Retention of Habitat Complexity Minimizes Disassembly of Reef Fish Communities following Disturbance: A Large-Scale Natural Experiment

High biodiversity ecosystems are commonly associated with complex habitats. Coral reefs are highly diverse ecosystems, but are under increasing pressure from numerous stressors, many of which reduce live coral cover and habitat complexity with concomitant effects on other organisms such as reef fishes. While previous studies have highlighted the importance of habitat complexity in structuring reef fish communities, they employed gradient or meta-analyses which lacked a controlled experimental design over broad spatial scales to explicitly separate the influence of live coral cover from overall habitat complexity. Here a natural experiment using a long term (20 year), spatially extensive (∼115,000 kms2) dataset from the Great Barrier Reef revealed the fundamental importance of overall habitat complexity for reef fishes. Reductions of both live coral cover and habitat complexity had substantial impacts on fish communities compared to relatively minor impacts after major reductions in coral cover but not habitat complexity. Where habitat complexity was substantially reduced, species abundances broadly declined and a far greater number of fish species were locally extirpated, including economically important fishes. This resulted in decreased species richness and a loss of diversity within functional groups. Our results suggest that the retention of habitat complexity following disturbances can ameliorate the impacts of coral declines on reef fishes, so preserving their capacity to perform important functional roles essential to reef resilience. These results add to a growing body of evidence about the importance of habitat complexity for reef fishes, and represent the first large-scale examination of this question on the Great Barrier Reef.

生物多样性丰富的生态系统通常与复杂栖息环境密切相关。珊瑚礁作为高度多样的生态系统，正面临日益增多的各类胁迫因子威胁：诸多胁迫因子会降低活珊瑚覆盖度与栖息环境复杂度，并对礁区鱼类等其他生物产生连锁效应。尽管既往研究已阐明栖息环境复杂度在塑造礁区鱼类群落中的核心作用，但此类研究多采用梯度分析或元分析方法，缺乏大空间尺度下的对照实验设计，无法明确区分活珊瑚覆盖度与整体栖息环境复杂度各自的独立影响。

本研究依托大堡礁（Great Barrier Reef）一项为期20年、空间覆盖范围约115000平方千米的长期数据集开展自然实验，明确了整体栖息环境复杂度对礁区鱼类的关键重要性。研究发现，与仅大幅降低活珊瑚覆盖度却未改变栖息环境复杂度时仅对鱼类群落产生轻微影响不同，同时降低活珊瑚覆盖度与栖息环境复杂度会对鱼类群落造成显著冲击。当栖息环境复杂度大幅下降时，鱼类种群丰度普遍降低，更多鱼类物种在局部区域绝迹，其中包括具有重要经济价值的鱼类；这进而导致物种丰富度下降，且功能类群内部的多样性也出现丧失。

本研究结果表明，在生态扰动后保留栖息环境复杂度，可缓解珊瑚退化对礁区鱼类的负面影响，从而维持鱼类发挥支撑珊瑚礁恢复力所需的关键功能角色的能力。上述结果进一步丰富了关于栖息环境复杂度对礁区鱼类重要性的实证证据，同时也是首次在大堡礁范围内针对该问题开展的大尺度系统性研究。