The functioning of coral reef communities along environmental gradients

Abstract

One of the primary challenges in ecology is to understand how environmental disturbance affects diversity and community structure, and what are the subsequent consequences on ecosystem functioning. Coral reefs are some of the most diverse ecosystems on the planet resulting in complex sets of interactions between benthic, habitat-forming constituents and mobile fish consumers. However, scleractinian corals, the primary habitat engineers, are dependent on high-light, low-nutrient water conditions and thus are highly responsive when the environment varies from this status. In Southeast Asia, an increase in human coastal populations centred around urban areas has resulted in extensive changes to the coastal environment such as degraded water quality and removal of fish consumers. This has resulted in highly varied abiotic and biotic conditions in relation with distance from the shore. Often, coral reefs closer to shore are much lower in benthic and fish diversity than those further from anthropogenic influences, with direct impacts on ecosystem functioning. Therefore the aim of this thesis was to explore coral reef ecosystem functioning with respect to changes in benthic community structure and fish diversity in relation to varying environmental conditions in the Spermonde Archipelago, Indonesia. A combination of observational, experimental and theoretical analyses were conducted on the functioning of coral reefs using eight islands on a transect of increasing distance from the mainland, varying from 1 to 55 km. At these eight sites, benthic and pelagic surveys identified variation in the status of coral reef communities, while recruitment and feeding assays identified variation in important ecological processes. Lastly, experimental observations were further qualified with stable isotope analysis and the application of contemporary indices of functional diversity. It was found that indeed, the coral reefs varied along a continuum of structure, assemblage and processes. Increasing distance from shore was associated with greater live coral cover and structural complexity, while sites closer to shore were dominated by turf algae and rubble. Furthermore, turf algae was observed as playing a particularly important role, as this group was dominant during recruitment and subsequent development of open benthic space as supplied by terracotta tiles. Fish diversity, along with redundancy in the important herbivore group, also increased with distance from shore, resulting in an increasingly diverse response to Sargassum and Padina assays. The functional composition of the fish assemblages became increasingly variable with loss in coral cover and structural complexity, suggesting communities become destabilised under habitat degradation. Furthermore, stable isotope analysis indicated that the trophic niche of a fish species can increase at sites with more degradation suggesting varying functional utility. However, functioning is not determined only by exposure to chronic, abiotic conditions. Outbreaks of the crown-of-thorns starfish (Acanthaster planci) and mechanical destruction (bomb fishing) resulted in extreme loss of live coral. At these sites, biological and functional diversity displayed some of the lowest values among all sites. Coral reefs can exist in systems with altered water condition if physiological and ecological capacity of the organisms allow for their continuation. Nevertheless, degraded water condition will select against many species, resulting not only in the observed lower biological diversity, but also in less species taking part in functional roles as reflected in higher functional variability. Combined, these results show that the functioning of coral reefs does not exist in discrete states; rather, their functioning is a result of abiotic stressors and biological feedbacks. It is becoming increasingly clear that pristine coral reefs are not a reality in many cases around the world. Thus understanding coral reef functioning at all stages of degradation will help with future management. This thesis adds to the ever-growing knowledge about disturbed coral reefs, but more importantly, it describes the changing relationship between diversity and functioning of coral reefs in relation to disturbance.