Most threatened coral reefs in the world are located in Southeast Asia

Thermal stress can cause coral reefs to bleach from losing their symbiotic algae giving them colour  (from imgarcade)

Coral reefs are not only a critical habitat for numerous species, they provide also essential ecosystem services such as:

• For food and livelihood a healthy, well-managed reef can yield between 5 and 15 tons of fish and seafood per square kilometer each year;

• Reefs are vital in many tropical countries for developing eco-tourism new revenues:  attracting divers, snorkelers, and recreational fishers, not to mention providing much of the white sand for beaches;

• Many reef-dwelling species have developed complex chemical compounds- such as venoms and chemical defenses- aiding their survival in highly competitive habitats which might offer the basis of life-saving new pharmaceuticals including treatments for cancer, HIV, malaria, and other diseases;

• Beyond their biological value, reefs dissipate wave energy, reducing erosion and lessening damage during storms with an estimated 150,000 km of reef bordered shoreline in more than 100 countries and territories; this function protects human settlements, infrastructure, and valuable coastal ecosystems such as sea grass meadows and mangrove forests.

Despite their importance, coral reefs face unprecedented threats throughout most of their range and many reefs are already degraded and unable to provide the vital services on which so many people depend.

Some threats are highly visible and occur directly on reefs. Levels of fishing are currently unsustainable on a large proportion of the world’s reefs, and have led to localized extinctions of certain fish species.

Many threats are also the result of human activities occuring far away from reefs, such as forest clearing, crop cultivation, intensive farming, polluted sewage runoff and poorly planned coastal development. Pollution and waste from ships and from oil and gas exploitation further exacerbate the situation.

Furthermore beyond these extensive and damaging local-scale impacts, reefs are increasingly at risk from the global threats associated with rising concentrations of greenhouse gases in the atmosphere.

Even in areas where local stress on reefs are relatively minimal, warming seas have caused mass coral bleaching occurring when corals become thermally stressed and are massively losing  the “zooxanthellae” symbiotic algae that live within their tissues and normally provide their specific colors.

It is rare for any reef to suffer only a single threat. More often the threats are compounded. For instance, overfishing eliminates a key herbivore such as “Parrot fish”, while runoff from agriculture supplies nutrients that cause a bloom in macro algae, impairing the growth of coral and ultimately reducing the competitive ability of coral communities.

Despite widespread recognition that reefs are severely threatened, information regarding threat assessment  to specific reefs are limited. Only a fraction of reefs have been studied or monitored consistently over time- such as Jamaica, Florida, and Australia’s Great Barrier- where changes in coral condition are well-documented. 

In most places, however, the availability of detailed information is limited, inhibiting effective management.

More than 15 years after the foundation of the World Reef Initiative (WRI), reef maps are showing clearly that the growth in threats has largely outpaced efforts to address those threats. 

In the mid-1990s, climate change was still perceived as a somewhat distant threat. However, in 1998, a powerful El Niño event further increased sea surface temperatures that were already rising due to climate change, triggering the most severe and expansive coral bleaching event on record.

As a result an update of the global coral reef threats analysis is clearly necessary in order to identify and understand the outcome  and implications of changes to the world’s reefs and to help guide targeted interventions aimed at mitigating existing threats .

Ranking of worldwide human threats to coral reefs

(see “WRI-Coral Reefs at risk revisited-2011”)

There is a difference between threats and damages. A threat analysis is an assessment of the likeliness (within 0-100%) of a specified type of damage.  A threat model measures threat rather than real conditions. 

Concerning coral reefs, harmful damage might result from the following threats:

• Overfishing and destructive fishing : such as destruction of fish habitat and species;
• Watershed land-based pollution : discharge of land base sediment or pollutant ;  
• Coastal development : sewage and pollutions from ports, industry, resorts;  
• Marine-based pollution and damage : discharge of marine waste and marine fuels;
• Thermal stress : damage from sea surface temperature increase.

Figure 1 : Worldwide human local threats to coral reefs  (WRI 2011)

Overfishing and destructive fishing

Threats to coral reefs from overfishing were evaluated based on coastal population density and extent of fishing areas, with adjustments to account for the increased demand due to proximity to large populations and market centers.

Areas where destructive fishing occurs (with explosives or poisons such as cyanide) were also included, based on observations from monitoring and mapping expert analysis.

The threat estimate was reduced inside marine protected areas rated by experts as having “effective” or “partially effective” management (meaning that a level of management is present that helps to guard ecological integrity).

Limitations: 

• Accurate, spatially referenced global data on fishing methods, catches, and number of fishers were not available; therefore, population pressure is used as a proxy for overfishing.

• The model fails to capture the targeting of very high value species, which affects most reefs globally, but has fewer ecosystem impacts than wider scale overfishing.

• Management effectiveness scores were only available for about 83% of the reefs within marine protected areas.

Reef regions ranking for overfishing and destructive fishing (see Figure 2 below): most threatened are Southeast Asia (N°1) & Atlantic (N°2); less threatened are Pacific (N°5) and Australia (N°6).

Watershed land-based pollution

The threat from land-based pollutants was modeled over 300,000 watersheds (catchments area drained by a river and its tributaries) discharging to coastal waters. Relative erosion rates were estimated across the landscape based on slope, land cover type, precipitation, and soil type.

Sediment delivery at the river mouth was estimated based on total erosion in the watershed, adjusted for the sediment delivery ratio (based on watershed size) and sediment trapping by dams and mangroves.

Sediment plume dispersion was modeled using a linear decay rate and was calibrated against actual sediment plumes observed from satellite data. The model represents a proxy for sediment, nutrient, and pollutant delivery.

Limitations: 

• Nutrient deliveries to coastal waters were probably underestimated due to a lack of spatial data on crop cultivation and fertilizer application. However, agricultural land is treated as a separate category of land cover, weighted for a higher influence.

• The model does not incorporate nutrient and pollutant inputs from industry, or from intensive livestock farming, which might be considerable.

Reef regions ranking for watershed land-based pollution (see Figure 2 below): most threatened are Southeast Asia (N°1) & India (N°2); less threatened are Middle East (N°5) and Australia (N°6)

Coastal development

The threat to coral reefs from coastal development was modeled based on size of cities, ports, and airports; size and density of hotels; and coastal population pressure (a combination of population density, growth, and tourism growth).

Limitations:

• Indicators were likely to miss some new constructions works and tourism locations.

• The model does not directly capture sewage discharge, but relies on population as a proxy for costal development threat.

Reef regions ranking for coastal development (see Figure 2 below): most threatened are Southeast Asia (N°1) & India (N°2); less threatened are Pacific (N°5) and Australia (N°6).

Figure 2 : Comparison of local and past thermal threats by region (WRI 2011) 

Marine-based pollution and damage

Threat from marine-based pollution and damage were based on the size and volume of commercial shipping ports, size and volume of cruise ship ports, intensity of shipping traffic, and the location of oil infrastructure.

Limitations:

• Threat associated with shipping intensity might be underestimated because the data were based on voluntary ship tracking, and does not include fishing vessels.

• The threat model does not account for marine waste (such as plastics), discarded fishing gear, recreational vessels or shipwrecks, due to a lack of global spatial data on these threats.

• The four local threats described above were then  combined to provide an integrated local threat index. Past thermal stress as described below were treated as an additional threat.

Reef regions ranking for marine-based pollution and damage (see Figure 2 above): most threatened are Atlantic (N°1) & Middle East (N°2); less threatened are Indian (N°5) and Southeast Asia (N°6).

Thermal stress

Estimation of thermal stress over the past 10 years (1998 to 2007) combining the following two data layers:

Past intense heating events. These were areas known to have had high temperature anomalies (scores of degree heating weeks > 8), based on satellite sea surface temperature data provided by NOAA Coral Reef Watch; and

Observations of severe bleaching from Reef Base (see ReefBase) .  

How to assess the outcome of threats which are real damage and true habitat condition on reefs?

A unique and important feature of the Reefs at Risk approach is its global coverage—assessing threats to all reefs, even those far from human habitation and scientific outreach. It is, however, a model, and it measures as explained above threat rather than condition. 

The only way to accurately assess condition is through direct measurement of fish, benthic cover (live coral, dead coral, algae, etc.), or other characteristics. Some reefs, including the Great Barrier Reef, have detailed and regular surveys covering numerous areas, but worldwide such observation or monitoring are sparse and irregular. 

Some threatened reefs may still be healthy, but many others might have already suffered some level of degradation.

Where are located the most threatened coral regions?

The Figure 3 & 4 below are showing that Southeast Asia (N°1 ranking) and Atlantic Reefs (N°2) regions are relatively the most threatened reef regions in the world; then slightly better:  Indian Ocean (N°3) and Middle East (N°4); then Pacific (N°5) less than the averaged global risk; Australia's Great barrier reefs being much less threatened (N°6).

Globally, after factoring by the coral reef regions' area, more than 60% of worldwide coral reefs (about 150,000 km2) are threatened by human local activities (see Figue 3 below) and about 75% (about 185,000 km2) are threatened when past thermal stress is included. 

Figure 3 : Integrated local threats consist of the four local threats—overfishing and destructive fishing, marine pollution and damage, coastal development, and watershed-based pollution

Figure 4 : Amount of reef area (in sq km) in each region classified by integrated local threat categorized as low, medium, and high; the four local threats are integrated plus the thermal stress during the past ten years; this figure summarizes current threats: future warming and acidification are not included

On all counts of relatively (see Figure 3 above) or absolute (see Figure 4 ) threats Shouteast Asia is always the most threatened reef regions in the world and Australia is always the best ranking.

This means that the Southeast Asia is definitely the place where the long-term conservation of coral reefs in the world is at stake, including the survival of many biological species that live there.