Effect of introduced Casuarina trees on the vulnerability of sea turtle nesting beaches to erosion

Abstract

Abstract The world s beaches are under severe pressure because of anthropogenic modification and climate change. The most common coastal management approaches have focused largely on the protection of infrastructure without considering the function of the beach and dune system as a natural protective barrier and a diverse ecosystem. Large-scale plantations of non-native vegetation have been initiated in many countries as a coastal protection tool. However, exotic trees commonly used for bio-shields, such as Casuarina equisetifolia, have been shown to have negative effects on the coastal environment, including the promotion of beach erosion in some instances and negative impacts on sea turtles. The aim of the study was to determine if exotic Casuarina spp. enhance the vulnerability of turtle nesting beaches by exacerbating erosion. The extent of use of exotic Casuarina spp. was assessed on 50 important sea turtle nesting beaches of the Indian Ocean and South-East Asia (IOSEA) region and used an adapted Coastal Vulnerability Index (CVI) to determine vulnerability of these beaches to erosion. Sites were assessed using long-term global data sets under four risk indicators (backshore width, beach exposure, modal beach energy, dune state) and three threat indicators (coastal development, sea-level rise and storminess). Beach vulnerability was then classified into four categories: 1) High Risk-High Threat; 2) High Risk-Low Threat; 3) Low Risk-High Threat and 4) Low Risk-Low Threat. Overall, Casuarina trees were present on 28% of beaches, and seem to be a popular management option in the IOSEA region. Lepidochelys olivacea rookeries were most affected by Casuarina, which is of concern because these populations are already considered at risk from other stressors. Overall, beaches tended to have higher risk than threat, and beaches with Casuarina present were not shown to be more vulnerable to erosion (Kruskal-Wallis Χ244\u202f=\u202f42.62, p\u202f=\u202f0.5308). However, these trees may pose other threats to sea turtles, e.g., providing obstructions to nesting females and altering nest incubation environments, which need to be carefully considered by decision makers. Ideally, natural dune cordons need to be maintained and restored, conferring benefits of protection to people, and benefits of ecological processes to biodiversity.