scholarly journals Exploiting delayed transitions to sustain semiarid ecosystems after catastrophic shifts

2017 ◽  
Author(s):  
Blai Vidiella, ◽  
Josep Sardanyés ◽  
Ricard V. Solé
Keyword(s):  
Computational Models ◽  
Warning Signal ◽  
Semiarid Ecosystems ◽  
Intervention Method ◽  
Catastrophic Shift ◽  
Future Potential ◽  
Desert Areas ◽  
Standard Models ◽  
Potential Shifts ◽  
Catastrophic Shifts

Semiarid ecosystems (including arid, semiarid and dry-subhumid ecosystems) span more than 40% of extant habitats and a similar percentage of human population. As a consequence of global warming, these habitats face future potential shifts towards the desert state characterized by an accelerated loss of diversity and stability leading to collapse. Such possibility has been raised by several mathematical and computational models, along with several early warning signal methods applied to spatial vegetation patterns. Here we show that just after a catastrophic shift has taken place an expected feature is the presence of a ghost, i.e., a delayed extinction associated to the underlying dynamical flows. As a consequence, a system might exhibit for very long times an apparent stationarity hiding in fact an inevitable collapse. Here we explore this problem showing that the ecological ghost is a generic feature of standard models of green-desert transitions including facilitation. If present, the ghost could hide warning signals, since statistical patterns are not be expected to display growing fluctuations over time. We propose and computationally test a novel intervention method based on the restoration of small fractions of desert areas with vegetation as a way to maintain the fragile ecosystem beyond the catastrophic shift caused by a saddle-node bifurcation, taking advantage of the delaying capacity of the ghost just after the bifurcation.

scholarly journals Exploiting delayed transitions to sustain semiarid ecosystems after catastrophic shifts

2018 ◽  
Vol 15 (143) ◽  
pp. 20180083 ◽  
Author(s):  
Blai Vidiella ◽  
Josep Sardanyés ◽  
Ricard Solé
Keyword(s):  
Human Population ◽  
Theoretical Models ◽  
Shift Condition ◽  
Critical Rate ◽  
Semiarid Ecosystems ◽  
Intervention Method ◽  
Stochastic Fluctuations ◽  
Catastrophic Shift ◽  
Desert Areas ◽  
Catastrophic Shifts

Semiarid ecosystems (including arid, semiarid and dry-subhumid ecosystems) span more than 40% of extant habitats and contain a similar percentage of the human population. Theoretical models and palaeoclimatic data predict a grim future, with rapid shifts towards a desert state, with accelerated diversity losses and ecological collapses. These shifts are a consequence of the special nonlinearities resulting from ecological facilitation. Here, we investigate a simple model of semiarid ecosystems identifying the so-called ghost, which appears after a catastrophic transition from a vegetated to a desert state once a critical rate of soil degradation is overcome. The ghost involves a slowdown of transients towards the desert state, making the ecosystem seem stable even though vegetation extinction is inevitable. We use this model to show how to exploit the ecological ghosts to avoid collapse. Doing so involves the restoration of small fractions of desert areas with vegetation capable of maintaining a stable community once the catastrophic shift condition has been achieved. This intervention method is successfully tested under the presence of demographic stochastic fluctuations.

scholarly journals An Overview of In Vitro, In Vivo, and Computational Techniques for Cancer-Associated Angiogenesis Studies

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
Heshu Sulaiman Rahman ◽  
Bee Ling Tan ◽  
Hemn Hassan Othman ◽  
Max Stanley Chartrand ◽  
Yashwant Pathak ◽  
...  
Keyword(s):  
Computational Models ◽  
Therapeutic Agents ◽  
Computational Techniques ◽  
Macroscopic Scale ◽  
Angiogenic Response ◽  
Physiological Processes ◽  
Alternative Approach ◽  
Standard Models

Angiogenesis is a crucial area in scientific research because it involves many important physiological and pathological processes. Indeed, angiogenesis is critical for normal physiological processes, including wound healing and embryonic development, as well as being a component of many disorders, such as rheumatoid arthritis, obesity, and diabetic retinopathies. Investigations of angiogenic mechanisms require assays that can activate the critical steps of angiogenesis as well as provide a tool for assessing the efficacy of therapeutic agents. Thus, angiogenesis assays are key tools for studying the mechanisms of angiogenesis and identifying the potential therapeutic strategies to modulate neovascularization. However, the regulation of angiogenesis is highly complex and not fully understood. Difficulties in assessing the regulators of angiogenic response have necessitated the development of an alternative approach. In this paper, we review the standard models for the study of tumor angiogenesis on the macroscopic scale that include in vitro, in vivo, and computational models. We also highlight the differences in several modeling approaches and describe key advances in understanding the computational models that contributed to the knowledge base of the field.

scholarly journals Trans-heteroclinic bifurcation: a novel type of catastrophic shift

2018 ◽  
Vol 5 (1) ◽  
pp. 171304 ◽  
Author(s):  
Josep Sardanyés ◽  
Regina Martínez ◽  
Carles Simó
Keyword(s):  
Steady State ◽  
Dynamical Systems ◽  
Fixed Points ◽  
Social Systems ◽  
Continuous Change ◽  
Heteroclinic Bifurcation ◽  
Mutation Matrix ◽  
Catastrophic Shift ◽  
Smooth Transitions ◽  
Catastrophic Shifts

Global and local bifurcations are extremely important since they govern the transitions between different qualitative regimes in dynamical systems. These transitions or tipping points, which are ubiquitous in nature, can be smooth or catastrophic. Smooth transitions involve a continuous change in the steady state of the system until the bifurcation value is crossed, giving place to a second-order phase transition. Catastrophic transitions involve a discontinuity of the steady state at the bifurcation value, giving place to first-order phase transitions. Examples of catastrophic shifts can be found in ecosystems, climate, economic or social systems. Here we report a new type of global bifurcation responsible for a catastrophic shift. This bifurcation, identified in a family of quasi-species equations and named as trans-heteroclinic bifurcation , involves an exchange of stability between two distant and heteroclinically connected fixed points. Since the two fixed points interchange the stability without colliding, a catastrophic shift takes place. We provide an exhaustive description of this new bifurcation, also detailing the structure of the replication–mutation matrix of the quasi-species equation giving place to this bifurcation. A perturbation analysis is provided around the bifurcation value. At this value the heteroclinic connection is replaced by a line of fixed points in the quasi-species model. But it is shown that, if the replication–mutation matrix satisfies suitable conditions, then, under a small perturbation, the exchange of heteroclinic connections is preserved, except on a tiny range around the bifurcation value whose size is of the order of magnitude of the perturbation. The results presented here can help to understand better novel mechanisms behind catastrophic shifts and contribute to a finer identification of such transitions in theoretical models in evolutionary biology and other dynamical systems.

scholarly journals Abrupt transitions to tumor extinction: A quasispecies phenotypic model

2016 ◽  
Author(s):  
Josep Sardanyes ◽  
Regina Martinez ◽  
Carles Simo ◽  
Ricard Sole
Keyword(s):  
Tumor Cell ◽  
Fixed Points ◽  
Tumor Cells ◽  
Computational Models ◽  
Cell Populations ◽  
Model Parameters ◽  
Abrupt Transition ◽  
Quasispecies Model ◽  
Catastrophic Shift ◽  
Tumor Clearance

Background: The dynamics of heterogeneous tumor cell populations competing with healthy cells is an important topic in cancer research with deep implications in biomedicine. Multitude of theoretical and computational models have addressed this issue, especially focusing on the nature of the transitions governing tumor clearance as some relevant model parameters are tuned. In this contribution, we analyze a mathematical model of unstable tumor progression using the quasispecies framework. Our aim is to define a minimal model incorporating the dynamics of competition between healthy cells and a heterogeneous population of cancer cell phenotypes involving changes in replication-related genes (i.e., proto-oncogenes and tumor suppressor genes), in genes responsible for genomic stability, and in house-keeping genes. Such mutations or loss of genes result into different phenotypes with increased proliferation rates and/or increased genomic instabilities. Also, lethal phenotypes with mutations or loss of house-keeping genes are present in our model. Results: Despite bifurcations in the classical deterministic quasispecies model are typically given by smooth, continuous shifts (i.e., transcritical bifurcations), we here identify an novel type of abrupt transition causing tumor extinction. Such a bifurcation, named as trans-heteroclinic, is characterized by the exchange of stability between two distant fixed points (that do not collide) involving, respectively, tumor persistence and tumor clearance. The increase of mutation and/or the decrease of the replication rate of tumor cells involves this catastrophic shift of tumor cell populations. The transient times near bifurcation thresholds are also characterized, showing a power law dependence of exponent −1 of the transients as mutation is changed near the bifurcation value. Conclusions: An abrupt transition involving tumor clearance has been identified with a phenotypic quasispecies cancer model. This result is discussed in the context of targeted cancer therapy as a possible therapeutic strategy to force a catastrophic shift by delivering mutagenic and cytotoxic drugs inside tumor cells. Our model also reveals a novel mechanism causing a discontinuous transition given by the stability exchange of two distant fixed points, which we name as a trans-heteroclinic bifurcation.

scholarly journals Synthetic soil crusts against green-desert transitions: a spatial model

2019 ◽  
Author(s):  
Blai Vidiella ◽  
Josep Sardanyés ◽  
Ricard V. Solé
Keyword(s):  
Spatial Model ◽  
Soil Degradation ◽  
Mean Field ◽  
Nonlinear Effects ◽  
Mean Field Model ◽  
Dispersal Capacity ◽  
Semiarid Ecosystems ◽  
Current Trends ◽  
Degradation Rates ◽  
Catastrophic Shifts

Semiarid ecosystems are threatened by global warming due to longer dehydration times and increasing soil degradation. Mounting evidences indicate that, given the current trends, drylands are likely to expand and possibly experience catastrophic shifts from vegetated to desert states. Here we explore a recent suggestion based on the concept of ecosystem terraformation, where a synthetic organism is used to counterbalance some of the nonlinear effects causing the presence of such tipping points. Using an explicit spatial model incorporating facilitation and considering a simplification of states found in semiarid ecosystems i.e., vegetation, fertile and desert soil, we investigate how engineered microorganisms can shape the fate of these ecosystems. Specifically, two different, but complementary, terraformation strategies are proposed: Cooperation-based: C-terraformation; and Dispersion-based: D-terraformation. The first strategy involves the use of soil synthetic microorganisms to introduce cooperative loops (facilitation) with the vegetation. The second one involves the introduction of engineered microorganisms improving their dispersal capacity, thus facilitating the transition from desert to fertile soil. We show that small modifications enhancing cooperative loops can effectively change the location of the critical transition found at increasing soil degradation rates, also identifying a stronger protection against soil degradation by using the D-terraformation strategy. The same results are found in a mean field model providing insights into the transitions and dynamics tied to these terraformation strategies. The potential consequences and extensions of these models are discussed.

Discontinuities in Regional Development

1986 ◽  
Vol 4 (1) ◽  
pp. 71-84 ◽  
Author(s):  
M S Gertler
Keyword(s):  
Regional Development ◽  
Capital Flows ◽  
Catastrophe Theory ◽  
Capital Mobility ◽  
Investment Decisions ◽  
Scale Economies ◽  
Division Of Labour ◽  
Alternative Conception ◽  
Catastrophic Shift ◽  
Catastrophic Shifts

Historically, geographers have been keen to adopt bodies of theory from other disciplines in a somewhat uncritical manner. This practice has surfaced again in the recent attempts to apply notions from catastrophe theory to the study of regional development. This paper is an examination of one such application to the modeling of interregional capital flows and critically evaluates it from a number of perspectives. On empirical grounds, there is little evidence that ‘catastrophic’ shifts in the geography of US manufacturing capital have indeed occurred. The presumption of a catastrophic shift is shown to be based on poor empirical indicators of capital mobility or on an incomplete understanding of the process of interregional investment decisions. The paper offers an alternative conception of this process in which the Kaldorian principles of scale economies, specialization, and division of labour are shown to be in continuing operation, albeit now in a spatially discontinuous manner.

scholarly journals Synthetic soil crusts against green-desert transitions: a spatial model

2020 ◽  
Vol 7 (8) ◽  
pp. 200161
Author(s):  
Blai Vidiella ◽  
Josep Sardanyés ◽  
Ricard V. Solé
Keyword(s):  
Spatial Model ◽  
Soil Degradation ◽  
Mean Field ◽  
Nonlinear Effects ◽  
Mean Field Model ◽  
Dispersal Capacity ◽  
Semiarid Ecosystems ◽  
Current Trends ◽  
Degradation Rates ◽  
Catastrophic Shifts

Semiarid ecosystems are threatened by global warming due to longer dehydration times and increasing soil degradation. Mounting evidence indicates that, given the current trends, drylands are likely to expand and possibly experience catastrophic shifts from vegetated to desert states. Here, we explore a recent suggestion based on the concept of ecosystem terraformation, where a synthetic organism is used to counterbalance some of the nonlinear effects causing the presence of such tipping points. Using an explicit spatial model incorporating facilitation and considering a simplification of states found in semiarid ecosystems including vegetation, fertile and desert soil, we investigate how engineered microorganisms can shape the fate of these ecosystems. Specifically, two different, but complementary, terraformation strategies are proposed: Cooperation -based: C -terraformation; and Dispersion -based: D -terraformation. The first strategy involves the use of soil synthetic microorganisms to introduce cooperative loops (facilitation) with the vegetation. The second one involves the introduction of engineered microorganisms improving their dispersal capacity, thus facilitating the transition from desert to fertile soil. We show that small modifications enhancing cooperative loops can effectively modify the aridity level of the critical transition found at increasing soil degradation rates, also identifying a stronger protection against soil degradation by using the D -terraformation strategy. The same results are found in a mean-field model providing insights into the transitions and dynamics tied to these terraformation strategies. The potential consequences and extensions of these models are discussed.

Sequential Difficulty Effects During Strategy Execution

2012 ◽  
Vol 59 (5) ◽  
pp. 295-301 ◽  
Author(s):  
Kim Uittenhove ◽  
Patrick Lemaire
Keyword(s):  
Computational Models ◽  
Preceding Trial ◽  
Human Cognition ◽  
Strategy Execution ◽  
Strategy Performance ◽  
Empirical Implications

In two experiments, we tested the hypothesis that strategy performance on a given trial is influenced by the difficulty of the strategy executed on the immediately preceding trial, an effect that we call strategy sequential difficulty effect. Participants’ task was to provide approximate sums to two-digit addition problems by using cued rounding strategies. Results showed that performance was poorer after a difficult strategy than after an easy strategy. Our results have important theoretical and empirical implications for computational models of strategy choices and for furthering our understanding of strategic variations in arithmetic as well as in human cognition in general.

Does Viotin Activate Violin More Than Viocin?

2014 ◽  
Vol 61 (1) ◽  
pp. 23-29 ◽  
Author(s):  
Manuel Perea ◽  
Victoria Panadero
Keyword(s):  
Word Recognition ◽  
Visual Word Recognition ◽  
Developmental Dyslexia ◽  
Computational Models ◽  
Error Rates ◽  
Visual Word ◽  
Base Word ◽  
Similar Response ◽  
Skilled Readers ◽  
Young Readers

The vast majority of neural and computational models of visual-word recognition assume that lexical access is achieved via the activation of abstract letter identities. Thus, a word’s overall shape should play no role in this process. In the present lexical decision experiment, we compared word-like pseudowords like viotín (same shape as its base word: violín) vs. viocín (different shape) in mature (college-aged skilled readers), immature (normally reading children), and immature/impaired (young readers with developmental dyslexia) word-recognition systems. Results revealed similar response times (and error rates) to consistent-shape and inconsistent-shape pseudowords for both adult skilled readers and normally reading children – this is consistent with current models of visual-word recognition. In contrast, young readers with developmental dyslexia made significantly more errors to viotín-like pseudowords than to viocín-like pseudowords. Thus, unlike normally reading children, young readers with developmental dyslexia are sensitive to a word’s visual cues, presumably because of poor letter representations.

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