Plant-mycorrhizal fungus co-occurrence network lacks substantial structure

The interactions between plants and arbuscular mycorrhizal fungi (AMF) maintain a crucial link between macroscopic organisms and the soil microbial world. These interactions are of extreme importance for the diversity of plant communities and ecosystem functioning. Despite this importance, only recently has the structure of plant–AMF interaction networks been studied. These recent studies, which used genetic data, suggest that these networks are highly structured, very similar to plant–animal mutualistic networks. However, the assembly process of plant–AMF communities is still largely unknown, and an important feature of plant–AMF interactions has not been incorporated: they occur at an extremely localized scale. Studying plant–AMF networks in a spatial context seems therefore a crucial step. This paper studies a plant–AMF spatial co-occurrence network using novel methodology based on information theory and a unique set of spatially explicit species-level data. We apply three null models of which only one accounts for spatial effects. We find that the data show substantial departures from null expectations for the two non-spatial null models. However, for the null model considering spatial effects, there are few significant co-occurrences compared with the other two null models. Thus, plant–AMF spatial co-occurrences seem to be mostly explained by stochasticity, with a small role for other factors related to plant–AMF specialization. Furthermore, we find that the network is not significantly nested or modular. We conclude that this plant–AMF spatial co-occurrence network lacks substantial structure and, therefore, plants and AMF species do not track each other over space. Thus, random encounters seem more important in the first step of the assembly of plant–AMF communities.

植物与丛枝菌根真菌（arbuscular mycorrhizal fungi, AMF）之间的互作关系，维系了大型生物与土壤微生物世界之间的关键纽带。这类互作对于植物群落多样性与生态系统功能具有至关重要的意义。尽管其重要性不言而喻，但学界直至近年才对植物-AMF互作网络的结构展开研究。这些依托遗传数据开展的近期研究显示，此类网络具备高度结构化的特征，与动植物互利共生网络高度相似。然而，植物-AMF群落的组装过程仍在很大程度上未被阐明，且植物-AMF互作的一项核心特征尚未被纳入研究框架：这类互作发生在极小的空间尺度上。因此，在空间框架下开展植物-AMF网络研究，堪称关键的研究环节。本研究基于信息论的新型分析方法，结合一套独特的空间显性物种水平数据（spatially explicit species-level data），对植物-AMF空间共现网络展开研究。我们采用了三种零模型（null models），其中仅一种纳入了空间效应。研究发现，相较于两种非空间零模型，实测数据与零模型的预期结果存在显著偏离。然而，针对纳入空间效应的零模型，相较另外两种模型，其检测到的显著共现事件极少。由此可见，植物-AMF的空间共现关系大多可由随机性（stochasticity）解释，其余与植物-AMF专化性（specialization）相关的因素仅发挥微弱作用。此外，本研究发现该网络既未呈现显著的嵌套结构（nested），也不具备显著的模块化结构（modular）。综上，本研究中的植物-AMF空间共现网络并无实质性的结构化特征，换言之，植物与AMF物种并未在空间尺度上彼此追踪匹配。因此，随机邂逅似乎在植物-AMF群落组装的初始阶段发挥着更为关键的作用。