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Volcanic type islands built by volcenic activity and therefore with substrateg derived from
lava (usuelly basalt) and volcanic ash.
Ebvated reefg - ielande or parts of islandg compoeed of raised coral platforms or limestone.
Lcnr islande - eomposed of sand and coral rubble accumulated m
sea-level. This is the typical type m atolls and barrier
occutr es coastal or beach areas on other island types.
The islands in turn rnay be flanked by one or a eombination of several reef structures.
Following Dahl (1980), these may be described as follows. An algal reef is a calcareoug
structure in which corelline algae are principal eontributors to reef Lonstruction end s:rface
eover' A coral reef, m the other hand, is a structure actively constructed by the skeletal
deposition of hermatypic corals and associated organisms. Corai reefe in turn take the form
of:
Atoll reefs - annular reefs generally with an internal lagoon unassociated with any major
landmass.
Barrier reefs - reefs offshore from a major land mass and
or navigable channel.
separaled from it by a deep lagoon
Fringing reefs - reefs growing directly out fnom the coastline and not separated from it by
more than a shallow depression.
Lagoon reefs or patch reefs - reef structures developing in the sheltered waters of a lagoon.
A rrnon-growing" reef is a calcareous structure now covered with organisms not
contributing significantly to skeletal accumulation or reef qrowth. Subrnerged reefs are those
"drowned" by zubsidence to depths below which reef qro;tn has been insufficient to regain
the zurface.
.
Similarly, the marine environment may be characterized by different structural criteria
(Dahl, 1980). Substrates in different areas may be rocky (calcareous or non-calcareous) or
uneonsolidated. In addition, reef community structure would depend on whether the island is
subrnerging, emerging or +parently stationary relative to sea level, and whether the setting
is exposed or protecLed with respect to lhe physical forces of lhe sea. These factors are all
part of lhe variability of the physical and chemical environnent of island ecosystems (whicfr
will be discussed in the following section), and which contribuLe signif icantly to the
uniqueness of each island.
,
a reef platform at or near
reefs, and also frequently
Descriptions of reef structune and nnrphology in the tropical Pacific are relatively
few. ssne studies that may be nrentioned are thosl m lhe cook lslands (Gibbs et al., r97l;
Stoddart and Pillai, I97J)i Fanning lsland (Maragos, 1974a and b; Roy and Shith,-197I);
French Polvnesia (chevalier, 1975b; Ricard 4 4.; thl); rhe Lau tslandi in Fiji (phipps and
Preobrazhensky, 1975); the Marshall Islands-(L-acid, 1975; MacNeil, Lgiz)i the New Hebrides
(now Vanuatu) in M_elanesia (Guilcher, 1974); New caledonia (chevalier, '197)a); p+ua r.rlw
Guinea (Weber, I975b; Whitehouse, I97)); Samoa (Mayor, L9Z4); and 'the iotornon Islands
(Jmes, 1977; Morton, 1974; Weber, L97Ja).
The physical structure of an island is a major determinant of the types of ecosystems
present (Dahl' f980)- A elear example may be found in the case of islands wilh exposed end
protected sides, such as atolls. Coral reefs tend to be npre robust and better developed on
the exposed sides due to rnore adequate wat.er circulation and greater availabiiity of
nutrients. In addition, reef forms and lagoon types are determined by the structure and
history of the island substrate (Dahl, I9S0). In general, Lherefore, a broad distinction can be
made among continental islands, high volcanic islands, elevated reefs, and low coral islands,
but it is also necessary to consider the delailed origin and structure of each, and its
geographic location in terms of the origin and evolution of flora and fauna (E1io[t, Lg;-J).
Dasmann (1971) attributes lhe uniqueness of island ecosystems to their relative isolation
which induces peculiar evolutionary patlerns in each separate setting. Differences among
similarly situated neef habitats are manifested, for example, in t6e differing rates oi
productiviiy of at.oll lagoons in the South pacific (Ricard and belesalle, lggl).