and the phosphate enrichment appears related to bird
guano.
In contrast to neighboring high islands, the atolls and
low islands of the Indian and Pacific Oceans have a relatively species-poor flora. There is a distinct pantropical
group of plants that are dispersed effectively across large
distances. Many of these plants are strand plants that have
seeds that float and can be easily washed up on the beaches
around the margins of atolls. Early descriptions of the
vegetation and floristics of atolls were undertaken by
Raymond Fosberg (1974, 1992), and a major compilation on the vegetation of the tropical Pacific outlines the
principal vegetation associations (Mueller-Dombois and
Fosberg, 1998).
Several creepers can occur on the beach, such as
Ipomoea pes-caprae and Vigna marina. A coastal fringe
of shrubs develops on those sand cays that are not ephemeral, and a broad belt of Scaevola taccada and Suriana
maritima is frequent, often with Tournefortia argentea
(Argusia [Messerschmidia] argentea), which can grow
into a small tree. On the larger islands a peripheral belt
of shrubs often grades into a forest with trees such as
Cordia subcordata, Calophyllum inophyllum, Hernandia
peltata, Guettarda speciosa, and Pisonia grandis. On
many atolls the interior is covered with coconut woodland,
but in most cases this has been planted and is maintained
because the coconut (Cocos nucifera) is important for subsistence and often as the basis of a commercial copra
industry. A zonation with increasing maturity of vegetation and soils into the interior of reef islands can be recognized, often representing a temporal succession (see Coral
Cays, Vegetational Succession).
The smaller islands tend to contain the least species,
and the relationship between number of plant species
and island area, particularly based on a study of the vegetation of Kapingamarangi Atoll (Niering, 1963) provided
support for the development of ideas on island biogeography by MacArthur and Wilson (1967). However, it is also
apparent that the smallest islands, particularly those less
than 100m wide, are too small to support a freshwater lens,
and only have a restricted strand flora (Whitehead and
Jones, 1969). Although on predominantly sandy reef
islands there are more species on the larger islands, two
further factors are important. On the remote atoll of
Suwarrow in the Cook Islands, it is evident that plants
are influenced by the substrate, with extensive stands of
Pemphis acidula sprawled across outcrops of conglomerate (Woodroffe and Stoddart, 1992). Mangrove habitats
are also restricted in extent to those few locations where
it is sheltered and there is brackish water. Mangroves
may occur around the margins of a lagoon (e.g., on Nui
in northern Tuvalu, or in the more enclosed embayments
as in Vaitupu, Tuvalu, and can also be found in completely
enclosed depressions in the center of table reefs, such
as Nanumanga, Tuvalu) (Figure 7). On the islands with
settlements there are generally many introduced plants
(Woodroffe, 1985), including crops such as breadfruit
(Artocarpus), pandanus, banana and taro (Calocasia,
Cyrtosperma) and ornamentals, such as the frangipani
(Plumeria) (Dawson, 1959). Giant taros provide the traditional stable food and are planted in excavated pits. These
pits are dug until ground water level is reached. Taros are
planted just above the groundwater level and surrounded
by woven coconut fronds or pandanus leaves that provide
a retaining wall for the plant compost.
On the most remote atolls, where human visitation is
minimal, reef islands can support huge colonies of seabirds. Noddy and sooty terns, boobies, and frigate birds
nest in enormous numbers, and if unused to human presence, sit unconcerned while the occasional visitor wanders
amongst them. Typically, the ecosystems which develop
on atolls are composed of widespread easily dispersed
species, and there tends to be a low degree of endemism,
presumably because islands have only appeared in the
past few millennia on most atolls. Land birds are generally
few in number, although there are flightless rails on several atolls. Endemic species are found where the atoll
has remained emergent over glacial-interglacial cycles,
and not undergone complete submergence during the
postglacial sea-level rise. Aldabra in the western Indian
Ocean is a noteworthy example with its population of
giant tortoises (Arnold, 1976; Stoddart, 1984).
Atoll hydrology
The small reef islands around the margin of atolls rarely
contain freshwater lakes (e.g., Washington in the Line
Islands). However, rainwater percolates through the soil
and can accumulate as a freshwater lens beneath the island
surface. The freshwater lens floats above seawater, and
has been characteristically described by the GhybenHerzberg principle, that is that the surface elevation of
the lens extends above the level of the sea by about 1/40
of the depth to which the lens occurs. The characteristic
shape of a Ghyben-Herzberg lens is rarely actually found
beneath reef islands because of significant variations in
the degree and extent of lithification of the reefal material
and in porosity of the limestone (see: Coral Cays-Geohydrology). A dual aquifer model has been proposed which
recognizes the significance of the greater porosity of the
Pleistocene limestone that is found beneath the rim of
most atolls (Buddemeier and Oberdorfer, 1986). The
hydrogeology of several atolls is summarized in reviews
by Falkland (1991) and Vacher and Quinn (1997). The
elevation of the surface topography influences the water
level, and the shape of the lens can also be constrained
by well-cemented conglomerate, which may confine the
aquifer, as seen in the example of the island of Deke on
Pingelap Atoll (Figure 8) in the Marshall Islands (Ayers
and Vacher, 1986).
Human impacts on atolls
The formation of habitable reef islands occurred in mid
Holocene, as described above. The history of human occupation therefore is also restricted to the late Holocene, and
depends on sea-level history and the accretion of islands
ATOLLS
63
guano.
In contrast to neighboring high islands, the atolls and
low islands of the Indian and Pacific Oceans have a relatively species-poor flora. There is a distinct pantropical
group of plants that are dispersed effectively across large
distances. Many of these plants are strand plants that have
seeds that float and can be easily washed up on the beaches
around the margins of atolls. Early descriptions of the
vegetation and floristics of atolls were undertaken by
Raymond Fosberg (1974, 1992), and a major compilation on the vegetation of the tropical Pacific outlines the
principal vegetation associations (Mueller-Dombois and
Fosberg, 1998).
Several creepers can occur on the beach, such as
Ipomoea pes-caprae and Vigna marina. A coastal fringe
of shrubs develops on those sand cays that are not ephemeral, and a broad belt of Scaevola taccada and Suriana
maritima is frequent, often with Tournefortia argentea
(Argusia [Messerschmidia] argentea), which can grow
into a small tree. On the larger islands a peripheral belt
of shrubs often grades into a forest with trees such as
Cordia subcordata, Calophyllum inophyllum, Hernandia
peltata, Guettarda speciosa, and Pisonia grandis. On
many atolls the interior is covered with coconut woodland,
but in most cases this has been planted and is maintained
because the coconut (Cocos nucifera) is important for subsistence and often as the basis of a commercial copra
industry. A zonation with increasing maturity of vegetation and soils into the interior of reef islands can be recognized, often representing a temporal succession (see Coral
Cays, Vegetational Succession).
The smaller islands tend to contain the least species,
and the relationship between number of plant species
and island area, particularly based on a study of the vegetation of Kapingamarangi Atoll (Niering, 1963) provided
support for the development of ideas on island biogeography by MacArthur and Wilson (1967). However, it is also
apparent that the smallest islands, particularly those less
than 100m wide, are too small to support a freshwater lens,
and only have a restricted strand flora (Whitehead and
Jones, 1969). Although on predominantly sandy reef
islands there are more species on the larger islands, two
further factors are important. On the remote atoll of
Suwarrow in the Cook Islands, it is evident that plants
are influenced by the substrate, with extensive stands of
Pemphis acidula sprawled across outcrops of conglomerate (Woodroffe and Stoddart, 1992). Mangrove habitats
are also restricted in extent to those few locations where
it is sheltered and there is brackish water. Mangroves
may occur around the margins of a lagoon (e.g., on Nui
in northern Tuvalu, or in the more enclosed embayments
as in Vaitupu, Tuvalu, and can also be found in completely
enclosed depressions in the center of table reefs, such
as Nanumanga, Tuvalu) (Figure 7). On the islands with
settlements there are generally many introduced plants
(Woodroffe, 1985), including crops such as breadfruit
(Artocarpus), pandanus, banana and taro (Calocasia,
Cyrtosperma) and ornamentals, such as the frangipani
(Plumeria) (Dawson, 1959). Giant taros provide the traditional stable food and are planted in excavated pits. These
pits are dug until ground water level is reached. Taros are
planted just above the groundwater level and surrounded
by woven coconut fronds or pandanus leaves that provide
a retaining wall for the plant compost.
On the most remote atolls, where human visitation is
minimal, reef islands can support huge colonies of seabirds. Noddy and sooty terns, boobies, and frigate birds
nest in enormous numbers, and if unused to human presence, sit unconcerned while the occasional visitor wanders
amongst them. Typically, the ecosystems which develop
on atolls are composed of widespread easily dispersed
species, and there tends to be a low degree of endemism,
presumably because islands have only appeared in the
past few millennia on most atolls. Land birds are generally
few in number, although there are flightless rails on several atolls. Endemic species are found where the atoll
has remained emergent over glacial-interglacial cycles,
and not undergone complete submergence during the
postglacial sea-level rise. Aldabra in the western Indian
Ocean is a noteworthy example with its population of
giant tortoises (Arnold, 1976; Stoddart, 1984).
Atoll hydrology
The small reef islands around the margin of atolls rarely
contain freshwater lakes (e.g., Washington in the Line
Islands). However, rainwater percolates through the soil
and can accumulate as a freshwater lens beneath the island
surface. The freshwater lens floats above seawater, and
has been characteristically described by the GhybenHerzberg principle, that is that the surface elevation of
the lens extends above the level of the sea by about 1/40
of the depth to which the lens occurs. The characteristic
shape of a Ghyben-Herzberg lens is rarely actually found
beneath reef islands because of significant variations in
the degree and extent of lithification of the reefal material
and in porosity of the limestone (see: Coral Cays-Geohydrology). A dual aquifer model has been proposed which
recognizes the significance of the greater porosity of the
Pleistocene limestone that is found beneath the rim of
most atolls (Buddemeier and Oberdorfer, 1986). The
hydrogeology of several atolls is summarized in reviews
by Falkland (1991) and Vacher and Quinn (1997). The
elevation of the surface topography influences the water
level, and the shape of the lens can also be constrained
by well-cemented conglomerate, which may confine the
aquifer, as seen in the example of the island of Deke on
Pingelap Atoll (Figure 8) in the Marshall Islands (Ayers
and Vacher, 1986).
Human impacts on atolls
The formation of habitable reef islands occurred in mid
Holocene, as described above. The history of human occupation therefore is also restricted to the late Holocene, and
depends on sea-level history and the accretion of islands
ATOLLS
63