The complex effect of heterogeneity and isolation in determining alpha and beta orchid diversity on islands in the Aegean archipelago

Knowledge of factors determining the distribution of plants on islands is of crucial importance in island biogeography. Here the drivers and mechanisms that best explain α- and β-diversity of orchids in the Aegean Archipelago were explored. The analyses were based on (a) matrix of the total number of orchid taxa recorded on each island, and (b) matrix containing the number of grid cells of each island in which each orchid has been recorded. Based on these matrices, generalized linear models (GLMs) were then used to determine the most important factors (geographical, climatic, geological) associated with orchid richness. Generalized dissimilarity modelling (GDM) was used to explore whether there are specific patterns in β-diversity and to evaluate contributions to individual factors to these patterns. To investigate the similarity of the Aegean islands in terms of their orchid flora, a β-diversity abundance matrix was analyzed using non-metric multidimensional scaling ordination method. GLM analysis revealed that maximum elevation, index of bioclimatic breadth (IBB), neighbour index (NI), island area, and the percentage of granite, limestone and quartz at each island were the best predictors of orchid α-diversity. Maximum elevation, habitat heterogeneity, IBB and island area were positively associated with α-diversity, whereas neighbour index and the percentage of granite, limestone and quartz at each island were negatively associated. Maximum elevation, NI, habitat heterogeneity and geographical distance were the best predictors of orchid turnover across the Aegean islands, whereas IBB, granite and limestone were less important. The NMDS ordination revealed that islands are structured from the southernmost to the northernmost, in a way that in general conforms to the phytogeographical division of the Aegean Archipelago. The drivers that account for the patterns in α- and β-diversity were associated with differences in the degrees of isolation and heterogeneity (climatic and habitat) of the islands. Among the predictors, elevation and IBB proved to be better surrogates of habitat heterogeneity than island area.岛屿植物分布决定因子的认知，在岛屿生物地理学（island biogeography）研究中具有至关重要的意义。本研究探究了能够最佳解释爱琴海群岛（Aegean Archipelago）兰科植物α多样性与β多样性的驱动因子与机制。 分析基于两类矩阵：其一为各岛屿记录到的兰科类群总数矩阵，其二为包含各岛屿内每种兰科植物被记录到的网格单元格数量的矩阵。基于上述矩阵，本研究采用广义线性模型（generalized linear models, GLMs）以确定与兰科植物丰富度关联最紧密的关键因子，包括地理、气候与地质因子。随后采用广义相异性建模（generalized dissimilarity modelling, GDM）探究β多样性是否存在特定分布模式，并评估各因子对该模式的贡献度。 为探究爱琴海群岛兰科植物区系的相似性，本研究采用非度量多维标度排序法（non-metric multidimensional scaling ordination method）对β多样性丰度矩阵进行分析。 GLM分析结果显示，最大海拔、生物气候广度指数（index of bioclimatic breadth, IBB）、邻域指数（neighbour index, NI）、岛屿面积，以及各岛屿的花岗岩、石灰岩与石英占比，是预测兰科植物α多样性的最佳指标。其中，最大海拔、生境异质性、IBB与岛屿面积与α多样性呈正相关，而邻域指数以及各岛屿的花岗岩、石灰岩与石英占比则与α多样性呈负相关。 针对爱琴海群岛兰科植物群落更替的最佳预测因子为最大海拔、NI、生境异质性与地理距离，而IBB、花岗岩与石灰岩占比的重要性相对较低。非度量多维标度排序结果显示，爱琴海群岛的兰科植物区系按从最南端到最北端的顺序形成聚类，整体符合爱琴海群岛的植物地理分区格局。 影响α与β多样性分布格局的驱动因子，与岛屿的隔离程度以及气候、生境异质性差异相关。在所有预测因子中，海拔与IBB相比岛屿面积，能更好地作为生境异质性的替代指标。