Invariant antagonistic network structure despite high spatial and temporal turnover of interactions

Recent work has suggested that emergent ecological network structure exhibits very little spatial or temporal variance despite changes in community composition. However, the changes in network interactions associated with turnover in community composition have seldom been assessed. Here we examine whether changes in ecological networks are best detected by standard emergent network metrics or by assessing internal network changes (i.e. interaction and composition turnover). To eliminate possible spatial or phylogenetic effects, that in large-scale studies may obscure mechanisms structuring networks and interactions, we sampled multiple antagonistic (plant-herbivore) networks for a single diverse plant family (the Restionaceae) in the hyperdiverse Cape Floristic Region. These are the first plant-herbivore networks constructed for this global biodiversity hotspot. We found invariant emergent network structure despite considerable changes in insect and plant composition across communities over time and space. In contrast, there was high interaction turnover between networks. Seasonally, this was driven by turnover in insect species and insect host switching. Spatially, this was driven by simultaneous turnover in plant and insect species, suggesting that many insects are host specific or that both groups exhibit parallel responses to environmental gradients. Spatial interaction turnover was also driven by turnover in plants, showing that many insects can utilise multiple (possibly closely related) hosts and this may create divergent selection gradients that promote insect speciation. Thus we show highly variable interaction fidelity, despite invariant emergent network structure. We suggest that evaluating internal network changes may be more effective at elucidating the processes structuring networks, and many fine-scale changes may be obscured when only calculating emergent network metrics.

近期研究表明，尽管群落组成发生改变，涌现的生态网络结构几乎不存在空间或时间上的变异。然而，与群落组成周转相关的网络相互作用变化却极少得到评估。本研究旨在探究：相较于评估网络内部变化（即相互作用与组成周转），标准涌现网络指标（emergent network metrics）是否最能有效检测生态网络的变化。为消除大型研究中可能掩盖网络与相互作用构建机制的空间或系统发育效应，我们在物种极度丰富的开普植物区系区（Cape Floristic Region），针对单一多样的植物科——帚灯草科（Restionaceae），采样了多个拮抗（antagonistic）植食昆虫-寄主植物网络。这是首个针对这一全球生物多样性热点区域构建的植食昆虫-寄主植物网络。研究结果显示，尽管昆虫与植物的组成在不同群落的时间与空间维度上均存在显著变化，但涌现的网络结构始终保持恒定。与之形成鲜明对比的是，网络间的相互作用周转（interaction turnover）程度极高。在季节尺度上，该周转现象由昆虫物种周转与昆虫寄主转换共同驱动；在空间尺度上，则由植物与昆虫物种的同步周转所驱动，这提示多数昆虫具有寄主专化性，或两类群对环境梯度呈现平行响应。空间尺度的相互作用周转还受到植物类群周转的影响，这表明许多昆虫可利用多种（可能为近缘的）寄主，而这一现象可能产生差异化的选择压力，进而推动昆虫物种形成。综上，本研究证实，尽管涌现网络结构保持恒定，但相互作用的保真度存在高度变异。我们提出，评估网络内部变化或许更有助于阐明构建网络的生态过程，而仅计算涌现网络指标时，诸多精细尺度的变化可能会被掩盖。