Landscape Ecology, Institute of Terrestrial Ecosystems, ETH Zürich, Zürich, Switzerland.
Swiss Federal Research Institute WSL, 8903, Birmensdorf, Switzerland.
Biol Rev Camb Philos Soc. 2018 May;93(2):785-800. doi: 10.1111/brv.12366. Epub 2017 Sep 22.
Knowledge of species composition and their interactions, in the form of interaction networks, is required to understand processes shaping their distribution over time and space. As such, comparing ecological networks along environmental gradients represents a promising new research avenue to understand the organization of life. Variation in the position and intensity of links within networks along environmental gradients may be driven by turnover in species composition, by variation in species abundances and by abiotic influences on species interactions. While investigating changes in species composition has a long tradition, so far only a limited number of studies have examined changes in species interactions between networks, often with differing approaches. Here, we review studies investigating variation in network structures along environmental gradients, highlighting how methodological decisions about standardization can influence their conclusions. Due to their complexity, variation among ecological networks is frequently studied using properties that summarize the distribution or topology of interactions such as number of links, connectance, or modularity. These properties can either be compared directly or using a procedure of standardization. While measures of network structure can be directly related to changes along environmental gradients, standardization is frequently used to facilitate interpretation of variation in network properties by controlling for some co-variables, or via null models. Null models allow comparing the deviation of empirical networks from random expectations and are expected to provide a more mechanistic understanding of the factors shaping ecological networks when they are coupled with functional traits. As an illustration, we compare approaches to quantify the role of trait matching in driving the structure of plant-hummingbird mutualistic networks, i.e. a direct comparison, standardized by null models and hypothesis-based metaweb. Overall, our analysis warns against a comparison of studies that rely on distinct forms of standardization, as they are likely to highlight different signals. Fostering a better understanding of the analytical tools available and the signal they detect will help produce deeper insights into how and why ecological networks vary along environmental gradients.
了解物种组成及其相互作用的知识，以相互作用网络的形式，对于理解它们在时间和空间上的分布过程至关重要。因此，比较生态网络沿环境梯度的变化是理解生命组织的一个很有前途的新研究方向。网络中链接的位置和强度随环境梯度的变化可能是由物种组成的周转率、物种丰度的变化以及物种相互作用的非生物影响驱动的。虽然调查物种组成的变化具有悠久的传统，但到目前为止，只有少数研究考察了网络之间物种相互作用的变化，而且通常采用不同的方法。在这里，我们回顾了沿环境梯度研究网络结构变化的研究，强调了关于标准化的方法决策如何影响它们的结论。由于其复杂性，生态网络的变化通常使用总结相互作用分布或拓扑的属性来研究，例如链接数量、连通度或模块性。这些属性可以直接比较或使用标准化程序。虽然网络结构的度量可以直接与环境梯度的变化相关，但标准化通常用于通过控制某些协变量或通过零模型来促进对网络属性变化的解释。零模型允许比较经验网络与随机预期的偏差，并有望在与功能特征耦合时提供对塑造生态网络的因素的更机械理解。作为说明，我们比较了量化特征匹配在驱动植物-蜂鸟共生网络结构中的作用的方法，即直接比较、通过零模型和基于假设的元网络标准化。总体而言，我们的分析警告不要比较依赖于不同标准化形式的研究，因为它们可能会突出不同的信号。促进对现有分析工具及其检测信号的更好理解将有助于深入了解生态网络如何以及为何沿环境梯度变化。
