Chapter 1. General Introduction
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.