GREAT BARRIER REEF
// Outlook Report 2014
Another way to consider the cumulative effect of threats is by examining a particular component of the ecosystem or a distinct heritage value, for example coral reef habitats (Figure 9.7). This has generally been done previously at a single species level.18,19 More recently, qualitative models developed for the Region have provided an initial assessment of some of the complexity associated with understanding cumulative effects on coral reef habitats.16 The model shows some cause-and-effect relationships are relatively simple, such as an increase in ocean warming making coral bleaching events more frequent, which then leads to a reduction in hard coral cover. However, other relationships are much more complex. For example, an increase in land-based run-off from agriculture drives four threats identified in the model (toxins, nutrients, turbidity and sedimentation), in turn affecting seven ecosystem variables (predatory fish, herbivorous fish, crown-of-thorns starfish, fish and invertebrates, macroalgae, crustose coralline algae and coral recruitment, and coral cover).
The accumulation of many threats increases the overall risk.
Figure 9.7 Cumulative effects on coral reef habitats
As noted in Chapter 2, the condition of coral reef habitats is declining. The major causes of this decline (land-based run-off, outbreaks of crown-of-thorns starfish, coral bleaching and cyclones) have been identified at a broad Region-wide scale.14,20,21 However, when considered at the local scale, there are many additional threats that directly affect coral reef habitats. Like any of the Region’s ecosystem and heritage values, consideration of all threats affecting coral reefs, regardless of the level of risk or the scale at which the threat operates, is essential to improving the habitat’s resilience. This is particularly important given the declining trends for coral reef habitats in much of the Region and the central role they play in the Reef ecosystem and its outstanding universal value as a world heritage property.