Southern Cross University
ePublications@SCU
School of Environment, Science and Engineering
Papers
School of Environment, Science and Engineering
2009
Mangrove islands
Peter Saenger
Southern Cross University
Publication details
Saenger, P 2009, 'Mangrove islands', in RG Gillespie & DA Clague (eds), Encyclopedia of Islands, University of California Press,
Berkeley, CA, pp. 591-593. ISBN: 9780520256491
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understanding the evolutionary history of mammalian
radiations, as well as new information on how we may
conserve these remarkable fauna.
SEE ALSO THE FOLLOWING ARTICLES
Convergence / Endemism / Lemurs and Tarsiers / Madagascar /
Philippines, Biology / Radiation Zone / Rodents
FURTHER READING
Burney, D. A., L. P. Burney, L. R. Godfrey, W. L. Jungers, S. M. Goodman,
H. T. Wright, and A. J. T. Jull. . A chronology for late Prehistoric
Madagascar. Journal of Human Evolution : –.
Goodman, S. M., ed. . Paysages naturels et biodiversité de Madagascar.
Paris: Muséum national d’Histoire Naturelle.
Goodman, S. M., and J. P. Benstead, eds. . The natural history of
Madagascar. Chicago: The University of Chicago Press.
Heaney, L. R. . Dynamic disequilibrium: a long-term, large-scale
perspective on the equilibrium model of island biogeography. Global
Ecology and Biogeography : –.
Jansa, S., K. Barker, and L. R. Heaney. . Molecular phylogenetics and
divergence time estimates for the endemic rodents of the Philippine
Islands: evidence from mitochondrial and nuclear gene sequences. Systematic Biology : –.
Steppan, S., C. Zawadski, and L. R. Heaney. . Molecular phylogeny
of the endemic Philippine rodent Apomys and the dynamics of diversification in an oceanic archipelago. Biological Journal of the Linnean
Society : –.
Woods, C. A., and Florence E. Sergile, eds. . Biogeography of the West
Indies: patterns and perspectives, nd ed. Boca Raton, FL: CRC Press.
MANGROVE ISLANDS
mangroves form a part of the island vegetation. Those mangrove islands consisting only of mangroves commonly occur
where sediments have built up sufficiently to form an island
at low tide, which has then been colonized by mangroves
(Fig. ). These are known as overwash mangroves as the
islands are completely submerged at high tide, and only the
mangroves are visible. Such overwash forests are commonly
associated with deltas of rivers carrying high terrigenous silt
loads (e.g., the Meghna–Brahmaputra delta of Bangladesh
or the Fly River delta of Papua New Guinea) or in shallow
waters where abundant sediments of biogenic carbonates
accumulate (e.g., the overwash mangrove islands of the
Florida Keys). Clearly, however, the mangrove vegetation
will differ structurally and functionally in the deltaic, terrigenous setting from that of the carbonate setting.
FIGURE 1 Overwash mangroves on carbonate shoals in the Ras
Mohammed National Park in the northern Red Sea, Egypt. These man-
PETER SAENGER
Southern Cross University, Lismore, Australia
Mangrove islands are islands composed entirely or partially of mangrove vegetation and comprise microcosms of
generally high productivity, high biodiversity, and structurally complex habitats in the nearshore environment.
Such mangrove islands are largely confined to the tropics, with their latitudinal limits occurring in those regions
where water temperatures never exceed  °C throughout
the year. In comparison to shoreline mangroves, the vegetation of mangrove islands is subjected to much greater
fluctuations in hydrological and meteorological conditions, resulting in its high dynamism and leading to its
enhanced susceptibility to drastic change.
MANGROVE ISLAND TYPES
Mangrove islands can be subdivided into two broad types:
islands consisting only of mangroves and islands where
groves consist of only one species, Avicennia marina, whose pneumatophores form a 20–30-cm-high dense fringe to the stand.
The second type of mangrove islands, where mangroves
form a significant but non-exclusive part of the island vegetation, are usually associated with coral reef islands. There
mangroves may grow directly over a reef flat or a fossil reefal
platform, or in the lee of shingle ramparts, which provide
protection from trade winds. Mangrove islands on reef flats
are common off the coast of Central America (e.g., Belize)
and on many islands of the Pacific (e.g., Palau and Ponape),
where low energy conditions and relatively low tidal ranges
occur. Although such mangroves are generally more extensive on “high islands”—which are not included here as
mangrove islands—they are also common on “low islands”
(e.g., the atoll islands of Nui, Vaitupu, and Nanumanga of
Tuvalu). On the Great Barrier Reef of Australia, persistent
southeasterly trade winds occur during April to November,
resulting in shingle ramparts on the windward, southeastern shores of these “low islands” and leeward sand cays with
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intervening mangroves in the lee of the shingle. These are
referred to as “low wooded islands” and occur commonly
in northeastern Queensland, north of ° S. Low wooded
islands display a complex range of features, which occupy
around –% of the reef flat. The windward shingle banks
are often partially colonized by mangrove scrub (e.g., Avicennia marina, Aegialitis annulata, Pemphis acidula) and other
halophytes (e.g., Sarcocornia, Sesuvium, Suaeda), and moats
form to the leeward of the shingle ramparts, which retain
water at low tide and allow micro-atoll growth (commonly
of Porites andrewsi) to occur. The micro-atolls grow upward
to mean low water level, and provide a convenient platform
for the anchoring of drifting propagules and the subsequent
growth of mangroves, which ultimately cover the entire moat
with a closed canopy mangrove forest. On the Great Barrier
Reef, the primary colonizing species is Rhizophora stylosa,
although other species, such as Avicennia marina and Lumnitzera racemosa, quickly follow. In the northern Great Barrier Reef, closed canopy mangroves on low wooded islands
consist of around  species. Because of the small catchment
areas of these islands, freshwater availability is restricted,
and mangrove communities on low wooded islands are
restricted to regions where the annual rainfall exceeds 
mm. Low Isles, off Cooktown in North Queensland, is the
most studied low wooded island on the Great Barrier Reef,
being originally surveyed by the – Great Barrier
Reef Expedition, which spent a year surveying the biota of
this island. The island and its mangroves have been resurveyed in , , , and , and over that period, the
Rhizophora woodland has increased markedly. Some, but
not all, other low wooded islands also showed mangrove
expansion, and it seems that reef-top topography and the
extent of micro-atoll formation appear to be the major factors in the extent of mangrove development. Cyclones are
an annual occurrence in this area, and destructive winds can
arrest or even reverse mangrove expansion on low wooded
islands for considerable periods.
flocculate and are deposited when mixed with seawater.
As a result, nutrient-rich sediments are deposited around
mangrove pneumatophores and stiltroots on mangrove
islands, and luxuriant and diverse mangrove communities rapidly develop. Organic-matter production is generally washed out of the system (strongly outwelling) and
replenished by the river-borne nutrient supply. In turn,
the outwelling of organic matter trophically supports
nearshore food webs, particularly for molluscs, penaeid
shrimp, and juvenile fish.
By way of contrast, reefal settings bear the brunt of
the tides, which are generally full-strength seawater.
Only biogenic sediments are available, brought in by
daily tides (bidirectional flux), and nutrients are generally deficient. As a consequence, mangroves are slowed
in terms of their organic-matter production, nutrient
coupling is usually tight, and organic matter tends to
accumulate amongst the vegetation. Reefal mangrove
islands are susceptible to erosion, particularly around
their periphery, and tend to have less luxuriant mangrove vegetation, often wind-pruned and limited in
species richness to those species capable of growing in
full-strength seawater. The tendency toward erosion,
however, may be offset in this setting, as root material
does not break down as rapidly as in deltaic systems and
may accumulate as mangrove peat, which facilitates vertical accretion and habitat stability. In Central America,
in particular, wherever coring of reefal mangrove islands
has been undertaken, peat layers of up to  m in depth
have been found, and carbon dating suggests that the
mangrove communities were initiated through rising sea
levels around  years ago and have accumulated peat
at a rate that allowed them to keep pace with the rising
sea levels of the late Holocene. On the Great Barrier
Reef, the mangrove communities of low wooded islands,
less reliant on peat accumulation than on micro-atoll
growth, were initiated around  years ago.
ENVIRONMENTAL SETTINGS
OF MANGROVE ISLANDS
ECOLOGICAL SERVICES PROVIDED
BY MANGROVE ISLANDS
As outlined above, mangrove islands occur in two contrasting settings: in river mouth deltas and on reefal platforms. Clearly, the deltaic setting is river-dominated, and
the reefal setting is tide-dominated. These dominating
factors result in significant differences in the structure
and functioning of mangrove islands.
In the deltaic setting, mangrove islands are flooded by
river water as well as by tides, so salinity remains moderate.
Silt and nutrients are carried by the river water, but they
Mangrove islands play an important role in stabilizing
and protecting shoaling sand and mud flats. Their roots
bind the sediments, and their stiltroots and pneumatophores reduce the water velocity around them, leading to
the further deposition of sediments.
Mangrove islands, particularly those associated with deltas, tend to be strongly outwelling. Organic matter is broken down by various biotic and physical process into small
particles, and this detritus supports a range of dependent
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Stoddart, D. R. . Mangroves as successional stages, inner reefs of the
northern Great Barrier Reef. Journal of Biogeography : –.
Woodroffe, C. D. . Pacific Island mangroves: distribution and environmental settings. Pacific Science : –.
MARIANAS, BIOLOGY
GORDON H. RODDA
U. S. Geological Survey, Fort Collins, Colorado
FIGURE 2 Brown pelicans and darters roost in the canopy of Avicennia
germinans on reefal mangrove islands in Quintana Roo, Mexico.
Although such roosting aggregations may provide significant nutrient enrichment, it does not offset the physical damage they do to the
upper canopy.
nearshore species, including penaeid shrimp (prawns) and
detritivorous fish, such as mullet (family Mugilidae) and
bream (family Sparidae). Many commercial and subsistence
fisheries are focused in and around such mangrove islands.
Finally, mangrove islands constitute structurally complex habitats that provide roosting and nesting sites, particularly for bats and seabirds. The surrounding waters,
the mangrove vegetation, and the mangrove-associated
invertebrates provide a diverse source of food for such nesting and roosting aggregations. On the Great Barrier Reef,
around  species of seabirds nest on low wooded islands;
one species, the Torres Strait pigeon (Myristicivora spilorrhoa), relies on such mangrove islands in its annual migrations between Australia and Papua New Guinea. Although
such nesting aggregations in the reefal setting bring much
needed nutrients, this comes at a physical cost in that dense
aggregations of seabirds damage the mangroves (Fig. ).
SEE ALSO THE FOLLOWING ARTICLES
Climate on Islands / Coral / Great Barrier Reef Islands,
Biology / Hurricanes and Typhoons / Hydrology / Tides
FURTHER READING
Cintrón, G., A. E. Lugo, D. J. Pool, and G. Morris. . Mangroves of
arid environments in Puerto Rico and adjacent islands. Biotropica :
–.
Hopley, D. . The geomorphology of the Great Barrier Reef: Quaternary
development of coral reefs. New York: John Wiley & Sons Inc.
Macintyre, I. G., M. A. Toscano, R. G. Lighty, and G. B. Bond. .
Holocene history of the Mangrove Islands of Twin Cays, Belize, Central America. Atoll Research Bulletin : –.
Saenger, P. . Mangrove ecology, silviculture and conservation. Dordrecht,
Netherlands: Kluwer Academic Publishers.
The Mariana archipelago is a line of small oceanic islands in
the tropical northern Pacific Ocean about  km east of
the Philippine Islands. Because of its remoteness from Melanesian and Asian source areas, its small land area, its limited
elevational range, and the sparse opportunities for faunal
exchange among islands, the native biota of the Mariana
Islands is relatively depauperate (species-poor), and native
vertebrates are limited to species that can fly (birds, bats) or
raft on floating vegetation and withstand contact with seawater (small lizards). Many of these vertebrates have been
extirpated, and all but one of the Mariana endemic vertebrates have experienced range reductions consequent to the
arrival of humans and human-introduced species.
BIOGEOGRAPHY
The Mariana island chain consists of  primary islands
stretching  km from north to south and ranging in
size from -km Guam (.° N) to -km Farallon de
Pajaros (.° N). Guam is more than four times the size
of the next larger Mariana island (Saipan, at  km) and
by itself makes up over half the land area of the Marianas.
Guam is also the largest island in Micronesia, the cluster
of mostly tiny islands (hence the “micro” in Micronesia)
stretching more than  km east–west across the vast
area of tropical northern Pacific Ocean west of the International Date Line.
The Mariana Islands arose from the depths of the
ocean, in response to melting of the Pacific plate as it
pushed under the Philippine plate. Accordingly, none
of the islands has ever been connected to a continental
land mass or to its neighboring islands. The southern
arc of islands (Guam to Farallon de Medinilla) arose at
about the same time, during a land-building period that
occurred from  to  million years ago. Although seismically active, the southern arc has experienced no volcanism in the modern era.
MARIANAS, BIOLOGY
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