A fresh approach reveals how dispersal shapes metacommunity structure in a human-altered landscape

To understand species losses from disturbed landscapes, it is important to distinguish the effects of degraded environmental conditions from those caused by barriers to dispersal between habitat patches. To assess the relative importance of these effects, we developed a new approach using permutation and association tests applied to rank abundance data, using the invertebrate fauna of two rivers in two seasons.
Our study streams were Hughes Creek and Seven Creeks, in south-eastern Australia, which have both been degraded by agriculture in downstream sections. We collected benthic invertebrates and also dispersing individuals (drift, terrestrial adults) during two seasons in 2007–2008. Study sites spanned strong environmental gradients as well as the main dispersal route (up- and down-channel). Environmental data were analysed to set up permutation tests on rank abundances. Survey and disperser data were contrasted using contingency table analyses.
The results suggest dispersal plays a strong role in community structure. Environmental effects were evident and strongest upstream, but evidence of environmental effects was weak over much of the gradient. Many species had different distributions in different data sets or dispersers that were abundant at locations distant from centres of benthic distribution.
Our results differ from many studies, but few have been able to evaluate dispersal effects directly. Our method provides a practical approach for evaluating the role dispersal plays in driving species abundance patterns across landscapes, thus bridging a gap between theory and practice.
Synthesis and applications. Managers typically use indices of ecosystem health that assume environmental conditions largely determine species diversity and abundance. Dispersal between habitat patches is known to be important, but there are no reliable methods to assess the role dispersal may play. We provide an approach that allows both dispersal and environmental effects on species distributions to be evaluated from survey data. This may open the way for dispersal information to be incorporated into management actions. Additionally, the approach should allow improved siting of restoration projects that depend greatly on successful dispersal of individuals for successful outcomes.

为解析受干扰景观中的物种丧失机制，区分环境条件退化与生境斑块间扩散障碍所引发的两类效应至关重要。为评估这两类效应的相对重要性，我们开发了一种全新研究方法：将置换检验（permutation test）与关联检验（association test）应用于秩丰度数据（rank abundance data），并以澳大利亚东南部两条河流的无脊椎动物类群为研究对象，分两个季节开展数据采集与分析。
本研究选取的溪流为休斯溪（Hughes Creek）与七溪（Seven Creeks），二者的下游河段均因农业活动发生了生态退化。我们于2007—2008年的两个季节中，采集了底栖无脊椎动物（benthic invertebrates）以及扩散个体，包括水流漂流个体与陆生成虫。研究样地覆盖了显著的环境梯度与主要扩散通道，即河道上下游区域。我们先通过环境数据分析构建了针对秩丰度数据的置换检验框架，随后采用列联表分析（contingency table analysis）对物种调查数据与扩散个体数据进行对比。
研究结果表明，扩散在群落结构构建中发挥着关键作用。环境效应在河道上游区域表现最为显著且强烈，但在多数环境梯度区间内，环境效应的相关证据较为薄弱。诸多物种在不同调查数据集间呈现出差异化的分布特征，或是其扩散个体在远离底栖类群分布中心的区域大量聚集。
本研究结果与多数同类研究存在差异，但此前鲜有研究能够直接评估扩散效应的具体作用。我们提出的方法为解析景观尺度下物种丰度格局的驱动机制提供了实用路径，填补了理论与实践间的空白。
【综合与应用】
生态系统管理者通常采用生态系统健康指数开展评估，这类指数默认环境条件是决定物种多样性与丰度的核心因素。生境斑块间的扩散作用虽已被证实具有重要意义，但目前尚无可靠方法可用于量化评估其具体影响。我们开发的方法能够通过野外调查数据同时解析扩散与环境效应对物种分布的作用，这或将推动扩散相关信息被纳入生态管理实践。此外，该方法还有助于优化修复工程的选址——此类修复工程的成功高度依赖物种个体的成功扩散。