Coral reef ecosystems are under assault from multiple stressors. These range from local to global disturbances, some falling into the realm of naturally-induced. Some of these disturbances can be exacerbated through anthropogenic means. Some of the most insidious threats to these ecosystems are completely human-induced, such as the current inextricable acidification of the global ocean.
There have been mechanisms proposed that corals could theoretically use to deal with a rapidly changing global environment, particularly a warming surface ocean. Existing coral reef ecosystems are built around a symbiosis between the corals themselves and an intracellular photosynthetic dinoflagellate symbiont, within the genus Symbiodinium. Shallow-sea corals are capable of capturing bits of organic matter and small plankton in the water column for subsidence, but these organisms garner most of their energy from their photosynthetic tenants. Bleaching occurs when this symbiosis is compromised–one common mechanism is heat stress. Other causes exist, but bleaching via heat stress is an increasing dominant phenomenon; stark bleaching predictions related to oceanic temperature anomalies were recently announced for most of the Caribbean. Scleractinian corals (stony, hypercalcifying, reef-building corals) show variability in terms of which species bleach at certain thermal thresholds, as well as their geographic locations around the world. This variability may be at least partially explained by the different symbiont types within the corals. All types of coral symbionts are apparently not equal in how they can deal with warmer temperatures. For example, evidence exists that type D symbionts are more abundant after severe bleaching episodes and the ensuing mortality1. One proposed hypothesis states that reef-building corals can switch out their existing internal symbionts in exchange for more heat-tolerant symbiont types. A study in the Public Library of Science2 warns that this mechanism may not the panacea hoped for. Colonies of a scleractinian coral, Porites divaricata, were bleached and then exposed to symbiont types not usually found associated with this species. Some colonies were actually able to uptake these new symbionts–the first time that this environmental acquisition has been shown experimentally for scleractinian corals–however, this was an evanescent acquisition. Only the original symbiont types proved to be stable during the bleaching recovery, implying that not all corals have the capacity to acclimate to a warming ocean by uptaking more heat-resistant symbionts.
However, a recent review of coral reef resilience3 by Dr Terry Hughes and collagues reveals guarded optimism for these heavily-impacted ecosystems, provided that action is taken. In this context, resilience is the ability of an ecosystem to absorb and adapt to disturbances, without ecosystem functions being profoundly altered. These disturbances can vary in their rates of change; for instance, when long-term stressors are at a low level, reefs can be acted upon point (acute) disturbances of greater magnitude and speed and still recover. The authors point out that natural dynamics are inherent in complex biological systems–Heron Island on the Great Barrier Reef loses almost all coral cover on a decadal basis due to cyclones, yet can recover due to their locally high resilience. However, this is proving to be the exception, rather then the rule: many reef systems have been pushed near the proverbial tipping-point, rending them ecologically unable to recover from acute, high-magnitude disturbances. Hurricanes are enormously powerful disturbances, yet resilient reefs will recover relatively quickly. However, even a gradual shift away from a coral-dominated ecosystem from long-term chronic impacts, say increased levels of pollution or sedimentation, can reduce resilience over time. These types of local stressors can accumulate and produce community changes without acute, large disturbances. Even some deep reef sites, below wave action and thus unaffected from hurricanes, have been gradually moved away from normally coral-dominated ecosystems, undergoing phase-shifts. Unfortunately, these local impacts can be compounded by global impacts such as invasive species, warming, and ocean acidification. Phase-shifts refer to a change in the species assemblage to one that is not normally dominant. Consider the Caribbean. Coral cover has declined by ~80-90% since the late 1970s at sites throughout the region, while macroalgae cover has increased markedly. In a nutshell, many grazing fishes were once present, keeping the macroalgae at bay and allowing for both the persistence of coral-dominated reefs and the settlement of coral larvae. Once these grazer populations were exploited by overfishing, a sea urchin became the dominant grazer until it was decimated from a disease epidemic in the early 1980s. This die-off was a pivotal event, and phase-shifts to algal systems become to become more common and many reefs are still operating under this alternative stable state today. Atolls in the Pacific are thought to be much more resilient to these types of ecological changes, due to lower fishing pressure and less directly-induced human impacts overall. On the other hand, the isolation of these oceanic oases means that these sites are much more self-dependent than other sites, due to reduced larval connectivity (e.g. coral reef organisms’ larvae have less success getting to isolated sites, simply due to distance and ocean currents).
So is there good news to be had? These phase-shifts have been shown to be reversible in some cases. Diadema antillarum populations in the Caribbean is recovering, albeit slowly and in spatially discrete locations, providing increased herbivory on some reefs–at these locations, the macroalae is being grazed back and coral-dominated systems are possible again. This is not widespread currently, as most Caribbean reefs continue to be algal-dominated. Another example of the reversibility of these trends involve No-Take-Zones (NTAs), a type of Marine Protected Area (MPA). In the Bahamas, NTAs have resulted in higher densities of herbivorous fishes, contributing to declining macroalgal cover and more young corals settling on the reef. MPAs have also been shown to increase coral cover on a global basis.
Rather than dealing with each threat to coral reefs individually, a common approach, they must be dealt with in an integrated, proactive fashion, rather than reactively after the damage is likely already done. Hughes et al. stress the need for: the enforcement of harvesting regulations, local education, integrative decision-making via better access to “international networks of expertise”, and the confrontation of climate change by rapidly reducing greenhouse gas emissions. Rapid climate change and ocean acidification will ultimately undo any management policies for coral reef ecosystems. It is imperative for the state of coral reefs worldwide to be recognized for what it is: “a crisis of governance.” Though the situation may be daunting, these systems can be saved.
[This article has been cross-posted, albeit with a different image 1, at The Urban Times, an online magazine.]
Images 1: Close-up of a scleractinian brain coral. From Flickr user Laszlo Ilyes (cc) 2: Scleractinian coral colonies shown before (A) and after (B) one month of increased temperatures (Coffroth et al 2010).
1. Baker AC, Starger CJ, McClanahan TR, & Glynn PW (2004). Coral reefs: corals’ adaptive response to climate change. Nature, 430 (7001) PMID: 15306799
2. Coffroth MA, Poland DM, Petrou EL, Brazeau DA, & Holmberg JC (2010). Environmental symbiont acquisition may not be the solution to warming seas for reef-building corals. PloS one, 5 (10) PMID: 20949064
3. Hughes TP, Graham NA, Jackson JB, Mumby PJ, & Steneck RS (2010). Rising to the challenge of sustaining coral reef resilience. Trends in ecology & evolution (Personal edition) PMID: 20800316