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Ecological connectivity in East African seascapes
Stockholm University, Faculty of Science, Department of Systems Ecology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Coral reefs, seagrass beds and mangroves constitute a complex mosaic of habitats referred to as the tropical seascape. Great gaps exist in the knowledge of how these systems are interconnected. This thesis sets out to examine ecological connectivity, i.e. the connectedness of ecological processes across multiple scales, in Zanzibar and Mafia Island, Tanzania. Paper I examined the current knowledge of interlinkages and their effect on seascape functioning, revealing that there are surprisingly few studies on the influences of cross-habitat interactions and food-web ecology. Furthermore, 50% of all fish species use more than one habitat and 18% of all coral reef fish species use mangrove or seagrass beds as juvenile habitat in Zanzibar. Paper II examined the seascape of Menai Bay, Zanzibar using a landscape ecology approach and studied the relationship between fish and landscape variables. The amount of seagrass within 750m of a coral reef site was correlated with increased invertebrate feeder/piscivore fish abundance, especially Lethrinidae and Lutjanidae, which are known to perform ontogenetic and feeding migrations. Within patch seagrass cover was correlated with nursery species abundance. Paper III focused on a seagrass-dominated seascape in Chwaka Bay, Zanzibar and showed that small-scale habitat complexity (shoot height and density) as well as large-scale variables such as distance to coral reefs affected abundance and distribution of a common seagrass parrotfish Leptoscarus vaigiensis. Paper IV studied the connectivity and functional role of two snappers (Lutjanus fulviflamma and L. ehrenbergii) using stable isotopes (δ15N and δ13C) and found that connectivity between habitats was maintained by ontogenetic and foraging migrations by these species. The thesis concludes that ecological connectivity and multi-habitat usage by fish is a general and important characteristic in the Western Indian Ocean and should be considered in management planning.

Place, publisher, year, edition, pages
Stockholm: Department of Systems Ecology, Stockholm University , 2012. , 39 p.
Keyword [en]
fish, functional group, food-web interactions, nursery, coral, seagrass, mangrove, landscape ecology
National Category
Ecology
Research subject
Marine Ecotoxicology
Identifiers
URN: urn:nbn:se:su:diva-75194ISBN: 978-91-7447-477-0 (print)OAI: oai:DiVA.org:su-75194DiVA: diva2:515890
Public defence
2012-05-25, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Submitted.

Available from: 2012-05-03 Created: 2012-04-11 Last updated: 2013-04-08Bibliographically approved
List of papers
1. Exploring 'knowns' and 'unknowns' in tropical seascape connectivity with insights from East African coral reefs
Open this publication in new window or tab >>Exploring 'knowns' and 'unknowns' in tropical seascape connectivity with insights from East African coral reefs
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2012 (English)In: Estuarine, Coastal and Shelf Science, ISSN 0272-7714, E-ISSN 1096-0015, Vol. 107, 1-21 p.Article in journal (Refereed) Published
Abstract [en]

Applying a broader landscape perspective to understand spatio-temporal changes in local populations and communities has been increasingly used in terrestrial systems to study effects of human impact and land use change. With today’s major declines in fishery stocks and rapid degradation of natural coastal habitats, the understanding of habitat configuration and connectivity over relevant temporal and spatial scales is critical for conservation and fisheries management of the seascape. Coral reefs, seagrass beds and mangroves are key-components of the tropical seascape. The spatial distribution of these habitat-types may have strong influences on cross-habitat migration and connectivity patterns among organisms. However, the consequences of seascape fragmentation and ecological connectivity are largely unknown. Here, we review the literature to provide an overview of current knowledge with regards to connectivity and food-web interactions within the tropical seascape. We show that information on fish acting as mobile links and being part of nutrient transfer and trophic interactions is scarce. We continue by making an in-depth analysis of the seascape around Zanzibar (Eastern Africa) to fill some of the knowledge gaps identified by the literature survey. Our analysis shows that (i) fifty percent of all fish species found within the Zanzibar seascape use two or multiple habitat-types, (ii) eighteen percent of all coral reef-associated fish species use mangrove and seagrass beds as juvenile habitat, and (iii) macrocarnivores and herbivores are highly represented among those coral reef fish species that use mangrove and seagrass beds as juvenile habitat. We argue that understanding the inter-linkages within and between habitat-types is essential for successful management of the tropical seascape.

Keyword
trophic interactions, fish, tropical, coral reef, seagrass, mangrove
National Category
Ecology
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-75186 (URN)10.1016/j.ecss.2012.03.020 (DOI)000306633900001 ()
Available from: 2012-04-16 Created: 2012-04-11 Last updated: 2017-12-07Bibliographically approved
2. Seascape configuration influences connectivity of reef fish assemblages
Open this publication in new window or tab >>Seascape configuration influences connectivity of reef fish assemblages
(English)In: Biological Conservation, ISSN 0006-3207, E-ISSN 1873-2917Article in journal (Refereed) Submitted
Abstract [en]

Shallow-water habitats within tropical seascapes are intimately connected through ontogenetic and/or feeding migrations of fish. Knowledge on connectivity in the Indo-Pacific region is however sparse. Landscape ecology has been suggested as a useful approach when studying seascape connectivity. In this study, we examine the influence of habitat connectivity on reef fish assemblages in shallow-water habitats surrounding Zanzibar (Tanzania), using a seascape approach. We tested the relationships between a set of landscape and habitat variables and fish diversity and density for different functional groups and life stages. Habitat data was collected at scales ranging from 1m to >2km using aerial photography and ground-truthing. Fish data was collected using a standardised point census method. Furthermore, semi-structured interviews with 127 fishers in the bay were conducted to account for different fishing activity. We show that coral reefs in a complex seascape of Zanzibar are connected to seagrass beds through migration of fish. Habitat connectivity of seagrass and seagrass/coral mix within a 750m radius of coral reefs had a positive influence on fish abundances in the functional group of invertebrate feeders/piscivores, especially within the family Lutjanidae and Lethrinidae. Within-patch seagrass cover had a positive influence on nursery species. Depth also had a positive influence on fish assemblages, highlighting the importance of considering a third dimension, not accounted for in terrestrial studies. Generally, fishing activity between sites did neither influence species richness nor abundance, except for the abundance of juvenile parrotfish. We demonstrate that a landscape ecology approach, combining connectivity and habitat variables, is important for understanding and managing the tropical seascape, although it must be applied at relevant scales, habitat metrics and seascape configurations to fully capture ecological connectivity.

National Category
Ecology
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-75187 (URN)
Note

Submitted article part of dissertation.

Available from: 2012-04-16 Created: 2012-04-11 Last updated: 2017-12-07Bibliographically approved
3. Scale-dependent patterns of variability of a grazing parrotfish (Leptoscarus vaigiensis) in a tropical seagrass-dominated seascape
Open this publication in new window or tab >>Scale-dependent patterns of variability of a grazing parrotfish (Leptoscarus vaigiensis) in a tropical seagrass-dominated seascape
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2011 (English)In: Marine Biology, ISSN 0025-3162, E-ISSN 1432-1793, Vol. 158, no 7, 1483-1495 p.Article in journal (Refereed) Published
Abstract [en]

Although herbivorous fish form critical linkages between primary producers and higher trophic levels, the major factors regulating their spatial structure in seagrass systems remain poorly understood. The present study examined the parrotfish Leptoscarus vaigiensis in seagrass meadows of a tropical embayment in the western Indian Ocean. Stomach content analysis and direct field observations showed that L. vaigiensis is an efficient grazer, feeding almost exclusively on seagrass leaves. Seagrass shoot density was highly correlated to all density variables (total, juvenile and subadult) and juvenile biomass of L. vaigiensis, while subadult biomass was predicted by distance to neighbouring coral habitat. Moreover, density and biomass of predatory fish (piscivores) were predicted by seagrass canopy height and the distribution patterns of predators followed those of L. vaigiensis. Hence, factors at local (seagrass structural complexity and feeding mode) and landscape scale levels (seascape context and distribution of piscivores) likely mutually structure herbivorous fish communities. The findings underscore the importance of incorporating multiple scale-dependent factors when managing coastal seagrass ecosystems and their associated key species.

National Category
Biological Sciences
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-66998 (URN)10.1007/s00227-011-1665-z (DOI)000291922200005 ()
Note
5Available from: 2011-12-28 Created: 2011-12-22 Last updated: 2017-12-08Bibliographically approved
4. Ecological connectivity and niche differentiation between two closely related fish species in the mangrove−seagrass−coral reef continuum
Open this publication in new window or tab >>Ecological connectivity and niche differentiation between two closely related fish species in the mangrove−seagrass−coral reef continuum
2013 (English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 477, 201-215 p.Article in journal (Refereed) Published
Abstract [en]

We aim to understand ontogenetic shifts in habitat use and feeding patterns by 2 fish species, Lutjanus fulviflamma and L. ehrenbergii, within a tropical seascape in East Africa. Stomach contents and stable isotope signatures of muscle tissues (δ13C and δ15N) were compared between and within species. Fish of all life stages and potential food items were sampled from mangrove creeks, seagrass beds, and coral reefs around Mafia Island, Tanzania. Due to similarities in morphology between species, correct species identity was confirmed using genetic barcoding (mtDNA, partial sequence of cytochrome oxidase subunit I [COI]). Stable isotope analysis in R  (based on mixing models) confirmed that δ13C and δ15N values in L. fulviflamma and L. ehrenbergii reflected those of prey items caught in different habitats. Diets and mean δ13C and δ15N values of muscle tissue differed between life stages of fish, indicating ontogenetic changes in habitat and diet. L. fulviflamma and L. ehrenbergii differed in diet and δ13C and δ15N values of muscle tissue, although they overlapped in habitat use, suggesting food resource partitioning between the 2 species. Furthermore, diet overlap indexes were low between subadult species in mangrove and seagrass or coral habitats. L. fulviflamma displayed a diet shift with decreasing importance of small crustaceans in juveniles and an increasing importance of prey fishes in subadults and adults. L. ehrenbergii showed the opposite pattern. The study verifies feeding interlinkage within the mangrove-seagrass-coral reef continuum in Mafia Island by providing strong evidence of ontogenetic migration. Understanding these connections will enhance our ability to manage tropical seascapes, and highlights the need to include multiple habitats in marine protected areas.

Keyword
Stable isotopes, Stomach content, Ontogenetic shifts, Connectivity, Resource partitioning, Coral reef, Seagrass, Mangrove
National Category
Ecology
Research subject
Marine Ecotoxicology
Identifiers
urn:nbn:se:su:diva-75190 (URN)10.3354/meps10171 (DOI)
Available from: 2012-04-16 Created: 2012-04-11 Last updated: 2017-12-07Bibliographically approved

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