Phase shifts in coral reef communities and their ecological significance

Abstract

Many coral reefs around the world have degraded to a degree that their present intrinsic value and utility are greatly reduced: (mass coral mortality followed by algal invasions; local depletions of reef fisheries; deficit of reef accretion compared to physical and biological erosion). Though we can sometimes identify proximal causes (outbreaks of coral predators and eroders; over-fishing; habitat destruction), we do not have a good understanding of how population, community and ecosystem structure and function differ in degraded from un-degraded reefs. The deficiencies in our understanding limit our ability to interpret the long-term significance of reef degradation, and therefore to develop scientifically based plans for conservation and management of reefs.A particular course of action, or lack of action, based on uncritical acceptance of any of the various views of temporal variability can lead to further deterioration of specific reefs. None of these viewsthat reefs are either inherently robust, inherently fragile, or inherently resilient -is true over all time-space scales. This presentation reviews various models and case studies which suggest that reefs can be knocked precipitously or move slowly from one phase (coral-dominated) to another (coral-depleted and/or algal dominated). Transitions in the other direction ('recovery') involve changes (e.g. succession) in populations and communities (of all reef-associated biota, not just sessile benthos), and in reef function (e.g. community metabolism, trophodynamics) which are of great intrinsic interest but only poorly understood.

Spatial distribution of epibenthic molluscs on a sandstone reef in the Northeast of Brazil

“…Fleshy algae are described in the literature as indicators of reef degradation, shown by the decline in the coral cover (Done, 1992;Hughes, 1994;McCook, 1999;McCook et al, 2001;Littler et al, 2006) and could indicate a poor state of conservation on the Maracajaú reef, reflected in the results of this study for the molluscan assemblages. Earlier investigations conducted by Laborel (1970) about 4 decades ago on the Maracajaú reefs may show evidence of the existence of environmental alterations on these reefs.…”

“…Therefore, the main macroalgae recorded in this study could avoid the occurrence of molluscan herbivores at Maracajaú by chemical and morphological defenses, although more studies are needed to investigate this process. Moreover, the great cover of fleshy algae on the reef could prevent the growth and settlement of reef building corals (Done, 1992;Hughes, 1994;McCook, 1999, McCook et al, 2001McManus and Polsenberg, 2004), which could indirectly influence the low molluscan richness on Maracajaú's reef.…”

“…This change in substrate cover may account for the altered coral reef community structure, which would result in loss of local biodiversity (Done, 1992;Hughes, 1994;McClanahan et al, 2002;Folk et al, 2004) and increased abundance in dominant generalist species. This substrate alteration may be correlated to the fishing activity that has taken place in this region for many years, which would explain the large fleshy alga cover in the entire reef area due to the lack of herbivore fish.…”

Recovery of Diadema antillarum reduces macroalgal cover and increases abundance of juvenile corals on a Caribbean reef

“…The best known examples of these changes are found in the Caribbean (1), where reefs that were formerly dominated by scleractinian corals and diminutive algal turfs have become overgrown by macroalgae. This transition is referred to often as a phase shift to an alternate state (1)(2)(3)(4), where the alternative state (i.e., one dominated by macroalgae) is considered stable unless, or until, a reversal of one or more of the causative agents favors another change. However, there are few examples of multiple phase shifts on coral reefs (5) and none, to our knowledge, of a reversal from macroalgal to scleractinian dominance.…”

The future of coral reefs

“…The fossil record clearly shows, however, that marine species and ecosystems have their limits (refs. 119 and 120 and other articles in this colloquium), and the recent record indicates that these limits may be approached with little warning (9,10). The broader ecological consequences of reducing biodiversity (11) remain essentially uninvestigated for coral reefs.…”