Dispersal mode mediates the effect of patch size and patch connectivity on metacommunity diversity

1. Metacommunity theory predicts that increasing patch size and patch connectivity can alter local species diversity by affecting either colonization rates, extinction rates or both. Although species’ dispersal abilities or ‘dispersal mode’ (e.g. gravity-, wind- or animal-dispersed seeds) can mediate the effects of patch size and connectivity on diversity, these important factors are frequently overlooked in empirical metacommunity work. 2. We use a natural metacommunity of aspen stands within a grassland matrix to determine whether dispersal mode alters the influence of stand size and connectivity on understorey plant diversity. We sampled the same area in each patch, controlled for the presence of matrix species in aspen stands, and tested for the effects of size, connectivity and dispersal mode on metacommunity richness. Because dispersal groups responded differently to patch size and connectivity, we created a null model and assessed ungulate activity to explore whether competitive dynamics or herbivory were driving diversity patterns. 3. Animal-dispersed species and species with no dispersal aid had higher diversity per unit area in larger stands, likely because large stands can both support larger populations that are less prone to extinction and may also attract seed-dispersing animals such as birds and small mammals that are sensitive to edge effects. Consistent with other empirical work, we found a positive relationship between diversity and connectivity for wind-dispersed species. However, we detected a negative effect of stand connectivity on the diversity of species with no dispersal aid, possibly due to the presence of other highly competitive species groups dominating well-connected patches, as our null model results suggest. We found no evidence for higher ungulate activity in highly connected patches, suggesting that herbivory may not be driving the decline in diversity of plants with no dispersal aid. 4. Synthesis. Overall, we see a positive effect of stand area on diversity for most groups despite sampling equal area in all stands, which is a prediction of metacommunity theory that is normally overlooked. Our results demonstrate the importance of considering variation in the dispersal modes of focal species for explaining the diversity patterns of natural metacommunities.1. 集合群落（metacommunity）理论预测，增大斑块面积与提升斑块连通性可通过改变定殖率、灭绝率或同时改变二者来调控局域物种多样性。尽管物种的扩散能力或“扩散模式（dispersal mode）”（例如依靠重力、风力或动物传播的种子）能够调节斑块面积与连通性对多样性的影响，但这类关键因素在实证集合群落研究中常被忽视。 2. 本研究以草原基质中的天然颤杨林集合群落为研究对象，旨在探明扩散模式是否会改变林分面积与连通性对林下植物多样性的影响。我们对每个斑块开展了统一面积的采样，控制了颤杨林中基质物种的存在情况，并检验了林分面积、连通性以及扩散模式对集合群落物种丰富度的效应。鉴于不同扩散类群对斑块面积与连通性的响应存在差异，我们构建了零模型，并评估了有蹄类动物活动情况，以探究竞争动态或植食作用是否为多样性格局的驱动因素。 3. 动物传播物种与无扩散辅助的物种在更大的林分中单位面积多样性更高，这可能是因为大林分既能支撑更不易灭绝的种群，还能吸引对边缘效应敏感的鸟类、小型哺乳类等种子传播动物。与其他实证研究结果一致，我们发现风传播物种的多样性与连通性呈正相关关系。但我们检测到林分连通性对无扩散辅助物种的多样性存在负向效应，这可能正如我们的零模型结果所示，源于竞争力更强的其他物种类群在连通性良好的斑块中占据主导。我们未发现连通性更高的斑块中有蹄类动物活动更强的证据，这表明植食作用可能并非导致无扩散辅助植物多样性下降的原因。 4. 综合分析。总体而言，尽管我们对所有林分采用了统一的采样面积，但多数物种类群的多样性仍受到林分面积的正向影响，这一结果契合集合群落理论的预测，而该预测通常被以往研究所忽视。本研究结果表明，在阐释天然集合群落的多样性格局时，聚焦物种的扩散模式差异具有重要意义。