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

The aim of this study is to estimate the erosion vulnerability of important sea turtle nesting beaches in the Indian Ocean and South East Asia (IOSEA) region to determine whether Casuarina trees enhance erosion vulnerability over and above other factors, such as urban development. The specific objectives are 1) to quantify the distribution of non-native Casuarina trees on sea turtle nesting beaches of the IOSEA region and 2) to create an erosion vulnerability score (based on risk and threat) for the nesting sites based on global datasets of erosion indicators. Then, 3) to apply the vulnerability index to 50 sea turtle nesting beaches, and 4) to assess whether the presence of Casuarina trees could be used as an indicator of enhanced beach erosion. We hypothesise that the characteristics of Casuarina trees are not conducive to them being an effective coastal protection tool and predict that beaches backed by these trees will have higher erosion vulnerability scores than those beaches without these trees. To calculate beach erosion vulnerability as a measure of risk and threat, the Coastal Vulnerability Index (CVI) developed by Gornitz et al. (1994) for site-specific studies was adapted for use at a regional scale. Risk indicators, as suggested by existing literature, evaluate physical beach features, including: backshore width; beach exposure; modal beach energy, as measured by wave height and tide range; and state of the dune system. Threat indicators focus on erosion threats to beaches, such as: development; sea-level rise; and storm frequency and intensity. The indicators included in this study included those above, modified by data availability at an ocean-basin scale. Scores were normalized for each indicator to yield a value between zero and one (i.e. allocated score was divided by the maximum score of that category) and beach vulnerability was visualised by plotting the sum of normalized scores for risk indicators (x-axis) against the sum of normalized scores for threat indicators (y-axis). Recognising that it might be impossible for a beach to attain the highest score in every category (i.e., have no backshore, experience rapid sea-level rise, and very frequent extreme storms) and that scoring the lowest value for every category gave a total that was not zero, we calculated the theoretical minimum and maximum values attainable, and visualised relative vulnerability based on those ranges. The theoretical minimum score was calculated as the lowest possible score attainable, and the theoretical maximum was calculated as the sum of the maximum value attained for each metric. The midpoint of each range (risk and threat) was used to divide the data into four quadrants: 1) High Risk-High Threat; 2) High Risk-Low Threat; 3) Low Risk-High Threat; 4) Low Risk-Low Threat, which correspond to four categories of erosion vulnerability.本研究旨在评估印度洋与东南亚（Indian Ocean and South East Asia, IOSEA）区域内重要海龟筑巢海滩的侵蚀脆弱性，以明确木麻黄树（Casuarina trees）是否会在城市开发等其他因素之外，进一步加剧海滩侵蚀脆弱性。具体研究目标如下：1）量化IOSEA区域海龟筑巢海滩上外来木麻黄树的分布情况；2）基于全球侵蚀指标数据集，为筑巢位点构建以风险与威胁为依据的侵蚀脆弱性评分；3）将该脆弱性指数应用于50处海龟筑巢海滩；4）评估木麻黄树的存在是否可作为海滩侵蚀加剧的指示因子。本研究提出假说：木麻黄树的特性并不利于其成为有效的海岸防护工具，并预测种植有该类树木的海滩，其侵蚀脆弱性评分将高于无木麻黄树的海滩。 为计算作为风险与威胁衡量指标的海滩侵蚀脆弱性，本研究对Gornitz等人1994年针对特定场地研究开发的海岸脆弱性指数（Coastal Vulnerability Index, CVI）进行适配，使其可应用于区域尺度。现有文献提出的风险指标用于评估海滩物理特征，包括：后滨宽度、海滩暴露度、以波高和潮差衡量的典型海滩能量，以及沙丘系统状态。威胁指标则聚焦于海滩面临的侵蚀威胁，例如：开发活动、海平面上升，以及风暴发生频率与强度。本研究纳入的指标包含上述内容，并结合大洋盆地尺度下的数据可获得性进行了调整。对每项指标的评分进行归一化处理，使其取值介于0至1之间（即分配得分除以该类别下的最高得分），随后以风险指标归一化得分之和为横轴、威胁指标归一化得分之和为纵轴绘图，实现海滩脆弱性的可视化。考虑到海滩不可能在所有类别中都取得最高评分（即无后滨、经历快速海平面上升且极端风暴频发），且所有类别均取最低值时总分并不为零，因此本研究计算了理论上可达到的最低与最高分值，并基于该分值范围可视化相对脆弱性。理论最低分值为所有指标可取的最低值之和，理论最高分值则为每项指标可取最高值之和。以该分值范围的中点为界，将数据划分为四个象限：1）高风险-高威胁；2）高风险-低威胁；3）低风险-高威胁；4）低风险-低威胁，对应四类侵蚀脆弱性等级。