The relevance and significance of corals being undeniable, it is of concern that coral reef abundance and diversity has rapidly declined over the past decades. This worldwide degradation is partially induced by natural causes like storms but is mainly driven and enhanced by anthropogenic (human-induced) disturbances.
Coral abundance is naturally limited by recruitment, which describes the successful growth of coral larvae into adults. However, high predation risk and spatial competition in the early life stage can lead to coral mortalities of up to 100%, thus making this early phase a true demographic bottleneck for coral populations.
Since corals are sessile animals and cannot move away from unfavourable conditions, the success of those that made it through the initial bottleneck phase depends on the environmental conditions surrounding them. A major factor is the sea surface temperature. Corals are found in tropical regions all around the world, where the temperatures are already elevated. In other words, most corals are already living close to their thermal threshold. Climate-change induced rise of temperatures, therefore, poses an incredible threat to our coral reefs worldwide. In the past decades, increased sea surface temperatures have led to coral bleaching events over extended periods. Coral bleaching is the dissociation of the coral-algal symbiosis that becomes unbeneficial when the temperature rises too high. As a consequence, the polyp expels the colourful algae resulting in the coral turning white. With the algae gone, the coral loses its primary energy source. Consequently, coral bleaching is generally followed by high levels of coral mortality.
Another major threat for coral reefs is the phenomenon of ocean acidification. About 40% of the carbon dioxide (CO2) that we release into the atmosphere through burning fossil fuels is taken up by the ocean. As a result, increased levels of CO2 in our oceans lead to a decrease in available carbonate ions (CO32-) in the water column. However, corals require CO32- to build their protective skeletons. Therefore, a reduction of available CO32- significantly diminishes the ability of corals to build their protective skeleton and inexorably leads to coral reef degradation.
Additional stressors such as eutrophication (i.e. increased nutrient levels in the ocean), sedimentation and unsustainable fishing have also contributed to the worldwide coral decline. Altogether, the disturbances mentioned above are direct causes of coral reef degradation and have indirect negative impacts since stress dramatically reduces the resilience of a reef and thereby facilitates global disease outbreaks, which ultimately can lead to coral bleaching and coral death.