Effective conservation requires rigorous baselines of pristine conditions to assess the impacts of human activities and to evaluate the efficacy of management. Most coral reefs are moderately to severely degraded by local human activities such as fishing and pollution as well as global change, hence it is difficult to separate local from global effects. To this end, we surveyed coral reefs on uninhabited atolls in the northern Line Islands to provide a baseline of reef community structure, and on increasingly populated atolls to document changes associated with human activities. We found that top predators and reef-building organisms dominated unpopulated Kingman and Palmyra, while small planktivorous fishes and fleshy algae dominated the populated atolls of Tabuaeran and Kiritimati. Sharks and other top predators overwhelmed the fish assemblages on Kingman and Palmyra so that the biomass pyramid was inverted (top-heavy). In contrast, the biomass pyramid at Tabuaeran and Kiritimati exhibited the typical bottom-heavy pattern. Reefs without people exhibited less coral disease and greater coral recruitment relative to more inhabited reefs. Thus, protection from overfishing and pollution appears to increase the resilience of reef ecosystems to the effects of global warming.

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Baselines and Degradation of Coral Reefs in the Northern Line Islands

Increased frequency of disturbances and anthropogenic activities are predicted to have a devastating impact on coral reefs that will ultimately change the composition of reef associated fish communities. We reviewed and analysed studies that document the effects of disturbance-mediated coral loss on coral reef fishes. Meta-analysis of 17 independent studies revealed that 62% of fish species declined in abundance within 3 years of disturbances that resulted in >10% decline in coral cover. Abundances of species reliant on live coral for food and shelter consistently declined during this time frame, while abundance of some species that feed on invertebrates, algae and/or detritus increased. The response of species, particularly those expected to benefit from the immediate loss of coral, is, however, variable and is attributed to erratic replenishment of stocks, ecological versatility of species and sublethal responses, such as changes in growth, body condition and feeding rates. The diversity of fish communities was found to be negatively and linearly correlated to disturbance-mediated coral loss. Coral loss >20% typically resulted in a decline in species richness of fish communities, although diversity may initially increase following small declines in coral cover from high coverage. Disturbances that result in an immediate loss of habitat complexity (e.g. severe tropical storms), have a greater impact on fishes from all trophic levels, compared with disturbances that kill corals, but leave the reef framework intact (e.g. coral bleaching and outbreaks of Acanthaster planci). This is most evident among small bodied species and suggests the long-term consequences of coral loss through coral bleaching and crown-of-thorn starfish outbreaks may be much more substantial than the short-term effects currently documented.

http://faculty.jsd.claremont.edu/emorhardt/159/pdfs/2007/wilson et al. 2006.pdf

Multiple disturbances and the global degradation of coral reefs: are reef fishes at risk or resilient?

The decline of many coral reef ecosystems in recent decades surprised experienced managers and researchers. It shattered old paradigms that these diverse ecosystems are spatially uniform and temporally stable on the scale of millennia. We now see reefs as heterogeneous, fragile, globally stressed ecosystems structured by strong positive or negative feedback processes. We review the causes and consequences of reef decline and ask whether management practices are addressing the problem at appropriate scales. We conclude that both science and management are currently failing to address the comanagement of extractive activities and ecological processes that drive ecosystems (e.g. productivity and herbivory). Most reef conservation efforts are directed toward reserve implementation, but new approaches are needed to sustain ecosystem function in exploited areas.

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Coral reef management and conservation in light of rapidly evolving ecological paradigms

Global climate change is having devastating effects on habitat structure in coral-reef ecosystems owing to extreme environmental sensitivities and consequent bleaching of reef-building scleractinian corals. Coral bleaching frequently causes immediate loss of live coral and may lead to longer-term declines in topographic complexity. This review identifies coral cover and topographic complexity as critical and distinct components of coral-reef habitats that shape communities of coral-reef fishes. Coral loss has the greatest and most immediate effect on fishes that depend on live corals for food or shelter, and many such fishes may face considerable risk of extinction with increasing frequency and severity of bleaching. Coral loss may also have longer-term consequences for fishes that require live corals at settlement, which are compounded by devastating effects of declining topographic complexity. Topographic complexity moderates major biotic factors, such as predation and competition, contributing to the high diversity of fishes on coral reefs. Many coral-reef fishes that do not depend on live coral are nonetheless dependent on the topographic complexity provided by healthy coral growth. Ecological and economic consequences of declining topographic complexity are likely to be substantial compared with selective effects of coral loss but both coral cover and topographic complexity must be recognised as a critical component of habitat structure and managed accordingly. Urgent action on the fundamental causes of climate change and appropriate management of critical elements of habitat structure (coral cover and topographic complexity) are key to ensuring long-term persistence of coral-reef fishes.

Effects of climate -induced coral bleaching on coral -reef fishes — ecological and economic consequences

Coral reefs are degrading worldwide at an alarming rate. Nutrient over-enrichment is considered a major cause of this decline because degraded coral reefs generally exhibit a shift from high coral cover (low algal cover) to low coral cover with an accompanying high cover and biomass of fleshy algae. Support for such claims is equivocal at best. Critical examination of both experimental laboratory and field studies of nutrient effects on corals and coral reefs, including the Elevated Nutrient on Coral Reefs Experiment (ENCORE) enrichment experiment conducted on the Great Barrier Reef, does not support the idea that the levels of nutrient enrichment documented at anthropogenically-enriched sites can affect the physiology of corals in a harmful way, or for most cases, be the sole or major cause of shifts in coral- algal abundance. Factors other than nutrient enrichment can be significant causes of coral death and affect algal cover, and include decreased abundance of grazing fishes by fishing, and of grazing sea urchins to disease; grazing preferences of remaining grazers; temperature stress that kills coral (i.e., coral bleaching) and creates more open substrate for algal colonization; sedimentation stress that can weaken adult corals and prevent coral recruitment; coral diseases that may be secondary to coral bleaching; and outbreaks of coral predators and sea urchins that may be secondary effects of overfishing. Any factor that leads to coral death or reduces levels of herbivory will leave more substrate open for algal colonization or make the effects of even low-level enrichment more severe. Factors that contribute to an imbalance between production and consumption will result in community structure changes similar to those expected from over- enrichment. Over-enrichment can be and has been the cause of localized coral reef degradation, but the case for widespread effects is not substantiated.

/pdf/nutrient-enrichment-on-coral-reefs-is-it-a-major-cause-of-1eu0r3ex3s.pdf

Nutrient Enrichment on Coral Reefs: Is It a Major Cause of Coral Reef Decline?

Widespread rises in the abundance of fleshy macroalgae on Caribbean reefs within the last 20 yr have variously been attributed to eutrophication, overfishing, or the 1983-4 mass mortality of the grazing sea urchin Diadema antillarum. However, none of those factors can satisfactorily explain why macroalgae are abundant today even on lightly fished mid-depth reefs in sparsely pop- ulated areas. Here we explore another explanation for rises in macroalgal cover on such reefs, namely that they are a side effect of declines in coral cover. We suggest that grazing by herbivorous fishes can exclude macroalgae from mid-depth reefs with high cover of hard corals, but that on low- cover reefs, the amount of space occupied by algae overwhelms the ability of grazing fishes to crop it down. We simulated the effect of 10% and 25% rises in coral cover by attaching 'pseudo-corals' (PVC tiles covered in a non-toxic anti-fouling coating) to reef substratum in 5 × 5 m plots (3 groups: control, 10%T and 25%T) on a 12 m deep forereef site in front of Ambergris Caye, Belize. Within 3 mo of attaching tiles, macroalgae had declined by approximately 10% in 10%T plots and 25% in 25%T plots compared with control plots, and these new states persisted for the duration of the experiment (macro-algae averaging 38.0% in control plots, 28.7% in 10%T plots, and 13.6% in 25%T plots). Dra- matic reductions in macroalgae may have made the 25%T plots more attractive to herbivorous fishes, as, in the later stages of the experiment, herbivorous fish biomass and acanthurid feeding rate were higher in those plots than in 10%T and control plots.

/pdf/limits-to-grazing-by-herbivorous-fishes-and-the-impact-of-419ie8moz9.pdf

Vicki J. Hendrick

Papers

Limits to grazing by herbivorous fishes and the impact of low coral cover on macroalgal abundance on a coral reef in Belize