Response of coral reef fish assemblages to a severe natural disturbance

Background/Question/Methods

While many studies focus on documenting the degradations occurring in species-rich ecosystems due to human activities, fewer efforts have been undertaken to understand how such ecosystems cope with natural disturbances. Yet, species-rich ecosystems such as coral reefs are generally subjected to recurrent natural disturbances (e.g., hurricanes). Because intensity and frequency of these disturbances are expected to increase, understanding how associated assemblages respond to them may help to determine the mechanisms underlying coral reef resilience.

In this study we focus on the response of coral reef fish assemblages from Moorea Island (French Polynesia) to an extremely severe crown-of-thorns starfish (COTS) outbreak, one of the most severe natural disturbances affecting coral reefs. Using data collected from 2004 to 2012 on both fish species and coral cover on 13 reefs surrounding the island, we analyzed changes in species composition (fish biomass) and functional composition (based on five functional traits) using β diversity. 

Results/Conclusions

Although the COTS outbreak caused a logistically declined of 90% in coral cover in a few years over all reefs, the fish biomass showed relative stability. Nonetheless, we showed that a temporal shift in fish species composition occurred at each reef. The shift was due to high temporal turnover in biomass among species which explained 66% of the total β diversity. Yet, interaction between space and time also significantly explained variation in fish species composition suggesting that these temporal changes were contrasted among reefs. Indeed, the set of species involved in this turnover markedly differed between western and northeastern reefs. Functional β diversity was, on the other hand, lower than taxonomic β diversity indicating that temporal turnover among species did not translate into major functional changes.

In conclusion, turnover among species is an important mechanism underlying the resilience of fish assemblages to natural disturbances. Functional redundancy between the species involved in this turnover may allow the system to maintain the same functioning. This study also suggests that natural disturbances can be an important driver of species-rich ecosystem temporal dynamics.