Large-scale variation in biodiversity–ecosystem functioning (BEF) relationships in aquatic metacommunities on terrestrial islands

Recent work has shown that the biodiversity of potential colonists in a landscape (the local species pool) may be more important for ecosystem functioning than the biodiversity in local habitat patches. However, it is unknown how such biodiversity-ecosystem functioning (BEF) relationships may change across different biomes. To explore such patterns, nested insular ecosystems where variation in local biodiversity and local species pool biodiversity can be reliably quantified can provide important insights. Study locations were rock pool metacommunities on isolated rocky outcrops (i.e., inselbergs) in Africa, Australia, Europe, and North America. The sampling time period was 2011-2019. Major taxa studied were freshwater invertebrates. We assembled a large-scale dataset of invertebrate metacommunities from replicated rock pool clusters on inselbergs as a model system to test the ability of local biodiversity and local species pool biodiversity to explain community biomass in organisms with different survival strategies (active or passive dispersers). To test our hypotheses, we used a combination of directed acrylic graph based path analyses and general linear mixed-effects models. The biodiversity of the local species pool was influenced by climate but did not significantly impact community biomass. Instead, local environmental gradients seem to override any species pool effects on community biomass. However, in line with expectations, the relationship between local biodiversity and biomass varied across inselbergs. Contrary to expectations, inselberg prominence did not influence the BEF slope. However, in drier conditions, the BEF relationship weakened for active dispersers, likely reflecting environmental limits on recolonisation. Thus, climate and dispersal strategy jointly shaped how biodiversity influenced community biomass. This study illustrates that even in a simple ecosystem there can be substantial geographical variation in the relationship between biodiversity and ecosystem functioning that may be partially explained by environmental conditions and by the survival strategy of the organisms considered.

Methods
The dataset consists of invertebrate communities sampled in 238 rock pools from ten different inselbergs across four continents. On each inselberg site, between 12 and 34 pools were sampled. After sampling, the invertebrates from each rock pool were identified to the lowest possible taxonomic resolution in the laboratory (genus or species, when possible), and the number of individuals of each group was counted. The community-level invertebrate biomass per litre was calculated for each pool by multiplying species-specific biomass estimates by density. Alpha diversity was calculated as the local-scale species richness of each rock pool, and gamma diversity was calculated as the inselberg-scale species richness representing the diversity of the local species pool.

已有研究表明，景观中潜在定居者的生物多样性（即区域物种库（local species pool）），相较于本地生境斑块内的生物多样性，对生态系统功能的影响更为关键。然而，此类生物多样性-生态系统功能（biodiversity-ecosystem functioning, BEF）关系如何随不同生物群落发生变化，目前仍不明确。
为探究此类规律，嵌套式岛屿生态系统可提供重要研究视角，因其能可靠量化本地生物多样性与区域物种库生物多样性的变异。本研究的采样地点为非洲、澳大利亚、欧洲及北美洲的孤立岩丘（inselberg，又称岛状山）上的岩石池集合群落（metacommunity），采样时段为2011-2019年，研究类群为淡水无脊椎动物。
我们以岛状山（inselberg）上重复布设的岩石池集群为模式系统，构建了无脊椎动物集合群落（metacommunity）的大规模数据集，用以检验本地生物多样性与区域物种库生物多样性能否解释具有不同生存策略（主动扩散者与被动扩散者）的生物的群落生物量。为验证研究假说，我们结合了基于有向无环图（directed acyclic graph）的路径分析与广义线性混合效应模型开展研究。
结果显示，区域物种库的生物多样性受气候调控，但并未对群落生物量产生显著影响。相较而言，本地环境梯度似乎掩盖了物种库对群落生物量的所有影响。但与预期一致，本地生物多样性与生物量之间的关系因岛状山而异。与预期相悖的是，岛状山的突出度并未对BEF斜率产生影响。不过在更为干旱的环境中，主动扩散者的BEF关系有所减弱，这可能反映了再定殖过程受到的环境限制。由此可见，气候与扩散策略共同决定了生物多样性对群落生物量的影响方式。
本研究表明，即便在简单的生态系统中，生物多样性与生态系统功能之间的关系也可能存在显著的地理变异，这种变异可部分由环境条件与研究对象生物的生存策略加以解释。

研究方法
本数据集涵盖了四大洲10座岛状山的238个岩石池中采集的无脊椎动物群落。每座岛状山采样点的岩石池数量为12至34个不等。采样完成后，在实验室中将每个岩石池中的无脊椎动物鉴定至尽可能低的分类阶元（若条件允许则鉴定至属或种），并统计每个类群的个体数量。针对每个岩石池，通过将物种特异性生物量估算值与密度相乘，计算得到每升水体的无脊椎动物群落生物量。α多样性以每个岩石池的本地尺度物种丰富度计算，γ多样性则以代表区域物种库多样性的岛状山尺度物种丰富度计算。