图1. 包含直接和间接相互作用的生态网络。灰色箭头为直接配对相互作用(箭头1-3), 黑色箭头为间接相互作用(箭头4-5)。在间接相互作用中, 箭头4表示高阶相互作用, 即物种k影响的是物种j和i之间的相互作用, 箭头5表示链式相互作用, 即物种k先影响物种j的密度进而影响物种i。可见, 物种k对物种i既存在直接相互作用(箭头1), 也存在间接相互作用(箭头4和5)。箭头表示作用方向, 为简单起见, 只绘出了单向作用。

Fig. 1. The ecological network including direct (arrows 1-3) and indirect interactions (arrows 4-5) between species. Arrow 4 indicates that species k may indirectly affect species i by modifying the per capita effect of species j on species i (higher-order interactions, HOIs). Arrow 5 indicates that species k may indirectly affect species i by changing population density of species j (interaction chains). Therefore, species k may have both direct (arrow 1) and indirect (arrows 4-5) effects on species i. For simplicity, we only display direct and indirect effects of species j and k on species i.