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Exploring the resilience in coral reefs
Stockholm University, Faculty of Science, Department of Systems Ecology. (Natural Resources Management)
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 [en]
Coral Reefs, Resilience, Functional Groups, Vulnerability, Overfishing, Ecosystem Indicators, Tanzania, Zanzibar
National Category
Ecology
Research subject
Natural Resources Management
Identifiers
URN: urn:nbn:se:su:diva-26659ISBN: 978-91-7155-857-2 (print)OAI: oai:DiVA.org:su-26659DiVA: diva2:210852
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
List of papers
1. A comparison of functional groups in coral reefs around Zanzibar Island (Tanzania)
Open this publication in new window or tab >>A comparison of functional groups in coral reefs around Zanzibar Island (Tanzania)
Show others...
(English)Manuscript (Other academic)
Abstract [en]

Species’ traits determine how biodiversity interacts with ecosystem processes and influence the capacity of ecosystems to respond to and recover from disturbances. Classifying species with regards to their traits, i.e. functional groups, and analyzing their distribution provides a mechanistic approach to investigate the resilience in ecosystems. This study investigates the status of functional groups of coral, fish and sea urchins on reefs on the west coast of Zanzibar Island, Tanzania. The classification of functional groups is based on traits reflecting important community processes or properties which underpin ecosystem resilience. 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. More specifically, highly accessible reefs display lower abundances of herbivorous fish (macroalgae browsers in particular), large-bodied fish, structurally complex corals and corals with certain reproduction strategies. Based on these findings we speculate what this means in terms of ecosystem functioning and vulnerability. This study also provides a first “baseline” of functional group distribution and although it represents an already degraded state it may serve as an important comparison when evaluating further degradation and effects of impending management interventions.

Keyword
Biodiversity, Coral reefs, Ecosystem resilience, Functional analysis, Functional groups, Vulnerability
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-26656 (URN)
Available from: 2009-04-06 Created: 2009-04-06 Last updated: 2010-01-14Bibliographically approved
2. Assessing potential vulnerability of coral reefs through functional groups of herbivores
Open this publication in new window or tab >>Assessing potential vulnerability of coral reefs through functional groups of herbivores
(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:nbn:se:su:diva-26658 (URN)
Available from: 2009-04-06 Created: 2009-04-06 Last updated: 2010-01-14Bibliographically approved
3. The non-linear relationship between body size and function in parrotfihes
Open this publication in new window or tab >>The non-linear relationship between body size and function in parrotfihes
2008 (English)In: Coral reefs (Print), ISSN 0722-4028, E-ISSN 1432-0975, Vol. 27, no 4, 967-974 p.Article in journal (Refereed) Published
Abstract [en]

Parrotfishes are a group of herbivores that play an important functional role in structuring benthic communities on coral reefs. Increasingly, these fish are being targeted by fishermen, and resultant declines in biomass and abundance may have severe consequences for the dynamics and regeneration of coral reefs. However, the impact of overfishing extends beyond declining fish stocks. It can also lead to demographic changes within species populations where mean body size is reduced. The effect of reduced mean body size on population dynamics is well described in literature but virtually no information exists on how this may influence important ecological functions. The study investigated how one important function, scraping (i.e., the capacity to remove algae and open up bare substratum for coral larval settlement), by three common species of parrotfishes (Scarus niger, Chlorurus sordidus, and Chlorurus strongylocephalus) on coral reefs at Zanzibar (Tanzania) was influenced by the size of individual fishes. There was a non-linear relationship between body size and scraping function for all species examined, and impact through scraping was also found to increase markedly when fish reached a size of 15-20 cm. Thus, coral reefs which have a high abundance and biomass of parrotfish may nonetheless be functionally impaired if dominated by small-sized individuals. Reductions in mean body size within parrotfish populations could, therefore, have functional impacts on coral reefs that previously have been overlooked.

National Category
Ecology
Identifiers
urn:nbn:se:su:diva-15616 (URN)10.1007/s00338-008-0394-3 (DOI)000260616400027 ()
Available from: 2008-12-06 Created: 2008-12-06 Last updated: 2017-12-13Bibliographically approved
4. Capturing the cornerstones of coral reef resilience: linking theory to practice
Open this publication in new window or tab >>Capturing the cornerstones of coral reef resilience: linking theory to practice
2008 (English)In: Coral reefs (Print), ISSN 0722-4028, E-ISSN 1432-0975, Vol. 27, no 4, 795-809 p.Article in journal (Refereed) Published
Abstract [en]

Coral reefs can undergo unexpected and dramatic changes in community composition, so called phase shifts. This can have profound consequences for ecosystem services upon which human welfare depends. Understanding of this behavior is in many aspects still in its infancy. Resilience has been argued to provide insurance against unforeseen ecosystem responses in the face of environmental change, and has become a prime goal for the management of coral reefs. However, diverse definitions of resilience can be found in the literature, making its meaning ambiguous. Several studies have used the term as a theoretical framework and concern regarding its practical applicability has been raised. Consequently, operationalizing theory to make resilience observable is an important task, particularly for policy makers and managers dealing with pressing environmental problems. Ultimately this requires some type of empirical assessments, something that has proven difficult due to the multidimensional nature of the concept. Biodiversity, spatial heterogeneity, and connectivity have been proposed as cornerstones of resilience as they may provide insurance against ecological uncertainty. The aim of this article is to provide an overview of the divergent uses of the concept and to propose empirical indicators of the cornerstones of coral reef resilience. These indicators include functional group approaches, the ratios of "good" and "bad" colonizers of space, measurements of spatial heterogeneity, and estimates of potential space availability against grazing capacity. The essence of these operational indicators of resilience is to use them as predictive tools to recognize vulnerability before disturbance occurs that may lead to abrupt phase shifts. Moving toward operationalizing resilience theory is imperative to the successful management of coral reefs in an increasingly disturbed and human-dominated environment.

Keyword
Diversity, Functional groups, Management, Phase shifts, Coral reefs, Resilience
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-15628 (URN)10.1007/s00338-008-0426-z (DOI)000260616400011 ()
Available from: 2008-12-07 Created: 2008-12-07 Last updated: 2017-12-13Bibliographically approved

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