristics as well as their general location. Red mangroves possess prop roots-large curved roots that surround and entangle the trees. Because of the three-dimensional structure provided by the prop roots, both above and below the water, they attract entire communities of organisms, including mangrove periwinkles (Littorina angulifera) and mangrove crabs (Aratus pisonii). Coffee-bean snails (Melampus coffeus) actually migrate up and down the prop roots with the rise and fall of the tides (Kaplan, 1988). In comparison, black mangroves, possess small "erect lateral branches of an otherwise horizontal root system" (Nelson, 1994) known as pneumatophores. These look like small pencils projecting out of the adjacent sediments in all directions. White and gray mangroves are located further inland, on more elevated sites with drier soil. In general, mangroves are distributed along a transect from the marine environment to land in the following order: red, black, white, and gray.

Biota

Mangrove communities are characterized by a diversity of flora and fauna. The saltwort (Batis maritime) is a mangrove plant that is located in close proximity to very salty, shallow ponds. Sea oxeye (Borrichia frutescens), glasswort (Salicornia virginica), and southern glasswort (Salicornia perennis) are a few of the prominent species found occupying mangrove communities. Southern glasswort is a reddish-stemmed, thin, asparagus-like plant that forms dense aggregations (Kaplan, 1988). In addition, mangrove communities are often skirted with large colonies of mangrove ferns (Acrosticum aureum).

Mangrove ForestMangrove forests contribute to an extremely complex food web. Mangrove leaves may be broken down either mechanically or biologically and are continually being colonized by bacteria, fungi, and protozoans. The resulting small particles of organic matter may then be eaten by small fish and crustaceans, which are in turn fed upon by larger fishes and invertebrates. The thick mud that accumulates around mangroves is also dominated by a variety of crabs which aid in the further breakdown of mangrove detritus. As a result, the mangrove crab Sesarma, the wharf crab Pachygrapsus, several species of fiddler crabs, the spotted mangrove or tree crab Goniopsis cruentata, the mangrove mud crab Eurytium limosum, the common mud crab Panopeus herbstii, and the mangrove land crab (Ucides cordatus) all play an important role in nutrient cycling (Kaplan, 1988).

The result is that this detrital food base helps support enormous number of birds, reptiles, mammals, fish, and invertebrates. Mangroves serve as nursery areas for many economically important species of fishes and invertebrates as well. Furthermore, many species of shore birds nest among the mangroves including brown pelicans (Pelicanus occidentalis), little blue and tricolored herons (Egretta tricolor), great (Casmerodius albus) and snowy (Egretta thula) egrets, white ibises (Eudocimus albus), and frigate birds (Campbell, 1988; Alden et al., 1998). Birds such as osprey, bald eagle, red-breasted merganser, and roseate spoonbill (Ajaia ajaja) forage near the mangroves, as do otters, mosquito fish, killifish, mangrove salt marsh snakes, snapping and soft-shelled turtles, snook, redfish, snapper, pink shrimp, and baby alligators.

Ecosystem Dynamics/Relationships With Other Ecosystems

Red mangroves apparently facilitate the process of succession within mangrove systems. Kaplan (1988) defines succession as, "the sequence of events by which a biotic community constantly changes in composition until it achieves near-perfect balance with its environment" (p. 173). The end point of succession is referred to as a climax community, and often takes years and even decades to reach. In mangrove communities, this end point is characterized by the colonization of white mangroves and buttonwoods.

Mangrove propagation (seed production) begins with a yellow waxy flower that transforms into a brown lima bean-shaped embryo known as a propagule. Once in water, the waxy end of the propagule causes it to float vertically, thereby allowing for the seed to be dispersed by water currents. When the propagule comes in sufficient contact with soft bottom, usually at low tide, it lodges in the substrate and begins to drop its roots. Within three years the growing prop-root system begins to alter local currents, slowing water movement and causing suspended particles to begin settling to the bottom. This accumulation of particulate matter and leaf litter slowly transforms the muddy substrate. One acre of mangrove trees drops approximately three tons of leaves per year, further altering the bottom (Kaplan, 1988; Alden et al., 1998).

The protein value of this leaf litter is continually increased as it is colonized by mold and bacteria. As the leaves are broken into smaller and smaller particles by both mechanical and biological processes, the surface area of individual particles is greatly increased. When mixed with sand, this coarse organic matter forms a rich, densely packed sediment. Algae move in and help further stabilize the mud. After extensive growth and soil accumulation, supplemented by leaf litter and detrital development, new shrubs and small trees begin to grow. Black mangrove colonization contributes additional leaf-litter, further elevating and drying the area until eventually the more elevated, drier ground encourages the influx of white mangrove and buttonwood, thereby completing the succession process.

Human Impacts/Restoration/Ecosystem Management

Mangrove Forest BirdThe above flora and fauna are as important to a mangrove ecosystem as the system is to the water bodies that it fringes. Mangroves serve to catch garbage such as bottles, cans, bags, etc. before it enters rivers, bays and the ocean. They also filter agricultural and urban runoff such as fertilizers, pesticides, fecal matter, and silt. Mangroves not only protect the water from the land, but they also protect the land from the water. Hurricane Andrew was one of the most devastating storms in Florida’s history. In many areas, the first line of defense against this storm was the coastal mangrove system which provided a buffer against wave action, storm surge, and coastal erosion.