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Potential for Sudden Shifts in Transient Systems: Distinguishing Between Local and Landscape-Scale Processes

机译：瞬态系统中突然发生变化的可能性：区分局部过程和景观过程

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Thorough understanding of the potential for threshold dynamics and catastrophic shifts to occur in natural systems is of great importance for ecosystem conservation and restoration. However, verifying the presence of alternative stable states, one of the theoretical explanations for sudden shifts in natural systems, has proven to be a major challenge. We examine processes on local and landscape scales in salt-marsh pioneer zones, to assess the presence of alternative stable states in this system. To that end, we investigated the presence of typical characteristics of alternative stable states: bimodality and threshold dynamics. We also studied whether vegetation patches remained stable over long time periods. Analysis of false-color aerial photographs revealed clear bimodality in plant biomass distribution. By transplanting Spartina anglica plants of three different biomass classes on three geographically different marshes, we showed that a biomass threshold limits the establishment of Spartina patches, potentially explaining their patchy distribution. The presence of bimodality and biomass thresholds points to the presence of alternative stable states and the potential for sudden shifts, at small, within-patch scales and on short time scales. However, overlay analysis of aerial photographs from a salt marsh in The Netherlands, covering a time span of 22 years, revealed that there was little long-term stability of patches, as vegetation cover in this area is slowly increasing. Our results suggest that the concept of alternative stable states is applicable to the salt-marsh pioneer vegetation on small spatio-temporal scales. However, the concept does not apply to long-term dynamics of decades or centuries of heterogeneous salt-marsh pioneer zones, as landscape-scale processes may determine the large-scale dynamics of salt marshes. Hence, our results provide the interesting perspective that threshold dynamics may occur in systems with, on the long term, only a single stable state.
机译：全面了解自然系统中阈值动态变化和灾难性变化的可能性对于生态系统的保护和恢复至关重要。但是，验证替代稳态的存在是对自然系统突然转变的理论解释之一，已证明是一项重大挑战。我们研究了盐沼先锋地区局部和景观尺度上的过程，以评估该系统中其他稳定状态的存在。为此，我们研究了替代稳态的典型特征：双峰态和阈值动力学。我们还研究了植被斑块在长时间内是否保持稳定。对假彩色航空照片的分析表明，植物生物量分布具有明显的双峰态。通过在三种地理上不同的沼泽地上移植三种不同生物量类的无花果草植物，我们发现生物量阈值限制了草草斑块的建立，可能解释了它们的斑片状分布。双峰态和生物量阈值的存在表明存在替代的稳定状态，并且在小规模，补丁内规模和短时间规模上可能会发生突然变化。但是，对荷兰盐沼的航拍照片进行的叠加分析（涵盖了22年的时间）显示，斑块的长期稳定性几乎没有，因为该地区的植被覆盖率正在缓慢增加。我们的结果表明，替代稳定状态的概念适用于小时空尺度上的盐沼先锋植被。但是，该概念不适用于数十年或几个世纪的非均质盐沼先锋地区的长期动态，因为景观规模过程可能决定盐沼的大规模动态。因此，我们的结果提供了一个有趣的观点，即从长远来看，只有一个稳定状态的系统中可能会发生阈值动态变化。

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