A Long-Term Study of Competition and Diversity of Corals
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Ecological Monographs, 74(2), 2004, pp. 179±210 q 2004 by the Ecological Society of America A LONG-TERM STUDY OF COMPETITION AND DIVERSITY OF CORALS JOSEPH H. CONNELL,1,4 TERENCE P. H UGHES,2 CARDEN C. WALLACE,3 JASON E. TANNER,2 KYLE E. HARMS,1 AND ALEXANDER M. KERR1,2 1Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106 USA 2Department of Marine Biology, James Cook University, Townsville, Queensland 4810, Australia 3Museum of Tropical Queensland, Townsville, Queensland 4810, Australia Abstract. Variations in interspeci®c competition, abundance, and alpha and beta di- versities of corals were studied from 1962 to 2000 at different localities on the reef at Heron Island, Great Barrier Reef, Australia. Reductions in abundance and diversity were caused by direct damage by storms and elimination in competition. Recovery after such reductions was in¯uenced by differences in the size of the species pools of recruits, and in contrasting competitive processes in different environments. In some places, the species pool of coral larval recruits is very low, so species richness (S) and diversity (D) never rise very high. At other sites, this species pool of recruits is larger, and S and D soon rise to high levels. After ®ve different hurricanes destroyed corals at some sites during the 38- year period, recovery times of S and D ranged from 3 to 25 years. One reason for the variety of recovery times is that the physical environment was sometimes so drastically changed during the hurricane that a long period was required to return it to a habitat suitable for corals. Once S and D have peaked during recolonization, they may either remain at a high level, or decline. In shallow water, with no deleterious changes in environmental conditions, S and D may not decline over time, because superior competitors cannot overtop inferior competitors without exposing themselves to deleterious aerial exposure at low tide. At other times and places, S and D did decline over time. One cause of this was a gradual deterioration of the physical environment, as corals grew upward into the intertidal region and died of exposure. S and D also fell because the wave action in hurricanes either killed colonies in whole or part, or changed the drainage patterns over the reef crest, leaving corals high and dry at low tide. At deeper sites, declines in S and D were sometimes caused by heavy wave action, or by interspeci®c competition, as some corals overgrew or over- topped their neighbors and eliminated them. Key words: algae; alpha and beta diversity; community dynamics; competition; corals; cyclones; disturbance; Great Barrier Reef, Australia; hurricanes; mortality; recruitment; species richness. INTRODUCTION different environmental conditions. Second, we will A fundamental question asked by ecologists and describe interspeci®c competition, species composi- those interested in conservation is: ``What factors de- tion, and diversity of corals. We measured species rich- termine the level of diversity?'' Both natural processes ness (S), the number of species per unit area, and di- and human activities that destroy habitats and organ- versity (D), an index based on the relative abundances isms may reduce diversity. To evaluate the effects of of the different species. We calculated diversity at a human impacts, it is ®rst necessary to understand the single site (alpha diversity), and also the difference in effects of natural processes. Natural disturbances such species composition among two or more sites or times as violent storms, temperature extremes, and outbreaks (beta diversity; Whittaker 1967). Third, we tested sev- of predators, pests, and diseases, as well as interspeci®c eral hypotheses that could explain some of the mech- competition that could eliminate competitively inferior anisms that produce these patterns in competition, al- species, all tend to reduce local abundance and diver- pha and beta diversity, and species composition. sity, while recruitment and growth tend to increase it. Given that assemblages of corals often have high spe- Mechanisms affecting interspeci®c competition cies diversity, the question of interest in the present Given that interspeci®c competition can eliminate study is: What factors determine it? colonies and so reduce abundance, S, and D, it is im- Our objectives were as follows. First, we will de- portant to understand the conditions that determine its scribe changes over 38 years in abundance, distribu- occurrence and strength. Interspeci®c competition for tion, and recruitment of corals and macroalgae, under space in sessile marine invertebrates such as corals, barnacles, ascidians, etc., may take either of two forms. Manuscript received 3 June 2002; revised 5 February 2003; accepted 22 March 2003; ®nal version received 28 May 2003. These are direct aggressive behavior, e.g., by over- Corresponding Editor: S. G. Morgan. growth, shading by overtopping, digestion by mesen- 4 E-mail: [email protected] terial ®laments, etc., or indirect defensive behavior, by 179 180 JOSEPH H. CONNELL ET AL. Ecological Monographs Vol. 74, No. 2 preemption of space, i.e., defending it against en- not apply if interspeci®c competition is not strong, or croachment by settling larvae or growth of neighbors is symmetrical (i.e., species are equivalent in compet- (Connell 1961, 1973, 1976, 1983, 1990, Connell and itive ability). In that case, S and D will not necessarily Keough 1985, Lang and Chornesky 1990). Coral col- fall as abundance increases to high levels. onies may win over their neighbors because of inherent Compensation among species.ÐCompensation is traits (e.g., age, size, ontogeny, growth rate, morphol- de®ned here as any process that counteracts the effects ogy, and aggressive ability), and/or because of the local of competitive superiority of some species over others environmental conditions, e.g., depth, time since last (Connell 1978). For example, if inferior competitors disturbance, recent weather, etc. (Connell 1973, 1976, are favored because predators preferentially attack the Johnston et al. 1981, Bak et al. 1982, Connell and superior competitors, this would tend to keep the for- Keough 1985, Alino et al. 1992, Genin et al. 1994). mer from being reduced to low levels, even to local We tested the following hypotheses concerning in- extinction, and so would tend to promote S and D. Any terspeci®c competition in corals: (1) Regardless of their reduction in recruitment, growth, survival, or abun- abundance, species may differ in their inherent ability dance of the superior species, relative to that of the to win in competition with neighbors. (2) Since com- inferior species, would be evidence of compensation. petition for space takes place only between near neigh- Frequency dependence among species.ÐIf the rel- bors, it would be expected to occur more often when ative abundances of the more rare species increase coral cover was high. (3) Larger colonies might be while those of the commoner species decrease over an expected to win over smaller ones more often than interval, S and D should be maintained. Such negative expected by chance. (4) Survival should be longest for frequency dependence equalizes relative abundances, those colonies that did not compete, less so for winners, and thus reduces both the probability of local extinction and shortest for losers. (5) Commoner species would of more rare species and the chance that commoner be expected to have more interactions with neighbors species will increasingly preempt space. The same ap- than would more rare ones. (6) Those species that en- plies if the recruitment rate of more rare species is gaged in more interactions should develop a greater greater than that of the commoner species. degree of competitive ability, and so win a higher pro- Recruitment limitation.ÐThe coral assemblage in portion of them. (7) If hypothesis 6 is true, then com- some areas, particularly at the Inner Flat site, has many moner species might be expected to win a greater pro- fewer species than at the other study sites. One possible portion of their encounters than rare species would. (8) explanation is that the recruitment rate of all species Because competition presumably requires the expen- is much lower in some sites, thereby constraining their diture of energy, it will reduce the growth rate of col- species pools. We also investigated the possibility that onies. (9) If competition is important in determining certain species might recruit less commonly at some abundance, species superior in competitive ability sites than at others, again implying that limitation of should be commoner than those of inferior competitive recruitment could explain low S and D in the former ability. sites. Beta diversity.ÐTo estimate variation in species Mechanisms affecting alpha and beta diversity composition over space and time, we calculated the The intermediate disturbance model of diversity.Ð degree of difference in species composition between The intermediate disturbance model (Horn 1975, Con- study areas, which Whittaker (1967) called beta di- nell 1978) predicts that diversity (S or D) will be lower versity. Two different locations could have the same in: (1) regimes of either high or low frequency of dis- alpha diversity, but if their species compositions were turbance, or (2) very soon after, or very long after, a very different, the total diversity of both locations com- severe disturbance. In contrast, S or D will be higher bined would be higher than if their species composi- in a disturbance regime of intermediate frequency, or tions were similar. We also extended the concept of at an intermediate time after a severe disturbance. The beta diversity to time, comparing the species compo- reasoning underlying this model is that abundance sition at different times at the same place. (both numbers of genets and cover), and therefore S and D, will be low in regimes of high disturbance fre- METHODS quency or very soon after a disturbance.