Ecology of Food Webs in Mangroves
INTRODUCTION
Food webs are models of flows of energy through a system. Mangroves are among
world’s most productive ecosystems - WHY? Mangrove systems tend to act as net
accumulator (sink) of various elements. (N,P, trace elements and heavy metals)
MANGROVE PRODUCTIVITY AND LITTER PRODUCTION
litter fall refers to leaves, wood, leaf scales, propagules, bracts, flowers and insect frass
(excrement.) Leaf litter fall in Sth Florida averages 2-3 g/m2/day (approx. 8 ton/ha/yr) dry
wt. Litter fall related to productivity with productivity of 6 forest types. Productivity
red>black>white mangroves.
HABITATS IN MANGROVES:
Habitats in mangrove forests are arranged
1. by forest type-overwash, fringe, riverine, basin, hammock, dwarf
2. horizontally - R. mangle zone seaward, A.geminans landward, with white and
buttonwood above tidal zone.
influenced by frequency of inundation.
3. vertically - roots, trunk, canopy (like rocky intertidal)
is influenced by tidal cycle.
Species inhabiting mangroves
Spatial zonation of habitats and faunal composition of habitats. Fauna of mangroves
derived from both terrestrial and marine sources and is therefore a mixture of both
1. MARINE SPECIES:
INVERTEBRATES
1. direct grazers - insects and mangrove tree crab, small inverts that feed on prop root and
mud algae directly
2. filter feeders - sessile, on prop roots that filter phytoplankton and detritus from water
column
3. deposit feeders - mobile inverts which skim detritus , algae and occasional small
animals from surface of substrate
4. motile invertebrates (=carnivores) - highly mobile species that feed on the three other
groups. Can occur in canopy to mud surface.
FISHES
are important components of the basin, fringe, overwash and riverine forest types. approx.
100 species in Florida.
In general although many aquatic species, only few are found at any one location ,and
also tend to be low in abundance , due to salinity stress imposed by environment
Adaptations to salinity stress: Adaptations of aquatic species requires them to have
ability to tolerate salinity differences, and intertidal life style.
1. morphological
2. behavioral
3, physiological
2. TERRESTRIAL SPECIES
AMPHIBIANS AND REPTILES -24 spp in Florida
BIRDS
highly diverse assemblage 50-200 species.
six groups:
1. wading birds
2, Probing birds
3. Floating and diving water birds
4. Aerially searching birds
5. Raptors
6. Arboreal birds
MAMMALS
rodents and bats mainly, also manatee.Many terrestrial animals that use mangrove
systems as a habitat are not necessarily intricately linked with the food web
process. These are transients especially from terrestrial side
Distribution by guilds:
Guild 1: Sublittoral/Littoral mangrove guild
Special habitat : PROP ROOTS, Aerial prop roots of R. mangle support a diverse
assemblage of species
Guild 2: Mangrove arboreal canopy guild
Guild 3: mangrove benthic and infuana community:
Guild 4: Mangrove tidal creek and Ditch community:
Guild 5: Mangrove basin community;
Guild 6: Upland Arboreal community
Guild 7: Upland terrestrial community
Food webs:
1. HERBIVORY-terrestrial, mainly insects, grazing estimated at 5-10% total leaf area.
2. DETRITUS BASED: CARBON EXPORT (OUTWELLING):
Mangroves export C to nearby water bodies (estuarine) in form of litter
Eric Heald studied the production of litter and detritus in a mangrove forest in ENP, and
William Odum followed detritus cycling in the food web organisms in the same
area.
Decomposition of leaf litter by:
leaching,
saprophytic decay (microbial decomposition)
fragmentation.
Two basic types of detrital food chains, abiotic (DOC) and biotic (POM).
POM- Fringe and riverine systems represent open systems with very high rate of export
and limited nutrient recycling. Dead plant structures, leaves mainly submerged
breakdown are processed into POM by several spp of amphipods and xanthid
crabs. They reduce leaves to detrital sized particles. These animals are themselves
consumed by fish. However does not constitute large portion of animal biomass
and are not major energy flow pathway. Leaf litter becomes nutritious due to
microbial enrichment process. Bacteria and fungi are able to breakdown
indigestible plant substances such as cellulose and lignin. Result of this microbial
breakdown is increase in N content of leaf.
DOC - Less well flushed basins, etc. have a higher degree of microbial recycling. Plant
nutrients may enter the trophic system as DOC due to abiotic leaching
processes.DOC is flocculated under changes in salinity. Colonization of these
formed flocs (POM) occurs in 1-3 d by bacteria, flagellates and ciliates which are
then consumed by detritivores.
OUTWELLING: some problems
Detrital feeders: the continuation of the aquatic food web story. Mangrove
dominated estuaries important nursery and feeding ground for pink shrimp,
mullet, gray snapper, red drum, spotted seatrout and blue crab (all commercially
important species.). The trophic relations leading to these species will be
presented.
3. Mosquito-linked food chain: an alternative food web
Mosquitoes breed in pools in section of the mangrove forest. The aquatic larval stage
provides food for mosquito-fish, which are adapted to exploit shallow and
ephemeral pools. As the waters dry up the fish migrate down to more permanent
tidal waters where they are consumed and enter the previously discussed pathway
of energy flow.
4. Sedimentary bacteria and further food webs
Sedimentary bacteria play an important role as mineralizers of organic detritus and
recyclers of essential nutrients.Anaerobic decomposition of mangrove tissue particularly root material may support an extensive web based on bacteria
associated with methanogenesis (reduction of sulfur cmpds).
NURSERY AND OTHER LINKAGES WITH AQUATIC HABITATS:
Nursery function:
Many fish and crustacean species use the mangroves as nursery ground for post-larval
and juvenile stages.
Linkage with other systems:
Linkage of mangroves with estuarine seagrass habitats and coral reefs is mainly by
trophic transfer to large fish via detrital export food web.