Scale-dependent shifts in functional and phylogenetic structure of Mediterranean island plant communities over two centuries

1. Since the Industrial Revolution, the rapid global population and economic expansion has had tremendous impacts on biodiversity across spatial scales, especially for islands. While changes in species richness are easily inferred, the impact of human activity on the underlying community assembly processes has been difficult to ascertain because of lack of long-term community data.

2. Here, we document how the manifestations of plant community assembly have changed over time and space in a Mediterranean archipelago, using a long-term dataset of plant species composition on 16 Tuscan islands sampled across two centuries. The community structure of Mediterranean island plant communities was assessed by integrating species’ trait and evolutionary distances.

3. We found that, with increasing island area, the functional and phylogenetic structure of plant communities shifted from clustered early (1830–1950) to overdispersed more recently (1951–2015). On large islands, extirpated species were generally more phylogenetically or functionally similar to remaining residents than expected by chance, while colonists were generally more distantly related to residents. The extinction of similar species and the colonization of dissimilar species drove plant communities towards overdispersion.

4. Synthesis. We provide evidence that plant community assembly on islands has dramatically changed following increased human impacts during the last two centuries, and that this change is shaped by the scale dependency of species extinctions and colonizations. Our results reveal accelerated species replacements of closely related residents by distant colonists on large islands over time, reflecting changes in community assembly and which could alter the functioning of island ecosystems in the future. 

Methods
These trait data were obtained from the Botanical Information and Ecology Network database (Enquist et al., 2016) using the R package BIEN (Maitner et al., 2018), TRY database (Kattge et al., 2020) and BROT 2.0 (Tavşanoğlu & Pausas 2018).

1. 自工业革命以来，全球人口与经济的快速扩张对不同空间尺度下的生物多样性造成了巨大冲击，对岛屿生态系统的影响尤为显著。尽管物种丰富度的变化易于推断，但由于缺乏长期群落监测数据，人类活动对群落底层组装（community assembly）过程的影响仍难以确定。
2. 本研究基于横跨两个世纪的16个托斯卡纳（Tuscan）群岛植物物种组成长期数据集，阐明了地中海群岛植物群落组装特征随时间与空间的演变规律。研究通过整合物种功能性状（functional trait）与进化距离，对地中海岛屿植物群落的结构进行了定量评估。
3. 研究发现，随着岛屿面积增大，植物群落的功能与系统发育结构（phylogenetic structure）从早期（1830–1950年）的聚集模式逐渐转变为近期（1951–2015年）的过度分散模式。在大型岛屿上，灭绝物种与现存群落中其余物种的系统发育或功能相似性通常高于随机预期水平；而迁入定植物种与现存群落物种的亲缘关系则通常更远。相似物种的灭绝与远缘物种的定植共同推动植物群落结构向过度分散方向发展。
4. 综合与结论：本研究证实，在过去两个世纪人类活动影响加剧的背景下，岛屿植物群落组装过程已发生显著改变，且这一变化由物种灭绝与定植的尺度依赖性所塑造。研究结果揭示，随着时间推移，大型岛屿上的近缘现存物种正被远缘定植物种快速替代，这反映了群落组装模式的改变，并可能在未来改变岛屿生态系统的功能。

研究方法
本研究的性状数据来源于植物信息与生态网络（Botanical Information and Ecology Network, BIEN）数据库（Enquist等，2016），并通过R语言包BIEN（Maitner等，2018）、TRY数据库（Kattge等，2020）以及BROT 2.0数据库（Tavşanoğlu & Pausas，2018）获取。