The challenges: Coral bleaching
Coral Reef Resilience to Climate Change
Coral reefs are highly diverse ecosystems that are vital to the welfare of large human populations throughout the tropical world. Although they only cover 0.2% of the ocean’s floor, they contain 25% of its species and are often dubbed the ‘tropical rainforests of the oceans’ (Roberts, 2003). Coral reef goods and services (tourism, fisheries, coastal protection and medicinal uses) provide an annual net benefit of US$30 billion to economies worldwide, and millions of humans depend on them for an income. A recent study estimates the potential global economic cost of severe bleaching over the next 50 years at US$104.8 billion (Cesar et al, 2003). Healthy coral reef ecosystems are therefore of paramount importance.
Unfortunately, coral reefs are also among the most vulnerable ecosystems in the world to anthropogenic disturbances, and they are especially vulnerable to climate change. Today, an estimated 19% of the original area of coral reefs worldwide have been destroyed by various anthropogenic disturbances, while 15% are seriously threatened with loss within the next 10 to 20 years and a further 20% are under threat of loss in 20 to 40 years(Wilkinson, 2008).
Diver recording coral bleaching, Australia
Photo: Paul Marshall, GBRMPA
The predicted effect of climate change is increased coral bleaching, which is caused by the disruption of the symbiotic relationship between polyps and zooxanthellae resulting in the expulsion of zooxanthellae and loss of photosynthetic pigments. Many different types of stresses can cause this, but the most important are increased light and sea surface temperature (SST). If stresses continue for long enough, corals and whole reefs can suffer mortality. Once sections of the coral reef die they become vulnerable to structural degradation by algal overgrowth and bioerosion (Douglas, 2003). Coral reefs are especially vulnerable to predicted climate change because they bleach rapidly and dramatically in response to increase SSTs, and even increases of 1 or 2°C above average over a sustained period of time (e.g. a month) can cause mass bleaching (Hoegh-Guldberg, 1999). The potential severity of the predicted increases of 1-3°C in SSTs by 2050 (Hoegh-Guldberg, 1999) and 1.4-5.8°C in Earth surface temperatures by 2100 (IPCC, 2001) thus becomes apparent.
To learn more about climate change, please see the Intergovernmental Panel on Climate Change website: www.ipcc.ch
Large-scale bleaching events have been recorded with higher frequency since the 1980s and are linked to El Niño events. During the 1997/1998 El Niño, the most severe bleaching global bleaching event ever recorded caused bleaching in over 50 countries, causing the mortality of 16% of the world’s coral reefs. As temperatures continue to increase, events such as this could become more frequent and climate change may now be the single greatest threat to coral reefs worldwide.
Increases in the concentration of atmospheric CO2 also pose a threat to coral reefs by causing changes in seawater chemistry, decreasing the pH and making the ocean more acidic. The concentration of carbonate ions decreases and this leads to decreases in calcification rates of calcifying organisms such as calcareous plankton and reef-building corals. Coral reefs will thus experience reduced growth rates and structural strength. Experimental work has shown that on average corals decrease their calcification rates by 30% with doubled atmospheric CO2 concentrations (Kleypas, 2006).
Resilience - A promising paradigm
Even though climate change and coral bleaching pose a serious threat to the future survival of coral reefs, there is still hope that these ecosystems will be able to survive increased SSTs. Some coral reefs are able to withstand stresses to a greater degree (are more resistant) while other coral reefs are able to recover from bleaching events more rapidly (are more resilient) depending on a number of oceanographic, ecological and physiological factors. The principles of resistance and resilience are emerging as a promising paradigm to aid the management of coral reefs in the face of climate change, and give hope in the face of adversity. The figure below illustrates the stages in the coral bleaching process where it is possible for a coral or coral reef to survive the disturbance. It illustrates four main processes that can allow a coral reef to survive: protection, resistance, tolerance and resilience.
Oceanographic and other environmental factors that create pockets of reduced or non-stressful conditions where ecosystems are protected from disturbances (Salm et al, 2001). A coral reef can be protected against increased SSTs or light levels and therefore against bleaching by local upwelling, fast water flow, shading and screening.
The ability of an organism or ecosystem to withstand disturbance without undergoing a phase shift or losing neither structure nor function (Odum, 1989). For example a coral reef’s ability to withstand bleaching and mortality. Coral morphology, different zooxanthellae clades and coral acclimatisation can all influence a coral reef's resistance to bleaching.
The ability of an organism to absorb a disturbance and not suffer mortality (Obura, 2006). For example, a coral’s ability to bleach, and then recover its zooxanthellae to become healthy again.
The ability of a system to absorb or recover from disturbance and change, while maintaining its functions and services (Adapted from Carpenter et al, 2001). For example a coral reef’s ability to recover from a bleaching event. Factors that can improve a coral reef's resilience to a mass bleaching event include good species and functional diversity, good connectivity to larval sources, appropriate substrates for larval settlement and protection from other anthropogenic impacts.
For more information on resilience principles and more detail on the individual factors, please see 'Coral reef resilience and resistance to coral bleaching' by Grimsditch and Salm (2006).
The concept of building resilience principles into Marine Protected Area (MPA) management is relatively new, and until recently resilience had never been explicitly defined or listed as a criterion for MPA selection or design. Yet the concept of resilience demonstrates that there are positive actions we can take to counter potentially devastating impacts of climate-related bleaching. Several principles have been identified by The Nature Conservancy (www.reefresilience.org) that are useful when planning resilience MPAs.
Principle 1: Representation and Replication (and risk-spreading) can help increase likelihood of reef survival. By ensuring that resilient species and habitats are well represented and replicated throughout an MPA network, coral reef managers can decrease risk of catastrophic events, like bleaching, from destroying entire reef ecosystems.
Principle 2: Critical Areas are vital to survival and sustainability of marine habitats. These areas may provide secure and essential sources of larvae to enhance replenishment and recovery of reefs damaged by bleaching, hurricanes or other events. They also include high-priority conservation targets, such as fish spawning aggregations and nursery habitats.
Principle 3: Connectivity influences the design of marine protected area networks. Preserving connectivity among reefs and their associated habitats ensures replenishment of coral communities and fish stocks from nearby healthy reefs, and may enhance recovery.
Principle 4: Effective Management is essential to meeting goals and objectives of an MPA, and ultimately keeping reefs vibrant and healthy. Reducing threats is the foundation for successful conservation and the core of our resilience-based strategies. Measuring effective management provides the foundation for adaptive management. Investments in human capacity and long-term financing are also crucial to sustaining effective management for the future.