Exploring the resilience in coral reefs
Abstract: Considering the unprecedented global decline of coral reefs concerns about their future existence are well-justified. Safeguarding ecological resilience (i.e. the capacity of ecosystems to absorb disturbance without changing their identity) has become a prime goal for management in order to combat further degradation of coral reefs. This thesis uses the concept of ecological resilience as the theoretical framework to analyze vulnerability of coral reefs exposed to human interventions. This thesis consists of four papers. Papers 1-3 are based on field data from Zanzibar Island, Tanzania, whereas Paper 4 is a synthesis that explores the use of the resilience concept in coral reefs, putting the first papers into a broader context.Paper 1 investigates the distribution and estimate the status of functional groups of coral, fish and sea urchins on five coral reefs outside the western coast of Zanzibar Island. The study provides a first ecological “baseline” that may help detect future degradation and evaluate the effects of impending management interventions. The results show that reefs with high accessibility, i.e. close to shore and open to fisheries, have lower abundance and diversity of functional groups of both coral and fish compared to more remote or protected reefs. Paper 2 analyzes the impact of artisanal fishing on three key functional groups of herbivorous (grazers, scrapers and bioeroders). The study shows a negative correlation between fishing pressure and fish biomass, abundance, and diversity. The study also demonstrates a negative influence of fishing on the demographic structure of functional groups. Paper 3 focuses on the scraping function (i.e. the capacity of fish to remove algae and open up bare substratum for coral larval settlement) and investigates how body size of individual fishes influences the function. The results reveal a non-linear relationship between body size and scraping function and suggest that fishes start to have a significant impact on the function only after reaching a certain size. The results from Paper 1-3 suggest that human interventions (fishing in particular) can have profound impacts on the distribution and composition of functional groups which influence the vulnerability of coral reefs. Paper 4 provides an overview of the divergent uses of the resilience concept and proposes a range of empirical indicators which can be helpful when assessing coral reef resilience, such as functional groups, the ratio of “good” and “bad” colonizers of space, demographic skewness, discontinuities, the distribution of local phase shifts in the seascape and estimates of potential space availability against grazing capacity.
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