Data from: 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.

为明晰受干扰景观中的物种丧失机制，亟需区分生境斑块间扩散（dispersal）障碍所引发的效应，与环境条件退化所带来的影响。为评估两类效应的相对重要性，我们开发了一种全新分析方法：针对秩丰度数据（rank abundance data）开展置换检验（permutation test）与关联检验（association test），并以澳大利亚东南部两条河流的无脊椎动物群落在两个季节的采样数据为案例进行验证。

本研究的采样溪流为澳大利亚东南部的休斯溪（Hughes Creek）与七溪（Seven Creeks），两处溪流的下游河段均因农业活动发生退化。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（包括溪流中的漂移个体与陆生成虫）。采样站点覆盖了显著的环境梯度，同时涵盖了溪流的主要扩散通道（河道上下游方向）。通过分析环境数据，构建针对秩丰度的置换检验框架；同时采用列联表分析（contingency table analysis），对比物种的野外调查数据与扩散个体数据。

研究结果表明，扩散过程对群落结构具有显著调控作用。环境效应在溪流上游表现得尤为显著且强度最高，但在多数环境梯度区间内，环境效应的相关证据较为薄弱。多数物种在不同数据集间呈现出迥异的分布格局，或是其扩散个体在远离底栖种群分布核心的区域大量出现。

本研究结果与多数已有研究存在差异，但目前鲜有研究能够直接评估扩散效应的作用。我们开发的分析方法为解析扩散过程如何驱动景观尺度下的物种丰度格局提供了实用路径，从而填补了理论与实践之间的研究空白。

综合与应用：当前生态管理者通常采用的生态系统健康指数，均默认环境条件是决定物种多样性与丰度的核心因素。尽管学界已认识到生境斑块间的扩散过程至关重要，但目前尚无可靠方法可用于评估扩散的具体作用。我们提出的分析方法可通过野外调查数据，同时评估扩散与环境效应对物种分布的影响，这或将推动扩散相关信息纳入生态管理决策流程。此外，该方法还有助于优化修复项目的选址——此类修复项目的成功实施高度依赖个体的成功扩散。