Classifying Hawaiian plant species along a habitat generalist-specialist continuum: Implications for species conservation under climate change

Plant communities on tropical high islands, such as the Hawaiian Islands, are predicted to experience rapid climate change, resulting in novel climates. If increased temperature and/or drought exceed plant species’ current tolerances, species that are unable to adapt or shift ranges risk extinction. By definition, habitat generalists have a wide niche breadth and thrive in a variety of habitats, whereas habitat specialists have a narrow niche breadth, and typically thrive under more specific climatic characteristics (e.g., cold). The objectives of this study were to: (1) classify plant species in the Hawaiian Islands along a habitat generalist-specialist continuum; (2) independently test the validity of species rankings, using environmental and biogeographic ranges; and (3) identify species’ life-history traits that predict species location along the continuum. We quantified specialization for 170 plant species using species co-occurrence data from over one thousand plots to rank species’ realized habitat niche breadth using the Jaccard index. The distribution of species along this continuum differed by species biogeographic origin, with endemic plant species ranked on the specialist end and non-native plant species ranked on the generalist end. Habitat specialization rankings also differed for four of nine tested variables (while controlling for biogeographic origin): number of habitat moisture types, minimum elevation, number of Hawaiian Islands, and life form. Life form was the only trait tested that differed across the continuum, with woody species ranked as stronger generalists than herbaceous species; this pattern was particularly evident for non-native species. This indirect method of estimating species’ potential climatic flexibility uses increasingly available large plant community data sets with output rankings which represent species’ realized habitat niches. Identifying species and plant communities that are on the habitat specialist end of the continuum allows for their prioritization in conservation planning, as globally the loss of specialists is an indication of degradation.

以夏威夷群岛为代表的热带高岛植被群落，预计将经历快速气候变化，进而形成全新的气候环境。若升温或干旱程度超出植物物种当前的耐受阈值，无法适应或无法迁移分布范围的物种将面临灭绝风险。根据定义，广适性生境物种（habitat generalist）拥有较宽的生态位宽度，可在多种生境中存活繁衍；而专性生境物种（habitat specialist）的生态位宽度较窄，通常仅能在特定气候条件（如寒冷环境）下存活。本研究的目标如下：（1）基于广适性-专性生境的连续谱对夏威夷群岛的植物物种进行分类；（2）利用环境与生物地理分布范围，独立检验物种排序结果的有效性；（3）识别可预测物种在该连续谱上位置的生活史性状。本研究借助超过1000个样地的物种共现数据，对170种植物的生境特化程度进行量化，并通过雅卡尔指数（Jaccard index）计算物种的实际生境生态位宽度，进而完成物种排序。物种在该连续谱上的分布因生物地理起源而异：本土特有植物物种多被归类至专性生境端，而外来植物物种则多归类至广适性生境端。在控制生物地理起源的前提下，9个测试变量中有4个与生境特化排序存在显著差异：生境湿度类型数量、最低海拔、分布的夏威夷群岛数量以及生活型。生活型是所有测试性状中唯一在该连续谱上呈现显著差异的指标：木本植物相较于草本植物，被归类为更强的广适性物种；这一规律在外来植物物种中尤为显著。本研究采用的间接估算物种潜在气候适应性的方法，依托日益丰富的大型植物群落数据集，其输出的排序结果可反映物种的实际生境生态位。识别处于该连续谱专性生境端的物种与植物群落，可将其纳入保护规划的优先名录——从全球范围来看，专性生境物种的丧失是生态系统退化的重要指示信号。