Portfolio effect and bet-hedging adaptations in sea turtles
- Authors: Hoekstra, Shaun
- Date: 2024-12
- Subjects: Wildlife conservation , Sea turtles -- South Africa , Animal behavior
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/68873 , vital:77153
- Description: A great proportion of marine species are at risk of going extinct as a result of profound changes in the world’s oceans, primarily driven by human activities. Sea turtles are not exempt from this fate. Despite sea turtles being exposed to a range of environmental conditions and threats that vary in frequency and intensity, these ancient marine reptiles have persisted for millennia. In the past, sea turtles have been able to adapt to natural perturbations while other animal groups have not. The ability of sea turtles to adapt to contemporary changes in conditions, produced by human activities, has become limited. The persistence of sea turtles may be explained by the application of a concept termed “portfolio effect”. An aggregate system is more stable and less prone to fluctuations over time (i.e. portfolio effect) if a diversity of finer components comprises the entire system and displays asynchronous fluctuations over time. Biological entities (e.g. species measured by the abundance trend) may achieve long-term stability even though the individual components (e.g. populations measured in abundance trend) display weak or negative correlations with each other in space and time. Different sea turtle populations may display independent trends in abundances over time, resulting in a portfolio effect at species levels. Trends in sea turtle abundances may be affected by individual behaviour. The adoption of bet-hedging adaptations may enable populations to thrive despite environmental changes or stochasticity, by lowering short-term fitness in exchange for maximising long-term fitness. Individual sea turtles may exhibit bet-hedging adaptations through changes in their temporal nesting behaviour, including the remigration interval. Since remigration interval length regulates the number of nesting females per nesting season, changes in remigration interval will affect abundance trends. The ability of individuals to change their remigration interval allows them to “skip” the breeding migration when environmental conditions seem unfavourable. The overall aim of this thesis was to investigate potential reasons for observed sea turtle abundance trends at species, population, and rookery levels. I tested whether the portfolio effect applies to sea turtles by analysing and interpreting spatiotemporal trends in loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) abundances at the population and species levels using global datasets. The application of bet-hedging strategies was tested at a local level using data from the long-term monitoring program for South African nesting sea turtles. The majority of loggerhead rookeries displayed an increase in abundance over time, while most leatherback rookeries declined (Chapter 2). Population growth rates also differed among rookeries within Regional Management Units (RMUs) (Chapter 2). This diversity in abundance trends among rookeries suggested spatiotemporal variation in environmental conditions, threats and/or level of protection accompanying sea turtles. The species-level growth rate for both species displayed an overall increase in abundance over time. However, larger (more turtles) leatherback rookeries/RMUs experienced more rapid declines in abundance compared to smaller rookeries/RMUs, resulting in a recent species-level decline. Uncorrelated trends in abundances at the RMU-level could have stabilised species-level trends. Differences in the remigration intervals as a possible bet-hedging adaptation could potentially explain differences in population recovery rates between South Africa’s loggerhead and leatherback sea turtles (Chapter 3), but found no evidence that individuals from either species exhibit bet-hedging adaptations. Loggerheads, however, have increased their remigration intervals over time, while leatherbacks have not. These increases were accompanied by declines in adult body size (previously established) which could be an effect of diminishing foraging area quality. The increased remigration interval shown by loggerheads may be a response rather than an adaptation to the prevailing environmental conditions that impact their food supply in their foraging areas. The population growth rate of leatherback sea turtles may surpass that of the loggerheads in the near future, although there is a global decline in abundance (Chapter 2). This dissertation provides information on possible reasons for observed trends in sea turtle abundances at various levels of biological organisation. While a variety of factors have the potential to influence trends in sea turtle abundance, the possible occurrence of portfolio effects and investigating nesting behaviour may resolve any uncertainties regarding the reasons for observed trends in abundance. , Thesis (MSc) -- Faculty of Science, School of Environmental Sciences, 2024
- Full Text:
- Date Issued: 2024-12
- Authors: Hoekstra, Shaun
- Date: 2024-12
- Subjects: Wildlife conservation , Sea turtles -- South Africa , Animal behavior
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/68873 , vital:77153
- Description: A great proportion of marine species are at risk of going extinct as a result of profound changes in the world’s oceans, primarily driven by human activities. Sea turtles are not exempt from this fate. Despite sea turtles being exposed to a range of environmental conditions and threats that vary in frequency and intensity, these ancient marine reptiles have persisted for millennia. In the past, sea turtles have been able to adapt to natural perturbations while other animal groups have not. The ability of sea turtles to adapt to contemporary changes in conditions, produced by human activities, has become limited. The persistence of sea turtles may be explained by the application of a concept termed “portfolio effect”. An aggregate system is more stable and less prone to fluctuations over time (i.e. portfolio effect) if a diversity of finer components comprises the entire system and displays asynchronous fluctuations over time. Biological entities (e.g. species measured by the abundance trend) may achieve long-term stability even though the individual components (e.g. populations measured in abundance trend) display weak or negative correlations with each other in space and time. Different sea turtle populations may display independent trends in abundances over time, resulting in a portfolio effect at species levels. Trends in sea turtle abundances may be affected by individual behaviour. The adoption of bet-hedging adaptations may enable populations to thrive despite environmental changes or stochasticity, by lowering short-term fitness in exchange for maximising long-term fitness. Individual sea turtles may exhibit bet-hedging adaptations through changes in their temporal nesting behaviour, including the remigration interval. Since remigration interval length regulates the number of nesting females per nesting season, changes in remigration interval will affect abundance trends. The ability of individuals to change their remigration interval allows them to “skip” the breeding migration when environmental conditions seem unfavourable. The overall aim of this thesis was to investigate potential reasons for observed sea turtle abundance trends at species, population, and rookery levels. I tested whether the portfolio effect applies to sea turtles by analysing and interpreting spatiotemporal trends in loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) abundances at the population and species levels using global datasets. The application of bet-hedging strategies was tested at a local level using data from the long-term monitoring program for South African nesting sea turtles. The majority of loggerhead rookeries displayed an increase in abundance over time, while most leatherback rookeries declined (Chapter 2). Population growth rates also differed among rookeries within Regional Management Units (RMUs) (Chapter 2). This diversity in abundance trends among rookeries suggested spatiotemporal variation in environmental conditions, threats and/or level of protection accompanying sea turtles. The species-level growth rate for both species displayed an overall increase in abundance over time. However, larger (more turtles) leatherback rookeries/RMUs experienced more rapid declines in abundance compared to smaller rookeries/RMUs, resulting in a recent species-level decline. Uncorrelated trends in abundances at the RMU-level could have stabilised species-level trends. Differences in the remigration intervals as a possible bet-hedging adaptation could potentially explain differences in population recovery rates between South Africa’s loggerhead and leatherback sea turtles (Chapter 3), but found no evidence that individuals from either species exhibit bet-hedging adaptations. Loggerheads, however, have increased their remigration intervals over time, while leatherbacks have not. These increases were accompanied by declines in adult body size (previously established) which could be an effect of diminishing foraging area quality. The increased remigration interval shown by loggerheads may be a response rather than an adaptation to the prevailing environmental conditions that impact their food supply in their foraging areas. The population growth rate of leatherback sea turtles may surpass that of the loggerheads in the near future, although there is a global decline in abundance (Chapter 2). This dissertation provides information on possible reasons for observed trends in sea turtle abundances at various levels of biological organisation. While a variety of factors have the potential to influence trends in sea turtle abundance, the possible occurrence of portfolio effects and investigating nesting behaviour may resolve any uncertainties regarding the reasons for observed trends in abundance. , Thesis (MSc) -- Faculty of Science, School of Environmental Sciences, 2024
- Full Text:
- Date Issued: 2024-12
The foraging ecology of Wedge-tailed Shearwaters (Ardenna Pacifica) in the tropical Western Indian Ocean
- Authors: Keys, Danielle Zuanda
- Date: 2018
- Subjects: Birds of prey -- Behavior -- South Africa -- Western Cape , Animal behavior , Wildlife conservation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/31251 , vital:31348
- Description: The distribution and abundance of prey largely influences how marine predators forage. Thus, understanding how marine predators respond to such changes becomes imperative when making decisions in marine spatial planning, especially in light of global climate change. Tropical marine systems are known for their relatively low productivity and high unpredictability of prey resources. Compared to higher latitudes, very few studies on seabirds have been implemented in the tropics. The Wedge-tailed Shearwater Ardenna pacifica is an Indo-Pacific tropical seabird with a wide distributional range. However, like many other seabirds, its population numbers are currently declining. Very little is known about the at-sea distribution of Wedge-tailed Shearwaters in the tropical western Indian Ocean. Therefore, this thesis investigates the foraging ecology of Wedge-tailed Shearwaters in the western Indian Ocean by tracking the fine scale movements of individuals at sea. This study was conducted in two different oceanic environments, namely D’Arros Island and Fouquet Island, St Joseph Atoll in the Amirantes Group in Seychelles archipelago as well as Réunion Island in the Mascarene archipelago during the Wedge-tailed Shearwaters breeding seasons. Here I aimed to investigate the foraging distribution of Wedge-tailed Shearwaters on Réunion Island in relation to different stages of the breeding period. I also compared the foraging ecology of all three populations, subjected to different abiotic and biotic factors. Furthermore, I investigated foraging site fidelity in individuals and repeatability within individual flight characteristics. To my knowledge, this is the first study to comprehensively investigate the foraging ecology of Wedge-tailed Shearwaters using global positioning system (GPS) loggers. This study demonstrated that the Wedge-tailed Shearwaters implement flexible foraging strategies throughout different stages of their breeding period and between different islands. In Réunion, during the shearwater breeding season, sexual segregation was prominent during early chick rearing and both sexes implemented dual foraging. As suggested by previous studies, it is likely that long trips were implemented for self-maintenance, whereas multiple short trips were implemented to meet the high energetic demands of the chicks. Several important foraging areas were identified for these Wedge-tailed Shearwaters. During incubation, shearwaters from Réunion faithfully foraged towards the south east of Madagascar near the East Madagascar Current and flew over large areas of the Mascarene Basin. During chick rearing, shearwaters significantly constricted their range closer to the colony and exploited deep waters on the south-west of Réunion Island. Shearwaters concentrated near oceanographic features such as the Pérouse Seamount (which is a known area of high productivity) as well as the Bourdonnais Ridge. There was no clear evidence for spatial or sexual segregation between individuals from the two study colonies in Seychelles during incubation and birds utilized a broader foraging range than birds from Réunion. This is likely to be the result of a large annual phytoplankton bloom which coincides with the shearwater breeding season in Seychelles, providing many foraging opportunities within the Seychelles Basin. The shearwaters from Seychelles exploited the Amirante Basin, East Somali Basin, and parts of the Seychelles archipelago. Shearwaters from D’Arros flew further east and foraged in the Mascarene Basin. Shearwaters from all populations foraged over deep, relatively unproductive waters with relatively high sea surface temperatures which are similar oceanographic features favoured by tuna. Foraging plasticity between individuals from different colonies was observed with varying flight bearings, flight durations and the maximum distances travelled from their respective colonies. Shearwaters displayed dissimilar levels of site fidelity, where shearwaters from Réunion faithfully foraged towards south east Madagascar, while shearwaters from Seychelles foraged over a wide range over the Seychelles Basin. Foraging effort for shearwaters from Fouquet was lower than shearwaters from Reunion and they spent less time at sea and foraged closer to their colony. These difference in behaviour responses between colonies may be advantageous to shearwaters, indicating that they may be able to respond to some level of environmental variability resulting from climate change and anthropogenic pressures. Lastly, during chick rearing, though shearwaters showed low levels of repeatability in flight characterising, foraging site fidelity was high amongst some of the individuals. Here we see that shearwaters continuously rely on permanent features, like seamounts and possibly fish aggregating devices, which are known to attract high abundances of tuna. Shearwaters may to some extent be relying on memory to find prey in a seemingly unpredictable environment. This study has helped us gain valuable insight about the foraging ecology of Wedge-tailed Shearwaters in the western Indian Ocean. Together with similar studies determining the at-sea behaviour of seabirds, better informed recommendations can be made to marine spatial planners as to where much needed pelagic marine protection should be implemented to preserve marine life in this fascinating part of the global ocean.
- Full Text:
- Date Issued: 2018
- Authors: Keys, Danielle Zuanda
- Date: 2018
- Subjects: Birds of prey -- Behavior -- South Africa -- Western Cape , Animal behavior , Wildlife conservation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/31251 , vital:31348
- Description: The distribution and abundance of prey largely influences how marine predators forage. Thus, understanding how marine predators respond to such changes becomes imperative when making decisions in marine spatial planning, especially in light of global climate change. Tropical marine systems are known for their relatively low productivity and high unpredictability of prey resources. Compared to higher latitudes, very few studies on seabirds have been implemented in the tropics. The Wedge-tailed Shearwater Ardenna pacifica is an Indo-Pacific tropical seabird with a wide distributional range. However, like many other seabirds, its population numbers are currently declining. Very little is known about the at-sea distribution of Wedge-tailed Shearwaters in the tropical western Indian Ocean. Therefore, this thesis investigates the foraging ecology of Wedge-tailed Shearwaters in the western Indian Ocean by tracking the fine scale movements of individuals at sea. This study was conducted in two different oceanic environments, namely D’Arros Island and Fouquet Island, St Joseph Atoll in the Amirantes Group in Seychelles archipelago as well as Réunion Island in the Mascarene archipelago during the Wedge-tailed Shearwaters breeding seasons. Here I aimed to investigate the foraging distribution of Wedge-tailed Shearwaters on Réunion Island in relation to different stages of the breeding period. I also compared the foraging ecology of all three populations, subjected to different abiotic and biotic factors. Furthermore, I investigated foraging site fidelity in individuals and repeatability within individual flight characteristics. To my knowledge, this is the first study to comprehensively investigate the foraging ecology of Wedge-tailed Shearwaters using global positioning system (GPS) loggers. This study demonstrated that the Wedge-tailed Shearwaters implement flexible foraging strategies throughout different stages of their breeding period and between different islands. In Réunion, during the shearwater breeding season, sexual segregation was prominent during early chick rearing and both sexes implemented dual foraging. As suggested by previous studies, it is likely that long trips were implemented for self-maintenance, whereas multiple short trips were implemented to meet the high energetic demands of the chicks. Several important foraging areas were identified for these Wedge-tailed Shearwaters. During incubation, shearwaters from Réunion faithfully foraged towards the south east of Madagascar near the East Madagascar Current and flew over large areas of the Mascarene Basin. During chick rearing, shearwaters significantly constricted their range closer to the colony and exploited deep waters on the south-west of Réunion Island. Shearwaters concentrated near oceanographic features such as the Pérouse Seamount (which is a known area of high productivity) as well as the Bourdonnais Ridge. There was no clear evidence for spatial or sexual segregation between individuals from the two study colonies in Seychelles during incubation and birds utilized a broader foraging range than birds from Réunion. This is likely to be the result of a large annual phytoplankton bloom which coincides with the shearwater breeding season in Seychelles, providing many foraging opportunities within the Seychelles Basin. The shearwaters from Seychelles exploited the Amirante Basin, East Somali Basin, and parts of the Seychelles archipelago. Shearwaters from D’Arros flew further east and foraged in the Mascarene Basin. Shearwaters from all populations foraged over deep, relatively unproductive waters with relatively high sea surface temperatures which are similar oceanographic features favoured by tuna. Foraging plasticity between individuals from different colonies was observed with varying flight bearings, flight durations and the maximum distances travelled from their respective colonies. Shearwaters displayed dissimilar levels of site fidelity, where shearwaters from Réunion faithfully foraged towards south east Madagascar, while shearwaters from Seychelles foraged over a wide range over the Seychelles Basin. Foraging effort for shearwaters from Fouquet was lower than shearwaters from Reunion and they spent less time at sea and foraged closer to their colony. These difference in behaviour responses between colonies may be advantageous to shearwaters, indicating that they may be able to respond to some level of environmental variability resulting from climate change and anthropogenic pressures. Lastly, during chick rearing, though shearwaters showed low levels of repeatability in flight characterising, foraging site fidelity was high amongst some of the individuals. Here we see that shearwaters continuously rely on permanent features, like seamounts and possibly fish aggregating devices, which are known to attract high abundances of tuna. Shearwaters may to some extent be relying on memory to find prey in a seemingly unpredictable environment. This study has helped us gain valuable insight about the foraging ecology of Wedge-tailed Shearwaters in the western Indian Ocean. Together with similar studies determining the at-sea behaviour of seabirds, better informed recommendations can be made to marine spatial planners as to where much needed pelagic marine protection should be implemented to preserve marine life in this fascinating part of the global ocean.
- Full Text:
- Date Issued: 2018
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