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Assessing potential vulnerability of coral reefs through functional groups of herbivores
Stockholm University, Faculty of Science, Department of Systems Ecology. (Natural Resouce Management)
Stockholm University, Faculty of Science, Department of Systems Ecology. (Natural Resouce Management)
Stockholm University, Faculty of Science, Department of Systems Ecology. (Natural Resouce Management)
ARC Centre of Excellence for Coral Reef Studies, James Cook University.
(English)Manuscript (Other academic)
Abstract [en]

Fishing can have major influences on ecological processes on coral reefs. Overfishing of herbivores is particularly detrimental, generating ecosystem-wide impacts where algae overgrow corals and become the dominant benthic organism. This can result in altered ecosystem functioning and subsequently, loss of important ecosystem services. Knowing when important processes, such as herbivory, are becoming fragile is important because it can provide an opportunity for managers to avoid such undesirable ecosystem changes. This study investigates the impact of artisanal fishing on three important functional groups of herbivores (grazers, scrapers and bioeroders) on five coral-dominated reefs outside Zanzibar, Tanzania. Fishing pressure was assessed through interviews with households and fishermen and compared with ecological data for each of the three focal functional groups. The ecological status of the groups were assessed through analysis of species richness, -abundance, -biomass and demographic structure; variables relating to functional performance and in a wider sense ecosystem resilience. The results showed a negative correlation between fishing pressure and fish biomass, -abundance, and diversity. Moreover, fishing had a negative influence on the demographic structure of functional groups, particularly for bioeroders, manifesting as a skewness towards smaller individuals within species populations. Fishing pressure also correlated positively with sea urchin abundances suggesting a compensatory response within the guild of herbivores, which could explain the low abundances of macro algae on all of the investigated reefs. This study shows that artisanal fishing can have significant impacts on herbivores which may erode the resilience on coral reefs. Moreover, it illustrates how functional groups may help to expose potential vulnerability of coral reefs by mechanistically linking the diversity and identity of species to ecosystem processes.

National Category
Ecology
Research subject
Natural Resources Management
Identifiers
URN: urn:nbn:se:su:diva-26658OAI: oai:DiVA.org:su-26658DiVA: diva2:210831
Available from: 2009-04-06 Created: 2009-04-06 Last updated: 2010-01-14Bibliographically approved
In thesis
1. Exploring the resilience in coral reefs
Open this publication in new window or tab >>Exploring the resilience in coral reefs
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

 

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University, 2009. 50 p.
Keyword
Coral Reefs, Resilience, Functional Groups, Vulnerability, Overfishing, Ecosystem Indicators, Tanzania, Zanzibar
National Category
Ecology
Research subject
Natural Resources Management
Identifiers
urn:nbn:se:su:diva-26659 (URN)978-91-7155-857-2 (ISBN)
Public defence
2009-05-08, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 8 A, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-04-17 Created: 2009-04-06 Last updated: 2009-04-06Bibliographically approved

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