Some Physical and Biological Determinants of Coral Community Structure in the Eastern Pacific

Abstract

Various physical and biological factors affecting coral community structure were investigated by direct observation and periodic censusing (supplemented with laboratory observations and experiments) on three coral reefs off the Pacific coast of Panama from 1970 to 1975. The physical environment has a strong control over coral growth at shallow depth; physical factors are also important subtidally (light, sediment transport). However, paralleling the pattern on temperate shores, biological processes (competition, predation, bioturbation, mutualism) assume an increasing influence on community structure in deeper and more diverse reef assemblages. Coral zonation is marked on these biologically simple and small reefs; the following assemblages are recognized: drying reef flat–live coral cover moderate, species diversity low; reef crest and upper reef slope–highest cover, lowest diversity; lower reef slope and reef base–cover moderate to low, diversity highest. Coral populations in the different zones, though spatially close, are affected by unique sets of conditions. Recurrent extreme tidal exposures devastate reef flat corals (Pocillopora mortality = 40%—60%). The mortality rate of pocilloporid corals is higher than for other corals; this has a diversifying effect on the reef flat assemblage. Acanthaster normally feeds in deep reef zones and numerically its major prey are the predominant pocilloporid corals (most often small colonies and broken branches). Electivity indices and prey choice experiments indicate that less abundant, nonbranching corals are preferred over Pocillopora. Large, branching pocilloporid colonies harbor crustacean symbionts (Trapezia and Alpheus) which can repulse Acanthaster and therefore protect this group of corals. Experimental removal of the symbionts results in a shift of prey preference from nonbranching corals towards the branching pocilloporids. Crustacean symbionts were present in all large Pocillopora colonies sampled, but the density of Trapezia in colonies on the reef flat was about twice that in colonies from deep zones where Acanthaster forages. Further, small pocilloporid colonies and fragments contained relatively few (and a high proportion of juvenile) symbionts. The variety of preferred coral prey present along the seaward reef flanks and the relatively low abundance of Pocillopora in this habitat are considered important factors affecting the distribution of Acanthaster. In addition, a continuous live cover of pocilloporid corals, which Acanthaster avoids, can protect reef zones (e.g., the reef flat) or preferred prey species from attack. The selective destruction of nonpocilloporid corals by Acanthaster tends to lower both live coral cover and species diversity (H'). This trend is evident on the Uva Island study reef where a significant decline in coral cover (47%—18%) and H' (1.06—0.58) occurred over a 4—mo period.

Climatic and biotic thresholds of coral-reef shutdown

“…Reefs in the TEP experience more frequent subaerial exposure during La Niña than at other times, which can result in high levels of coral mortality 41,42 . Additionally, enhanced rainfall during La Niña events in some parts of the TEP, including Pacific Panamá [36][37][38] , can lead to highly turbid conditions because of enhanced coastal runoff 41 , potentially resulting in limited light availability and reduced coral growth 43 . Finally, La Niña is associated with a strengthening of the northeast trade winds, which increases upwelling in some parts of the TEP 30,44 .…”

The Coral Reefs of Eilat — Past, Present and Future: Three Decades of Coral Community Structure Studies

“…Coral mass mortality on reefs may result from a variety of causes such as (below selected references) mechanical destruction during tropical storms (Woodley et al 1981;Rogers 1993), abnormally low or high seawater temperatures (Glynn 1981;Jokiel and Coles 1990), sedimentation (Loya 1976e;Rogers 1990) exposure to air during midday air temperature maxima (Glynn 1976;Loya 1976a), bleaching (Glynn 1993;Brown 1997;Hoegh-Guldberg 1999;Ostrander et al 2000;Loya et al 2001), diseases (Kushmaro et al 1996;Harvell et al 1999), Acanthaster predation (Lourey et al 2000) and anthropogenic activities (Brown and Howard 1985;Nystrom et al 2000). The period of time required for recruitment and recovery of reefs after such disturbances varies from very short (2 years; Shinn 1972) to prolonged (50 years or more; Grigg and Maragos 1974;Pearson 1981) and largely depends on local settings.…”

“…They hypothesized that the decline in diversity in older flows was due to space limitation and competitive exclusion by dominant species, a process that took over 50 years. Glynn (1976) attributed the diversifying effect to recurrent and extreme tidal exposures of reef flat corals off the Pacific coast of Panama. Tide-induced mortality of pocilloporid corals, which are prime-space monopolists at that site, resulted in increased coral species diversity.…”

“…I have suggested (Loya 1976a) that the unpredictable low tides at Eilat act as a diversifying force in a way similar to storm and swell damage in Hawaii (Grigg and Maragos 1974;Grigg 1983), and the extreme tidal exposures of reef flats off the Pacific coast of Panama (Glynn 1976). Connell's (1978) 'intermediate-disturbance' hypothesis, stating that the highest diversity of natural communities will be maintained in areas suffering intermediate disturbances on the scales of frequency and intensity, fits these examples.…”

Carbonate production of an emergent reef platform, Warraber Island, Torres Strait, Australia

“…Minimum and maximum estimates of 60 and 250 g m -2 yr -1 of calcification per 15% cover (400 and 1,667 g m -2 yr -1 for 100% cover) were used to test the sensitivity of the total carbonate production figures to Halimeda production rates, taking into account potential variation in local growth rates and in the exact ratio of rhipsalian to lithophytic species present in the different reef flat subenvironments. Knutson et al 1972;Buddemeier et al 1975;Highsmith 1979;Grigg 1982;Wellington 1982;Wellington and Glynn 1983;Charuchinda and Chansang 1985;Hughes 1987;Guzman and Cortes 1989;Scoffin et al 1992;Klein et al 1993;Glynn et al 1996;Stimson 1996 ;Glynn 1976;Neudecker 1977;Glynn and Stewart 1973;Wellington 1982; 0.165 Eakin 1992 1,394 ± 627 b …”