Predicting biodiversity loss in island and countryside ecosystems through the lens of taxonomic and functional biogeography

We investigate how variation in patch area and forest cover quantified for three different spatial scales (buffer size of 500, 1500 and 3000 m radius) affects species richness and functional diversity of bat assemblages in two ecosystems differing in fragment-matrix contrast: a landbridge island system in Panama and a countryside ecosystem in the Brazilian Amazon. Bats were sampled on 11 islands and the adjacent mainland in Panama, and in eight forest fragments and nearby continuous forest in Brazil. Species–area relationships (SAR) were assessed based on Chao1 species richness estimates, and functional diversity–area relationships (FAR) were quantified using Chao1 functional diversity estimates measured as the total branch length of a trait dendrogram. FARs were calculated using three trait sets: considering five species functional traits (FARALL), and trait subsets reflecting “diet breadth” (FARDIET) and “dispersal ability” (FARDISPERSAL). We found that in both study systems, FARALL was less sensitive to habitat loss than SAR, in the sense that an equal reduction in habitat loss led to a disproportionately smaller loss of functional diversity compared to species richness. However, the inhospitable and static aquatic matrix in the island ecosystem resulted in more pronounced species loss with increasing loss of habitat compared to the countryside ecosystem. Moreover, while we found a significant FARDISPERSAL for the island ecosystem in relation to forest cover within 500 m landscape buffers, FARDIET and FARDISPERSAL were not significant for the countryside ecosystem. Our findings highlight that species richness and functional diversity in island and countryside ecosystems scale fundamentally differently with habitat loss, and suggest that key bat ecological functions, such as pollination, seed dispersal, and arthropod suppression, may be maintained in fragments despite a reduction in species richness. Our study reinforces the importance of increasing habitat availability for decreasing the chances of losing species richness in smaller fragments.

本研究探讨了在三种不同空间尺度（缓冲半径分别为500、1500、3000米）下量化的斑块面积与森林覆盖度的变化，如何影响两种生境片段-基质对比度存在差异的生态系统中蝙蝠群落的物种丰富度与功能多样性：其一为巴拿马的陆桥岛屿系统，其二为巴西亚马逊地区的乡村型生态系统。研究团队对巴拿马11座岛屿及其邻近大陆，以及巴西8处森林片段与周边连续森林内的蝙蝠群落开展了采样调查。本研究基于Chao1物种丰富度估计量评估了种-面积关系（Species-area relationship, SAR），并以性状聚类树的总分支长度作为度量指标，通过Chao1功能多样性估计量量化了功能多样性-面积关系（Functional diversity-area relationship, FAR）。本次研究共基于三类性状集合计算FAR：其一为涵盖5个物种功能性状的全性状集合（FARALL），其二为反映“食性广度”的性状子集（FARDIET），其三为反映“扩散能力”的性状子集（FARDISPERSAL）。研究结果显示，在两个研究系统中，FARALL对生境丧失的敏感性均低于SAR；具体而言，当生境丧失程度相同时，功能多样性的损失幅度显著小于物种丰富度的损失幅度。然而，与乡村型生态系统相比，岛屿生态系统中严苛且静态的水生基质，使得随着生境丧失程度加剧，物种丢失现象更为显著。此外，本研究发现岛屿生态系统中，500米景观缓冲范围内的森林覆盖度与FARDISPERSAL存在显著相关性，但乡村型生态系统中的FARDIET与FARDISPERSAL均未呈现显著相关性。本研究结果表明，岛屿与乡村型生态系统中的物种丰富度和功能多样性，其随生境丧失的尺度响应规律存在本质差异；同时研究显示，即便物种丰富度有所下降，森林片段中仍可维持蝙蝠的关键生态功能，如传粉、种子传播以及节肢动物抑制。本研究进一步证实，提升生境可获得性，有助于降低小型森林片段中物种丰富度丧失的风险。