Long-term empirical evidence, early warning signals and multiple drivers of regime shifts in a lake ecosystem 东湖生态系统长期演变中稳态转换和早期预警信号研究
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Catastrophic regime shifts in various ecosystems are increasing with the intensification of anthropogenic pressures. Understanding and predicting critical transitions are thus a key challenge in ecology. Previous studies have mainly focused on single environmental drivers (e.g. eutrophication) and early warning signals (EWSs) prior to population collapse. However, how multiple environmental stressors interact to shape ecological behaviour and whether EWSs were detectable prior to the recovery process in lake ecosystems are largely unknown. We present long-term empirical evidence of the critical transition and hysteresis with the combined pressures of climate warming, eutrophication and trophic cascade effects by fish stocking in a subtropical Chinese lake in the Yangtze floodplain. The catastrophic regime shifts are cross-validated by 64-year multi-trophic level monitoring data and paleo-diatom records. We show that EWSs are detectable in both the collapse and recovery trajectories and that including body size information in composite EWSs requires shorter time-series data and can improve the predictive ability of regime shifts. Although full recovery has not yet been observed, EWSs prior to recovery provide us with the opportunity to take measures for a clear-water regime. Climate warming and top-down cascade effects have a negative influence on water clarity by altering lower trophic level abundance and body size, which, in turn, have a negative effect on macrophyte abundance. Furthermore, we identify a shift in the dominant driving forces from bottom-up to top-down after regime shifts, decoupling the relationships between nutrients and biological components and thus decreasing the efficiency of nutrient reduction. Synthesis. This study provides new insights into ecological hysteresis under multiple external stressors and improves our understanding of trait-based early warning signals in both the collapse and recovery processes in natural freshwater ecosystems. For management practice, our work suggests that slowing down climate warming and weakening the fish predation pressure on food webs are necessary to increase the effectiveness of nutrient reduction in the restoration of lakes.
随着人为活动压力加剧，各类生态系统中的灾难性状态转换（catastrophic regime shifts）正愈发频发。理解并预测临界转变（critical transitions）是生态学领域的核心挑战之一。过往研究多聚焦于单一环境驱动因子（如富营养化）以及种群崩溃前的早期预警信号（early warning signals, EWSs）。然而，多重环境胁迫因子如何共同作用塑造生态系统行为，以及湖泊生态系统的恢复过程前是否可检测到EWSs，目前仍不明晰。本研究针对长江中下游平原的一处亚热带湖泊，获取了气候变暖、富营养化以及鱼类放养引发的营养级联效应多重胁迫下，临界转变与滞后效应（hysteresis）的长期实证证据。本研究通过64年的多营养级监测数据与古硅藻记录，对上述灾难性状态转换进行了交叉验证。研究结果表明，种群崩溃与恢复轨迹中均可检测到EWSs；且在复合EWSs中纳入体型信息后，所需的时间序列数据更短，同时可提升状态转换的预测能力。尽管目前尚未观测到完全恢复，但恢复前的EWSs为我们采取措施以维持清水态生态系统（clear-water regime）提供了契机。气候变暖与下行营养级联效应（top-down cascade effects）通过改变低营养级生物的丰度与体型，对水体透明度产生负面影响，进而抑制大型水生植物（macrophytes）的丰度。此外，本研究发现状态转换后，主导驱动因子由上行调控（bottom-up）转为下行调控（top-down），打破了营养盐与生物组分间的关联，进而降低了营养盐削减的效率。综合来看，本研究为多重外部胁迫下的生态滞后效应提供了新视角，并加深了我们对天然淡水生态系统（natural freshwater ecosystems）中，种群崩溃与恢复过程中基于功能性状的早期预警信号的理解。对于湖泊修复的管理实践而言，本研究表明，减缓气候变暖、削弱食物网中鱼类的捕食压力，是提升营养盐削减效率的必要举措。
