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Axel Brandenburg - One of the best experts on this subject based on the ideXlab platform.
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the inverse Cascade and nonlinear alpha Effect in simulations of isotropic helical hydromagnetic turbulence
The Astrophysical Journal, 2001Co-Authors: Axel BrandenburgAbstract:A numerical model of isotropic homogeneous turbulence with helical forcing is investigated. The resulting flow, which is essentially the prototype of the α2 dynamo of mean field dynamo theory, produces strong dynamo action with an additional large-scale field on the scale of the box (at wavenumber k = 1; forcing is at k = 5). This large-scale field is nearly force free and exceeds the equipartition value. As the magnetic Reynolds number Rm increases, the saturation field strength and the growth rate of the dynamo increase. However, the time it takes to build up the large-scale field from equipartition to its final superequipartition value increases with magnetic Reynolds number. The large-scale field generation can be identified as being due to nonlocal interactions originating from the forcing scale, which is characteristic of the α-Effect. Both α and turbulent magnetic diffusivity ηt are determined simultaneously using numerical experiments where the mean field is modified artificially. Both quantities are quenched in an Rm-dependent fashion. The evolution of the energy of the mean field matches that predicted by an α2 dynamo model with similar α and ηt quenchings. For this model an analytic solution is given that matches the results of the simulations. The simulations are numerically robust in that the shape of the spectrum at large scales is unchanged when changing the resolution from 303 to 1203 mesh points, or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity) from 1 to 100. Increasing the forcing wavenumber to 30 (i.e., increasing the scale separation) makes the inverse Cascade Effect more pronounced, although it remains otherwise qualitatively unchanged.
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the inverse Cascade and nonlinear alpha Effect in simulations of isotropic helical hydromagnetic turbulence
arXiv: Astrophysics, 2000Co-Authors: Axel BrandenburgAbstract:A numerical model of isotropic homogeneous turbulence with helical forcing is investigated. The resulting flow, which is essentially the prototype of the alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with an additional large scale field on the scale of the box (at wavenumber k=1; forcing is at k=5). This large scale field is nearly force-free and exceeds the equipartition value. As the magnetic Reynolds number R_m increases, the saturation field strength and the growth rate of the dynamo increase. However, the time it takes to built up the large scale field from equipartition to its final super-equipartition value increases with magnetic Reynolds number. The large scale field generation can be identified as being due to nonlocal interactions originating from the forcing scale, which is characteristic of the alpha-Effect. Both alpha and turbulent magnetic diffusivity eta_t are determined simultaneously using numerical experiments where the mean-field is modified artificially. Both quantities are quenched in a R_m-dependent fashion. The evolution of the energy of the mean field matches that predicted by an alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an analytic solution is given which matches the results of the simulations. The simulations are numerically robust in that the shape of the spectrum at large scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints, or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity) from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the scale separation) makes the inverse Cascade Effect more pronounced, although it remains otherwise qualitatively unchanged.
Xinxing He - One of the best experts on this subject based on the ideXlab platform.
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functional redundancy dampens the trophic Cascade Effect of a web building spider in a tropical forest floor
Soil Biology & Biochemistry, 2016Co-Authors: Jocelyn E Behm, Xinxing He, Shenglei Fu, Jing Hu, Jin Chen, Douglas SchaeferAbstract:Abstract The trophic Cascade Effect of predators on ecosystem functioning is generally believed to be less frequent and weaker in detritus-based than primary producer-based food webs, in part because of functional redundancy among soil fauna. Despite this view, no empirical studies have explicitly examined roles of different soil fauna within trophic levels in mediating cascading Effects of predators in detritus food webs. Here we manipulated the density of a dominant funnel-web building spider Macrothele yunnanica in permanent plots (1 m 2 ) for one year. Three spider treatments were applied: 0 spiders, 6 spiders (natural density) and 10 spiders (high density). We found that although changes in spider densities caused large shifts in litter-dwelling Collembola community composition on average, modifying spider densities did not generate a trophic Cascade Effect and alter litter decomposition in litter bags with coarse mesh (2 mm). Our data supports the hypothesis that functional redundancy among Collembola species may weaken the strength of spider-initiated cascading Effects. Consequently, changes in Collembola diversity occupying the same trophic level may not significantly alter ecosystem function in tropical forest-floor ecosystems.
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trophic Cascade of a web building spider decreases litter decomposition in a tropical forest floor
European Journal of Soil Biology, 2014Co-Authors: Jin Chen, Xinxing He, Jing HuAbstract:Abstract Trophic Cascade Effects on detritus-based food webs of forest floor have the potential to alter ecosystem functioning, but due to the complexity of detrital food web no general pattern of cascading Effects has emerged. The goal of this study was to evaluate trophic Cascade Effects of a dominant funnel-web building spider on leaf-litter decomposition rate and microbial biomass in a tropical forest floor. We manipulated the density of a dominant funnel-web building spider Macrothele yunnanica in permanent plots (1 m 2 ) for one year. Three spider treatments were applied: 0 spider, 6 spiders (natural density) and 10 spiders (high density). We found that the high density of M. yunnanica led to significant negative cascading Effects on litter decomposition rate, which is probably due to decreased density of one dominant collembolan, Entomobrya , in the litter layer. We detected no cascading Effects from the natural density of M. yunnanica , and changes in spider densities had no cascading Effects on microbial biomass. Modifying spider densities can generate a trophic Cascade Effect and alter leaf litter decomposition, with a potential to influence ecosystem function in tropical forest floors. This study suggests that trophic interactions in detritus-based food webs should be considered to better understand soil organic matter dynamics in tropical forests.
Douglas Schaefer - One of the best experts on this subject based on the ideXlab platform.
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functional redundancy dampens the trophic Cascade Effect of a web building spider in a tropical forest floor
Soil Biology & Biochemistry, 2016Co-Authors: Jocelyn E Behm, Xinxing He, Shenglei Fu, Jing Hu, Jin Chen, Douglas SchaeferAbstract:Abstract The trophic Cascade Effect of predators on ecosystem functioning is generally believed to be less frequent and weaker in detritus-based than primary producer-based food webs, in part because of functional redundancy among soil fauna. Despite this view, no empirical studies have explicitly examined roles of different soil fauna within trophic levels in mediating cascading Effects of predators in detritus food webs. Here we manipulated the density of a dominant funnel-web building spider Macrothele yunnanica in permanent plots (1 m 2 ) for one year. Three spider treatments were applied: 0 spiders, 6 spiders (natural density) and 10 spiders (high density). We found that although changes in spider densities caused large shifts in litter-dwelling Collembola community composition on average, modifying spider densities did not generate a trophic Cascade Effect and alter litter decomposition in litter bags with coarse mesh (2 mm). Our data supports the hypothesis that functional redundancy among Collembola species may weaken the strength of spider-initiated cascading Effects. Consequently, changes in Collembola diversity occupying the same trophic level may not significantly alter ecosystem function in tropical forest-floor ecosystems.
Eleanor M. Slade - One of the best experts on this subject based on the ideXlab platform.
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Woodland Recovery after Suppression of Deer: Cascade Effects for Small Mammals, Wood Mice (Apodemus
2013Co-Authors: Bank Voles Glareolus, Emma R Bush, Christina D Buesching, Eleanor M. Slade, David W. MacdonaldAbstract:Over the past century, increases in both density and distribution of deer species in the Northern Hemisphere have resulted in major changes in ground flora and undergrowth vegetation of woodland habitats, and consequentially the animal communities that inhabit them. In this study, we tested whether recovery in the vegetative habitat of a woodland due to Effective deer management (from a peak of 0.4–1.5 to,0.17 deer per ha) had translated to the small mammal community as an example of a higher order Cascade Effect. We compared deer-free exclosures with neighboring open woodland using capture-mark-recapture (CMR) methods to see if the significant difference in bank vole (Myodes glareolus) and wood mouse (Apodemus sylvaticus) numbers between these environments from 2001–2003 persisted in 2010. Using the multi-state Robust Design method in program MARK we found survival and abundance of both voles and mice to be equivalent between the open woodland and the experimental exclosures with no differences in various metrics of population structure (age structure, sex composition, reproductive activity) and individual fitness (weight), although the vole population showed variation both locally and temporally. This suggests that the vegetative habitat- having passed some threshold of complexity due to lowered deer density- has allowed recovery of the small mammal community, although patch dynamics associated with vegetation complexity still remain. We conclude that the response of small mammal communities to environmental disturbance such as intense browsing pressure can be rapidly reversed once the disturbing agent has bee
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woodland recovery after suppression of deer Cascade Effects for small mammals wood mice apodemus sylvaticus and bank voles myodes glareolus
PLOS ONE, 2012Co-Authors: Emma R Bush, Christina D Buesching, Eleanor M. SladeAbstract:Over the past century, increases in both density and distribution of deer species in the Northern Hemisphere have resulted in major changes in ground flora and undergrowth vegetation of woodland habitats, and consequentially the animal communities that inhabit them. In this study, we tested whether recovery in the vegetative habitat of a woodland due to Effective deer management (from a peak of 0.4–1.5 to <0.17 deer per ha) had translated to the small mammal community as an example of a higher order Cascade Effect. We compared deer-free exclosures with neighboring open woodland using capture-mark-recapture (CMR) methods to see if the significant difference in bank vole (Myodes glareolus) and wood mouse (Apodemus sylvaticus) numbers between these environments from 2001–2003 persisted in 2010. Using the multi-state Robust Design method in program MARK we found survival and abundance of both voles and mice to be equivalent between the open woodland and the experimental exclosures with no differences in various metrics of population structure (age structure, sex composition, reproductive activity) and individual fitness (weight), although the vole population showed variation both locally and temporally. This suggests that the vegetative habitat - having passed some threshold of complexity due to lowered deer density - has allowed recovery of the small mammal community, although patch dynamics associated with vegetation complexity still remain. We conclude that the response of small mammal communities to environmental disturbance such as intense browsing pressure can be rapidly reversed once the disturbing agent has been removed and the vegetative habitat is allowed to increase in density and complexity, although we encourage caution, as a source/sink dynamic may emerge between old growth patches and the recently disturbed habitat under harsh conditions.
Jing Hu - One of the best experts on this subject based on the ideXlab platform.
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functional redundancy dampens the trophic Cascade Effect of a web building spider in a tropical forest floor
Soil Biology & Biochemistry, 2016Co-Authors: Jocelyn E Behm, Xinxing He, Shenglei Fu, Jing Hu, Jin Chen, Douglas SchaeferAbstract:Abstract The trophic Cascade Effect of predators on ecosystem functioning is generally believed to be less frequent and weaker in detritus-based than primary producer-based food webs, in part because of functional redundancy among soil fauna. Despite this view, no empirical studies have explicitly examined roles of different soil fauna within trophic levels in mediating cascading Effects of predators in detritus food webs. Here we manipulated the density of a dominant funnel-web building spider Macrothele yunnanica in permanent plots (1 m 2 ) for one year. Three spider treatments were applied: 0 spiders, 6 spiders (natural density) and 10 spiders (high density). We found that although changes in spider densities caused large shifts in litter-dwelling Collembola community composition on average, modifying spider densities did not generate a trophic Cascade Effect and alter litter decomposition in litter bags with coarse mesh (2 mm). Our data supports the hypothesis that functional redundancy among Collembola species may weaken the strength of spider-initiated cascading Effects. Consequently, changes in Collembola diversity occupying the same trophic level may not significantly alter ecosystem function in tropical forest-floor ecosystems.
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trophic Cascade of a web building spider decreases litter decomposition in a tropical forest floor
European Journal of Soil Biology, 2014Co-Authors: Jin Chen, Xinxing He, Jing HuAbstract:Abstract Trophic Cascade Effects on detritus-based food webs of forest floor have the potential to alter ecosystem functioning, but due to the complexity of detrital food web no general pattern of cascading Effects has emerged. The goal of this study was to evaluate trophic Cascade Effects of a dominant funnel-web building spider on leaf-litter decomposition rate and microbial biomass in a tropical forest floor. We manipulated the density of a dominant funnel-web building spider Macrothele yunnanica in permanent plots (1 m 2 ) for one year. Three spider treatments were applied: 0 spider, 6 spiders (natural density) and 10 spiders (high density). We found that the high density of M. yunnanica led to significant negative cascading Effects on litter decomposition rate, which is probably due to decreased density of one dominant collembolan, Entomobrya , in the litter layer. We detected no cascading Effects from the natural density of M. yunnanica , and changes in spider densities had no cascading Effects on microbial biomass. Modifying spider densities can generate a trophic Cascade Effect and alter leaf litter decomposition, with a potential to influence ecosystem function in tropical forest floors. This study suggests that trophic interactions in detritus-based food webs should be considered to better understand soil organic matter dynamics in tropical forests.