Data from: Microhabitat heterogeneity and a non-native avian frugivore drive the population dynamics of an island endemic shrub, Cyrtandra dentata

Understanding of the role of environmental change in the decline of endangered species is critical to designing scale-appropriate restoration plans. For locally endemic rare plants on the brink of extinction, frugivory can drastically reduce local recruitment by dispersing seeds away from geographically isolated populations. Dispersal of seeds away from isolated populations can ultimately lead to population decline. For localized endemic plants, fine-scale changes in microhabitat can further limit population persistence. Evaluating the individual and combined impact of frugivores and microhabitat heterogeneity on the short-term (i.e. transient) and long-term (i.e. asymptotic) dynamics of plants will provide insight into the drivers of species rarity.
In this study, we used four years of demographic data to develop matrix projection models for a long-lived shrub, Cyrtandra dentata (H. St. John & Storey) (Gesneriaceae), which is endemic to the island of O'ahu in Hawai'i. Furthermore, we evaluated the individual and combined influence of a non-native frugivorous bird, Leiothrix lutea, and microhabitat heterogeneity on the short-term and long-term C. dentata population dynamics.
Frugivory by L. lutea decreased the short-term and long-term population growth rates. However, under the current level of frugivory at the field site the C. dentata population was projected to persist over time. Conversely, the removal of optimum microhabitat for seedling establishment (i.e. rocky gulch walls and boulders in the gulch bottom) reduced the short-term and long-term population growth rates from growing to declining.
Survival of mature C. dentata plants had the greatest influence on long-term population dynamics, followed by the growth of seedlings and immature plants. The importance of mature plant survival was even greater when we simulated the combined effect of frugivory and the loss of optimal microhabitat, relative to population dynamics based on field conditions. In the short-term (10 years), however, earlier life stages had the greatest influence on population growth rate.
Synthesis and applications. This study emphasizes how important it is to decouple rare plant management strategies in the short versus long-term in order to prioritize restoration actions, particularly when faced with multiple stressors not all of which can be feasibly managed. From an applied conservation perspective, our findings also illustrate that the life stage that, if improved by management, would have the greatest influence on population dynamics is dependent on the timeframe of interest and initial conditions of the population.

明晰环境变化在濒危物种衰退过程中的作用，是制定尺度适配型修复方案的核心前提。对于濒临灭绝的本土特有稀有植物而言，食果作用（frugivory）可通过将种子从地理隔离的种群中扩散出去，大幅降低本地种群的补充量；而种子远离隔离种群的扩散行为，最终会加剧种群衰退。对于局域特有植物而言，微生境的细微变化会进一步限制种群存续。评估食果动物与微生境异质性各自及联合作用对植物短期（即瞬时）与长期（即渐近）种群动态的影响，将有助于揭示物种稀有性的驱动机制。

本研究依托四年的种群动态数据，针对夏威夷瓦胡岛（O'ahu）特有多年生灌木钝齿尖舌苣苔（Cyrtandra dentata (H. St. John & Storey)，苦苣苔科（Gesneriaceae)）构建了矩阵投影模型（matrix projection models）。此外，本研究还评估了非原生食果鸟类红嘴相思鸟（Leiothrix lutea）与微生境异质性，分别及联合作用对钝齿尖舌苣苔种群短期与长期动态的影响。

红嘴相思鸟的食果作用降低了种群的短期与长期增长率。但在当前研究样地的食果作用水平下，钝齿尖舌苣苔种群预计可长期存续。与之相反，若移除幼苗定居所需的最优微生境（即沟谷岩壁与沟谷底部的巨石），种群的短期与长期增长率将从增长转为下降。

成熟钝齿尖舌苣苔的存活对种群长期动态的影响最大，其次为幼苗与未成熟植株的生长。相较于基于野外观测条件的种群动态，当模拟食果作用与最优微生境丧失的联合效应时，成熟植株存活的重要性进一步提升。但在短期（10年）尺度下，早期生活史阶段对种群增长率的影响最为显著。

综合与应用。本研究强调，需区分稀有植物的短期与长期管理策略，以优先确定修复行动的优先级——尤其当面临多个难以全部实施可行管理的胁迫因子时。从应用保护的视角来看，本研究结果还表明：若通过管理改善某一生活史阶段以最大化提升种群动态效益，其选择取决于关注的时间尺度与种群初始状态。