Department of Biosciences, Centre for Sustainable Aquatic Research (CSAR), Swansea University, Swansea, UK.
Kent Business School, University of Kent, Canterbury, UK; Durrell Institute of Conservation and Ecology, University of Kent, Canterbury, UK.
Sci Total Environ. 2022 Nov 20;848:157471. doi: 10.1016/j.scitotenv.2022.157471. Epub 2022 Jul 19.
Barrier removal can be an efficient method to restore river continuity but resources available for defragmenting rivers are limited and a prioritization strategy is needed. We review methods for prioritizing barriers for removal and report on a survey asking practitioners which barrier prioritization methods they use. Opportunities for barrier removal depend to a large extent on barrier typology, as this dictates where barriers are normally located, their size, age, condition, and likely impacts. Crucially, river fragmentation depends chiefly on the number and location of barriers, not on barrier size, while the costs of barrier removal typically increase with barrier height. Acting on many small barriers will often be more cost-efficient than acting on fewer larger structures. Barriers are not randomly distributed and a small proportion of barriers have a disproportionately high impact on fragmentation, therefore targeting these 'fragmentizers' can result in substantial gains in connectivity. Barrier prioritization methods can be grouped into six main types depending on whether they are reactive or proactive, whether they are applied at local or larger spatial scales, and whether they employ an informal or a formal approach. While mathematical optimization sets the gold standard for barrier prioritization, a hybrid approach that explicitly considers uncertainties and opportunities is likely to be the most effective. The effectiveness of barrier removal can be compromised by inaccurate stream networks, erroneous barrier coordinates, and underestimation of barrier numbers. Such uncertainties can be overcome by ground truthing via river walkovers and predictive modelling, but the cost of collecting additional information must be weighed against the cost of inaction. To increase the success of barrier removal projects, we recommend that barriers considered for removal fulfill four conditions: (1) their removal will bring about a meaningful gain in connectivity; (2) they are cost-effective to remove; (3) they will not cause significant or lasting environmental damage, and (4) they are obsolete structures. Mapping barrier removal projects according to the three axes of opportunities, costs, and gains can help locate any 'low hanging fruit.'
去除障碍物可以有效地恢复河流的连续性，但用于河流碎片化治理的资源是有限的，因此需要制定一个优先化策略。我们回顾了障碍物去除的优先级方法，并报告了一项调查，询问从业者他们使用哪些障碍物优先级方法。去除障碍物的机会在很大程度上取决于障碍物的类型，因为这决定了障碍物通常的位置、大小、年龄、状况和可能的影响。至关重要的是，河流的碎片化主要取决于障碍物的数量和位置，而不是障碍物的大小，而障碍物去除的成本通常会随着障碍物高度的增加而增加。对许多小障碍物采取行动通常比对少数较大的结构采取行动更具成本效益。障碍物不是随机分布的，一小部分障碍物对碎片化的影响极大，因此针对这些“碎片化制造者”可以在连通性方面取得实质性的进展。障碍物优先级方法可以根据其是反应性的还是前瞻性的、是在局部还是更大的空间尺度上应用、以及是否采用非正式或正式的方法，分为六类。虽然数学优化是障碍物优先级的黄金标准，但考虑不确定性和机会的混合方法可能是最有效的。障碍物去除的有效性可能会因不准确的河流网络、错误的障碍物坐标和障碍物数量的低估而受到影响。通过河流步行和预测建模进行实地核实，可以克服这些不确定性，但必须权衡收集额外信息的成本与不作为的成本。为了提高障碍物去除项目的成功率，我们建议考虑去除的障碍物应满足以下四个条件：（1）去除它们将带来有意义的连通性增益；（2）去除它们具有成本效益；（3）它们不会造成重大或持久的环境破坏；（4）它们是过时的结构。根据机会、成本和收益这三个轴来对障碍物去除项目进行映射，可以帮助确定任何“容易达到的目标”。
