to remain viable for at least 6 months. More often than not
these pantropical species are part of the initial beachfront
colonizing vegetation (Figure 5). Sauer (1982) noted 11
pantropical species on the Caymans, but many more occur
in the Pacific. For example, of the 380 species listed for
the northern GBR (Fosberg and Stoddart, 1991), 25 also
grow on the Cayman Islands though a number may be
introductions. Twenty-seven GBR species were also
found on Jamaican cays (Stoddart and Fosberg, 1991)
and 33 on the cays of Belize (Fosberg et al., 1982).
Many questions remain unanswered about coral cay
vegetation. Although apparently simple, with a classic
pattern of ecological succession, the highly successful
mechanisms of dispersal reinforce at the species level,
the global physiognomic similarities. However, even this
ability does not answer all questions. As Sauer (1982)
noted, the fossil record for this vegetation is very poor,
though clearly going back until at least the Tertiary. It
has thus become involved in plate tectonics and changes
to global land masses, especially the isthmus of Panama,
which now closes the link between Caribbean and the
Indo-Pacific. While there are possibilities of some flora
migrating around the southern tip of Africa, this would
entail a long route outside the tropics. Today’s pantropical
species may be the last vestige of the more open Tertiary
geography of meso-America.
Age, evolution, and relationships to Holocene
sea level
A foundation, usually a reef flat at or near to sea level, is
necessary for reef-island formation, but it may be small
and not yet sea level constrained (Hopley, 1997; Kench
et al. 2005). The relationship between reef growth, sea
level, and reef flat formation on a global scale is complicated by various factors including postglacial isostatic
adjustments of the continents and ocean basins, which
have produced broad geographic differences in relative
sea-level history and especially when reef tops first
approached the sea surface (Clark et al., 1978; Lambeck
et al., 2002). Pirazzoli (1991) presents detail on these patterns (see Glacio-Hydro Isostasy), but the relative sealevel curves for most of the Pacific and the Caribbean
and thus the potential onset of cay formation in these
two provinces are very different. Modern sea level in the
former was reached 6,000–5,000 years ago, in places rose
1–2 m higher and has then fallen to present, whereas in the
Caribbean it has been rising throughout the Holocene but,
slowing over the past few thousand years. Modern sea
level has only recently been reached (Toscano and
Macintyre, 2003). Further, at a more regional scale, isostatic influences driven by differential loading by
postglacial transgressive seas can produce variable relative sea-level histories as deeper shelf edge areas subside
under water load (delaying reef flat formation) and
shallower inshore areas possibly upwarp (Hopley, 1983;
Lambeck and Nakada, 1990). Other factors that influence
when a reef reaches sea level and provides a base for
reef-island development include the depth from which
the reef has grown and the size of the reef and its lagoon.
Typically, reef islands are oldest where current sea level
was reached early – in the mid-Holocene, where the reefs
grew from shallow substrates and were thus sea level
constrained earlier, and where they grew on relatively
small reefs with smaller lagoons that were infilled more
rapidly than those of larger reefs.
The relationships described above are demonstrated on
the GBR, where stable vegetated cays are strongly associated with planar reefs; 41 of 43 are on planar reefs that
grow above shallow pre-Holocene foundations (mean:
10.8 m) and are relatively small (mean: 4.1 km
2
). Planar
reefs also have the oldest mean reef top age ($6,000
years), so that depending on where reef geometry and
hydrodynamic conditions initially focus sediment deposition, cay formation could have begun 6,000 years ago
(Hopley et al., 2007). In Hopley’s (1982) evolutionary
classification of reefs [see Reef Classification, Hopley
(1982)], planar reefs represent a senile stage approaching
the end of their growth trajectories. Significantly, most
reef islands with any degree of permanence are found on
these senile reefs though at the time of their initiation the
reef could have been at an earlier state (even reef patches)
and persisted through the mid to late Holocene period of
lagoon infilling and reef flat extension. Thus, at the geological time scale the factors that promote the advance of
reefs through the reef growth sequence are also important
drivers of cay formation. Spender’s (1930) hypothesis that
reef-island formation is strongly influenced by relative
sea-level fall and reef platform emergence was dismissed
for more than five decades (Steers, 1937; Stoddart,
1965). However, the importance of hydroisostatic
upwarping of the inner shelf in accelerating the development of reefs to planar stage and as a consequence improving reef-island accumulation and, possibly, preservation is
now generally accepted for the GBR.
Most reef islands on the GBR and all low wooded
islands are inside the zero hydroisostatic isobase. Emergent reef occurs beneath reef islands in many other settings (discussed below). Kench et al. (2005) suggested
that some Maldivian cays began to develop prior to reef
flat formation, and West Indian cays have formed where
sea level has risen gradually to present since the midHolocene (Woodroffe, 2003). Radiocarbon-dated fossil
microatolls underlie many of the reef islands which have
been investigated on the GBR, suggesting that most of
them developed after the reefs had reached sea level (see
Hopley et al., 2007 chapter 10), but the occurrence of
$75 unvegetated cays on platforms close to sea level suggests that emergent reef platforms are not absolutely necessary. Reef islands on the Cocos (Keeling) Islands, Indian
Ocean are also deposited over fossil reef flat that are made
emergent by the falling late-Holocene sea levels
(Woodroffe et al., 1999). Dickinson (2004) revisited the earlier ideas of Schofield (1977) and proposed that waveresistant emergent palaeoreef flats strongly influenced the
development of stable reef islands on many Pacific atolls,
246
CORAL CAY CLASSIFICATION AND EVOLUTION
these pantropical species are part of the initial beachfront
colonizing vegetation (Figure 5). Sauer (1982) noted 11
pantropical species on the Caymans, but many more occur
in the Pacific. For example, of the 380 species listed for
the northern GBR (Fosberg and Stoddart, 1991), 25 also
grow on the Cayman Islands though a number may be
introductions. Twenty-seven GBR species were also
found on Jamaican cays (Stoddart and Fosberg, 1991)
and 33 on the cays of Belize (Fosberg et al., 1982).
Many questions remain unanswered about coral cay
vegetation. Although apparently simple, with a classic
pattern of ecological succession, the highly successful
mechanisms of dispersal reinforce at the species level,
the global physiognomic similarities. However, even this
ability does not answer all questions. As Sauer (1982)
noted, the fossil record for this vegetation is very poor,
though clearly going back until at least the Tertiary. It
has thus become involved in plate tectonics and changes
to global land masses, especially the isthmus of Panama,
which now closes the link between Caribbean and the
Indo-Pacific. While there are possibilities of some flora
migrating around the southern tip of Africa, this would
entail a long route outside the tropics. Today’s pantropical
species may be the last vestige of the more open Tertiary
geography of meso-America.
Age, evolution, and relationships to Holocene
sea level
A foundation, usually a reef flat at or near to sea level, is
necessary for reef-island formation, but it may be small
and not yet sea level constrained (Hopley, 1997; Kench
et al. 2005). The relationship between reef growth, sea
level, and reef flat formation on a global scale is complicated by various factors including postglacial isostatic
adjustments of the continents and ocean basins, which
have produced broad geographic differences in relative
sea-level history and especially when reef tops first
approached the sea surface (Clark et al., 1978; Lambeck
et al., 2002). Pirazzoli (1991) presents detail on these patterns (see Glacio-Hydro Isostasy), but the relative sealevel curves for most of the Pacific and the Caribbean
and thus the potential onset of cay formation in these
two provinces are very different. Modern sea level in the
former was reached 6,000–5,000 years ago, in places rose
1–2 m higher and has then fallen to present, whereas in the
Caribbean it has been rising throughout the Holocene but,
slowing over the past few thousand years. Modern sea
level has only recently been reached (Toscano and
Macintyre, 2003). Further, at a more regional scale, isostatic influences driven by differential loading by
postglacial transgressive seas can produce variable relative sea-level histories as deeper shelf edge areas subside
under water load (delaying reef flat formation) and
shallower inshore areas possibly upwarp (Hopley, 1983;
Lambeck and Nakada, 1990). Other factors that influence
when a reef reaches sea level and provides a base for
reef-island development include the depth from which
the reef has grown and the size of the reef and its lagoon.
Typically, reef islands are oldest where current sea level
was reached early – in the mid-Holocene, where the reefs
grew from shallow substrates and were thus sea level
constrained earlier, and where they grew on relatively
small reefs with smaller lagoons that were infilled more
rapidly than those of larger reefs.
The relationships described above are demonstrated on
the GBR, where stable vegetated cays are strongly associated with planar reefs; 41 of 43 are on planar reefs that
grow above shallow pre-Holocene foundations (mean:
10.8 m) and are relatively small (mean: 4.1 km
2
). Planar
reefs also have the oldest mean reef top age ($6,000
years), so that depending on where reef geometry and
hydrodynamic conditions initially focus sediment deposition, cay formation could have begun 6,000 years ago
(Hopley et al., 2007). In Hopley’s (1982) evolutionary
classification of reefs [see Reef Classification, Hopley
(1982)], planar reefs represent a senile stage approaching
the end of their growth trajectories. Significantly, most
reef islands with any degree of permanence are found on
these senile reefs though at the time of their initiation the
reef could have been at an earlier state (even reef patches)
and persisted through the mid to late Holocene period of
lagoon infilling and reef flat extension. Thus, at the geological time scale the factors that promote the advance of
reefs through the reef growth sequence are also important
drivers of cay formation. Spender’s (1930) hypothesis that
reef-island formation is strongly influenced by relative
sea-level fall and reef platform emergence was dismissed
for more than five decades (Steers, 1937; Stoddart,
1965). However, the importance of hydroisostatic
upwarping of the inner shelf in accelerating the development of reefs to planar stage and as a consequence improving reef-island accumulation and, possibly, preservation is
now generally accepted for the GBR.
Most reef islands on the GBR and all low wooded
islands are inside the zero hydroisostatic isobase. Emergent reef occurs beneath reef islands in many other settings (discussed below). Kench et al. (2005) suggested
that some Maldivian cays began to develop prior to reef
flat formation, and West Indian cays have formed where
sea level has risen gradually to present since the midHolocene (Woodroffe, 2003). Radiocarbon-dated fossil
microatolls underlie many of the reef islands which have
been investigated on the GBR, suggesting that most of
them developed after the reefs had reached sea level (see
Hopley et al., 2007 chapter 10), but the occurrence of
$75 unvegetated cays on platforms close to sea level suggests that emergent reef platforms are not absolutely necessary. Reef islands on the Cocos (Keeling) Islands, Indian
Ocean are also deposited over fossil reef flat that are made
emergent by the falling late-Holocene sea levels
(Woodroffe et al., 1999). Dickinson (2004) revisited the earlier ideas of Schofield (1977) and proposed that waveresistant emergent palaeoreef flats strongly influenced the
development of stable reef islands on many Pacific atolls,
246
CORAL CAY CLASSIFICATION AND EVOLUTION