Coral reefs seem to defy many of the paradigms which
characterise less complex biological communities. While there is
general agreement that the biota of coral reefs exhibit high
species diversity, some authors have characterised coral reef
assemblages by selecting species with high population densities
(Sale, 1974; 1976; 1977; 1984; Sale and Dybdahl, 1975; Connell,
1978). Other authors have included rarer species (Kohn, 1959;
1968; Den Boer, 1971; Grassle, 1973) and Endean and Cameron (1990
a) have emphasised the importance of the role of these rarer
species and stated that rarity is virtually ignored in most
ecological models of the coral reef ecosystem. They suggest that
our understanding of coral-reef ecology is influenced strongly by
the constraints of many of the analytical tools being used in
reef studies. As a result they believe that most analyses have
dealt primarily with species that are sufficiently numerous to
provide statistically satisfactory numbers of records and that
most studies have excluded rare species which, in fact comprise
the majority of coral-reef species.
The complexity of coral reef ecosystems is not surprising given
the great length of time that these ecosystems have been in
existence. While the shallow water distribution of coral reefs
has varied with the alternation of glacial and interglacial
periods (Hays, Imbrie and Shackleton, 1976), in their broad
biological form, coral reefs have existed since the Precambrian
and reefs similar to present reefs have existed for around 50
million years (Newell, 1972). While stating that there is no
general rule for coral-reef organisms, Endean and Cameron (1990
a) have suggested that the attribute of persistence possessed by
most of the rarer species characterises the majority of
coral-reef species and is responsible for both structuring and
perpetuating this ecosystem. They regard the coral reef ecosystem
as being an ordered and predictable system. However, other
authors (Sale, 1977; 1991; Connell, 1978) have different views.
Sale (1991) regards reef fish communities as open non-equilibrial
systems with living space determined in a random manner. Connell
(1978) regards intermediate levels of disturbance as essential to
the maintenance of diversity in this and other highly diverse and
complex ecosystems. There has been much discussion of the meaning
of stability (MacArthur, 1955; Dunbar, 1960; Leigh, 1965; May,
1972; Jacobs, 1974; Margalef, 1974; Goodman, 1975; Peters, 1976;
Pimm, 1984).
Endean and Cameron (1990 a) have put forward the hypothesis that
complex, high diversity assemblages of coral-reef animals are
characterised by a preponderance of rare but long-lived species
that they have termed persisters. These persistent species
exhibit low recruitment, low adult mortality and relative
constancy of adult population numbers and population structure.
They occur in association with opportunist species that have high
recruitment, a high adult mortality and varying adult population
numbers and population structure. While individuals belonging to
opportunist species are more abundantly represented than those
belonging to persistent species, Endean and Cameron believe that
the majority of species in the coral reef ecosystem are
persistent species. This hypothesis has not been tested in the
field.
As no general consensus relating to the organisation of coral
reefs has been reached in the literature, the persister /
opportunist distinction is examined in this thesis, rather than a
deep analysis of the opposing views relating to stability. Events
that are stochastic and unpredictable at one spatial or temporal
scale may be predictable at another. In addition, the stability
or otherwise of any system may be determined, amongst other
things, by the particular set of species that is chosen to
characterise the system.
The starfish fauna of coral reefs can be distinguished from the
starfish fauna of surrounding waters (Endean, 1953; 1965) and
coral-reef starfish may be regarded as an ecological entity.
During studies of Queensland echinoderms, Endean (1953; 1957;
1961; 1965) found 18 species of starfish on Heron Reef. Although
reference was made to the habitat, general abundance and
biogeography of each of the species, no detailed study of the
Heron Reef starfish assemblage was made. This study will compare
a number of ecological parameters in several species of starfish
occurring on this coral reef. The population stability of the
less abundantly represented, persistent species will be
contrasted with that of the more abundantly represented
opportunistic species. For the purposes of this study, the
population stability of each species refers to the constancy of
its population size structure over time.
Clark and Rowe (1971) and Yamaguchi (1975 b) reviewed the
geographic distribution of many coral-reef starfish. It is clear
that specimens of some species are frequently encountered and
appear to be relatively common while others are known from very
few specimens and appear to be extremely rare. The ecological
requirements of coral-reef starfish, as well as the role of both
rare and common species, are not understood and it is not known
whether rarity is a survival strategy, an abundance limit imposed
by predators or a failure in competitive ability of a species on
its path to extinction. These problems have not been addressed
for asteroids or any other taxonomic group within the highly
diverse and complex coral reef ecosystem.
It has been suggested that longevity may characterise species of
predictable environments (Frank, 1968; Grassle, 1973) or species
with unpredictable pre-reproductive survival (Ebert, 1982;
Goodman, 1974; Murphy, 1968). Several authors (Frank, 1969;
Grassle, 1973; Ebert, 1982) have found many coral-reef animals to
be long lived and Endean and Cameron (1990 a) regard the
long-term persistence of individuals at given sites as an
ordering phenomenon in the coral reef ecosystem. Little
information is available on the longevity of coral-reef starfish.
Ebert (1983), Kenchington (1976), Cameron and Endean (1982) and
Endean and Cameron (1990 b), believe that Acanthaster planci is a
long lived species, but Lucas (1984) suggested individual
senescence in this species at an age of approximately five years.
Stump and Lucas (1990) reported a linear growth pattern in aboral
spine ossicles of this species which supported this suggestion,
however the maximum age of this species has now been re-evaluated
to at least 12-15 years (R.Stump, Ph.D. thesis). Yamaguchi and
Lucas (1984) demonstrated a short lived population structure in
the small and cryptic starfish Ophidiaster granifer, but little
is known of the longevity of other species of coral-reef
starfish.
The severe effects of Acanthaster planci predation are well
documented (Chesher, 1969 a,b; Endean, 1969) and the change in
coral population structure following an A. planci population
outbreak was reported by Cameron, Endean and Devantier (1991).
Moran (1986) has compiled a bibliography on the Acanthaster
planci population outbreak phenomenon. Research on temperate
starfish species that undergo population outbreaks has been
reviewed by Loosanoff (1961).
Little is known of the other coral-reef starfish species, and the
reproductive patterns, population stability and diversity of
starfish assemblages on reefs that have not carried population
outbreaks of Acanthaster planci are poorly understood. Heron Reef
is such a reef. It is a Marine National Park and is situated near
the southern end of the Great Barrier Reef.
It should be appreciated that the number of species recorded in
any study is determined by both the spatial and temporal scales
of sampling as well as by the distribution and composition of the
species in the assemblage (species richness or diversity). To
allow some degree of standardisation for collection effort, the
rate at which the number of species in a sample increases with
area of the sample (the species-area relationship) and the range
of abundances within this assemblage (species relative abundance)
have been chosen as a more representative measure of species
richness than the total number of species. The species-area
relationship, relative species abundance, and constancy of
numbers and mean size (population stability) of coral-reef
starfish are unknown elsewhere.
The fact that this study was undertaken on a reef that appeared
to have low starfish abundance (and was not known to have carried
an Acanthaster outbreak) generally precluded small-scale analyses
that were dependent on high population densities. Large-scale
traverse sampling is analogous to manta tows that have been used
to monitor populations of Acanthaster. This scale of sampling is
useful to establish a general pattern of starfish abundance, but
is not capable of providing detailed data on either microhabitat
partitioning or small-scale abundance. It should provide a basis
for future comparison with data from Heron and other reefs.
Sexual reproductive patterns have been studied in some of the
coral-reef starfish species known to occur throughout the
Indo-West Pacific. Most of these studies have been conducted on
reefs that are known to have carried population outbreaks of
Acanthaster planci (Yamaguchi, 1973 a,b; 1974; 1975 b; 1977 a;
Yamaguchi and Lucas, 1984). In addition to providing reproductive
data for these species from a reef that does not undergo such
population outbreaks, this study will examine the reproductive
patterns of previously unstudied species. When the timing and
extent of sexual reproduction along with the type of larval
development exhibited by the various species studied are
correlated, inferences can be drawn regarding the reproductive
effort and dispersal capacity of each species involved. Endean
and Cameron (1990 a) have suggested that opportunists and
persisters are basically different with respect to their rates of
recruitment, and a pattern should emerge when data on
reproduction and population structure for a number of coral-reef
starfish species are compared.
Several species of coral-reef starfish are known to exhibit
asexual reproduction. The extent of asexual reproduction in the
population maintenance of each species is an indication of the
adaptive significance of this low-dispersal reproductive
strategy. Many authors have commented on the role that may be
played by this form of reproduction (Rideout, 1978; Yamaguchi,
1975 b; Ottesen and Lucas, 1982; Yamaguchi and Lucas, 1984) and
Endean and Cameron (1990 a) have suggested that this mode of
reproduction may assist species to withstand disturbance. Most
species of starfish cannot reproduce asexually but are still
capable of great powers of regeneration. Missing limbs in species
that do not reproduce asexually may indicate sub-lethal
predation.
Recruitment, migration and mortality ultimately determine the
spatial and temporal distributions of the starfish populations in
the Heron Reef assemblage. There is a distinction to be drawn
between reproduction and recruitment as well as between predation
and mortality. Recruitment is a process that is complete only
when an offspring reaches maturity and reproduces itself.
Similarly, predation may only be sub-lethal and autotomised limbs
may be regenerated or may become asexual recruits. Starfish
mortality occurs only when all fragments of a starfish have died.
For logistical reasons, it was decided not to examine potential
predators in this study. Likewise, a detailed examination of
larval settlement processes was not undertaken. Migration of
starfish is poorly understood as there is considerable difficulty
in relocating tagged specimens particularly in autotomous
species.
However, the interaction between the major determinants of
population size mentioned above will influence the size-frequency
distributions of each species. These distributions will be
compared over time at Heron Reef and with size-frequency data
from other localities. Mean individual size will vary with
periods of recruitment and mortality, and size-frequency
distributions that are constant over a study period of several
years will suggest stability within the age structure.
Alternately, such a finding could reflect the apparently static
nature of a long-lived species when observed on a comparatively
short time scale, even one of several years. However, study of
the latter alternative could not be pursued beyond the time frame
of this study which embraced five years.
Knowledge of the spatial pattern, fecundity and population
dynamics of each of the coral-reef starfish species represented
is essential to an understanding of the stability or otherwise of
the populations of species comprising the coral-reef asteroid
assemblages of Indo-West Pacific reefs. This knowledge is also
essential to an understanding of outbreak phenomena, such as
population outbreaks of Acanthaster planci. The obtaining of
comparative distribution and reproductive data on many starfish
species from Heron Reef will clarify the factors that influence
diversity and stability within this assemblage.
With these broad aims in mind, this study focused on Heron Reef
and sought answers to the following questions:
What starfish species are present at Heron Reef?
What is the spatial pattern for each species?
What is the population structure of each species?
What is the reproductive mode of each species?
Is the mean individual size stable for each species?
How is abundance distributed within this assemblage?
Thesis to be defended
In this study of the shallow-water asteroid assemblage of Heron
Island reef, an Indo-West Pacific coral reef that has not been
known to carry an outbreak of Acanthaster planci and hence can be
regarded as a reef that has not been subject to a major
disturbance at least in the immediate past, the thesis to be
defended is:
1. The asteroid assemblage is comprised of numerous persistent
species and a smaller number of opportunistic species.
2. The persistent species are relatively uncommon (rare) and
possess relatively stable population densities and population
size structures and have low rates of recruitment.
3. The opportunistic species exhibit localised high density,
significant population fluctuations and are characterised by high
recruitment (either sexual or asexual).
Site of study
Heron Reef (23° 27' S, 151° 57' E) lies in the Capricorn Group
which is towards the southern end of the Great Barrier Reef. It
is a lagoonal platform reef with a vegetated cay at its western
end (Figure 1). The cay supports a tourist resort and research
station. Heron Reef has been zoned as Marine Park A within the
Capricornia Section of the Great Barrier Reef Marine Park, and
prior to this was protected, from over-collection, by a
regulation of Queensland State Fisheries. The western end of the
reef is easily accessible from the cay but access to the eastern
end requires the use of a small boat.
The major habitat zones used in the present study are described
in detail by Jell and Flood (1978).
These zones are: 1. Reef flat (with lagoon)
2. Reef crest or reef rim
3. Reef slope
4. Off-reef floor
At the western end of Heron Reef, where studies were made, the
reef flat is the sub-tidal habitat nearest to the cay. It is
chiefly comprised of dead and living coral clumps which vary in
size from a few centimetres in diameter to dead coral boulders or
living micro-atolls with diameters of several metres. The dead
coral clumps can, at certain times of the year, be obscured by a
prolific growth of algae. The chief physical parameter that
separates the reef flat from the lagoon is the water depth at low
water spring tides. The water depth can vary from less than half
a metre at the western end of the reef where sedimentation is
great to more than a metre at its transition into lagoon east of
the cay. The lagoon is up to six meters in depth at Heron Reef
and has scattered coral outcrops which may reach the surface. It
is regarded as an extension of the reef flat for the purposes of
this study. At the innermost part of the reef flat (adjacent to
the cay) a series of strata composed of cemented sand and coral
fragments occurs. The strata are called beachrock.
The reef crest is the outer region of intertidal coral growth and
is shallower than the previous zone. It is the most turbulent of
all coral-reef zones being exposed to direct wave action at all
stages of the tide. It has little fine sediment other than that
which is trapped within the algal turf and which has accumulated
under boulders. Living coral growth is usually low in profile and
the general substrate is comprised of cemented reef rock strewn
with broken coralline material. This material ranges in size from
single coral fragments which are a few centimetres in diameter,
to large boulders that are greater than two meters in diameter.
The reef slope is subtidal and supports extensive coral growth to
a depth of approximately 20 meters. The coral growth tapers off
to almost negligible coral cover at a depth of approximately 30
meters where the slope merges with the off-reef floor. This
transition may be sudden on some reefs which possess almost
vertical reef slopes, but at Heron Reef the transition is
gradual. This zone is less physically controlled than are the
previous zones. After periods of severe swell there may be areas
of broken coral colonies but generally, as depth increases, the
direct effect of wave action decreases. The substrate is of
poorly sorted sediments as well as living and dead coral
colonies, together with their epibiota.
The off-reef floor between Heron Reef and the adjacent reefs is
over 40 meters deep and in places supports a well developed fauna
of alcyonarians and solitary hard corals along with their
associated epibiota. The off-reef floor is the deepest of the
reef zones and provides habitats that are clearly different from
the shallow water habitats provided by the other three zones. The
sediment found on the off-reef floor is varied and its
composition is dependent on currents as well as on surge effects
during heavy wave action.
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