Disturbance and the Dynamics of Coral Cover on the Great Barrier Reef (1995–2009)

Coral reef ecosystems worldwide are under pressure from chronic and acute stressors that threaten their continued existence. Most obvious among changes to reefs is loss of hard coral cover, but a precise multi-scale estimate of coral cover dynamics for the Great Barrier Reef (GBR) is currently lacking. Monitoring data collected annually from fixed sites at 47 reefs across 1300 km of the GBR indicate that overall regional coral cover was stable (averaging 29% and ranging from 23% to 33% cover across years) with no net decline between 1995 and 2009. Subregional trends (10–100 km) in hard coral were diverse with some being very dynamic and others changing little. Coral cover increased in six subregions and decreased in seven subregions. Persistent decline of corals occurred in one subregion for hard coral and Acroporidae and in four subregions in non-Acroporidae families. Change in Acroporidae accounted for 68% of change in hard coral. Crown-of-thorns starfish (Acanthaster planci) outbreaks and storm damage were responsible for more coral loss during this period than either bleaching or disease despite two mass bleaching events and an increase in the incidence of coral disease. While the limited data for the GBR prior to the 1980's suggests that coral cover was higher than in our survey, we found no evidence of consistent, system-wide decline in coral cover since 1995. Instead, fluctuations in coral cover at subregional scales (10–100 km), driven mostly by changes in fast-growing Acroporidae, occurred as a result of localized disturbance events and subsequent recovery

Retention of habitat complexity minimizes disassembly of reef fish communities following disturbance: a large-scale natural experiment.

High biodiversity ecosystems are commonly associated with complex habitats. Coral reefs are highly diverse ecosystems, but are under increasing pressure from numerous stressors, many of which reduce live coral cover and habitat complexity with concomitant effects on other organisms such as reef fishes. While previous studies have highlighted the importance of habitat complexity in structuring reef fish communities, they employed gradient or meta-analyses which lacked a controlled experimental design over broad spatial scales to explicitly separate the influence of live coral cover from overall habitat complexity. Here a natural experiment using a long term (20 year), spatially extensive (∼ 115,000 kms(2)) dataset from the Great Barrier Reef revealed the fundamental importance of overall habitat complexity for reef fishes. Reductions of both live coral cover and habitat complexity had substantial impacts on fish communities compared to relatively minor impacts after major reductions in coral cover but not habitat complexity. Where habitat complexity was substantially reduced, species abundances broadly declined and a far greater number of fish species were locally extirpated, including economically important fishes. This resulted in decreased species richness and a loss of diversity within functional groups. Our results suggest that the retention of habitat complexity following disturbances can ameliorate the impacts of coral declines on reef fishes, so preserving their capacity to perform important functional roles essential to reef resilience. These results add to a growing body of evidence about the importance of habitat complexity for reef fishes, and represent the first large-scale examination of this question on the Great Barrier Reef

Macroalgal feedbacks and substrate properties maintain a coral reef regime shift

Coral reefs are among the world's most diverse and productive ecosystems, yet they are also one of the most threatened. The combined effects of local human activities and climate change have led to corals being replaced by macroalgae in various tropical settings, lessening the ecological, social, and economic value of these reefs. Once established, macroalgal regimes are maintained by a range of physical, chemical, and biological feedback mechanisms that suppress the settlement, survival, growth, and hence recovery of coral populations. Our understanding of these feedbacks has come largely from small‐scale experimental studies, but their relative importance in sustaining a regime shift has rarely been examined in situ. We investigated the role of macroalgae in limiting coral recovery on an inshore reef on Australia's Great Barrier Reef that shifted to macroalgal dominance in 2001. Coral recruitment on terracotta tiles in habitats with low cover of macroalgae at the regime‐shifted reef and at comparable habitats at an adjacent coral‐dominated reef was similar, suggesting that neither larval supply nor reef‐wide “avoidance” by coral larvae was contributing to the lack of coral recovery at the regime‐shifted reef. However, within the regime‐shifted reef, recruitment of corals on tiles, and their survival in the first two months post‐settlement, was substantially lower in habitats characterized by dense beds of the brown macroalga Lobophora than in habitats just meters away that were relatively free of macroalgae. Despite the negative effects of Lobophora on recruitment and early recruit survival, there was no effect of Lobophora on the persistence of juvenile corals (1–50 mm diameter). Juvenile coral persistence in beds of Lobophora (50%) was comparable to that in neighboring habitats free of Lobophora (60%) over nine months. Rather, the persistence of juvenile corals was lowest (10%) in unconsolidated rubble habitat, where photographs of fixed quadrats showed that, over nine months, rubble substrate had been redistributed. Our results highlight two bottlenecks to coral recovery; inhibition of coral recruitment and recruit survival by macroalgae, and reduced juvenile coral persistence in patches of loose rubble substrate. Importantly, these processes appear to be habitat‐specific and are unlikely to constrain coral recovery at a reef‐wide scale

Location of the study reefs in each of the three treatments (Major Decline, Minor Decline and Control).

<p>Small panels display trends in hard coral cover and habitat complexity, along with shaded periods of time when disturbances (COTS = <i>Acanthaster planci</i> outbreaks, storms & coral disease) occurred. Points are raw data means, while solid lines indicate modelled average trends and dotted lines show 2 x standard errors from a linear mixed effects model fitted separately to hard coral cover and habitat complexity. Arrows mark the years of greatest and least hard coral cover.</p

Differences in hard coral cover, habitat complexity, total species richness of fishes and species richness of eight fish families and five broad functional groups for each of the three treatments (Major Decline, Minor Decline and Control).

<p>Data are average effect sizes from generalized linear mixed effects model expressed as a per cent change from the time of greatest to least coral cover. Inferences about temporal changes were based on 95% Bayesian Highest Posterior Density (HPD) intervals of cell means predicted from posterior distributions of model parameters derived via Markov-chain Monte Carlo (MCMC) sampling. Effects are considered significant if the HPD intervals do not intersect zero.</p

Multi-dimensional plot based on Bray-Curtis similarity coefficients of (a) square-root transformed percent benthic cover and (b) fourth-root transformed fish species abundances.

<p>Each panel presents changes to communities following disturbances for the three treatments (Major Decline, Minor Decline and Control). A full model ADONIS analysis revealed a significant interaction for both benthic communities (ADONIS Treatment*Time: F = 14.293, d. f. = 2, Pr(>F) = 0.001) and fish communities (ADONIS Treatment*Time: F = 4.9225, d. f. = 2, Pr(>F) = 0.001). Changes from times of greatest to least coral cover were further examined by separate ADONIS for each individual Treatment (Major Decline, Minor Decline and Control), and the small inset bar graphs display the effect sizes (Sums of Squares) from these individual analyses. ***: Pr(>F) = <0.001</p

Expectations and outcomes of reserve network performance following re-zoning of the Great Barrier Reef Marine Park

Networks of no-take marine reserves (NTMRs) are widely advocated for preserving exploited fish stocks and for conserving biodiversity. We used underwater visual surveys of coral reef fish and benthic communities to quantify the short- to medium-term (5 to 30 years) ecological effects of the establishment of NTMRs within the Great Barrier Reef Marine Park (GBRMP). The density, mean length, and biomass of principal fishery species, coral trout (Plectropomus spp., Variola spp.), were consistently greater in NTMRs than on fished reefs over both the short and medium term. However, there were no clear or consistent differences in the structure of fish or benthic assemblages, non-target fish density, fish species richness, or coral cover between NTMR and fished reefs. There was no indication that the displacement and concentration of fishing effort reduced coral trout populations on fished reefs. A severe tropical cyclone impacted many survey reefs during the study, causing similar declines in coral cover and fish density on both NTMR and fished reefs. However, coral trout biomass declined only on fished reefs after the cyclone. The GBRMP is performing as expected in terms of the protection of fished stocks and biodiversity for a developed country in which fishing is not excessive and targets a narrow range of species. NTMRs cannot protect coral reefs directly from acute regional-scale disturbance but, after a strong tropical cyclone, impacted NTMR reefs supported higher biomass of key fishery-targeted species and so should provide valuable sources of larvae to enhance population recovery and long-term persistence