[Show abstract][Hide abstract] ABSTRACT: Forest cover is notoriously sparse across neotropical southeastern South America. In particular, the practically treeless landscapes of the Campos, as they are locally known, have puzzled ecologists since Darwin's time. We used remote-sensing information and spatial regression models to relate tree cover to resource availability (i.e. climate, soil fertility, soil water holding capacity), disturbances (i.e. fire occurrence, cattle grazing) and landscape features that can mediate the effects of both (i.e. topography, distance to rivers). To better understand these relationships, we conducted the analysis at different spatial scales across non-cultivated areas of southeastern South America. Overall, tree cover in southeastern South America increases with precipitation but is limited by livestock densities and fire occurrence. Forests are concentrated close to rivers, especially in the Campos region, where cattle grazing seems to prevent tree expansion into the grasslands.
Full-text · Article · Mar 2016 · Forest Ecology and Management
[Show abstract][Hide abstract] ABSTRACT: It has been widely debated whether Arctic sea-ice loss can reach a tipping point, beyond which a large sea-ice area disappears abruptly. An important argument for this scenario is the destabilising role of the ice-albedo feedback. The theory of dynamical systems predicts a "slowing down" when a system destabilises towards a tipping point. In simple stochastic systems this can result in increasing variance and autocorrelation, potentially yielding an early warning of an abrupt change. Here we aim to establish whether the loss of Arctic sea ice would follow these conceptual predictions, an d which trends in sea ice variability can be expected from pre-industrial conditions toward an Arctic that is ice-free during the whole year.
To this end, we apply a model hierarchy consisting of two box models and one comprehensive Earth system model. We find a consistent and robust decrease of the ice volume's annual relaxation time before summer ice is lost because thinner ice can adjust more quickly to perturbations. Thereafter, the relaxation time increases, mainly because the system becomes dominated by the ocean water's large heat capacity when the ice-free season becomes longer. Both trends carry over to the autocorrelation of sea ice thickness in time series. Also accounting for the geometric effect of increasing open-water formation efficiency for thinning ice, we obtain an increasing variability in sea-ice area fraction, but a decreasing variability in sea-ice thickness.
These changes are robust to the nature and origin of climate variability in the models and hardly depend on the balance of feed backs. Therefore, characteristic trends can be expected in the future. As these trends are not specific to the existence of abrupt ice loss, the prospects for early warnings seem very limited. This result also has implications for statistical indicators in other systems whose effective "mass" changes over time, affecting the trend of their relaxation time. However, the robust relation between state and variability would allow an estimate of sea-ice variability from only short observations. This could help one to estimate the likelihood and persistence of extreme events in the future.
Full-text · Article · Jan 2016 · The Cryosphere Discussions
[Show abstract][Hide abstract] ABSTRACT: Objective:
We propose a novel paradigm to predict acute attacks and exacerbations in chronic episodic disorders such as asthma, cardiac arrhythmias, migraine, epilepsy, and depression. A better generic understanding of acute transitions in chronic dynamic diseases is increasingly important in critical care medicine because of the higher prevalence and incidence of these chronic diseases in our aging societies.
PubMed, Medline, and Web of Science.
We selected studies from biology and medicine providing evidence of slowing down after a perturbation as a warning signal for critical transitions.
Recent work in ecology, climate, and systems biology has shown that slowing down of recovery upon perturbations can indicate loss of resilience across complex, nonlinear biologic systems that are approaching a tipping point. This observation is supported by the empiric studies in pathophysiology and controlled laboratory experiments with other living systems, which can flip from one state of clinical balance to a contrasting one. We discuss examples of such evidence in bodily functions such as blood pressure, heart rate, mood, and respiratory regulation when a tipping point for a transition is near.
We hypothesize that in a range of chronic episodic diseases, indicators of critical slowing down, such as rising variance and temporal correlation, may be used to assess the risk of attacks, exacerbations, and even mortality. Identification of such early warning signals over a range of diseases will enhance the understanding of why, how, and when attacks and exacerbations will strike and may thus improve disease management in critical care medicine.
Full-text · Article · Jan 2016 · Critical care medicine
[Show abstract][Hide abstract] ABSTRACT: Ecological resilience is the ability of a system to persist in the face of perturbations. Although resilience has been a highly influential concept, its interpretation has remained largely qualitative. Here we describe an emerging family of methods for quantifying resilience on the basis of observations. A first set of methods is based on the phenomenon of critical slowing down, which implies that recovery upon small perturbations becomes slower as a system approaches a tipping point. Such slowing down can be measured experimentally but may also be indirectly inferred from changes in natural fluctuations and spatial patterns. A second group of methods aims to characterize the resilience of alternative states in probabilistic terms based on large numbers of observations as in long time series or satellite images. These generic approaches to measuring resilience complement the system-specific knowledge needed to infer the effects of environmental change on the resilience of complex systems.
Full-text · Article · Dec 2015 · Annual Review of Ecology Evolution and Systematics
[Show abstract][Hide abstract] ABSTRACT: Publicly available remote sensing products have boosted science in many ways. The openness of these data sources suggests high reproducibility. However, as we show here, results may be specific to versions of the data products that can become unavailable as new versions are posted. We focus on remotely-sensed tree cover. Recent studies have used this public resource to detect multi-modality in tree cover in the tropical and boreal biomes. Such patterns suggest alternative stable states separated by critical tipping points. This has important implications for the potential response of these ecosystems to global climate change. For the boreal region, four distinct ecosystem states (i.e., treeless, sparse and dense woodland, and boreal forest) were previously identified by using the Collection 3 data of MODIS Vegetation Continuous Fields (VCF). Since then, the MODIS VCF product has been updated to Collection 5; and a Landsat VCF product of global tree cover at a fine spatial resolution of 30 meters has been developed. Here we compare these different remote-sensing products of tree cover to show that identification of alternative stable states in the boreal biome partly depends on the data source used. The updated MODIS data and the newer Landsat data consistently demonstrate three distinct modes around similar tree-cover values. Our analysis suggests that the boreal region has three modes: one sparsely vegetated state (treeless), one distinct 'savanna-like' state and one forest state, which could be alternative stable states. Our analysis illustrates that qualitative outcomes of studies may change fundamentally as new versions of remote sensing products are used. Scientific reproducibility thus requires that old versions remain publicly available.
[Show abstract][Hide abstract] ABSTRACT: After a period of heavy emphasis on negative interactions, such as predation and competition, the past two decades have seen an explosion of literature on the role of positive interactions in ecological communities. Such positive interactions can take many forms. One possibility is that amelioration of environmental stress by plants or sessile animals enhances growth, reproduction and survival of others, but many more intricate patterns exist. Importantly such positive interactions may contribute to creating a positive feedback. For instance, biomass can lead to improved environmental conditions causing better growth and therefore leading to more biomass. A positive feedback is a necessary (but not sufficient) condition for the emergence of alternative stable states at the community scale. However, the literature on positive interactions in plant and animal communities rarely addresses this connection. Here, we address this gap, asking the question of when positive interactions may lead to alternative stable states, and hence set the stage for catastrophic transitions at tipping points in ecosystems. We argue that, although there are a number of now classical examples in the literature for which positive interactions are clearly the main actors of positive feedback loops, more empirical and theoretical research scaling up from the individual-level interactions to the community and the ecosystem scale processes is needed to further understand under which conditions positive interactions can trigger positive feedback loops, and thereby alternative stable states.
No preview · Article · Nov 2015 · Functional Ecology
[Show abstract][Hide abstract] ABSTRACT: While species fulfill many different roles in ecosystems, it has been suggested that numerous species might actually share the same function in a near neutral way. So-far, however, it is unclear whether such functional redundancy really exists. We scrutinize this question using extensive data on the world’s 4168 species of diving beetles. We show that across the globe these animals have evolved towards a small number of regularly-spaced body sizes, and that locally co-existing species are either very similar in size or differ by at least 35%. Surprisingly, intermediate size differences (10–20%) are rare. As body-size strongly reflects functional aspects such as the food that these generalist predators can eat, these beetles thus form relatively distinct groups of functional look-a-likes. The striking global regularity of these patterns support the idea that a self-organizing process drives such species-rich groups to self-organize evolutionary into clusters where functional redundancy ensures resilience through an insurance effect.
[Show abstract][Hide abstract] ABSTRACT: Variable flows of food, water, or other ecosystem services complicate planning. Management strategies that decrease variability and increase predictability may therefore be preferred. However, actions to decrease variance over short timescales (2-4 y), when applied continuously, may lead to long-term ecosystem changes with adverse consequences. We investigated the effects of managing short-term variance in three well-understood models of ecosystem services: lake eutrophication, harvest of a wild population, and yield of domestic herbivores on a rangeland. In all cases, actions to decrease variance can increase the risk of crossing critical ecosystem thresholds, resulting in less desirable ecosystem states. Managing to decrease short-term variance creates ecosystem fragility by changing the boundaries of safe operating spaces, suppressing information needed for adaptive management, cancelling signals of declining resilience, and removing pressures that may build tolerance of stress. Thus, the management of variance interacts strongly and inseparably with the management of resilience. By allowing for variation, learning, and flexibility while observing change, managers can detect opportunities and problems as they develop while sustaining the capacity to deal with them.
No preview · Article · Oct 2015 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Abrupt transitions of regional climate in response to the gradual rise in atmospheric greenhouse gas concentrations are notoriously difficult to foresee. However, such events could be particularly challenging in view of the capacity required for society and ecosystems to adapt to them. We present, to our knowledge, the first systematic screening of the massive climate model ensemble informing the recent Intergovernmental Panel on Climate Change report, and reveal evidence of 37 forced regional abrupt changes in the ocean, sea ice, snow cover, permafrost, and terrestrial biosphere that arise after a certain global temperature increase. Eighteen out of 37 events occur for global warming levels of less than 2°, a threshold sometimes presented as a safe limit. Although most models predict one or more such events, any specific occurrence typically appears in only a few models. We find no compelling evidence for a general relation between the overall number of abrupt shifts and the level of global warming. However, we do note that abrupt changes in ocean circulation occur more often for moderate warming (less than 2°), whereas over land they occur more often for warming larger than 2°. Using a basic proportion test, however, we find that the number of abrupt shifts identified in Representative Concentration Pathway (RCP) 8.5 scenarios is significantly larger than in other scenarios of lower radiative forcing. This suggests the potential for a gradual trend of destabilization of the climate with respect to such shifts, due to increasing global mean temperature change.
No preview · Article · Oct 2015 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Recent global crises reveal an emerging pattern of causation that could increasingly characterize the birth and progress of future global crises. A conceptual framework identifies this pattern's deep causes, intermediate processes, and ultimate outcomes. The framework shows how multiple stresses can interact within a single social-ecological system to cause a shift in that system's behavior, how simultaneous shifts of this kind in several largely discrete social-ecological systems can interact to cause a far larger intersystemic crisis, and how such a larger crisis can then rapidly propagate across multiple system boundaries to the global scale. Case studies of the 2008-2009 financial-energy and food-energy crises illustrate the framework. Suggestions are offered for future research to explore further the framework's propositions.
Full-text · Article · Sep 2015 · ECOLOGY AND SOCIETY
[Show abstract][Hide abstract] ABSTRACT: Positive biotic interactions play a significant role in shaping ecological communities. We used an individual-based model to demonstrate that plant facilitation on a microscale may cause ecosystem shifts on a landscape scale that can be announced by generic earlywarning indicators. Recruitment of woody plants in harsh environments such as drylands often depends on nurse plants that ameliorate stressful conditions and facilitate the establishment of seedlings under their canopy. We found that these facilitative interactions may cause a treeless and a woodland state to be alternative stable states on a landscape scale if nurse plant effects are strong and if the environment is harsh enough to make facilitation necessary for seedling survival. A corollary is that under such conditions environmental change can bring drylands to tipping points for woody plant encroachment or woodland collapse. We show that the proximity of tipping points may be indicated by slowness of recovery of woody vegetation cover from small perturbations as well as by elevated temporal and spatial autocorrelation and variance. These signs are known to be indicators of critical slowing down. This is the first demonstration that the systemic phenomena of tipping points, announced by critical slowing down as a warning signal, may plausibly arise from microscale individual interactions, such as plant facilitation.
Full-text · Article · Aug 2015 · The American Naturalist
[Show abstract][Hide abstract] ABSTRACT: Social justice concerns but also perceived business advantage are behind a widespread drive to increase workplace diversity. However, dominance in terms of ethnicity, gender or other aspects of diversity has been resistant to change in many sectors. The different factors which contribute to low diversity are often hotly contested and difficult to untangle. We propose that many of the barriers to change arise from self-reinforcing feedbacks between low group diversity and inclusivity. Using a dynamic model, we demonstrate how bias in employee appointment and departure can trap organizations in a state with much lower diversity than the applicant pool: a workforce diversity "poverty trap". Our results also illustrate that if turnover rate is low, employee diversity takes a very long time to change, even in the absence of any bias. The predicted rate of change in workforce composition depends on the rate at which employees enter and leave the organization, and on three measures of inclusion: applicant diversity, appointment bias and departure bias. Quantifying these three inclusion measures is the basis of a new, practical framework to identify barriers and opportunities to increasing workforce diversity. Because we used a systems approach to investigate underlying feedback mechanisms rather than context-specific causes of low workforce diversity, our results are applicable across a wide range of settings.
[Show abstract][Hide abstract] ABSTRACT: Solving one environmental problem may often invoke or intensify another one. Such environmental problem shifting (EPS) is a neglected topic in global sustainability research. Indeed, it is difficult to study as it requires the merging of insights from various research areas. Here we identify relevant studies, and provide an illustration and guidelines for the systematic study of EPS. As a modest thought experiment to illustrate the relevance of EPS, we consider solutions to scarcity of energy resources and climate change that, due to their extreme nature, may lead to considerable environmental problem shifting. We qualitatively assess the likely environmental and socioeconomic impacts of three hypothetical energy futures to highlight the possibility that as we resolve one environmental problem, another may be aggravated. We further present a set of guidelines to study EPS in a systematic and focused way. Here we stress that shifting can be mediated by biophysical as well as socioeconomic mechanisms, which means that its analysis requires a genuine interdisciplinary effort.
Full-text · Article · Jun 2015 · Current Opinion in Environmental Sustainability
[Show abstract][Hide abstract] ABSTRACT: Recent studies provide compelling evidence for the idea that creative thinking draws upon two kinds of processes linked to distinct physiological features, and stimulated under different conditions. In short, the fast system-I produces intuition whereas the slow and deliberate system-II produces reasoning. System-I can help see novel solutions and associations instantaneously, but is prone to error. System-II has other biases, but can help checking and modifying the system-I results. Although thinking is the core business of science, the accepted ways of doing our work focus almost entirely on facilitating system-II. We discuss the role of system-I thinking in past scientific breakthroughs, and argue that scientific progress may be catalyzed by creating conditions for such associative intuitive thinking in our academic lives and in education. Unstructured socializing time, education for daring exploration, and cooperation with the arts are among the potential elements. Because such activities may be looked upon as procrastination rather than work, deliberate effort is needed to counteract our systematic bias.
Full-text · Article · Jun 2015 · ECOLOGY AND SOCIETY
[Show abstract][Hide abstract] ABSTRACT: Facilitation is a major force shaping the structure and diversity of plant communities in terrestrial ecosystems. Detecting positive plant-plant interactions relies on the combination of field experimentation and the demonstration of spatial association between neighboring plants. This has often restricted the study of facilitation to particular sites, limiting the development of systematic assessments of facilitation over regional and global scales. Here we explore whether the frequency of plant spatial associations detected from high-resolution remotely sensed images can be used to infer plant facilitation at the community level in drylands around the globe. We correlated the information from remotely sensed images freely available through Google Earth with detailed field assessments, and used a simple individualbased model to generate patch-size distributions using different assumptions about the type and strength of plant-plant interactions. Most of the patterns found from the remotely sensed images were more right skewed than the patterns from the null model simulating a random distribution. This suggests that the plants in the studied drylands show stronger spatial clustering than expected by chance. We found that positive plant co-occurrence, as measured in the field, was significantly related to the skewness of vegetation patch-size distribution measured using Google Earth images. Our findings suggest that the relative frequency of facilitation may be inferred from spatial pattern signals measured from remotely sensed images, since facilitation often determines positive co-occurrence among neighboring plants. They pave the road for a systematic global assessment of the role of facilitation in terrestrial ecosystems.
Full-text · Article · May 2015 · Ecological Applications
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The statistical association between temperature and greenhouse gases over glacial cycles is well documented, but causality behind this correlation remains difficult to extract directly from the data. A time lag of CO 2 behind Antarctic temperature - originally thought to hint at a driving role for temperature - is absent at the last deglaciation, but recently confirmed at the last ice age inception and the end of the earlier termination II (ref.). We show that such variable time lags are typical for complex nonlinear systems such as the climate, prohibiting straightforward use of correlation lags to infer causation. However, an insight from dynamical systems theory now allows us to circumvent the classical challenges of unravelling causation from multivariate time series. We build on this insight to demonstrate directly from ice-core data that, over glacial-interglacial timescales, climate dynamics are largely driven by internal Earth system mechanisms, including a marked positive feedback effect from temperature variability on greenhouse-gas concentrations.
No preview · Article · Mar 2015 · Nature Climate Change