Bistability and regular spatial patterns in arid ecosystems.

A variety of patterns observed in ecosystems can be explained by resource–concentration mechanisms. A resource–concentration mechanism occurs when organisms increase the lateral flow of a resource toward them, leading to a local concentration of this resource and to its depletion from areas farther away. In resource–concentration systems, it has been proposed that certain spatial patterns could indicate proximity to discontinuous transitions where an ecosystem abruptly shifts from one stable state to another. Here, we test this hypothesis using a model of vegetation dynamics in arid ecosystems. In this model, a resource– concentration mechanism drives a positive feedback between vegetation and soil water availability. We derived the conditions leading to bistability and pattern formation. Our analysis revealed that bistability and regular pattern formation are linked in our model. This means that, when regular vegetation patterns occur, they indicate that the system is along a discontinuous transition to desertification. Yet, in real systems, only observing regular vegetation patterns without identifying the pattern-driving mechanism might not be enough to conclude that an ecosystem is along a discontinuous transition because similar patterns can emerge from different ecological mechanisms

Nutrients and Hydrology Indicate the Driving Mechanisms of Peatland Surface Patterning

Peatland surface patterning motivates studies that identify underlying structuring mechanisms. Theoretical studies so far suggest that different mechanisms may drive similar types of patterning. The long time span associated with peatland surface pattern formation, however, limits possibilities for empirically testing model predictions by field manipulations. Here, we present a model that describes spatial interactions between vegetation, nutrients, hydrology, and peat. We used this model to study pattern formation as driven by three different mechanisms: peat accumulation, water ponding, and nutrient accumulation. By on-and-off switching of each mechanism, we created a full-factorial design to see how these mechanisms affected surface patterning (pattern of vegetation and peat height) and underlying patterns in nutrients and hydrology. Results revealed that different combinations of structuring mechanisms lead to similar types of peatland surface patterning but contrasting underlying patterns in nutrients and hydrology. These contrasting underlying patterns suggest that the presence or absence of the structuring mechanisms can be identified by relatively simple short-term field measurements of nutrients and hydrology, meaning that longer-term field manipulations can be circumvented. Therefore, this study provides promising avenues for future empirical studies on peatland patternin

Hoogveenherstel in Nederland: meer dan een droom

Vernatten, de belangrijkste herstelmaatregel in hoogveenrestanten, heeft niet altijd het gewenste effect. Waarom dat zo is, is grotendeels nog onbekend. De auteurs beargumenteren dat recente inzichten uit empirische en experimentele studies in hoogveenherstelprojecten moeten worden gebruikt om de bestaande theoretische modellen uit te breiden, om deze vervolgens te kunnen gebruiken in het ontwikkelen van effectieve, gebiedspecifieke herstelplannen. Hierdoor kan de droom van zelfregulerende hoogvenen in Nederland wellicht op niet al te lange termijn werkelijkheid worden

Особливості правової моделі ханафітського мазгабу

Статья Исмагилова С.В. «Особенности правовой модели ханафитского мазхаба» исследует возникновение и развитие ханафитского мазхаба – правовой школы имама Абу Ханифы. В работе автор исследует источники фикха, феномен возникновения мазхабов и особенности правовой модели ханафитского мазхаба. В статье подчеркивается, что мазхаб ханафитов явился результатом научно-правовой деятельносты не только самого Абу Ханифы, но также и его учеников. Ключевые слова: ханафитский мазхаб, мусульманское право, иджтихад.The article by Ismagilov S.V. "Features of the jural model of the Hanafi school of thought" investigates the origin and development of the Hanafi school of thought - the juridical school of Imam Abu Hanifa. In this paper, the author analyses the sources of fiqh, the phenomenon of emergence of schools of thought and peculiarities of the juridical model of the Hanafi school of thought. The paper stresses that Madh'hab Hanafi is a product of scientific and jural activities not only of Abu Hanifa, but his disciples also. Key-words: juridical school of Abu Hanifa, Moslem jury, idgtikhad

Гоголевские традиции в творчестве М. Булгакова

Model studies suggest that semiarid ecosystems with patterned vegetation can respond in a nonlinear way to climate change. This means that gradual changes can result in a rapid transition to a desertified state. Previous model studies focused on the response of patterned semiarid ecosystems to changes in mean annual rainfall. The intensity of rain events, however, is projected to change as well in the coming decades. In this paper, we study the effect of changes in rainfall intensity on the functioning of patterned semiarid ecosystems with a spatially explicit model that captures rainwater partitioning and runoff-runon processes with simple event-based process descriptions. Analytical and numerical analyses of the model revealed that rainfall intensity is a key parameter in explaining patterning of vegetation in semiarid ecosystems as low mean rainfall intensities do not allow for vegetation patterning to occur. Surprisingly, we found that, for a constant annual rainfall rate, both an increase and a decrease in mean rainfall intensity can trigger desertification. An increase negatively affects productivity as a greater fraction of the rainwater is lost as runoff. This can result in a shift to a bare desert state only if the mean rainfall intensity exceeds the infiltration capacity of bare soil. On the other hand, a decrease in mean rainfall intensity leads to an increased fraction of rainwater infiltrating in bare soils, remaining unavailable to plants. Our findings suggest that considering rainfall intensity as a variable may help in assessing the proximity to regime shifts in patterned semiarid ecosystems and that monitoring losses of resource through runoff and bare soil infiltration could be used to determine ecosystem resilience. Key Points Rainfall intensity controls patterning and the resilience of arid ecosystems Both an increase and decrease in rainfall intensity can trigger desertification In line with observations, three types of rain events were identified in our mode

Striped pattern selection by advective reaction-diffusion systems: resilience of banded vegetation on slopes

Analysis and Stochastic

Spatial ecology of peatland ecosystems: Spatial self-organization and catastrophic shifts in bogs

Positive feedback interactions, as between plants and their abiotic environment, may have the consequence that an ecosystem has alternate stable equilibrium states. As a result, a gradual change in environmental conditions may lead to discontinuous, catastrophic shifts in such ecosystems. Until now, the occurrence of catastrophic shifts is hardly predictable. However, a recently developed idea is that self-organized vegetation patterns might serve as an indicator for imminent catastrophic shifts. Because self-organized vegetation patterns are a common feature of northern peatland bogs, we examine to what extent these causally related concepts (positive feedbacks, alternate stable states, spatial self-organization and catastrophic shifts) might contribute to a better understanding of this type of ecosystems. Empirical and theoretical studies reveal that many positive feedbacks can be present in bogs, which may generate alternate stable states. Furthermore, stratigraphic peat analysis has shown that large and abrupt transitions do occur in bogs. However, the relative contribution of each of these feedbacks to the self-organized vegetation patterns in bogs and the possible relation with catastrophic shifts remain unclear. Conclusively, a research design is discussed that addresses the current gaps in knowledge about the way positive feedbacks may lead to alternate stable states and spatial self-organization in bogs, and its possible indication of catastrophic shifts

aMazing pattern: spatial self-organization in peatlands

Predicting how gradual changes in abiotic conditions affect ecosystem functioning is a key challenge in ecology and environmental science. For many ecosystems, the response to gradual changes may not be smooth, but rapid and almost irreversible shifts in ecosystem states may occur. Early warning signals for such catastrophic shifts are difficult to obtain. Recent research suggests that so-called self-organized patchiness (regular spatial vegetation patterning) can serve as an indicator for such sudden changes. Self-organized patchiness has been observed in a variety of ecosystems, including peatlands. Most research has focused on linear patterns along the contours of peatland slopes. More recently, aerial photographs from relatively flat ground in Siberia revealed peatlands with so-called maze-patterning, because this type of patchiness somewhat resembles the corridors of a maze. The striking self-organized patchiness has amazed many peatland scientists and has lead to considerable attention for peatland patterning in the literature. Until now, however, the driving mechanisms of peatland patchiness still remain elusive, despite more than a century of research on this phenomenon. This thesis investigates underlying mechanisms that explain self-organized patchiness in peatlands, and whether this patchiness could serve as an indicator for proximity to catastrophic shifts in peatland ecosystem states. A combination of theoretical and empirical approaches is used. We conclude that the potential importance of different driving mechanisms for peatland patterning depends on climatic conditions. Increased evapotranspiration in vegetation patches with high density may be particularly important in peatlands where most water leaves the system through evapotranspiration. Alternatively, in peatlands where water is lost via drainage or overland flow, a positive feedback between the thickness of the upper aerobic peat layer and the rate of peat formation the peat accumulation mechanism may be more important. Global climate models project for most peatland regions an increasing importance of evapotranspiration during the coming century, with the strongest increases being projected for Siberia and Canada. Based on the results in this thesis, we speculate that evapotranspiration may become the main driver of pattern formation in parts of these regions. We also conclude that a shift from an unpatterned state without hummocks and hollows toward a patterned state with hummocks and hollows already comprises a catastrophic shift in ecosystem functioning that is difficult to reverse. This means that a pattern cannot be used as an indicator of proximity to a catastrophic shift, but rather indicates that the shift has already happened. Moreover, the very slow development of peatlands calls for caution when applying equilibrium concepts, which are used in most mathematical models of pattern formation, to peatland dynamics. Investigation of the mechanisms that drive self-organized patchiness in ecosystems is a promising approach to increase our understanding of ecosystem functioning, and the response of ecosystems to changing abiotic conditions. This thesis exemplifies that studies on pattern formation need to include both theoretical and empirical approaches, because the driving mechanisms of self-organized patchiness may change with climatic regions and may therefore be site-specific

Spatial ecology of peatland ecosystems: Spatial self-organization and catastrophic shifts in bogs

Positive feedback interactions, as between plants and their abiotic environment, may have the consequence that an ecosystem has alternate stable equilibrium states. As a result, a gradual change in environmental conditions may lead to discontinuous, catastrophic shifts in such ecosystems. Until now, the occurrence of catastrophic shifts is hardly predictable. However, a recently developed idea is that self-organized vegetation patterns might serve as an indicator for imminent catastrophic shifts. Because self-organized vegetation patterns are a common feature of northern peatland bogs, we examine to what extent these causally related concepts (positive feedbacks, alternate stable states, spatial self-organization and catastrophic shifts) might contribute to a better understanding of this type of ecosystems. Empirical and theoretical studies reveal that many positive feedbacks can be present in bogs, which may generate alternate stable states. Furthermore, stratigraphic peat analysis has shown that large and abrupt transitions do occur in bogs. However, the relative contribution of each of these feedbacks to the self-organized vegetation patterns in bogs and the possible relation with catastrophic shifts remain unclear. Conclusively, a research design is discussed that addresses the current gaps in knowledge about the way positive feedbacks may lead to alternate stable states and spatial self-organization in bogs, and its possible indication of catastrophic shifts

Eco-evolutionary litter feedback as a driver of exotic plant invasion

Many studies have examined positive feedbacks between invasive plant traits and nutrient cycling, but few have investigated whether feedbacks arise from introduction of pre-adapted species or from ecoevolutionary feedback that develops after introduction. Eco-evolutionary feedback could occur between an invader’s leaf tissue C:N ratio and its response to litter accumulation. Previous modeling predicts that occurrence of this feedback would be reflected by: (1) field data showing higher litter:biomass ratios in the invasive range; (2) high C:N genotypes benefiting more from experimental litter additions than low C:N genotypes; (3) this beneficial effect on high C:N genotypes inducing a critical transition toward invader dominance when a critical amount of litter is added to a native species-dominated community experiencing low nutrient conditions. Here, we empirically tested these predictions for the invasive grass Phalaris arundinacea, which has undergone post-introduction evolutionary change toward attaining higher C:N ratios under high nutrient conditions. We performed a biogeographical comparison of litter:biomass ratios in the native (Europe) and invasive (USA) range, and an experiment with mesocosms from the invasive range under low nutrient conditions. Low and high C:N Phalaris genotypes were introduced into native-dominated and bare mesocosms, to which varying litter amounts were added. The biogeographical comparison revealed that litter:biomass ratios were higher in the invasive range. The mesocosm experiment showed that when grown in isolation, only high C:N genotypes responded positively to litter. This effect, however, was not strong enough to stimulate Phalaris when exposed to competition with native species. Our results suggest that eco-evolutionary feedback between Phalaris’ C:N ratio and litter accumulation could occur, but only under high nutrient conditions. Our experiments suggest that eco-evolutionary feedback may select for specialist rather than superior genotypes. Hence, genotypic variation induced by post-introduction admixture may be subject to context-dependent selection due to eco-evolutionary feedback, increasing trait variation within invasive populations