Data from: Dispersal in dendritic networks: ecological consequences on the spatial distribution of population densities

1. Understanding the consequences of spatial structure on ecological dynamics is a central theme in ecology. Recently, research has recognized the relevance of river and river-analogue network structures, because these systems are not only highly diverse but also rapidly changing due to habitat modifications or species invasions. 2. Much of the previous work on ecological and evolutionary dynamics in metapopulations and metacommunities in dendritic river networks has been either using comparative approaches or was purely theoretical. However, the use of microcosm experiments provides the unique opportunity to study large-scale questions in a causal and experimental framework. 3. We conducted replicated microcosm experiments, in which we manipulated the spatially explicit network configuration of a landscape and addressed how linear versus dendritic connectivity affects population dynamics, specifically the spatial distribution of population densities, and movement behavior of the protist model organism Tetrahymena pyriformis. We tracked population densities and individual-level movement behavior of thousands of individuals over time. 4. At the end of the experiment, we found more variable population densities between patches in dendritic networks compared to linear networks, as predicted by theory. Specifically, in dendritic networks, population densities were higher at nodes that connected to headwaters compared to the headwaters themselves and to more central nodes in the network. These differences follow theoretical predictions and emerged from the different network topologies per se. These differences in population densities emerged despite weakly density-dependent movement. 5. We show that differences in network structure alone can cause characteristic spatial variation in population densities. While such differences have been postulated by theoretical work and are the underlying precondition for differential dispersal evolution in heterogeneous networks, our results may be the first experimental demonstration thereof. Furthermore, these population-level dynamics may affect extinction risks and can upscale to previously shown metacommunity level diversity dynamics. Given that many species in natural river systems exhibit strong spatio-temporal patterns in population densities, our work suggests that abundance patterns should not only be addressed from a local environmental perspective, but may be the outcome of processes that are inherently driven by the respective habitat network structure.

1. 解析空间结构对生态动态的影响是生态学的核心研究主题。近年来，学界逐渐认识到河流及类河网络结构的重要性：这类生态系统不仅物种多样性极高，还会因生境改造或物种入侵而快速发生演变。
2. 此前针对树状河网中集合种群（metapopulation）与集合群落（metacommunity）的生态及进化动态研究，大多采用比较研究方法，或仅为纯理论研究。然而，微观宇宙实验（microcosm experiments）为在因果实验框架下探究大尺度科学问题提供了独特契机。
3. 本研究开展了重复微观宇宙实验，通过操控景观的空间显式网络配置，探究线性与树状连通性如何影响种群动态——具体包括种群密度的空间分布，以及原生生物模式生物梨形四膜虫（Tetrahymena pyriformis）的运动行为。我们实时追踪了数千个个体的种群密度与个体水平运动行为随时间的变化。
4. 实验结束后，我们发现树状网络中斑块间的种群密度变异程度显著高于线性网络，这与理论预测相符。具体而言，在树状网络中，连接源头水域的节点处种群密度高于源头节点本身，也高于网络中更靠近中心的节点。这类差异符合理论预测，且仅由不同的网络拓扑结构（network topology）本身所导致；即便运动仅存在微弱的密度依赖性，这类种群密度差异仍会显现。
5. 本研究证实，仅网络结构差异便可引发种群密度的特征性空间变异。尽管理论研究早已提出这类差异，并将其作为异质网络中扩散演化分化的潜在前提，但本研究结果或许是此类现象的首个实验验证。此外，这类种群水平的动态可能会影响物种灭绝风险，并可推演至此前已被证实的集合群落水平多样性动态。鉴于自然河流系统中的诸多物种均表现出显著的种群密度时空格局，本研究提示，物种丰度格局不应仅从局域环境视角展开探讨，也可能是由对应生境网络结构内在驱动的过程所造就的结果。