生态系统状态跃变的早期预警信号及检测

生态系统在气候变化和土地利用及人类活动等的影响下其状态会由某一稳态转变到另一稳态。由于环境压力的复杂性、非线性、随机性等特征，往往导致状态转变表现为非线性、突变、跃变等特点。准确界定系统状态跃变的拐点或阈值点存在很大的挑战，而捕捉接近临界拐点前的生态系统结构和属性上的变化特征作为早期预警信号是切实可行的。早期预警信号理论经历理论框架构建、方法确立、机理认知等近半个多世纪的探索，已经由最初的通过仅依赖检测临界点恢复力的速率减慢、方差增加、系统自相关增强等统计学信号过度到更加多样化的检测方法，如检测系统组分属性的变化特征，诊断系统组分各属性之间的关系变化，系统组分的性状变化、系统组分网络结构变化等等，并且试图整合多信号提高预警的精确性。利用来自自然生态系统的长时间高密度数据集和空间代替时间的数据集，基于多度及性状信号的早期预警，结合稳定性、临界恢复力的减速、以及统计参数的指示作用对系统跃变进行早期诊断和预警是预测生态学的主旨。早期预警信号的深入研究不仅能够完善已有理论的不足，同时还能够为生态系统的保护和管理提供切实有效的理论指导。     

Shifts in the dynamics of productivity signal ecosystem state transitions at the biome‐scale

Understanding ecosystem dynamics and predicting directional changes in ecosystem in response to global changes are ongoing challenges in ecology. Here we present a framework that links productivity dynamics and ecosystem state transitions based on a spatially continuous dataset of aboveground net primary productivity (ANPP) from the temperate grassland of China. Across a regional precipitation gradient, we quantified spatial patterns in ANPP dynamics (variability, asymmetry and sensitivity to rainfall) and related these to transitions from desert to semi‐arid to mesic steppe. We show that these three indices of ANPP dynamics displayed distinct spatial patterns, with peaks signalling transitions between grassland types. Thus, monitoring shifts in ANPP dynamics has the potential for predicting ecosystem state transitions in the future. Current ecosystem models fail to capture these dynamics, highlighting the need to incorporate more nuanced ecological controls of productivity in models to forecast future ecosystem shifts.     

生态系统的多稳态与突变

许多生态系统可能在短时间内发生难以预料的状态突变, 其中一些生态系统突变的机理可以用多稳态理论进行解释。近年来生态系统的多稳态和突变现象及其机理吸引了研究者和管理者的广泛关注。本文重点对生态系统多稳态的理论基础、识别方法及稳态转换发生的早期预警信号进行综述, 并基于典型生态系统过程对现实世界中可能观测到的稳态转换进行实例分析, 最后对多稳态概念框架和理论应用中的潜在争议进行讨论, 以期为非线性生态系统动态的理论研究、管理实践和生物多样性保护等提供参考。     

On kinetics of phase transitions in cell membranes

Phase transitions in a bicomponent lipid membrane are considered. It is shown that in this case metastable states practically do not arise and phase transitions are smooth and hysteresisless. An elastic frame on the surface of the membrane changes the character of phase transitions: they become sharp and hysteretic. The role of membrane phase transitions for regulation of cell processes is considered.     

Hysteresis in an experimental phytoplankton population

The road to recovery of a deteriorated system is often different, and fraught with more barriers, than the path to the system's deterioration. This phenomenon is called hysteresis, and is inherent to systems presenting alternative stable states. In such systems, the stability of a given state is the product of positive feedback loops. A broad range of natural systems have been predicted to show hysteretic behaviour, but hysteresis has so far only been unambiguously demonstrated at cellular or metabolic levels, not yet at the population or ecosystem level. To extend our understanding of hysteresis at the population level, we performed an experiment on light‐stressed cyanobacteria and found hysteresis between alternative stable states. Furthermore, during the experiment, the cyanobacteria adapted physiologically to high light levels, and deviated from their theoretically predicted pathway of hysteresis, therewith also avoiding extinction. Our experiment confirmed that a population that loses resilience due to deteriorating external conditions can show a delayed – hysteretic – recovery‐response when conditions are improved. This population‐level study also indicates that the slowness of these systems may obscure the true state they are in, which is important to factor into ecosystem monitoring. Additionally, we show that adaptation can drastically alter the systems’ predicted behaviour to ecosystem management. Flexibility of species and slowness should, therefore, be included in the monitoring and prediction of ecosystem responses to environmental changes.     

Oscillating trophic control induces community reorganization in a marine ecosystem

Understanding how climate regulates trophic control may help to elucidate the causes of transitions between alternate ecosystem states following climate regime shifts. We used a 34-year time series of the abundance of Pacific cod ( Gadus macrocephalus ) and five prey species to show that the nature of trophic control in a North Pacific ecosystem depends on climate state. Rapid warming in the 1970s caused an oscillation between bottom–up and top–down control. This shift to top–down control apparently contributed to the transition from an initial, prey-rich ecosystem state to the final, prey-poor state. However, top–down control could not be detected in the final state without reference to the initial state and transition period. Complete understanding of trophic control in ecosystems capable of transitions between alternate states may therefore require observations spanning more than one state.     

Regime shifts in shallow lakes: the importance of seasonal fish migration

Shallow eutrophic lakes commonly exist in two alternative stable states: a clear-water state and a turbid water state. A number of mechanisms, including both abiotic and biotic processes, buffer the respective states against changes, whereas other mechanisms likely drive transitions between states. Our earlier research shows that a large proportion of zooplanktivorous fish populations in shallow lakes undertake seasonal migrations where they leave the lake during winter and migrate back to the lake in spring. Based on our past research, we propose a number of scenarios of how feedback processes between the individual and ecosystem levels may affect stability of alternative stable states in shallow lakes when mediated by fish migration. Migration effects on shallow lakes result from processes at different scales, from the individual to the ecosystem. Our earlier research has shown that ecosystem properties, including piscivore abundance and zooplankton productivity, affect the individual state of zooplanktivorous fish, such as growth rate or condition. Individual state, in turn, affects the relative proportion and timing of migrating zooplanktivorous fish. This change, in turn, may stabilize states or cause runaway processes that eventually lead to state shifts. Consequently, such knowledge of processes coupled to seasonal migration of planktivorous fish should increase our understanding of shallow lake dynamics.     

The effect of climate change on the resilience of ecosystems with adaptive spatial pattern formation

In a rapidly changing world, quantifying ecosystem resilience is an important challenge. Historically, resilience has been defined via models that do not take spatial effects into account. These systems can only adapt via uniform adjustments. In reality, however, the response is not necessarily uniform, and can lead to the formation of (self‐organised) spatial patterns – typically localised vegetation patches. Classical measures of resilience cannot capture the emerging dynamics in spatially self‐organised systems, including transitions between patterned states that have limited impact on ecosystem structure and productivity. We present a framework of interlinked phase portraits that appropriately quantifies the resilience of patterned states, which depends on the number of patches, the distances between them and environmental conditions. We show how classical resilience concepts fail to distinguish between small and large pattern transitions, and find that the variance in interpatch distances provides a suitable indicator for the type of imminent transition. Subsequently, we describe the dependency of ecosystem degradation based on the rate of climatic change: slow change leads to sporadic, large transitions, whereas fast change causes a rapid sequence of smaller transitions. Finally, we discuss how pre‐emptive removal of patches can minimise productivity losses during pattern transitions, constituting a viable conservation strategy.     

Precursors of state transitions in stochastic systems with delay

Ecosystem dynamics may exhibit alternative stable states induced by positive feedbacks between the state of the system and environmental drivers. Bistable systems are prone to abrupt shifts from one state to another in response to even small and gradual changes in external drivers. These transitions are often catastrophic and difficult to predict by analyzing the mean state of the system. Indicators of the imminent occurrence of phase transitions can serve as important tools to warn ecosystem managers about an imminent transition before the bifurcation point is actually reached. Thus, leading indicators of phase transitions can be used either to prepare for or to prevent the occurrence of a shift to the other state. In recent years, theories of leading indicators of ecosystem shift have been developed and applied to a variety of ecological models and geophysical time series. It is unclear, however, how some of these indicators would perform in the case of systems with a delay. Here, we develop a theoretical framework for the investigation of precursors of state shift in the presence of drivers acting with a delay. We discuss how the effectiveness of leading indicators of state shift based on rising variance may be affected by the presence of delays. We apply this framework to an ecological model of desertification in arid grasslands.     

Review on detection of critical transition in ecosystems

当一个存在多稳态的生态系统临近突变阈值点时, 外界条件即使发生一个微小变化, 也会引发生态系统的剧烈响应, 使之进入结构和功能截然不同的另一稳定状态, 这种现象称为重大突变(critical transition)。重大突变所导致的稳态转换总是伴随着生态系统服务的急剧变化, 可能对人类可持续发展产生重大影响。预测生态系统突变的发生非常困难, 但科学家在此领域的大量研究结果表明, 通过监测一些通用指标可以判断生态系统是否不断临近重大突变阈值点, 进而可以进行生态系统重大突变预警。该文对近年来生态系统重大突变检测领域所取得的成果进行总结与归纳, 论述了生态系统重大突变的产生机制及其后果, 介绍了生态系统突变预警信号提取的理论基础, 从时间和空间两个维度总结了近年来生态系统重大突变预警信号的提取方法, 概述了当前研究面临的挑战, 指出生态系统突变预警信号的检测应充分利用时空动态数据, 并且联合多个指标, 从多个角度进行综合预警, 此外, 还应重视生态系统结构与重大突变之间的关系, 增强生态系统突变预警能力。     

Do herbivores cause habitat degradation or vegetation state transition? Evidence from the tundra

Range expansion and increasing densities of large herbivores are held responsible for large-scale habitat degradation in a wide range of natural and semi-natural ecosystems. Herbivore-driven ecosystem changes frequently represent predictable transitions from one vegetation state to another. Whether such predictable changes justify the value judgement 'habitat degradation' may be debatable as this strongly depends on individual perspective.
To further the debate on herbivore-driven habitat degradation, I apply the concept of alternative stable states to arctic tundra as a framework to capture predictable stepwise vegetation transitions in which the productivity and hence herbivore-carrying capacity increases with grazing pressure. Specifically, evidence is provided that large parts of the tundra biome can be in either of three relatively discrete vegetation states and that changes in reindeer/caribou density are responsible for sudden, predictable but often reversible state transitions. From this, it appears that the relatively rapidly emerging vegetation changes do not necessarily equate to habitat degradation, but in many cases reflect predictable vegetation change. Acknowledgement of the existence of predictable state transitions in tundra ecosystems may help to evaluate the observed radical vegetation changes occurring throughout the reindeer/caribou range.     

Early detection of ecosystem regime shifts: a multiple method evaluation for management application

Critical transitions between alternative stable states have been shown to occur across an array of complex systems. While our ability to identify abrupt regime shifts in natural ecosystems has improved, detection of potential early-warning signals previous to such shifts is still very limited. Using real monitoring data of a key ecosystem component, we here apply multiple early-warning indicators in order to assess their ability to forewarn a major ecosystem regime shift in the Central Baltic Sea. We show that some indicators and methods can result in clear early-warning signals, while other methods may have limited utility in ecosystem-based management as they show no or weak potential for early-warning. We therefore propose a multiple method approach for early detection of ecosystem regime shifts in monitoring data that may be useful in informing timely management actions in the face of ecosystem change.     

Beyond Turing: The response of patterned ecosystems to environmental change

Spatially periodic patterns can be observed in a variety of ecosystems. Model studies revealed that patterned ecosystems may respond in a nonlinear way to environmental change, meaning that gradual changes result in rapid degradation. We analyze this response through stability analysis of patterned states of an arid ecosystem model. This analysis goes one step further than the frequently applied Turing analysis, which only considers stability of uniform states. We found that patterned arid ecosystems systematically respond in two ways to changes in rainfall: (1) by changing vegetation patch biomass or (2) by adapting pattern wavelength. Minor adaptations of pattern wavelength are constrained to conditions of slow change within a high rainfall regime, and high levels of stochastic variation in biomass (noise). Major changes in pattern wavelength occur under conditions of either low rainfall, rapid change or low levels of noise. Such conditions facilitate strong interactions between vegetation patches, which can trigger a sudden loss of half the patches or a transition to a degraded bare state. These results highlight that ecosystem responses may critically depend on rates, rather than magnitudes, of environmental change. Our study shows how models can increase our understanding of these dynamics, provided that analyses go beyond the conventional Turing analysis.     

Ecosystem structure, function, and restoration success: Are they related?

A direct relationship between ecosystem structure and function has been widely accepted by restoration ecologists. According to this paradigm, ecosystem degradation and aggradation represent parallel changes in structure and function, restoration following the same path as spontaneous succession. But the existence of single bidirectional trajectories and endpoints is not supported by empirical evidence. On the contrary, multiple meta-stable states, irreversible changes and hysteresis are common in nature. These situations are better described by state-and-transition models. Merging those models into the structure–function framework may help to develop new hypotheses on ecosystem dynamics, and may provide a suitable framework for planning restoration activities. We use the relationship between ecosystem function and the effort needed to restore a degraded ecosystem (i.e. restorability) as an example. A linear relationship between ecosystem structure and function suggests that ecosystem degradation and restorability are directly related. This may not be true when multiple states, not necessarily connected, are considered. We show two case studies that support this point, and discuss the implications of the incorporation of state-and-transition models into the structure–function framework on relevant topics of restoration ecology and conservation biology, such as the choice of reference ecosystems, the evaluation of restoration actions, and the identification of priority areas for conservation and restoration.     

Slow Recovery from Local Disturbances as an Indicator for Loss of Ecosystem Resilience

A range of indicators have been proposed for identifying the elevated risk of critical transitions in ecosystems. Most indicators are based on the idea that critical slowing down can be inferred from changes in statistical properties of natural fluctuations and spatial patterns. However, identifying these signals in nature has remained challenging. An alternative approach is to infer changes in resilience from differences in standardized experimental perturbations. However, system-wide experimental perturbations are rarely feasible. Here we evaluate the potential to infer the risk of large-scale systemic transitions from local experimental or natural perturbations. We use models of spatially explicit landscapes to illustrate how recovery rates upon small-scale perturbations decrease as an ecosystem approaches a tipping point for a large-scale collapse. We show that the recovery trajectory depends on: (1) the resilience of the ecosystem at large scale, (2) the dispersal rate of organisms, and (3) the scale of the perturbation. In addition, we show that recovery of natural disturbances in a heterogeneous environment can potentially function as an indicator of resilience of a large-scale ecosystem. Our analyses reveal fundamental differences between large-scale weak and local-scale strong perturbations, leading to an overview of opportunities and limitations of the use of local disturbance-recovery experiments.     

A hysteretic cycle in glucose 6-phosphate metabolism observed in a cell-free yeast extract

The dynamics of a partial glycolytic reaction sequence which converts glucose 6-phosphate to triose phosphates is described. The study was performed with cell-free extracts from baker's yeast harvested in the logarithmic and stationary growth phases. The experiments are based on a flow-through reactor supplied with the desalted cell-free extract as well as glucose 6-phosphate, ATP and phosphoenolpyruvate. In the reaction system the quasi-irreversible reactions catalyzed by 6-phosphofructo-1-kinase, pyruvate kinase, and fructose-1,6-bisphosphatase are involved. When substrate is supplied continuously, only stable stationary states can be observed. With transient perturbations of the substrate supply, multiple stationary states appear. Cyclic transitions between unique stable stationary states were induced by appropriate changes of the rate of substrate supply. A hysteretic cycle could then be demonstrated when, during reverse transitions, a parameter region of multistability was passed. The presence (in resting yeast) or absence (in growing yeast) of fructose-1,6-bisphosphatase did not significantly influence the dynamic capabilities of the investigated reaction sequence. The kinetic properties of the cell-free extracts fit mathematical models developed for in vitro systems reconstituted from purified enzymes.     

Grassland to shrubland state transitions enhance carbon sequestration in the northern Chihuahuan Desert

The replacement of native C₄‐dominated grassland by C₃‐dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m^?2 yr^?1 on average, while nearby native C₄ grassland was a net source of 31 g C m^?2 yr^?1 over this same period. Differences in C exchange between these ecosystems were pronounced – grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.     

Ecosystems off track: rate‐induced critical transitions in ecological models

Theory suggests that gradual environmental change may erode the resilience of ecosystems and increase their susceptibility to critical transitions. This notion has received a lot of attention in ecology in recent decades. An important question receiving far less attention is whether ecosystems can cope with the rapid environmental changes currently imposed. The importance of this question was recently highlighted by model studies showing that elevated rates of change may trigger critical transitions, whereas slow environmental change would not. This paper aims to provide a mechanistic understanding of these rate‐induced critical transitions to facilitate identification of rate sensitive ecosystems. Analysis of rate sensitive ecological models is challenging, but we demonstrate how rate‐induced transitions in an elementary model can still be understood. Our analyses reveal that rate‐induced transitions 1) occur if the rate of environmental change is high compared to the response rate of ecosystems, 2) are driven by rates, rather than magnitudes, of change and 3) occur once a critical rate of change is exceeded. Disentangling rate‐induced transitions from classical transitions in observations would be challenging. However, common features of rate‐sensitive models suggest that ecosystems with coupled fast–slow dynamics, exhibiting repetitive catastrophic shifts or displaying periodic spatial patterns are more likely to be rate sensitive. Our findings are supported by experimental studies showing rate‐dependent outcomes. Rate sensitivity of models suggests that the common definition of ecological resilience is not suitable for a subset of real ecosystems and that formulating limits to magnitudes of change may not always safeguard against ecosystem degradation. Synthesis Understanding and predicting ecosystem response to environmental change is one of the key challenges in ecology. Model studies have suggested that slow, gradual environmental change beyond some critical threshold can trigger so‐called critical transitions and abrupt ecosystem degradation. An important question remains however whether ecosystems can cope with the ongoing rapid anthropogenic environmental changes to which they are currently imposed. In this study we demonstrate that in some ecological models elevated rates of change can trigger critical transitions even if slow environmental change of the same magnitude would not. Such rateinduced critical transitions in models suggest that concepts like resilience and planetary boundaries may not always be sufficient to explain and prevent ecosystem degradation.     

汶川县生态系统服务对景观格局及其变化的响应

景观格局及其变化是影响生态系统服务供应与维持的主要原因之一,也是生态系统服务管理与景观空间配置优化的基础。基于30 m分辨率土地利用数据,统计汶川县2000—2015年多种景观格局指数和碳固定、水源涵养、土壤保持3种生态系统服务的变化,划分等级探究生态系统服务对景观格局及其变化的响应。研究结果表明:1)汶川县整体景观趋于破碎化、斑块形状复杂化,斑块异质性增加,且主要是在林地、草地和耕地之间转换;2)对于生态系统服务,碳固定在全域内有所减少,而大部分区域水源涵养和土壤保持呈增长趋势;3)不同生态系统服务对景观格局的响应存在差异,不仅表现在指标上,也体现在程度上,如碳固定对斑块形状指数(Landscape Shape Index,LSI)响应较为明显,而对蔓延度指数(Contagion Index,CONTAG)敏感性较差;4)并非所有景观格局变化均会引起生态系统服务变化,尺度也会对结果造成影响。本研究通过生态系统服务对景观格局的响应分析,可为区域景观管理的相关评估提供借鉴。     

In vitro demonstration of alternate stationary states in an open enzyme system containing phosphofructokinase

The kinetic properties of an open reconstituted enzyme system are investigated with the aim of demonstrating experimentally hysteretic transitions between alternate stationary states. The approach is based on a stirred flow-through reactor containing phosphofructokinase and pyruvate kinase entrapped in polyacrylamide gel. Through the reactor is pumped a solution containing fructose 6-phosphate, ATP, and phosphoenol pyruvate as well as adenylate kinase and glucose-6-phosphate isomerase. The latter two enzymes are in excess in respect to phosphofructokinase and pyruvate kinase. According to theoretical predictions the existence of multiple stationary states could be shown experimentally within precisely definable parameters. Switches between alternate stationary states have been caused by perturbations of flow rates and of reactant concentrations.