- Title
- The effect of casuarina trees on sea turtles nesting beaches throughout the Indian Ocean and South-East Asia regions: A beach vulnerability assessment
- Creator
- De Vos, Deidre
- Subject
- Conservation biology
- Subject
- Casuarina Sea turtles
- Date Issued
- 2018
- Date
- 2018
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10948/22420
- Identifier
- vital:29968
- Description
- Rapid global change and anthropogenic modification of natural habitats are threatening ecosystems across the globe. Many of these threats and adverse impacts are concentrated on the world’s coastlines, with a large portion of the global population choosing to live next to the coast. To accommodate coastal populations and their related activities, the dynamic littoral active zone is often stabilized with hard engineering structures such as sea walls, which disrupt coastal ecological processes. Most natural coastal processes have since become natural disaster risks to the anthropogenically-modified coasts. Coastal stabilization has led to “coastal squeeze” with modified shorelines unable to respond naturally to changes in sea level. Furthermore, global change through global ‘warming’ exacerbates sea level rise. The combined impacts result in permanent/episodic inundation and coastal erosion, which may damage human settlements and degrade or destroy coastal habitat. As an alternative to hard armouring structures like sea walls, ‘soft’ options like large-scale plantations of trees as bio-shields are used as a ‘green’ option to protect developed coasts. Indeed, green shelter belts have rapidly increased since the December 2004 tsunami event in the Indian Ocean and South-East Asian (IOSEA) region. Extensive portions of the coastal habitat, including sensitive sea turtle nesting habitat, have been modified by planting exotic vegetation. Two invasive species, Casuarina equisetifolia and Casuarina glauca, are regularly used for coastal dune stabilization or as bio-shields because they are adapted to arid conditions, high salinity and low soil fertility typical of beach systems. The functionality of Casuarina trees as a ‘comprehensive’ coastal protection tool, however, has yet to be tested and the effects of Casuarina on sandy beach ecosystems remain largely unquantified. There is no empirical evidence to support the claim that Casuarina trees adequately function as a storm protection measure, and other studies have suggested that Casuarina species have adverse effects on sandy beach ecosystems, including the promotion of erosion in some instances and negative impacts on fauna, including sea turtles. This study investigated the potential value of non-native Casuarina trees as a coastal protection measure on sea turtle nesting beaches taking into consideration the potential impacts of global change. First, the study estimated local scale effects of Casuarina trees within the South African sea turtle nesting rookery in the iSimangaliso Wetland Park by measuring impacts on back-beach substrate characteristics and shape. A few metrics were unaffected, such as grain size, sand penetrability and sand moisture, but sand pH and sand temperature were significantly lower underneath Casuarina stands. A mean pH of 8.2 and a median temperature of 26 ± SD1.5oC were recorded under Casuarina trees and a pH of 9 and a median temperature of 27.2 ± SD1.9oC under native vegetation. The significantly lower, more stable average temperature underneath Casuarina trees may have significant implications for sex ratios of sea turtles as the sex of sea turtles is determined by temperature; Temperatures above 32°C will yield all females and temperatures below 28°C will produce only males, with a pivotal temperature at which a 1:1 sex ratio is developed is about 29°C. Furthermore, sand temperature underneath Casuarina trees is less likely to reach lethal temperatures (above ~33°C) that could lead to hatchling mortality than in areas with no vegetation. So, in terms of hatchling success, the thermal environment created by Casuarina trees might be favourable in extremely hot climate areas, especially when considering the effects of climate change. Casuarina trees, however, affect back-beach and dune dynamics by altering sand movement. Semi-quantitative and qualitative analyses of beach profiles suggested that Casuarina trees and shrubs result in wind scouring directly in front of the foredunes as evidenced by the concave shape in front of Casuarina stands at some sites. Sites with natural vegetation had mainly convex beach shapes, suggesting sand accretion. Either way, vegetation had an important effect on sand dynamics (both positive and negative) where areas with no vegetation (bare sand) only had a neutral/straight shape. There was, however, a significant interaction between vegetation type and site, indicating that vegetation type alone cannot account for all the differences in beach profile dynamics and that site-specific conditions like wave height, sand budget, precipitation and orientation differences among locations may also have an effect. The second part of the study had a regional perspective, investigating the effect of Casuarina trees on 50 sea turtle nesting beaches across the Indian Ocean South East Asia (IOSEA) region. The study first derived an appropriate (novel) beach vulnerability index using features that drive beach erosion. The index was then applied to 50 turtle rookeries in the presence/absence of Casuarina trees. The study noted the extensive distribution of exotic Casuarina trees throughout the IOSEA region and established that 28% of the 50 selected sea turtle nesting beaches had non-native Casuarina present. There was no significant difference in the erosion vulnerability of beaches with and without Casuarina based on the existing indicators. However, the study did show that seven important sea turtle rookeries were highly vulnerable to erosion, some with extensive exotic Casuarina stands. Olive ridley (Lepidochelys olivacea) nesting beaches in India, were dominated by Casuarina trees and were highly vulnerable to erosion. The study showed that the presence of Casuarina trees alone could not be viewed as an indicator of erosion but could be a contributing factor, as several other drivers like narrow back-beach width, absence of dunes and development on exposed coasts prone to storms, also contribute to the high erosion vulnerability of beaches. The study questions the use of exotic vegetation, such as Casuarina species, for coastal protection by demonstrating local scale effects on the back-beach environment and provides a comprehensive overview of the erosion potential of sea turtle nesting beaches throughout the IOSEA region.
- Format
- viii, 203 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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