Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Neutral competition boosts cycles and chaos in simulated food webs
    Rodriguez Sanchez, Pablo ; Nes, E.H. van; Scheffer, M. - \ 2020
    Royal Society Open Science 7 (2020)6. - ISSN 2054-5703
    Similarity of competitors has been proposed to facilitate coexistence of species because it slows down competitive exclusion, thus making it easier for equalizing mechanisms to maintain diverse communities. On the other hand, previous studies suggest that chaotic ecosystems can have a higher biodiversity. Here, we link these two previously unrelated findings, by analysing the dynamics of food web models. We show that near-neutrality of competition of prey, in the presence of predators, increases the chance of developing chaotic dynamics. Moreover, we confirm that chaotic dynamics correlate with a higher biodiversity.
    A comparative analysis of human adult testicular cells expressing stem Leydig cell markers in the interstitium, vasculature, and peritubular layer
    Eliveld, Jitske ; Daalen, Saskia K.M. van; Winter-Korver, Cindy M. de; Veen, Fulco van der; Repping, Sjoerd ; Teerds, Katja ; Pelt, Ans M.M. van - \ 2020
    Andrology (2020). - ISSN 2047-2919
    human testis - markers - propagation - stem Leydig cells

    Background: Origin of human adult Leydig cells (ALCs) is not well understood. This might be partly due to limited data available on the identification and location of human precursor and stem Leydig cells (SLCs) which hampers the study on the development of ALCs. Objectives: The aim of the present study was to investigate whether described human (PDGFRα, NGFR) and rodent (NES, PDGFRα, THY1, NR2F2) SLC markers are expressed by a common cell population within human adult testicular interstitial cells in vivo and before and after in vitro propagation. Materials and methods: Immunohistochemical analyses were used to identify localization of human adult testicular interstitial cells expressing described SLC markers. Next, interstitial cells were isolated and cultured. The percentage of cells expressing one or more SLC markers was determined before and after culture using flow cytometry. Results: NR2F2 and PDGFRα were present in peritubular, perivascular, and Leydig cells, while THY1 was expressed in peritubular and perivascular cells. Although NES and NGFR were expressed in endothelial cells, co-localization with PDGFRα was found for both in vitro, although for NGFR only after culture. All marker positive cells were able to undergo propagation in vitro. Discussion: The partly overlap in localization and overlap in expression in human testicular cells indicate that PDGFRα, NR2F2, and THY1 are expressed within the same ALC developmental lineage from SLCs. Based on the in vitro results, this is also true for NES and after in vitro propagation for NGFR. Conclusion: Our results that earlier described SLC markers are expressed in overlapping human interstitial cell population opens up further research strategies aiming for a better insight in the Leydig cell lineage and will be helpful for development of strategies to cure ALC dysfunction.

    Cycles and interactions : A mathematician among biologists
    Rodríguez-Sánchez, Pablo - \ 2020
    Wageningen University. Promotor(en): M. Scheffer, co-promotor(en): E.H. van Nes. - Wageningen : Wageningen University - ISBN 9789463953931 - 144

    The backbone of this thesis is the interdisciplinary interaction between dynamic systems theory and a selection of biological problems. Each chapter focuses in one problem, namely plankton dynamics, cell development paths and sleep-wake dynamics. Despite these topics may seem disconnected, they share an important feature: all of them show cyclic behaviour under certain circumstances.
    In the present thesis we show that cyclic (or chaotic) behaviour is deeply related with plankton biodiversity. We also use cycles to show, in an intuitive way, that Waddington’s epigenetic landscapes (a common visual tool in stem cell research) are poorly defined, and we provide a practical solution to this. Lastly, we provide an algorithm to forecast a transition between synchronized and non-synchronized cyclic systems (such as normal sleep – insomnia, or normal hearth functioning – arrhythmia), with potential applications in medical sciences.

    Climbing Escher’s stairs: A way to approximate stability landscapes in multidimensional systems
    Rodriguez-Sanchez, Pablo ; Nes, E.H. van; Scheffer, M. - \ 2020
    PLoS Computational Biology 16 (2020)4. - ISSN 1553-734X
    Stability landscapes are useful for understanding the properties of dynamical systems. These landscapes can be calculated from the system’s dynamical equations using the physical concept of scalar potential. Unfortunately, it is well known that for most systems with two or more state variables such potentials do not exist. Here we use an analogy with art to provide an accessible explanation of why this happens and briefly review some of the possible alternatives. Additionally, we introduce a novel and simple computational tool that implements one of those solutions: the decomposition of the differential equations into a gradient term, that has an associated potential, and a non-gradient term, that lacks it. In regions of the state space where the magnitude of the non-gradient term is small compared to the gradient part, we use the gradient term to approximate the potential as quasi-potential. The non-gradient to gradient ratio can be used to estimate the local error introduced by our approximation. Both the algorithm and a ready-to-use implementation in the form of an R package are provided.
    Systemic risk in ecosystems
    Lever, J.J. - \ 2020
    Wageningen University. Promotor(en): M. Scheffer; J. Bascompte, co-promotor(en): E.H. van Nes. - Wageningen : Wageningen University - ISBN 9789463952637 - 189

    It is common knowledge that the millions of species that inhabit the Earth have adaptations that enable them to survive in different environments. Fish have gills which allow them to breath under water, while the wings of birds allow them to fly. These adaptations are, as different as they may be, a different solution to the same problem: the problem of staying alive and reproduce in a world where species are under the constant pressure of natural selection. Perhaps less well known, but maybe not surprising when thought about carefully, is that the often complex networks of interactions between species, e.g. between plants and pollinators or between predators and prey, have certain non-random properties as well. These ‘network structural properties’, i.e. specific ways in which the interactions within networks are arranged most likely allow the often large numbers of species in ecosystems to coexist. Just like similar adaptations may be found in a wide variety of species, e.g. gills or gill-like organs in aquatic animals and wings on birds, insects, and bats, similar network structural properties may be found in a wide variety of ecosystems. Similarities that may occur simply because they are, like adaptations, a solution to the same problem: the problem of coexistence in systems where species heavily influence each other’s probability of survival.

    While we are beginning to understand more about the structural properties of ecological networks, i.e. the networks of interactions between species, and how they might allow large numbers of species to coexist in complex ecosystems, the Earth and its ecosystems are changing at increasingly rapid rates due to human activities. In some cases, these changes are relatively simple in the sense that they affect a large group of species similarly, e.g. the effect of pesticides on a large group of insect pollinators, while in other cases these changes may be complex, e.g. the effects of climate change on the phenology and distribution of species which in turn leads to alterations in strengths of interspecific interactions in a way that is unique for each interaction. Ecosystems may respond in various ways to such changes (regardless of whether their effects are simple or complex). When conditions change gradually, the state of some ecosystems (e.g. the size of populations) may change likewise, in a smooth, gradual manner. Other systems may respond strongly to change within a narrow range of environmental conditions, but are relatively insensitive to change outside of this range. Particularly sudden shifts may occur when ecosystems have multiple alternative states. Such systems cannot change smoothly from one state (e.g. large population sizes) to an alternative state (e.g. a state in which some or all species are extinct). Instead, a sudden shift or ‘critical transition’ occurs when environmental conditions pass a critical point. To return back to the original state after such a transition, a return to conditions prior to the transition is often not sufficient; instead, a larger change in conditions is needed until another critical point is reached at which the system shifts back to the original state, a phenomenon called ‘hysteresis’.

    While the outcome of critical transitions is relatively predictable when a few leading species or species groups determine the state of an ecosystem, this may not be the case when ecosystem dynamics are determined by many interacting species. The consequences of critical transitions in such complex ecosystems might be severe, for example, when leading to the extinction of a large number of species. Not all critical transitions, however, will have dramatic consequences. Complex ecosystems may potentially shift to many different, alternative states. Some of those may imply minor, harmless changes in the state of a system, or invoke positive change, whereas others may have catastrophic con- sequences. The amount and type of change needed to cause a transition and a system’s future state after an impending critical transition depends in complex and often unknown ways on how ecosystems are organized, i.e. on the feedback mechanisms within it, and thus on the structure of ecological networks and/or how this structure might be changed by changing environmental conditions. Assessing or mitigating the risks associated with critical transitions in complex ecosystems thus requires a fundamental insight in the interrelationships between the structural properties of ecological networks, the dynamics of ecosystems, and the way in which these properties and dynamics might be affected by changing environmental conditions.

    Despite a longstanding interest in ecological networks and more recent advances in detecting commonalities in the structure of ecological networks, the common ground between studying the structure of ecological networks and the potential causes and consequences of critical transitions in complex ecosystems remains largely unexplored. In this thesis, I hope to have provided novel ideas and insights that might help to address the question of whether changing environmental conditions are likely to lead to large-scale systemic regime shifts in complex ecosystems. An emerging property of complex ecosystems that may be referred to as ‘systemic risk’.

    Complexity in inequality
    Mirza, Usman - \ 2020
    Wageningen University. Promotor(en): M. Scheffer, co-promotor(en): E.H. van Nes; A.P. Richter. - Wageningen : Wageningen University - ISBN 9789463951852 - 130

    This thesis explores three interrelated research streams. First, I am interested in the very fundamental and ancient question about the functioning of our societies – why are some individuals or countries rich and others poor? Under this broad question, I aim to understand the drivers of inequality, that traverse the artificial boundaries between economic, social and natural sciences. Second, I study inequality, as being complex and governed by highly interconnected processes such as technology, globalization, trade and migration. These processes can be highly nonlinear causing cyclical behaviour and/or critical transitions such as tipping points. Third, using dynamic models and empirical data, I explain the flow of information between heterogeneous agents, including individuals, households or countries, and how these interactions define the emergent dynamics of inequality. Heterogeneous individuals interact not only inside economic systems but also across systems, such as with the environment and climate. Furthermore, these interactions are governed by formal and informal rules, such as institutions, social norms and agreements.

    Times are changing : Implications of climate change for the occurrence and predictability of tipping points
    Bolt, Bregje van der - \ 2020
    Wageningen University. Promotor(en): M. Scheffer, co-promotor(en): E.H. van Nes. - Wageningen : Wageningen University - ISBN 9789463951807 - 146

    Complex systems in ecology and the climate can have tipping points. The term ‘tipping point’ is loosely defined as a threshold point in the conditions after which runaway change brings the system to a new stable state. Such a transition can have long-term dire consequences, and therefore it is of critical importance to understand why and when these transitions occur. Under the ongoing climate change, these critical transitions are projected to increase. However, little is understood about how relative timescales of the rate of environmental change and variability affect the occurrence and detectability of these critical transitions in nature and society. The aim of this thesis is to provide some insights in how differences in these timescales may affect critical transitions.

    This thesis starts with the analysis of bistability of marine anoxic events in the Mediterranean Sea. Reconstructed time series have been used to detect changes in resilience indicators prior to several abrupt shifts in the past climate. This is, however, only possible under a limited set of conditions. For the past marine anoxic events in the Mediterranean Sea, these conditions are met. Recent technological advances made it possible to construct high-resolution and (almost) evenly spaced time series of past widespread anoxic events in the Mediterranean Sea. In Chapter 2, we analysed whether past transitions in the Mediterranean Sea could have been predicted using the resilience indicators autocorrelation and variance. We show that the repeated shifts into marine anoxia in the Mediterranean Sea had the character of critical transitions, because there was a gradual increase in the temporal autocorrelation and variance in the deep cores (>1600 meter depth) before the onset of most events. Our results imply that future widespread anoxia in marine systems might be recognizable using an appropriate statistical approach and high-resolution records.

    These shifts to an anoxic state occurred relatively fast, but not all shifts to an alternative stable state unfold rapidly. Slowly responding systems show a gradual shift to the alternative state, once the tipping point has been passed. As the rate of the current environmental change is unprecedented, more system respond relatively slow to changes in the environment. The current resilience indicators are based on the theoretical finding that the system slows down close to the tipping point. But in these relatively slow systems, the recovery rates are always slow. Therefore, it is the question whether these resilience indicators flag that a relatively slow system is approaching a tipping point. In Chapter 3, we show that it is more difficult to quantify the resilience of a system that responds relatively slow. These results indicate that as the rates of environmental change keep increasing, it become more and more difficult to detect whether systems are approaching a tipping point. 

    Another risk under current rates of environmental change is that the rate of change in the conditions triggers a shift to an alternative stable state, whereas a change of the same magnitude but at slower rates would not. Only few studies describe this so-called ‘rate-tipping’ in ecological systems, but understanding rate-tipping is needed to understand and predict ecosystem response to the ongoing rapid environmental change. Therefore, we show in Chapter 4 that there can be rate-induced tipping for a range of initial conditions in a model of cyanobacteria with realistic parameter settings. A pulse in the environmental conditions, for example as a result of an extreme event, can cause a temporary collapse, depending on both the rate and the duration of the pulse. In addition, we showed that the type of environmental variability can influence the probability of inducing rate-tipping. These results imply that we need to incorporate critical rates of change in our ecosystems assessment and management.

    In addition to affecting the probability of inducing rate-tipping, environmental variability itself can bring a system past a tipping point. The variability is different in different parts of the climate, but because of climate change, the climatic variability is changing systematically in different parts of the world. Therefore, we analysed in Chapter 5 what the effect of changes in the memory of the climatic variability is on the chance of undergoing a critical transition. We show that chances of invoking such critical transitions are strongly affected by the climate memory as measured for instance by temporal autocorrelation in climatic variables. We illustrate the implications of this prediction with evidence from forests, corals reefs, poverty traps, violent conflict and ice-sheet instability. In all of these examples, the duration of anomalous dry or warm events increases the chance of invoking a critical transition. Our results imply that understanding the effects of altered climate variability requires research on climate memory.

    In the Afterthoughts I conclude that fast rates of environmental change and changes in environmental variability can affect the detectability and predictability of critical transitions. While the exact impacts of climate change are likely system-specific, interacting timescales make it difficult to untangle system dynamics from external forcing. The relations I describe throughout this thesis, however, are relative. This means that it is impossible to make general rules about whether resilience indicators can be observed, or if the conditions can be restored. Therefore, we should not give up on a priori detecting and reversing critical transitions driven by climate change.

    Foreseeing the future of mutualistic communities beyond collapse
    Lever, J.J. ; Leemput, I.A. van de; Weinans, E. ; Quax, R. ; Dakos, V. ; Nes, E.H. van; Bascompte, J. ; Scheffer, M. - \ 2020
    Ecology Letters 23 (2020)1. - ISSN 1461-023X - p. 2 - 15.
    Changing conditions may lead to sudden shifts in the state of ecosystems when critical thresholds are passed. Some well‐studied drivers of such transitions lead to predictable outcomes such as a turbid lake or a degraded landscape. Many ecosystems are, however, complex systems of many interacting species. While detecting upcoming transitions in such systems is challenging, predicting what comes after a critical transition is terra incognita altogether. The problem is that complex ecosystems may shift to many different, alternative states. Whether an impending transition has minor, positive or catastrophic effects is thus unclear. Some systems may, however, behave more predictably than others. The dynamics of mutualistic communities can be expected to be relatively simple, because delayed negative feedbacks leading to oscillatory or other complex dynamics are weak. Here, we address the question of whether this relative simplicity allows us to foresee a community's future state. As a case study, we use a model of a bipartite mutualistic network and show that a network's post‐transition state is indicated by the way in which a system recovers from minor disturbances. Similar results obtained with a unipartite model of facilitation suggest that our results are of relevance to a wide range of mutualistic systems.
    Finding the direction of lowest resilience in multivariate complex systems
    Weinans, Els ; Lever, Jelle ; Bathiany, Sebastian ; Quax, Rick ; Bascompte, Jordi ; Nes, Egbert H. Van; Scheffer, Marten ; De Leemput, Ingrid A. Van - \ 2019
    Journal of the Royal Society, Interface 16 (2019)159. - ISSN 1742-5689
    Complex networks - Resilience - Stability

    The dynamics of complex systems, such as ecosystems, financial markets and the human brain, emerge from the interactions of numerous components. We often lack the knowledge to build reliable models for the behaviour of such network systems. This makes it difficult to predict potential instabilities. We show that one could use the natural fluctuations in multivariate time series to reveal network regions with particularly slow dynamics. The multidimensional slowness points to the direction of minimal resilience, in the sense that simultaneous perturbations on this set of nodes will take longest to recover. We compare an autocorrelation-based method with a variance-based method for different time-series lengths, data resolution and different noise regimes. We show that the autocorrelation-based method is less robust for short time series or time series with a low resolution but more robust for varying noise levels. This novel approach may help to identify unstable regions of multivariate systems or to distinguish safe from unsafe perturbations.

    Superorganisms or loose collections of species? A unifying theory of community patterns along environmental gradients
    Liautaud, Kevin ; Nes, Egbert H. van; Barbier, Matthieu ; Scheffer, Marten ; Loreau, Michel - \ 2019
    Ecology Letters 22 (2019)8. - ISSN 1461-023X - p. 1243 - 1252.
    Alternative stable states - community organisation - competition theory - critical transitions - environmental gradient - Lotka–Volterra model

    The question whether communities should be viewed as superorganisms or loose collections of individual species has been the subject of a long-standing debate in ecology. Each view implies different spatiotemporal community patterns. Along spatial environmental gradients, the organismic view predicts that species turnover is discontinuous, with sharp boundaries between communities, while the individualistic view predicts gradual changes in species composition. Using a spatially explicit multispecies competition model, we show that organismic and individualistic forms of community organisation are two limiting cases along a continuum of outcomes. A high variance of competition strength leads to the emergence of organism-like communities due to the presence of alternative stable states, while weak and uniform interactions induce gradual changes in species composition. Dispersal can play a confounding role in these patterns. Our work highlights the critical importance of considering species interactions to understand and predict the responses of species and communities to environmental changes.

    White Adipose Tissue Response of Obese Mice to Ambient Oxygen Restriction at Thermoneutrality: Response Markers Identified, but no WAT Inflammation
    Hoevenaars, Femke P.M. ; Keijer, Jaap ; Stelt, Inge van der; Duivenvoorde, Loes P.M. ; Herreman, Laure ; Nes, Robin van; Friedecký, David ; Hegeman, Maria A. ; Schothorst, Evert M. Van - \ 2019
    Genes 10 (2019)5. - ISSN 2073-4425
    Obesity is associated with white adipose tissue (WAT) hypoxia and inflammation. We aimed to test whether mild environmental oxygen restriction (OxR, 13% O2), imposing tissue hypoxia, triggers WAT inflammation in obese mice. Thirteen weeks diet-induced obese male adult C57BL/6JOlaHsd mice housed at thermoneutrality were exposed for five days to OxR versus normoxia. WAT and blood were isolated and used for analysis of metabolites and adipokines, WAT histology and macrophage staining, and WAT transcriptomics. OxR increased circulating levels of haemoglobin and haematocrit as well as hypoxia responsive transcripts in WAT and decreased blood glucose, indicating systemic and tissue hypoxia. WAT aconitase activity was inhibited. Macrophage infiltration as marker for WAT inflammation tended to be decreased, which was supported by down regulation of inflammatory genes S100a8, Ccl8, Clec9a, Saa3, Mgst2, and Saa1. Other down regulated processes include cytoskeleton remodelling and metabolism, while response to hypoxia appeared most prominently up regulated. The adipokines coiled-coil domain containing 3 (CCDC3) and adiponectin, as well as the putative WAT hormone cholecystokinin (CCK), were reduced by OxR on transcript (Cck, Ccdc3) and/or serum protein level (adiponectin, CCDC3). Conclusively, our data demonstrate that also in obese mice OxR does not trigger WAT inflammation. However, OxR does evoke a metabolic response in WAT, with CCDC3 and adiponectin as potential markers for systemic or WAT hypoxia
    Inferring causation from time series in Earth system sciences
    Runge, Jakob ; Bathiany, Sebastian ; Bollt, Erik ; Camps-Valls, Gustau ; Coumou, Dim ; Deyle, Ethan ; Glymour, Clark ; Kretschmer, Marlene ; Mahecha, Miguel D. ; Muñoz-Marí, Jordi ; Nes, Egbert H. van; Peters, Jonas ; Quax, Rick ; Reichstein, Markus ; Scheffer, Marten ; Schölkopf, Bernhard ; Spirtes, Peter ; Sugihara, George ; Sun, Jie ; Zhang, Kun ; Zscheischler, Jakob - \ 2019
    Nature Communications 10 (2019)1. - ISSN 2041-1723

    The heart of the scientific enterprise is a rational effort to understand the causes behind the phenomena we observe. In large-scale complex dynamical systems such as the Earth system, real experiments are rarely feasible. However, a rapidly increasing amount of observational and simulated data opens up the use of novel data-driven causal methods beyond the commonly adopted correlation techniques. Here, we give an overview of causal inference frameworks and identify promising generic application cases common in Earth system sciences and beyond. We discuss challenges and initiate the benchmark platform causeme.net to close the gap between method users and developers.

    Technology driven inequality leads to poverty and resource depletion
    Usman Mirza, M. ; Richter, Andries ; Nes, Egbert H. van; Scheffer, Marten - \ 2019
    Ecological Economics 160 (2019). - ISSN 0921-8009 - p. 215 - 226.
    Critical transitions - Dynamic systems - Inequality - Poverty trap - Social-ecological systems - Technology

    The rapid rise in inequality is often seen to go in-hand with resource overuse. Examples include water extraction in Pakistan, land degradation in Bangladesh, forest harvesting in Sub-Saharan Africa and industrial fishing in Lake Victoria. While access to ecosystem services provided by common pool resources mitigates poverty, exclusive access to technology by wealthy individuals may fuel excessive resource extraction and deplete the resource, thus widening the wealth gap. We use a stylised social-ecological model, to illustrate how a positive feedback between wealth and technology may fuel local inequality. The resulting rise in local inequality can lead to resource degradation and critical transitions such as ecological resource collapse and unexpected increase in poverty. Further, we find that societies may evolve towards a stable state of few wealthy and many poor individuals, where the distribution of wealth depends on how access to technology is distributed. Overall, our results illustrate how access to technology may be a mechanism that fuels resource degradation and consequently pushes most vulnerable members of society into a poverty trap.

    Inferring ecosystem states and quantifying their resilience : linking theories to ecological data
    Arani, Babak M.S. - \ 2019
    Wageningen University. Promotor(en): M. Scheffer, co-promotor(en): E.H. van Nes. - Wageningen : Wageningen University - ISBN 9789463435765 - 108

    The core of my thesis concerns addressing the ecosystem resilience in a data-driven manner. In this direction, I have tried to make a bridge between advanced mathematical models and existing ecological data. I could come up with some quantitative measures of resilience and applied them to some ecological field and experimental data. These measures are more exact compared with the classical measures mentioned by Holling. I show that Holling measures are just two extremes of the measure I introduced and they do not necessarily capture the notion of resilience in its real sense of the word. Furthermore, I could also address the resilience of low-resolution tropical satellite data across the tropics (South America, Africa, south east Asia and, Australia).

    Besides, my thesis also sheds more light on the concept of ‘alternative stable states’ which is an important concept in ecology. I argue that advanced ‘system reconstruction’ approaches should be applied first, from where one can better justify weather or not an ecosystem has alternative stable states.

    Seeing a global web of connected systems
    Scheffer, Marten ; Nes, Egbert H. Van - \ 2018
    Science 362 (2018)6421. - ISSN 0036-8075 - 1 p.
    Quantifying resilience of humans and other animals
    Scheffer, Marten ; Bolhuis, J.E. ; Borsboom, Denny ; Buchman, Timothy G. ; Gijzel, Sanne M.W. ; Goulson, Dave ; Kammenga, Jan E. ; Kemp, Bas ; Leemput, Ingrid A. van de; Levin, Simon ; Martin, Carmel Mary ; Melis, René J.F. ; Nes, Egbert H. van; Romero, L.M. ; Olde Rikkert, Marcel G.M. - \ 2018
    Proceedings of the National Academy of Sciences of the United States of America 115 (2018)47. - ISSN 0027-8424 - p. 11883 - 11890.
    All life requires the capacity to recover from challenges that are as inevitable as they are unpredictable. Understanding this resilience is essential for managing the health of humans and their livestock. It has long been difficult to quantify resilience directly, forcing practitioners to rely on indirect static indicators of health. However, measurements from wearable electronics and other sources now allow us to analyze the dynamics of physiology and behavior with unsurpassed resolution. The resulting flood of data coincides with the emergence of novel analytical tools for estimating resilience from the pattern of microrecoveries observed in natural time series. Such dynamic indicators of resilience may be used to monitor the risk of systemic failure across systems ranging from organs to entire organisms. These tools invite a fundamental rethinking of our approach to the adaptive management of health and resilience.
    Resilience of tropical tree cover : The roles of climate, fire, and herbivory
    Staal, Arie ; Nes, Egbert H. van; Hantson, Stijn ; Holmgren, Milena ; Dekker, Stefan C. ; Pueyo, Salvador ; Xu, Chi ; Scheffer, Marten - \ 2018
    Global Change Biology 24 (2018)11. - ISSN 1354-1013 - p. 5096 - 5109.
    alternative stable states - bistability - forest - grasslands - livestock - model - regime shifts - remote sensing - tipping points - wildfire

    Fires and herbivores shape tropical vegetation structure, but their effects on the stability of tree cover in different climates remain elusive. Here, we integrate empirical and theoretical approaches to determine the effects of climate on fire- and herbivore-driven forest-savanna shifts. We analyzed time series of remotely sensed tree cover and fire observations with estimates of herbivore pressure across the tropics to quantify the fire–tree cover and herbivore–tree cover feedbacks along climatic gradients. From these empirical results, we developed a spatially explicit, stochastic fire-vegetation model that accounts for herbivore pressure. We find emergent alternative stable states in tree cover with hysteresis across rainfall conditions. Whereas the herbivore–tree cover feedback can maintain low tree cover below 1,100 mm mean annual rainfall, the fire–tree cover feedback can maintain low tree cover at higher rainfall levels. Interestingly, the rainfall range where fire-driven alternative vegetation states can be found depends strongly on rainfall variability. Both higher seasonal and interannual variability in rainfall increase fire frequency, but only seasonality expands the distribution of fire-maintained savannas into wetter climates. The strength of the fire–tree cover feedback depends on the spatial configuration of tree cover: Landscapes with clustered low tree-cover areas are more susceptible to cross a tipping point of fire-driven forest loss than landscapes with scattered deforested patches. Our study shows how feedbacks involving fire, herbivores, and the spatial structure of tree cover explain the resilience of tree cover across climates.

    Resilience of tropical forest and savanna: bridging theory and observation
    Staal, Arie - \ 2018
    Wageningen University. Promotor(en): M. Scheffer, co-promotor(en): E.H. van Nes; S.C. Dekker. - Wageningen : Wageningen University - ISBN 9789463438353 - 221

    This thesis explores the hypothesis that tropical forest and savanna can be alternative stable states. Feedbacks of tree cover with fire across the tropics, and with rainfall in the Amazon basin, are studied by linking modelling to the analysis of broad-scale remote-sensing data. Time series of tree cover and fire observations are used to quantify the strength of the fire-tree cover feedback loop along climatic gradients. From these empirical results a spatially explicit and stochastic fire-tree cover model is developed. The model predicts that forest and savanna are fire-driven alternative stable states across rainfall conditions, but the exact rainfall range depends strongly on rainfall seasonality. Next, regional-scale effects of tree cover on rainfall in the Amazon basin are presented. Forest transpiration is estimated and the trajectories of that transpired water through the atmosphere are simulated. It is found that one-third of all rainfall in the Amazon basin originates from the basin; two-thirds of that water has been transpired by trees at least once. Forests in the southern half of the basin contribute most to the resilience of other forests, whereas forests in the south-western Amazon are most dependent on transpiration from forests elsewhere in the basin. The relative contribution of forest transpiration to rainfall is higher in drier months and in drier years. In conclusion, tree cover will not change smoothly with climate change and possible transitions between forest and savanna will likely be relatively abrupt. The main mechanism behind such tipping points is a feedback between tree cover and fire. Increasing rainfall seasonality will strengthen that feedback and in the Amazon at least, reduced forest transpiration resulting from tree-cover loss will enhance that seasonality.

    A global climate niche for giant trees
    Scheffer, Marten ; Xu, Chi ; Hantson, Stijn ; Holmgren, Milena ; Los, Sietse O. ; Nes, Egbert H. van - \ 2018
    Global Change Biology 24 (2018)7. - ISSN 1354-1013 - p. 2875 - 2883.
    alternative ecosystem state - canopy height - LiDAR - precipitation temperate rainforest - remote sensing - resilience - threshold - tropical rainforest

    Rainforests are among the most charismatic as well as the most endangered ecosystems of the world. However, although the effects of climate change on tropical forests resilience is a focus of intense research, the conditions for their equally impressive temperate counterparts remain poorly understood, and it remains unclear whether tropical and temperate rainforests have fundamental similarities or not. Here we use new global data from high precision laser altimetry equipment on satellites to reveal for the first time that across climate zones ‘giant forests’ are a distinct and universal phenomenon, reflected in a separate mode of canopy height (~40 m) worldwide. Occurrence of these giant forests (cutoff height > 25 m) is negatively correlated with variability in rainfall and temperature. We also demonstrate that their distribution is sharply limited to situations with a mean annual precipitation above a threshold of 1,500 mm that is surprisingly universal across tropical and temperate climates. The total area with such precipitation levels is projected to increase by ~4 million km2 globally. Our results thus imply that strategic management could in principle facilitate the expansion of giant forests, securing critically endangered biodiversity as well as carbon storage in selected regions.

    Corrigendum to “The structuring role of submerged macrophytes in a large subtropical shallow lake : Clear effects on water chemistry and phytoplankton structure community along a vegetated-pelagic gradient” [Limnologica (2018) 142–154]
    Finkler Ferreira, T. ; Crossetti, Luciane O. ; Motta Marques, David M.L. ; Cardoso, Luciana ; Fragoso, Carlos Ruberto ; Nes, Egbert H. van - \ 2018
    Limnologica 71 (2018). - ISSN 0075-9511 - p. 89 - 89.
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