Climate driven regime shifts vs rebound potential

Conditions under which coral reefs show resilience and regenerate from bleaching events or dramatically shift in state to less diverse and productive algal states are not well known in ecology yet. This is a severe drawback if one wants to investigate the impacts of climate change and associated bleaching events on coral reef fish. As I have come to understand and explain through previous posts, the potential for coral reef fish to survive bleaching events depends on the frequency and severity of these events. There are also a wide range of biological and behavioral factors that influence this, such as the specificity of diet, range and distribution of these species. We already know that a loss of coral cover significantly impacts juveniles that rely on cover to survive and evade predators.

A letter published to Nature this year by Graham et al., one of the leading researchers on this issue looks to understand the dynamics by which a >90% loss of coral cover can lead to both recovery and  a shifting of regime, two completely different responses. And these two different reef responses translate to two distinctive fish species responses, with a return to predistrubance structure on recovering reefs yet becoming significantly and progressively altered on those that shifted regimes. Using a 17 year dataset the paper assesses the long term ecosystem dynamics of 21 reef sites across Seychelles, the most severely affected area by the 1998 el Nino.

Figure 1. Response or recovery of coral reef assemblages post disturbance (source: Graham et al.,2015)

Results from the study found 12 of 21 sites recovered but this was laboured over the first 7-10 years, speeding up after this period as local recruitment levels increased. The main point to take from the paper is that a trajectory towards recovery occurred when reefs were structurally complex and in deeper water. In these area juvenile corals and herbivorous fish populations were high and nutrient loads low. This is interesting as it suggests a level of symbiotism between coral reef fish and coral reefs; while the fish species rely on the reef for protection and for survival, the coral species depend on the herbivorous fish species to recover. Whilst the paper does give an interesting insight into reef responses to disturbance it is important to remember this was only one rather local study, there could be significant regional differences in factors that influence such response.