The amounts and ratios of nutrients (nitrogen and phosphorus) are important determinants of producer community biodiversity and composition and their responses to climate and dispersal. However, the nutrient effects on co-occurrence network topology, particularly in freshwaters, are understudied. Here, we investigate 1) whether nutrient supply and ratio constrain topological properties of algal co-occurrence networks in streams and 2) to what extent climate and space (a surrogate for dispersal) affect co-occurrence network topology versus metacommunity composition across nutrient supply and ratio contexts. We used a subcontinental dataset of benthic algae from 840 stream localities in the conterminous US. We constructed co-occurrence networks representing nutrient supply contexts (oligotrophic versus eutrophic) and nutrient ratio contexts (N-limited versus P-limited) and statistically assessed topological variability within each pair via randomization. We then used a null model framework and direct gradient analysis to ascertain the importance of climate and space in driving, respectively, network topology and metacommunity composition. Nutrient supply was only positively related to network size (species node counts), which was driven by motile species, while other topological differences were non-significant. Climatic and spatial variables had pronounced and for the most part comparable effects on network topology that further depended on nutrient context. A comparative assessment of topological versus compositional responses to climate and space across nutrient contexts identified both similarities and differences. While climate and space contributed to both network topology and metacommunity composition, space was a stronger predictor of compositional variability than climate, regardless of nutrient context. Our findings highlight the need for developing integrative multi-level approaches (from metacommunities to co-occurrence networks) to fully understand biological responses to complex and interactive abiotic forces.

中文翻译：

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养分供应和失衡对次大陆共生网络和河流藻类群落组成的影响

养分（氮和磷）的数量和比例是生产者群落生物多样性和组成及其对气候和扩散的反应的重要决定因素。然而，营养对共生网络拓扑结构的影响，特别是在淡水中，尚未得到充分研究。在这里，我们调查 1) 养分供应和比率是否限制了溪流中藻类共生网络的拓扑特性，以及 2) 气候和空间（扩散的替代品）在多大程度上影响共生网络拓扑与跨养分供应的元群落组成比率上下文。我们使用了来自美国本土 840 个河流地区的底栖藻类的次大陆数据集。我们构建了代表养分供应环境（贫营养与富营养）和养分比例环境（N 限制与 P 限制）的共现网络，并通过随机化对每对中的拓扑变异性进行了统计评估。然后，我们使用零模型框架和直接梯度分析来确定气候和空间分别在驱动网络拓扑和元社区组成中的重要性。营养供应仅与网络大小（物种节点数）呈正相关，而网络大小（物种节点数）受运动物种驱动，而其他拓扑差异不显着。气候和空间变量对网络拓扑结构产生了显着的影响，并且在大多数情况下具有可比性，这进一步取决于营养环境。对不同营养环境对气候和空间的拓扑与成分响应的比较评估确定了相似之处和不同之处。虽然气候和空间对网络拓扑结构和元群落组成都有贡献，但无论营养环境如何，空间都是比气候更能预测组成变异性的因素。我们的研究结果强调需要开发综合的多层次方法（从元社区到共现网络），以充分了解生物对复杂和互动的非生物力的反应。