Emergence of Structural Patterns in Neutral Trophic Networks

Interaction networks are central elements of ecological systems and have very complex structures. Historically, much effort has focused on niche-mediated processes to explain these structures, while an emerging consensus posits that both niche and neutral mechanisms simultaneously shape many features of ecological communities. However, the study of interaction networks still lacks a comprehensive neutral theory. Here we present a neutral model of predator-prey interactions and analyze the structural characteristics of the simulated networks. We find that connectance values (complexity) and complexity-diversity relationships of neutral networks are close to those observed in empirical bipartite networks. High nestedness and low modularity values observed in neutral networks fall in the range of those from empirical antagonist bipartite networks. Our results suggest that, as an alternative to niche-mediated processes that induce incompatibility between species (“niche forbidden links”), neutral processes create “neutral forbidden links” due to uneven species abundance distributions and the low probability of interaction between rare species. Neutral trophic networks must be seen as the missing endpoint of a continuum from niche to purely stochastic approaches of community organization.

相互作用网络（interaction networks）是生态系统的核心组成部分，其结构极为复杂。长期以来，学界多聚焦于生态位（niche）介导的过程以阐释此类网络结构；当前新兴的共识指出，生态位与中性（neutral）两种机制共同塑造了生态群落的诸多特征。然而，针对相互作用网络的研究至今仍缺乏一套完整的中性理论（neutral theory）。本研究提出了捕食者-猎物相互作用（predator-prey interactions）的中性模型，并对模拟生成的网络的结构特征展开分析。研究发现，中性网络的连接度（connectance）数值（即复杂度）以及复杂度-多样性关系，与经验观测得到的二元网络（bipartite network）特征高度吻合。中性网络所展现出的高嵌套性（nestedness）与低模块化（modularity）数值，与经验观测到的对抗性二元网络的对应参数范围一致。本研究结果表明，相较于会引发物种间不相容性的生态位介导过程（即"生态位禁止连接（forbidden links）"），中性过程会因物种丰度分布不均，以及稀有种间相互作用概率极低，而形成"中性禁止连接（forbidden links）"。中性营养网络（trophic network）应当被视为群落组织研究谱系中，从生态位方法到纯随机方法这一连续统一体上缺失的关键端点。