Mangrove ecosystems are widespread in estuarine and coastal regions of the subtropics and tropics. They are characterized by mangroves, various types of trees and shrubs that grow in saline or brackish water. Whether fringing a sandy key or bristling along a jungle seacoast river, mangrove swamps rank among the planet’s most biologically productive communities. The decomposition of these swamps’ huge accumulations of organic litter is key to that fertility.
Ecosystems are defined by a through-flow of energy -- derived in nearly all cases from sunlight -- and a cycling of matter. Matter is inherently limited on the planet, and must be recycled continually to support the growth and activity of Earth’s organisms. Primary producers such as plants and algae harness energy directly from the sun. They provide energy and nutrients to primary consumers, which in turn nourish secondary consumers -- predators and scavengers. Decomposers get nutrients and energy from dead animals and plants, and in the process mineralize or release nutrients that primary producers can then use. The microbes and invertebrates that provide decomposition services are often collectively called “saprophages.”
The massive quantities of detritus produced in a mangrove swamp -- the litter of twigs, bark and leaves from mangroves themselves and the organic waste of animals -- form the foundation of the ecosystem’s food web, along with nutrients washed in by rivers and tides. This organic litter can be prodigious: A riverine red-mangrove swamp can produce some 4 tons of detritus per acre each year. The physical environment aids the work of decomposers: the rise and fall of tides exposes litter to alternating wetting and drying, which hastens its breakdown.
A variety of organisms set upon organic detritus the moment it’s shed into the mangrove ecosystem. Fungi affix to it, sharing space with bacteria and algae; soon crustaceans and other larger organisms join the miniature community. Crabs, amphipods, small fish and other creatures may slice apart leaf bits, providing large-scale dismantling that contributes to decomposition.
Mangrove Food Web
The cycling of nutrients by decomposers supports the growth of algae, plankton and other tiny organisms as well as mangroves themselves. Vast arrays of fish use mangrove swamps as nurseries and foraging grounds; some of these eat decomposing litter and, in turn, are eaten by predatory fish that ultimately sustain top-level consumers such as herons, ospreys, crocodiles, sharks and humans. Indeed, the detritus-based food web of mangrove communities is key to fisheries throughout the subtropics and tropics: Florida’s mangrove swamps, for example, are reckoned as the direct foundation of 90 percent of that state’s commercial and recreational fishing industries.