Trophic relationships in mangrove ecosystems
The successful integrated management of mangrove wood and non-wood resources depends on an understanding of, firstly, the ecological and silvicultural parameters for forest management (primary production) and secondly, the biological role that the primary production from the forests plays in the mangrove food web of aquatic resources (secondary production). An understanding of the role of key species in maintaining the equilibrium of a particular ecosystem is likewise essential.
The mangrove food web
Our present knowledge on energy flow in mangrove ecosystem is mainly based on the pioneering work on food chains in Florida (Heald, 1971; Heald and Odum, 1970; Odum, 1971; Odum and Heald, 1972; 1975; and Odum et al., 1982). Briefly, the principal energy flow follows the path:
Mangrove leaf detritus - Bacteria and fungi - Detritus consumers (herbivores and omnivores) - Lower carnivores - Higher carnivores.
The chain begins with the production of carbohydrates and carbon by plants through photosynthesis.
Leaf litter is then fragmented by the grazing action of amphipods and crabs (Heald, 1971; Sasekumar, 1984). Decomposition continues through microbial and fungal decay of leaf detritus (Fell et al., 1975; Cundell et al., 1979) and use and reuse of detrital particles (in the form of faecal material) by a variety of detritivores (Odum and Heald, 1975), beginning with very small sized invertebrates (meiofauna) and ending with such species as worms, molluscs, prawns and crabs, who in turn are preyed upon by lower carnivores. The food chain ends with higher carnivores such as large fish, birds of prey, wild cats or man himself.
The earlier findings have now been extended to include other energy and carbon sources to consumers in mangrove ecosystems. (e.g. Carter et al., 1973; Lugo and Snedaker, 1974; 1975 and Pool et al.,1975). Odum et al. (1982) enlarged the earlier basic trophic model to include inputs from phytoplankton, benthic algae and sea grasses, and root epiphytes. For example, phytoplankton may be important as an energy source in mangroves with large bodies of relative clear deep water.
On this basis, the benthic algal contribution in estuaries with high levels of suspended sediments is likely to be lower. Similarly, where the continental shelf is truncated or very steep sloping, combined with high energy coastline and tidal amplitude, there is little sea grass or turtle grass. Where shading is not excessive, mangrove prop root epiphytes may also be highly productive. Values for periphyton production on prop roots of 0.14 and 1.1 gcal/m2/d have been reported. (Lugo et al. 1975; Hoffman and Dawes, 1980). A generalized food web in mangrove ecosystem is depicted in the figure below.
However, Odum et al. (1982) have stressed that in spite of innumerable studies, the Florida food chain model remains hypothetical and qualitative. Indeed, some recent data from the Indo-West Pacific region suggests that the Caribbean model requires some modifications.
As our knowledge on trophic relationships and interactions improves, foresters will be able to manage their resources better without harming the environment.
In mangrove management it is thus essential to take a holistic approach and to secure the survival of the entire ecosystem. Conserving or promoting biodiversity through the selection of species to be felled and regenerated and the protection of habitats for various marine and terrestrial animals is an imperative as is the maintenance of the protective role the mangroves play along river banks and coastlines.
Riverine vegetation should therefore never be felled indiscriminately, as bank erosion will increase water turbidity and adversely affect aquatic fauna, particularly shrimp larvae, molluscs and the breeding of important estuarine species. Protective areas should also be set aside in the mangrove area proper for the conservation of wildlife and plants of special interest.
Where the demand for land for agriculture or aquaculture necessitates the conversion of mangrove areas, the sites should be properly evaluated prior to the conversion in order to minimize the damage to the mangrove ecosystem as a whole.
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Sasekumar, A. 1984. Secondary productivity in mangrove forests. In J.E. Ong & W.K. Gong, eds. Productivity of the mangrove ecosystem: Management implications. p. 20-28. UNESCO/UNDP (RAS/79/002/G/01/13). Penang.
FAO. 1994. Mangrove forest management guidelines. FAO Forestry Paper No. 117. Rome.