Data from: Are all forms of defense lost on islands? Persistence of an indirect defensive trait in six island colonists from New Zealand

(a) Data collection Data collection was conducted between June 2018 and April 2019. The six study taxa (Coprosma repens [Rubiaceae], C. rhamnoides, C. robusta, C. lucida, Elaeocarpus dentatus [Elaeocapraceae] and Vitex lucens [Lamiaceae]) were chosen as they are the most widely distributed across New Zealand’s north-eastern islands. All taxa produce pit domatia, except E. dentatus which produces tent domatia. Mainland field sampling was conducted in the Kaimai-Mamaku Forest Park in Tauranga (37°41ʹS, 175°45ʹE). This site was chosen because the Kaimai Ranges span a large latitudinal extent of the north-eastern corner of New Zealand, and therefore represent the probable source pool for many island populations. Island field sampling was conducted on seven islands: Cuvier, Great Mercury, Red Mercury, Otata, Ruamahuaiti, Ruamahuanui, and Waiheke. Individuals were chosen haphazardly while walking through easily accessible forest sections. Only fully expanded, mature leaves were measured (1-3 leaves per individual from at least 30 individuals per site per species). Leaf length was measured as the longest linear distance from the most proximal to the most distal point of the leaf lamina using a digital calliper. Leaf width was measured as the widest distance across the leaf lamina perpendicular to the leaf length measurement. Leaf area was calculated as the product of leaf length and leaf width. Although more accurate methods of estimating leaf area exist [e.g., leaf scanners, image recognition software, or using ad hoc leaf shape correction factors 43], leaf x width calculations sufficed for the purpose of this analysis as it is not concerned with among species differences in leaf size per se. Couched in other terms, any errors associated with leaf area estimates are consistent between island-mainland comparisons. In a prior study incorporating the same species. I trialled ad hoc correction factors and found they provided little utility. Further, a leaf scanner could not be used as most of the islands included in this study are protected by the Department of Conservation, which does not allow destructive sampling protocols. Domatia were counted systematically in a basipetal direction with the aid of a USB microscope (Toolcraft DigiMicro 2.0 Scale). It should be noted that while this study quantifies defense investment by domatia count, experimental work with Grape cultivars has demonstrated that mite abundance on the phylloplane can scale with domatia size. To expand upon data gathered in the field, pressed herbarium specimens from a further 53 islands and 58 mainland sites from the Auckland War Memorial Museum Herbarium (AK) were measured. The Herbarium houses an extensive collection of high-quality, preserved specimens of both indigenous and exotic plants spanning the full geographic extent of the New Zealand landmass – including its offshore islands. To standardize sampling across specimens, 3 leaves per specimen were measured using the same methodology outlined above. Leaves were chosen haphazardly, and care was taken to not damage specimens (i.e., gloves and minimal calliper–specimen contact). 1,129 observations from 60 islands and 59 mainland sites comprised the final dataset. (b) Data analysis To test for mainland-island differences in domatia production, while simultaneously accounting for leaf size and exploring differences in domatia–leaf size scaling, I performed a linear mixed effects models of number of domatia production (total count per leaf) against leaf area (cm2), insularity (island, mainland), and their interaction. Both domatia count and leaf area were logarithm transformed to conform to linearity. One was added to counts of domatia before transformation to avoid biologically nonsensical values (i.e., negative infinity). Species was included as a random effect permitting both intercept and slope to vary among species. Analysis and data visualization were conducted in R version 4.2.2 (R Core Team 2022) using the ‘lme4’ and ‘tidyverse’ packages.

(a) 数据采集
本研究的数据采集工作于2018年6月至2019年4月间开展。本次研究选取的6个研究类群（taxa）分别为：*Coprosma repens*（茜草科Rubiaceae）、*C. rhamnoides*、*C. robusta*、*C. lucida*、*Elaeocarpus dentatus*（[Elaeocapraceae]）以及*Vitex lucens*（唇形科Lamiaceae），上述类群均为新西兰东北诸岛分布最广泛的物种。所有类群均产生凹坑型叶室（pit domatia），仅*E. dentatus*产生帐篷型叶室（tent domatia）。
陆地区域的野外采样设于陶朗加市的凯迈-马马库森林公园（Kaimai-Mamaku Forest Park，37°41′S，175°45′E）。选取该采样点的原因在于，凯迈山脉横跨新西兰东北部的广阔纬度范围，可作为诸多岛屿种群的潜在种源库。岛屿野外采样覆盖7座岛屿：库维尔岛（Cuvier）、大汞岛（Great Mercury）、红汞岛（Red Mercury）、奥塔塔岛（Otata）、鲁阿马胡艾蒂岛（Ruamahuaiti）、鲁阿马胡阿努伊岛（Ruamahuanui）以及怀赫科岛（Waiheke）。
采样个体为在易通行林区随机选取的植株，仅选取完全舒展的成熟叶片进行测量：每个物种在每个采样点至少选取30株植株，每株采集1~3片叶片。叶长定义为叶片叶肉组织从最近端（叶基部）至最远端（叶尖）的最长线性距离，采用数显游标卡尺完成测量；叶宽则定义为与叶长测量方向垂直的叶肉组织最宽处的距离。叶面积通过叶长与叶宽的乘积计算得到。
尽管当前存在更精准的叶面积估算方法（例如叶片扫描仪、图像识别软件，或采用定制化叶形校正因子43），但本分析无需关注物种间叶大小本身的差异，因此叶长宽乘积法已足够满足研究需求。换而言之，叶面积估算带来的任何误差在岛屿与陆地区域的对比中均保持一致。在一项针对相同类群的前期研究中，笔者曾测试过定制化校正因子，结果发现其应用价值有限。此外，本研究无法使用叶片扫描仪，因为本次研究涉及的多数岛屿由新西兰保护部（Department of Conservation）管辖，该部门不允许破坏性采样方案。
借助USB显微镜（Toolcraft DigiMicro 2.0 Scale），按向基方向（basipetal direction）系统计数叶室（domatia）数量。需说明的是，尽管本研究通过叶室数量量化防御投入，但针对葡萄栽培品种的实验研究已表明，叶面上的螨类丰度可与叶室大小呈比例关系。
为补充野外采集的数据，本研究还测量了奥克兰战争纪念博物馆标本馆（Auckland War Memorial Museum Herbarium，AK）馆藏的53座岛屿及58个陆地区域的压制蜡叶标本。该标本馆馆藏丰富，涵盖了原产及外来植物的高质量压制标本，覆盖新西兰全境（包括其离岸岛屿）的所有地理区域。为统一标本采样标准，每份标本选取3片叶片，采用前述相同的测量方法。叶片选取遵循随机原则，同时需注意避免损坏标本（例如佩戴手套，尽量减少游标卡尺与标本的接触）。最终数据集共包含来自60座岛屿及59个陆地区域的1129条观测记录。
(b) 数据分析
为检验岛屿与陆地区域在叶室产生量上的差异，同时控制叶大小的影响并探究叶室-叶大小的缩放关系差异，本研究构建了线性混合效应模型：以每片叶片的叶室总数为因变量，以叶面积（单位：cm²）、生境类型（岛屿/陆地）及其交互项为自变量。为满足线性模型的前提假设，对叶室数量及叶面积数据进行对数转换；为避免出现无生物学意义的数值（即负无穷），在对数转换前对叶室计数数据加1。将物种设定为随机效应，允许不同物种的截距和斜率存在差异。数据分析及可视化工作基于R 4.2.2版本（R Core Team 2022）完成，使用了"lme4"和"tidyverse"扩展包。