Catastrophic Shifts Research Articles

EARLY WARNINGS FOR CATASTROPHIC SHIFTS IN ECOSYSTEMS: COMPARISON BETWEEN SPATIAL AND TEMPORAL INDICATORS

The task of providing leading indicators of catastrophic regime shifts in ecosystems is fundamental in order to design management protocols for those systems. Here we address the problem of lake eutrophication (that is, nutrient enrichment leading to algal blooms) using a simple spatial lake model. We discuss and compare different spatial and temporal early warning signals announcing the catastrophic transition of an oligotrophic lake to eutrophic conditions. In particular, we consider the spatial variance and its associated patchiness of eutrophic water regions. We found that spatial variance increases as the lake approaches the point of transition to a eutrophic state. We also analyze the spatial and temporal early warnings in terms of the amount of information required by each and their respective forewarning times. From the consideration of different remedial procedures that can be followed after these early signals we conclude that some of these indicators are not early enough to avert the undesired impending shift.

How Vegetation and Sediment Transport Feedbacks Drive Landscape Change in the Everglades and Wetlands Worldwide

Mechanisms reported to promote landscape self-organization cannot explain vegetation patterning oriented parallel to flow. Recent catastrophic shifts in Everglades landscape pattern and ecological function highlight the need to understand the feedbacks governing these ecosystems. We modeled feedback between vegetation, hydrology, and sediment transport on the basis of a decade of experimentation. Results from more than 100 simulations showed that flows just sufficient to redistribute sediment from sparsely vegetated sloughs to dense ridges were needed for an equilibrium patterned landscape oriented parallel to flow. Surprisingly, although vegetation heterogeneity typically conveys resilience, in wetlands governed by flow/sediment feedbacks it indicates metastability, whereby the landscape is prone to catastrophic shifts. Substantial increases or decreases in flow relative to the equilibrium condition caused an expansion of emergent vegetation and loss of open-water areas that was unlikely to revert upon restoration of the equilibrium hydrology. Understanding these feedbacks is critical in forecasting wetland responses to changing conditions and designing management strategies that optimize ecosystem services, such as carbon sequestration or habitat provision. Our model and new sensitivity analysis techniques address these issues and make it newly apparent that simply returning flow to predrainage conditions in the Everglades may not be sufficient to restore historic landscape patterns and processes.

Multiple stressors and regime shifts in shallow aquatic ecosystems in antipodean landscapes

Summary1. Changes in land management (land use and land cover) and water management (including extraction of ground water and diversion of surface waters for irrigation) driven by increases in agricultural production and urban expansion (and fundamentally by population growth) have created multiple stressors on global freshwater ecosystems that we can no longer ignore.2. The development and testing of conceptual ecological models that examine the impact of stressors on aquatic ecosystems, and recognise that responses may be nonlinear, is now essential for identifying critical processes and predicting changes, particularly the possibility of catastrophic regime shifts or ‘ecological surprises’.3. Models depicting gradual ecological change and three types of regime shift (simple thresholds, hysteresis and irreversible changes) were examined in the context of shallow inland aquatic ecosystems (wetlands, shallow lakes and temporary river pools) in southwestern Australia subject to multiple anthropogenic impacts (hydrological change, eutrophication, salinisation and acidification).4. Changes in hydrological processes, particularly the balance between groundwater‐dominated versus surface water‐dominated inputs and a change from seasonal to permanent water regimes appeared to be the major drivers influencing ecological regime change and the impacts of eutrophication and acidification (in urban systems) and salinisation and acidification (in agricultural systems).5. In the absence of hydrological change, urban wetlands undergoing eutrophication and agricultural wetlands experiencing salinisation appeared to fit threshold models. Models encompassing alternative regimes and hysteresis appeared to be applicable where a change from a seasonal to permanent hydrological regime had occurred.6. Irreversible ecological change has potentially occurred in agricultural landscapes because the external economic driver, agricultural productivity, persists independently of the impact on aquatic ecosystems.7. Thematic implications: multiple stressors can create multiple thresholds that may act in a hierarchical fashion in shallow, lentic systems. The resulting regime shifts may follow different models and trajectories of recovery. Challenges for ecosystem managers and researchers include determining how close a system may be to critical thresholds and which processes are essential to maintaining or restoring the system. This requires an understanding of both external drivers and internal ecosystem dynamics, and the interactions between them, at appropriate spatial and temporal scales.

Fishing effects on age and spatial structures undermine population stability of fishes

Overfishing has caused dramatic changes in structures of exploited populations as well as ecosystems. In this article, we focus on fishing effects on age (size) and spatial structures of exploited fishes. Accumulating evidence has shown that large and experienced spawning individuals are able to produce higher quality and quantity of eggs, known as maternal effects, and that individuals of different age classes tend to spawn in different locations and times. These behaviors are associated with a healthy age structure and contribute to bet-hedging capacity that is important in smoothing out short-term environmental variability. Here, we document a widespread phenomenon of age (size)-truncation of exploited populations driven by size-selective fishery removals. Such size-selective fishing may have evolutionary consequence and may be difficult to reverse. In addition, fishing often reduces population spatial heterogeneity that also contributes importantly to bet-hedging. We review studies showing that the effects of age truncation and reduction of spatial heterogeneity have reduced resilience and elevated the fluctuation amplitude of exploited populations facing a changing environment. Recent analyses indicated that fish populations often exhibit nonlinear nature and have potential to shift dramatically in a short time. All the evidence suggests that fishing, by altering age or spatial structures, may make exploited fishes, more prone to catastrophic shifts. Therefore, to achieve sustainable fisheries, management should conserve the age and spatial structure in addition to viable spawning biomass.

Spatial correlation as leading indicator of catastrophic shifts

Generic early-warning signals such as increased autocorrelation and variance have been demonstrated in time-series of systems with alternative stable states approaching a critical transition. However, lag times for the detection of such leading indicators are typically long. Here, we show that increased spatial correlation may serve as a more powerful early-warning signal in systems consisting of many coupled units. We first show why from the universal phenomenon of critical slowing down, spatial correlation should be expected to increase in the vicinity of bifurcations. Subsequently, we explore the applicability of this idea in spatially explicit ecosystem models that can have alternative attractors. The analysis reveals that as a control parameter slowly pushes the system towards the threshold, spatial correlation between neighboring cells tends to increase well before the transition. We show that such increase in spatial correlation represents a better early-warning signal than indicators derived from time-series provided that there is sufficient spatial heterogeneity and connectivity in the system.

Catastrophic phase transitions and early warnings in a spatial ecological model

Gradual changes in exploitation, nutrient loading, etc produce shifts between alternativestable states (ASS) in ecosystems which, quite often, are not smooth but abrupt orcatastrophic. Early warnings of such catastrophic regime shifts are fundamentalfor designing management protocols for ecosystems. Here we study the spatialversion of a popular ecological model, involving a logistically growing single speciessubject to exploitation, which is known to exhibit ASS. Spatial heterogeneity isintroduced by a carrying capacity parameter varying from cell to cell in a regularlattice. Transport of biomass among cells is included in the form of diffusion.We investigate whether different quantities from statistical mechanics—like thevariance, the two-point correlation function and the patchiness—may serve asearly warnings of catastrophic phase transitions between the ASS. In particular,we find that the patch-size distribution follows a power law when the system isclose to the catastrophic transition. We also provide links between spatial andtemporal indicators and analyse how the interplay between diffusion and spatialheterogeneity may affect the earliness of each of the observables. We find thatpossible remedial procedures, which can be followed after these early signals,become more effective as the diffusion becomes lower. Finally, we comment onsimilarities of and differences between these catastrophic shifts and paradigmaticthermodynamic phase transitions like the liquid–vapour change of state for a fluid likewater.

Catastrophic regime shifts in coral communities exposed to physical disturbances: Simulation results from object-oriented 3-dimensional coral reef model

A 3-dimensional individual-based model, the ReefModel, was developed to simulate the dynamical structure of coral reef community using object-oriented techniques. Interactions among functional groups of reef organisms were simulated in the model. The behaviours of these organisms were described with simple mechanistic rules that were derived from their general behaviours (e.g. growing habits, competitive mechanisms, response to physical disturbance) observed in natural coral reef communities. The model was implemented to explore the effects of physical disturbance on the dynamical structure of a 3-coral community that was characterized with three functional coral groups: tabular coral, foliaceous coral and massive coral. Simulation results suggest that (i) the integration of physical disturbance and differential responses (disturbance sensitivity and growing habit) of corals plays an important role in structuring coral communities; (ii) diversity of coral communities can be maximal under intermediate level of acute physical disturbance; (iii) multimodality exists in the final states and dynamic regimes of individual coral group as well as coral community structure, which results from the influence of small random spatial events occurring during the interactions among the corals in the community, under acute and repeated physical disturbances. These results suggest that alternative stable states and catastrophic regime shifts may exist in a coral community under unstable physical environment.

Origin of Pareto-like Spatial Distributions in Ecosystems

Recent studies of cluster distribution in various ecosystems revealed Pareto statistics for the size of spatial colonies. These results were supported by cellular automata simulations that yield robust criticality for endogenous pattern formation based on positive feedback. We show that this patch statistics is a manifestation of the law of proportionate effect. Mapping the stochastic model to a Markov birth-death process, the transition rates are shown to scale linearly with cluster size. This mapping provides a connection between patch statistics and the dynamics of the ecosystem; the "first passage time" for different colonies emerges as a powerful tool that discriminates between endogenous and exogenous clustering mechanisms. Imminent catastrophic shifts (such as desertification) manifest themselves in a drastic change of the stability properties of spatial colonies.

Spatial variance and spatial skewness: leading indicators of regime shifts in spatial ecological systems

Ecosystems can undergo large-scale changes in their states, known as catastrophic regime shifts, leading to substantial losses to services they provide to humans. These shifts occur rapidly and are difficult to predict. Several early warning signals of such transitions have recently been developed using simple models. These studies typically ignore spatial interactions, and the signal provided by these indicators may be ambiguous. We employ a simple model of collapse of vegetation in one and two spatial dimensions and show, using analytic and numerical studies, that increases in spatial variance and changes in spatial skewness occur as one approaches the threshold of vegetation collapse. We identify a novel feature, an increasing spatial variance in conjunction with a peaking of spatial skewness, as an unambiguous indicator of an impending regime shift. Once a signal has been detected, we show that a quick management action reducing the grazing activity is needed to prevent the collapse of vegetated state. Our results show that the difficulties in obtaining the accurate estimates of indicators arising due to lack of long temporal data can be alleviated when high-resolution spatially extended data are available. These results are shown to hold true independent of various details of model or different spatial dispersal kernels such as Gaussian or heavily fat tailed. This study suggests that spatial data and monitoring multiple indicators of regime shifts can play a key role in making reliable predictions on ecosystem stability and resilience.

Contrasting impacts of invasive engineers on freshwater ecosystems: an experiment and meta-analysis

Invasion by common carp (Cyprinus carpio) and red swamp crayfish (Procambarus clarkii) in shallow lakes have been followed by stable-state changes from a macrophyte-dominated clear water state to a phytoplankton-dominated turbid water state. Both invasive carp and crayfish are, therefore, possible drivers for catastrophic regime shifts. Despite these two species having been introduced into ecosystems world-wide, their relative significance on regime shifts remains largely unexplored. We compared the ecological impacts of carp and crayfish on submerged macrophytes, water quality, phytoplankton, nutrient dynamics, zooplankton and benthic macroinvertebrates by combining an enclosure experiment and a meta-analysis. The experiment was designed to examine how water quality and biological variables responded to increasing carp or crayfish biomass. We found that even at a low biomass, carp had large and positive impacts on suspended solids, phytoplankton and nutrients and negative impacts on benthic macroinvertebrates. In contrast, crayfish had a strong negative impact on submerged macrophytes. The impacts of crayfish on macrophytes were significantly greater than those of carp. The meta-analysis showed that both carp and crayfish have significant effects on submerged macrophytes, phytoplankton, nutrient dynamics and benthic macroinvertebrates, while zooplankton are affected by carp but not crayfish. It also indicated that crayfish have significantly greater impacts on macrophytes relative to carp. Overall, the meta-analysis largely supported the results of the experiment. Taken as a whole, our results show that both carp and crayfish have profound effects on community composition and ecosystem processes through combined consequences of bioturbation, excretion, consumption and non-consumptive destruction. However, key variables (e.g. macrophytes) relating to stable-state changes responded differently to increasing carp or crayfish biomass, indicating that they have differential ecosystem impacts.

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

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.

Governance and Complexity—Emerging Issues for Governance Theory

Unexpected epidemics, abrupt catastrophic shifts in biophysical systems, and economic crises that cascade across national borders and regions are events that challenge the steering capacity of governance at all political levels. This article seeks to extend the applicability of governance theory by developing hypotheses about how different governance types can be expected to handle processes of change characterized by nonlinear dynamics, threshold effects, cascades, and limited predictability. The first part of the article argues the relevance of a complex adaptive system approach and goes on to review how well governance theory acknowledges the intriguing behavior of complex adaptive systems. In the second part, we develop a typology of governance systems based on their adaptive capacities. Finally, we investigate how combinations of governance systems on different levels buffer or weaken the capacity to govern complex adaptive systems.

Commentary on “The role of the large airways on smooth muscle contraction in asthma”

LETTERS TO THE EDITORCommentary on “The role of the large airways on smooth muscle contraction in asthma”Hans Christian Haverkamp, and Lennart Karl Alf LundbladHans Christian Haverkamp, and Lennart Karl Alf LundbladPublished Online:01 Oct 2007https://doi.org/10.1152/japplphysiol.00703.2007MoreSectionsPDF (25 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat to the editor: In view of the Viewpoint by Solbert Permutt (3) in this issue of the Journal of Applied Physiology, it is becoming clear that the extent of decline in pulmonary function in asthmatics is not a simple function of the degree of smooth muscle shortening, but is the manifestation of complicated relationships among baseline airway structure, the degree of airway smooth muscle shortening, and compensatory changes in lung volumes. In this case, increases in total lung capacity and functional residual capacity minimized the impact of the observed increase in residual volume on the decline in pulmonary function (1).We showed that in a mouse model of asthma, acute increases in airway wall thickness due to airway inflammatory effects leading to airway closure (and presumably change in lung volume) is the cause of airways hyperresponsiveness (AHR), not increased contractility of airway smooth muscle (2, 5). The study by Brown et al. (1) shows that airway structure and lung volumes are important determinants of AHR and pulmonary function in humans as well. Furthermore, Venegas et al. (4) showed that asthmatics have disturbed ventilation distribution that can be understood in terms of airway closure. These findings suggest a link between airway structure and its function that manifests as a change in lung volumes, irrespective of the model system used. Hence, we concur with Dr. Permutt that lung volumes are highly related to airway function in asthmatics and we believe an important determinant of AHR in both mice and humans.REFERENCES1 Brown RH, Pearse DB, Pyrgos G, Liu MC, Togias A, Permutt S. The structural basis of airways hyperresponsiveness in asthma. J Appl Physiol 101: 30–39, 2006.Link | ISI | Google Scholar2 Lundblad LK, Thompson-Figueroa J, Allen GB, Rinaldi L, Norton RJ, Irvin CG, Bates JH. Airways hyperresponsiveness in allergically inflamed mice: the role of airway closure. Am J Resp Crit Care Med 175: 768–774, 2007.Crossref | PubMed | ISI | Google Scholar3 Permutt S. The role of the large airways on smooth muscle contraction in asthma. J Appl Physiol; doi:10.1152/japplphysiol.00568–2007.Google Scholar4 Venegas JG, Winkler T, Musch G, Vidal Melo MF, Layfield D, Tgavalekos N, Fischman AJ, Callahan RJ, Bellani G, Harris RS. Self-organized patchiness in asthma as a prelude to catastrophic shifts. Nature 434: 777–782, 2005.Crossref | PubMed | ISI | Google Scholar5 Wagers S, Haverkamp HC, HC, Bates JHT, Norton RJ, Thompson-Figueroa JA, Sullivan MJ, Irvin CG. Intrinsic and antigen-induced airway hyperresponsiveness are the result of diverse physiological mechanisms. J Appl Physiol 102: 221–230, 2007.Link | ISI | Google ScholarAUTHOR NOTESAddress for reprint requests and other correspondence: H. C. Haverkamp, Univ. of Vermont, College of Medicine, 149 Beaumont Ave., HSRF 226, Burlington, VT 05405 (e-mail: [email protected]) Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 103Issue 4October 2007Pages 1463-1463 Copyright & PermissionsCopyright © 2007 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00703.2007PubMed17916687History Published online 1 October 2007 Published in print 1 October 2007 Metrics

Linking habitat modification to catastrophic shifts and vegetation patterns in bogs

Paleoecological studies indicate that peatland ecosystems may exhibit bistability. This would mean that these systems are resilient to gradual changes in climate, until environmental thresholds are passed. Then, ecosystem stability is lost and rapid shifts in surface and vegetation structure at landscape scale occur. Another remarkable feature is the commonly observed self-organized spatial vegetation patterning, such as string-flark and maze patterns. Bistability and spatial self-organization may be mechanistically linked, the crucial mechanism being scale-dependent (locally positive and longer-range negative) feedback between vegetation and the peatland environment. Focusing on bogs, a previous model study shows that nutrient accumulation by vascular plants can induce such scale-dependent feedback driving pattern formation. However, stability of bog microforms such as hummocks and hollows has been attributed to different local interactions between Sphagnum, vascular plants, and the bog environment. Here we analyze both local and longer-range interactions in bogs to investigate the possible contribution of these different interactions to vegetation patterning and stability. This is done by a literature review, and subsequently these findings are incorporated in the original model. When Sphagnum and encompassing local interactions are included in this model, the boundaries between vegetation types become sharper and also the parameter region of bistability drastically increases. These results imply that vegetation patterning and stability of bogs could be synergistically governed by local and longer-range interactions. Studying the relative effect of these interactions is therefore suggested to be an important component of future predictions on the response of peatland ecosystems to climatic changes.

Effect of rainfall interannual variability on the stability and resilience of dryland plant ecosystems

Positive feedbacks between vegetation and soil moisture may induce, in arid ecosystems, the emergence of two alternative stable states, corresponding to bare and completely vegetated soil. A new analytical model is developed to investigate this behavior and to study the effects of interannual rainfall variability on the dynamics of bistable ecosystems. When dichotomic Markov noise is used to account for the effect of random rainfall fluctuations, the long‐term dynamics of the system can be investigated through an analytical solution of the model. It is found that in a broad class of bistable ecosystems, random rainfall fluctuations may induce an ordered state in the dynamics, i.e., by turning the bistable deterministic system into a stochastic system with only one statistically stable state. This effect is enhanced by increases in noise intensity, whereas the stochastic dynamics become bistable (i.e., the noise‐induced ordered state disappears) as the noise intensity decreases below a critical (nonnull) value. This effect of noise‐induced stability is found in association with an enhancement of ecosystem resilience, indicating that the likelihood of catastrophic shifts to the desert state decreases as the noise intensity increases.

Vegetation loss alters soil nitrogen dynamics in an Arctic salt marsh

Summary Plant and microbial nitrogen (N) dynamics were examined in soils of an Arctic salt marsh beneath goose‐grazed swards and in degraded soils. The degraded soils are the outcome of intensive destructive foraging by geese, which results in vegetation loss and near‐irreversible changes in soil properties. The objective of the study was to determine whether vegetation loss led to a decline in microbial activity and a redistribution of N amongst the different soil N pools that potentially could adversely affect plant regrowth. In situ N allocation between plants, microbes and soil was determined based on injection of 15NH4Cl into soil cores; changes in isotopic ratios and N concentrations in the different pools were measured after 24 h. Degraded soils, in contrast to vegetated soils, were characterized by a decline in microbial biomass, reduced microbial 15N excess, reduced rates of gross N immobilization and an increased microbial residency time of 15N. In vegetated soils, both microbes and the forage grass, Puccinellia phryganodes, accumulated 15N such that little remained in the soil abiotic phase after 24 h, unlike in degraded soils. The decline in microbial activity in degraded soils may be linked to a low availability of soil carbon in the absence of plants and to deteriorating abiotic conditions, including the occurrence of hypersalinity in summer. Not all biotic processes respond similarly to the change in ecosystem state. Unlike vascular plant productivity and goose foraging which are absent in degraded soils, soil microbial activity is maintained, albeit at a lower level. In spite of this activity, the greater proportion of 15N in degraded soils is in the abiotic pool rather than in microbial biomass with an increased potential for soil N loss from leaching and soil erosion. Disturbance linked to herbivory often triggers catastrophic shifts in ecosystem properties resulting in vegetation loss and changes in soil biogeochemical cycling that are irreversible, at least on a decadal time scale, and lead to the loss of N and other nutrients required for plant growth. These results clearly illustrate that microbial activity does not compensate for the effects of plant removal on soil N retention.

Impact of noise on bistable ecological systems

The existence and implications of alternative stable states in ecological systems have been studied extensively within deterministic models. In this paper, we study the influence of random fluctuations in environmental parameters (e.g. nutrient input and rainfall) on the behavior of two simple bistable, ecological models with a single dynamical variable. We describe the changes in the bifurcation diagram of the bistable system: we find that the region of bistability is reduced for small amounts of noise while for noise beyond a critical width bistability vanishes completely. In the regime where bistability is eliminated by the noise, the time series of the solution shows that the system can undergo frequent catastrophic regime shifts. We also discuss how some of the results depend on model-specific details. We comment on the robustness of our results to changes in the noise characteristics and the implications for modeling real systems.

Self-Organized Patchiness in Asthma as a Prelude to Catastrophic Shifts

Purpose of the Study. To reveal self-organized small airway constriction contributing to large ventilation defects in asthmatics. Study Population. Mild and moderate asthmatics. Methods. Ventilation defects in asthmatics were studied during methacholine bronchoprovocation by using serial dynamic positron emission tomography with a positron-emitter nitrogen-13 tracer and a single terminal-airway model. Results. Heterogeneity of ventilation defects in asthmatics was demonstrated. In this model, constriction of terminal bronchioles was the main feature of bronchoconstriction, contributing to nonuniform ventilation defects. Consequently, on the basis of the mechanical interdependence in expansion between airways and surrounding parenchyma, clusters of constricted terminal bronchioles fed by a common tree branch developed and led to large ventilation defects. Conclusion. Clustered groups of self-organized terminal bronchiolar constriction, not large airways obstruction, contribute to large ventilation defects in acute asthma. Reviewer Comments. The nature of functional changes of both small and large airways affecting ventilation during acute asthma attacks has been unclear. Previously, MRIs of asthmatic lungs during bronchoprovocation suggested large-airway obstruction as a major cause of large ventilation defects (eg, J Allergy Clin Immunol. 2003;111:1205–1211). In this study, Venegas et al demonstrated the role of clustered terminal bronchiolar constriction resulting in ventilation defects in acute asthma. On the basis of this model, inhaled bronchodilators could be ineffective because the inhaled form might reach only well-ventilated regions and could further impede lung expansion of problematic regions and exacerbate regional ventilation defects. The concept of catastrophic shifts might account for sudden, unexplained, and severe asthma attacks in some patients. Systemic bronchodilators may be needed in some asthmatics to bypass this problem. This line of investigation is further elucidated elsewhere (J Appl Physiol. 2005;99:2388–2397).

When are alternative stable states more likely to occur?

T. Fukami and W. G. Lee argue that the logical expectation from ecological theory is that competitively‐structured assemblages will be more likely to exhibit alternative stable states than abiotically‐structured assemblages. We suggest that there are several important misinterpretations in their arguments, and that the substance of their hypothesis has both a weak basis in ecological theory and is not supported by empirical evidence which shows that alternative stable states occur more frequently in natural systems subject to moderate‐ to harsh abiotic extremes. While this debate is founded in ecological theory, it has important applied implications for restoration management. Sound theoretical predictions about when to expect alternative stable states can only aid more effective restoration if theoretical expectations can be shown to translate into predictable empirical outcomes. If strongly abiotically‐ or disturbance‐structured systems are more likely to exhibit catastrophic phase shifts in community structure that can be resilient to management efforts, then restoration ecologists will need to treat these systems differently in terms of the types of management inputs that are required.

Association between competition and facilitation processes and vegetation spatial patterns in alpha steppes

In semiarid ecosystems, the self-organized spatial patterns of plants associated with catastrophic shifts can emerge from a variety of processes. In this study, on moderate slopes where Stipa tenacissima cover was high, the selforganization of some of the typical species of semiarid Mediterranean matorral ( Phlomis purpurea , Sideritis oxteosylla, Helianthemum almeriense , and Brachypodium retusum ) was negatively correlated with Stipa cover. The extent of Stipa cover did not affect desert pioneer species, such as Artemisia herba-alba , Fagonia cretica , and Launaea lanifera . On pronounced slopes, the self-organizing structure of brushwood vegetation did not vary predictably with the amount of Stipa cover. We examined the competition/facilitation processes associated with self-organizing patterns in the dwarf shrub ( Phl. purpurea ) and the half shrub ( H. almeriense ). The developmental stability of H. almeriense was positively correlated with Stipa cover, which was expected because they are associated species in this seral thyme brushwood community. Indeed, facilitation processes were manifested by the developmental stability increases under the Stipa canopy, particularly on high slope areas, where Stipa is less competitive. In Phl. purpurea , negative feedback processes from competition with Stipa were manifested where Stipa cover was high and on low slopes (developmental instability increased). In general, competition with Stipa on low slopes tended to decrease plant self-organization. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 103‐113. ADDITIONAL KEYWORDS: developmental instability ‐ fractal ‐ Stipa tenacissima .