Introduction to initial thesis of 1985.
The primary hindrance to any creative synthesis is the failure to recognize the causes of dissension in science. In some cases a critic may doubt the integrity of either the scientist or the data. In these cases further repetition of experiments may alleviate skepticism. In other cases, there may be a philosophical disagreement which has its basis in differing beliefs held by scientists. These preconceptions, which all scientists possess, can severely limit the process of conciliation. Dissension, however caused, is not easily resolved, and often leads to polarization within science. The failure of science to recognize and acknowledge explicitly, the validity of differing viewpoints based on the same data demonstrates confusion over these two causes of dissension.
Additionally, our aversion to implausible conjecture has limited the diversity of models that can be open to empiric testing. It must be remembered that, when seen through the eyes of a different culture, a model held to be true universally might appear implausible. An example is our astonishment at the complexity of Micronesian fishhooks which have been individually crafted to capture specific types of fish (Johannes, 1981). The pattern of relative planetary movements in our solar system was simplified greatly when the geo-centric model was discarded, in favor of a model in which the planets moved in orbits about the sun (Whitlam, 1975). This and other conceptual leaps have occurred when cultural limitations were waived temporarily, and the consequently greater insight justified the cultural changes that followed. The phrase, “that is only conjecture”, seems to imply that many scientists regard conjecture as superfluous in the day to day running of science. In the pursuit of scientific rigor, we often overlook this necessary component of synthesis and find it increasingly difficult to model our thoughts in a manner that enables others to see a more distant horizon by standing on our shoulders.
Finally, the scientific process might not progress beyond the accumulation of facts. Often it appears impossible, from the data, to do more than simplify nature’s variability by statistics, to categorize its variety by description or to model its behavior mathematically. These techniques should be tools to further understanding of the interrelationships between the elements under study. To many workers the apparently stochastic nature of many phenomena prohibits a more detailed analysis.
In recent years there have been many studies in coral reef ecology, biology and biogeography. Often, they are related, one to another, by a common principle or factor (e.g. population outbreaks of Acanthaster planci, plans for reef management or theoretical questions about the causes of diversity). Whenever several independent researchers study similar phenomena or ask similar questions, the possibility of dissension arises. The ensuing debates, and even hostility, can appear as a failure in communication between the scientists concerned. More often the observed behavior results from philosophic disagreement taking its natural course. Philosophic changes in both science and society are often inseparable; they rarely occur, figuratively speaking, without bloodshed.
Philosophic clashes are likely to occur whenever the capacity to acquire data, strategic to the opposing viewpoints, approaches a limit imposed by logistics or technology. This is certainly the case with much of the ecological debate over population outbreaks of Acanthaster planci. The failure to recognize the symptoms of this fundamental disagreement can lead to much wasted effort, funds and, most of all, time. Often, we need to determine which observations are required to swing general support from one model to another. However, the competing models may have reached the same level of logistic untestability and there may be no logistically possible observation or series of experiments which could distinguish between the validities of either model.
There has been much debate about the relative roles of disturbance, stability and niche specialization, as factors contributing to the co- existence of the large number of species in some communities (e.g. Brown, 1981; Connell, 1978; Levins, 1963; MacArthur, 1955, 1969; May, 1972, 1974; Paine, 1966). By contrast, there has been little consideration of the possibility that these factors and associated models may be of secondary importance to community order or structure itself. The process of community succession follows a path of increasing complexity towards an hypothesized relatively static, climax community, the composition of which is determined by prevailing environmental as well as historical parameters (e.g. Dunbar, 1972). Throughout this process, as early (rapid) colonists are excluded by species that are competitively superior, the composition of the community changes. The number of species present in the community increases to a maximum at some stage of succession, prior to the climax and subsequently decreases as a result of the exclusion of inferior competitors (Connell, 1978). The extent to which disturbance, by creating spatial and temporal patches of early succession, acts to prevent the monopolization of available resources by a small number of superior competitors has been discussed extensively (e.g. Dayton, 1971; Levin and Paine, 1974). Other authors have either proposed or implied that high diversity communities are at equilibrium, and that species coexistence is mediated by the complex processes of interdependence and specialization that have evolved, in a physically benign environment, over long periods (e.g. Fischer, 1960; Sanders, 1969; and reviews by Goodman, 1975; Osman and Whitlatch, 1978; Pianka, 1966). Additionally, Jacobs (1974) and Peters (1976) have pointed out that the correspondence between stability in species composition and stage of community succession is tautological because a successional climax is defined in terms of its temporal stability.
The progressive increase in diversity, biomass, complexity and structure, resulting from the succession process, has been the focus of much discussion (e.g. Dunbar, 1972; Margalef, 1963; Odum,1969). In some instances (e.g. Sale, 1984), the co-existence of numerous species can be explained without resorting to complex, pattern oriented, models that require numerous assumptions which are testable only by prolonged, rigorous and exacting field observation. The almost universally accepted null hypothesis of “chance” does very little to enlighten biological scientists who want to understand or observe any existing inter-relationships between the species they study; that hypothesis, however, has gained a reverence totally unbecoming a statement that claims to say nothing at all (Dunbar, 1980; Roughgarden, 1983).
Any function which may be played by community order or structure has, in the past, been so secondary to the aims or objectives of the “experimental approach”, as to be uninteresting or considered logistically untestable (i.e. impractical or too difficult). However, there is a fundamental difference between a model being logistically untestable and it being logically untestable (for discussion compare Connell, 1980; Dunbar, 1980; Kuhn, 1970; Popper, 1983; Quinn and Dunham, 1983; Roughgarden, 1983; Simberloff, 1982). I do not propose that all the myths laid to rest by “Occam’s Razor” be resurrected and considered as reasonable explanations of available evidence. However, I do suggest that, in the field of coral reef ecology, our attempts to simplify the system under study have produced models that bear little relationship to reality. The rigid adherence to the least complex and ramifying hypothesis, has made it difficult to see beyond the generally accepted view of nature based on probability theory and chance
While there have been many taxonomic and biogeographic works dealing with the coral reef asteroid community (Clark, 1921, 1938, 1946; Clark and Rowe, 1971; Marsh, 1974; Marsh, 1974, 1976, 1977; Yamaguchi, 1975b), the ecological requirements of asteroid species occurring within the Indo-West Pacific region have not been studied extensively. It is known that many species occur on coral reefs throughout the region (Clark and Rowe, 1971), while others possess a more restricted distribution. Several asteroid species are known from only a few specimens and are considered to be rare (Clark, 1921; Yamaguchi, 1975b). The habitat requirements of coral reef asteroid species, and the ecological roles of rare as well as of more common species are not understood. 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 questions have not been answered for this or any other taxonomic group within the highly diverse and complex ecosystem of the coral reef.
Competition and other ecological models and corollaries draw their scientific context often, by analogy, from the corresponding pattern of interaction observed within contemporary human society. The influence of one’s cultural background in the initial perception and subsequent acceptance of the ecological generality of these analogies is overlooked often. A model is an abstraction only, but in common with all scientific models, socially analogous models can be raised, by consensus, to the status of paradigms, such that, observations which contradict the model are considered either inaccurate or implausible. Assume that the population size of some organism is limited by the level of juvenile recruitment in such a way that the density of adults is never sufficiently high for one individual to interact significantly with another (e.g. Dale, 1978; Doherty, 1982). If these assumptions were true but unknown, the interactions between adults and their ecological significance could be modeled incorrectly using competition theory. Observations, which are categorized within a severely limited body of theory, cannot be regarded as empiric support for any hypothesis, as biased observations can provide support for any model. It is possible that many organisms live presently at adult population densities, which are sufficiently low to preclude both, inter and intra-specific competition. In such species, the adult population density may be limited always, at some previous stage of the life cycle, and the adult populations may be free from density dependent interactions.
A range of reproductive strategies is found in coral reef asteroids. Sexual recruitment can follow either planktotrophic or lecithotrophic larval development (Yamaguchi, 1977b). The occurrence of parthenogenetic development (Yamaguchi and Lucas, 1984), hermaphroditism (Achituv, 1972) or asexual reproduction (Rideout, 1978) may be correlated with survival at low population density and the consequential low probability of locating an opposite sexed conspecific at breeding time. Within coral reef asteroids, asexual reproduction has been observed in Linckia guildingii, Linckia multifora, Ophidiaster robillardi, Echinaster luzonicus and Asterina anomala (Emson and Wilkie, 1980). This provides evidence that, in some species under certain conditions, genetic variability and potential dispersal are less important to the maintenance of population numbers, than is continuity of recruitment.
The larvae of coral reef asteroids generally require a solid substrate to complete their development, and a coralline algal substrate has been observed as the chosen settling surface for many species (Yamaguchi, 1973b). More complex species-specific optima, located by sensitive chemo-sensory receptors might ensure settlement in habitats which are conducive to survival of post-settlement stages (Morse, 1984). Yamaguchi (1977c) showed that some juvenile asteroids have exponential growth during the period following settlement and proposed that juveniles are subject to high mortality during this period. The juveniles transform to adult morphology at a certain size and before this may look quite different from adults (e.g. Culcita novaeguineae illustrated by Clark, 1921).
A general paucity of information about juveniles characterizes available data on population structures of large bodied, coral-reef asteroids (Yamaguchi, 1973a). It is possible that populations are maintained either by continual low recruitment or occasional high recruitment, each coupled with iteroparity. The juveniles are cryptic and their apparent absence or rarity indicates that reproductive success is either constantly low, sporadic or both. Sporadic success may depend on factors such as availability of planktonic food, level of planktonic predation or mortality of settled larvae. These may average out over the life span of the adult resulting in stability of adult numbers. Population increases of coral reef asteroid species have been well documented for Acanthaster planci, and apparent population increases of Linckia laevigata following Acanthaster outbreaks have been described (Laxton, 1974).
The differing requirements for growth and successful recruitment of juveniles, within the coral reef asteroid community, will have resulted in diverse life history strategies. A conceptual dichotomy exists in our perception of the life history of all organisms, and is referred to as r- versus K- strategy (Pianka, 1972; Stearns, 1976). These different survival characteristics are thought to have evolved in response to specific types of environments (Hairston, Tinkle and Wilbur, 1970; Murphy, 1968; Wilbur, Tinkle and Collins, 1974). The spectrum of existing life history attributes, apparent in any community study (see e.g. Menge, 1975; Vance, 1973), represents many points on a continuum between the conceptually ideal r- strategists and K- strategists.
Goodman (1974) proposed that if a population’s size is limited mainly by competition then natural selection will result in an increased competitive ability (K-selection) and, in populations which are not resource limited, selection will result in an increased reproductive rate (r-selection). The reproductive effort (energy used for reproduction compared with the energy used for non-reproductive purposes) and age specific mortality schedule are an indication of the type of selection which has occurred during the evolution of a species (Pianka, 1972).
The longevity of a species is determined by the relative probability of juvenile and adult survivorship. In the simplest case, if the probability of a sexually mature organism’s survival from one reproductive season to the next is greater than the probability of one of the offspring reaching sexual maturity, then the species will exhibit iteroparity (see Goodman, 1974; Murphy, 1968). The weighting of selective attributes is arbitrary (e.g. niche specialization, number and size of eggs, longevity, possession of toxin), as the absolute ends of the r-K continuum do not exist in reality. Unpredictable environmental factors (e.g. perils of larval life and enormous potential dispersion) can result in a high numerical fecundity, and consequentially, most marine benthic invertebrates have a high energy cost associated with reproduction (Mileikovsky, 1971).
The dispersal stage of a population spreads the risk of local extinction in space and time (den Boer, 1968; Scheltema, 1971; Strathmann, 1974). The early stages of succession survive by being able to colonize regions quickly following disturbance. The resultant spatial and temporal variation in population size seems to characterize the typical r- strategists. Their populations are stable only when viewed on a larger scale. The spatial and temporal scale at which a species must be viewed for its numbers to be stable is an indication of its position on the r-K continuum.
The life history strategy, of each species, will be viewed in this context and a variety of strategies should be observed within the coral reef asteroid community. Early succession species would be expected to have large fluctuations while late succession species should have smaller ones. Stable, climax communities should be characterized by small fluctuations of their component species. The apparent stability of any biological system is dependent on the scale of observation (see Bradbury and Reichelt, 1982; Sale, 1984; Weiss, 1969). At the organismic scale, there would be neither temporal nor spatial abundance variation if an individual exactly replaced itself, without dispersing, then died. At the population scale, a level of numerical stability, consistent with a model of community equilibrium and climax, could be achieved if dispersion, larval survival and settlement phenomena did not result in greatly differing adult numbers from one year to the next.
Since the late 1950’s, coral reefs of the Indo-West Pacific region have experienced population outbreaks of the corallivorous asteroid Acanthaster planci (e.g. Bligh and Bligh, 1971; Branham, 1973; Chesher, 1969; Endean and Chesher, 1973; Endean and Stablum, 1975; Goreau, 1963; Heydorn, 1972; Kenchington, 1976; Marsh and Tsuda, 1973; Pearson, 1972). The resultant loss of hard coral cover on some reefs of the Great Barrier Reef was studied during the period of outbreak, and subsequently, so that both the short and long term effects of this predator would be known (Endean and Stablum, 1973; Pearson, 1981). The role of this predator in the elevation or lowering of coral species diversity on the Great Barrier Reef has not been studied adequately. It is apparent that some reefs become reinfested with Acanthaster planci about 15 years following the initial infestation (Cameron and Endean, 1982). It would appear, that when the quantity (not necessarily diversity) of a reef’s hard coral cover has regrown, the asteroid can recruit again in high numbers.
Although the Acanthaster planci population outbreak phenomenon has puzzled scientists for a quarter of a century, and although many explanatory hypotheses and models have been proposed (e.g. Birkeland, 1982; Endean, 1969; Flanigan and Lamberts, 1981; Randall, 1972; Sale, Potts and Frankel, 1976), there remains disagreement about the causes of the phenomenon. Additionally, there is disagreement about the need for reef management strategies that might mitigate the widespread effects of this coral predator. The extent of present population outbreaks and the possibility of past outbreaks (prior to 1960) have not been studied in sufficient detail to allow critical evaluation of either the problem itself, or the risks associated with incorrect management. We do not know what factors allow high recruitment of this asteroid on some reefs when, on other reefs, it maintains a low population density. The natural life expectancy, larval dispersal and adult migration of this asteroid, while central to an understanding of the phenomenon, are not understood sufficiently (Moore, 1978). The role of natural predators in maintaining high diversity and the possible survival strategy of rarity in the coral reef community have not been studied adequately.
Comparative data on other coral reef asteroids might contribute usefully to an understanding, or at least, enlarge our perspective of the Acanthaster planci outbreak phenomenon. With this broad aim in mind, the present study focuses on the community ecology of asteroid species at Heron Island. The population dynamics of other coral reef asteroids might show patterns of high recruitment similar to those of Acanthaster planci. The study of the abundance, longevity, population density, diet and reproduction of other coral reef asteroid species will allow comparison with Acanthaster planci as well as provide information on the mechanisms that maintain diversity within this coral reef community.