scholarly journals Patterns in sets of positive density in trees and affine buildings

2021 ◽  
Author(s):  
Michael Björklund ◽  
Alexander Fish ◽  
James Parkinson
Keyword(s):  
Positive Density ◽  
Affine Buildings

The preserved plume conduits of the Caribbean Large Igneous Plateau and their relation with the Galápagos hotspot back to 90 Ma

2021 ◽  
Author(s):  
Angela Maria Gomez Garcia ◽  
Eline Le Breton ◽  
Magdalena Scheck-Wenderoth ◽  
Gaspar Monsalve ◽  
Denis Anikiev
Keyword(s):  
Reference Frames ◽  
Thermal Anomaly ◽  
Caribbean Region ◽  
Positive Density ◽  
South American ◽  
Kinematic Models ◽  
Tomographic Data ◽  
The Caribbean ◽  
Moho Depths ◽  
South American Continent

<p>Remnants of the Caribbean Large Igneous Plateau (C-LIP) are found as thickened zones of oceanic crust in the Caribbean Sea, that formed during strong pulses of magmatic activity around 90 Ma. Previous studies have proposed the Galápagos hotspot as the origin of the thermal anomaly responsible for the development of this igneous province. Particularly, geochemical signature relates accreted C-LIP fragments along northern South America with the well-known hotspot material.</p><p>In this research, we use 3D lithospheric-scale structural and density models of the Caribbean region, in which up-to-date geophysical datasets (i.e.: tomographic data, Moho depths, sedimentary thickness, and bathymetry) have been integrated. Based on the gravity residuals (modelled minus observed EIGEN6C-4 dataset), we reconstruct density heterogeneities both in the crust and the uppermost oceanic mantle (< 50km).</p><p>Our results suggest the presence of two positive mantle density anomalies in the Colombian and the Venezuelan basins, interpreted as the preserved plume material which migrated together with the Proto-Caribbean plate from the east Pacific. Such bodies have never been identified before, but a positive density trend is also observed in the mantle tomography, at least down to 75 km depth.</p><p>Using recently published regional plate kinematic models and absolute reference frames, we test the hypothesis of the C-LIP origin in the Galápagos hotspot. However, misfits of up to ~3000 km between the present hotspot location and the mantle anomalies, reconstructed back to 90 Ma, is observed, as other authors reported in the past.</p><p>Therefore, we discuss possible sources of error responsible for this offset and pose two possible interpretations: 1. The Galápagos hotspot migrated (~1200-3000 km) westward while the Proto-Caribbean moved to the northeast, or 2. The C-LIP was formed by a different plume, which – if considered fixed - would be nowadays located below the South American continent.</p>

scholarly journals A cryptic Allee effect: spatial contexts mask an existing fitness–density relationship

2015 ◽  
Vol 2 (6) ◽  
pp. 150034 ◽  
Author(s):  
Akira Terui ◽  
Yusuke Miyazaki ◽  
Akira Yoshioka ◽  
Shin-ichiro S. Matsuzaki
Keyword(s):  
Allee Effect ◽  
Local Level ◽  
Freshwater Mussel ◽  
Fertilization Rate ◽  
Positive Density ◽  
Allee Effects ◽  
Flow Conditions ◽  
Conspecific Density ◽  
Sperm Density ◽  
Density Relationship

Current theories predict that Allee effects should be widespread in nature, but there is little consistency in empirical findings. We hypothesized that this gap can arise from ignoring spatial contexts (i.e. spatial scale and heterogeneity) that potentially mask an existing fitness–density relationship: a ‘cryptic’ Allee effect. To test this hypothesis, we analysed how spatial contexts interacted with conspecific density to influence the fertilization rate of the freshwater mussel Margaritifera laevis . This sessile organism has a simple fertilization process whereby females filter sperm from the water column; this system enabled us to readily assess the interaction between conspecific density and spatial heterogeneity (e.g. flow conditions) at multiple spatial levels. Our findings were twofold. First, positive density-dependence in fertilization was undetectable at a population scale (approx. less than 50.5 m 2 ), probably reflecting the exponential decay of sperm density with distance from the sperm source. Second, the Allee effect was confirmed at a local level (0.25 m 2 ), but only when certain flow conditions were met (slow current velocity and shallow water depth). These results suggest that spatial contexts can mask existing Allee effects.

scholarly journals Variability of the Atlantic Meridional Overturning Circulation in CCSM4

2012 ◽  
Vol 25 (15) ◽  
pp. 5153-5172 ◽  
Author(s):  
Gokhan Danabasoglu ◽  
Steve G. Yeager ◽  
Young-Oh Kwon ◽  
Joseph J. Tribbia ◽  
Adam S. Phillips ◽  
...  
Keyword(s):  
Low Frequency ◽  
Surface Flux ◽  
Atlantic Meridional Overturning Circulation ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Positive Density ◽  
Overturning Circulation ◽  
Climate System Model ◽  
Control Simulation ◽  
Meridional Overturning

Abstract Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

Population growth and density dependence

2019 ◽  
pp. 63-80
Author(s):  
Gary G. Mittelbach ◽  
Brian J. McGill
Keyword(s):  
Population Growth ◽  
Density Dependence ◽  
Growth Model ◽  
Species Interactions ◽  
Single Species ◽  
Positive Density ◽  
Allee Effects ◽  
Species Population ◽  
Exponential Growth Model ◽  
Single Species Population

This chapter reviews the basic mathematics of population growth as described by the exponential growth model and the logistic growth model. These simple models of population growth provide a foundation for the development of more complex models of species interactions covered in later chapters on predation, competition, and mutualism. The second half of the chapter examines the important topic of density-dependence and its role in population regulation. The preponderance of evidence for negative density-dependence in nature is reviewed, along with examples of positive density dependence (Allee effects). The study of density dependence in single-species populations leads naturally to the concept of community-level regulation, the idea that species richness or the total abundance of individuals in a community may be regulated just like abundance in a single-species population. The chapter concludes with a look at the evidence for community regulation in nature and a discussion of its importance.

scholarly journals Resolving the Paradox of Clumped Seed Dispersal: Positive Density and Distance Dependence in a Bat-Dispersed Species

2019 ◽  
Vol 100 (1) ◽  
pp. e01469
Author(s):  
Anna Sugiyama ◽  
Liza S. Comita ◽  
Takashi Masaki ◽  
Richard Condit ◽  
Stephen P. Hubbell
Keyword(s):  
Seed Dispersal ◽  
Positive Density ◽  
Distance Dependence

scholarly journals Hybrid Flux Method in Monotonicity-Preserving Scheme for Accurate and Robust Simulation in Supersonic Flow

2019 ◽  
Vol 2019 ◽  
pp. 1-19
Author(s):  
Myeong-Hwan Ahn ◽  
Duck-Joo Lee
Keyword(s):  
Blow Up ◽  
Supersonic Jet ◽  
Positive Density ◽  
Computational Domain ◽  
Flux Method ◽  
Present Scheme ◽  
Local Extrema ◽  
Volume Method ◽  
Fifth Order ◽  
Monotonicity Preserving

The fifth-order monotonicity-preserving (MP5) scheme is an accurate and low dissipative numerical method. As a finite-volume method, MP5 adopts the Roe-flux scheme for solving the numerical flux in the compressible Euler equation. However, due to the deficiency of the MP limiter and Roe-flux in maintaining positive density and pressure, the calculation could fail in cases of extreme flow involving small values of density and pressure. In this study, to overcome such a limitation but still to achieve a high-accuracy of MP5, we propose a hybrid flux method: the Roe-flux is used in the global computational domain, but the first-order Lax-Friedrich (LF)-flux is adopted only for trouble grids. The numerical results of shock-tube and complicated interaction problems indicate that the present scheme is more accurate at discontinuities and local extrema compared to the previous scheme, maintaining positive density and pressure values. For two-dimensional applications, a supersonic jet is explored with different Mach numbers and temperature conditions. As a result, small vortices induced by the shear layer can be clearly captured by the proposed scheme. Furthermore, a simulation was successfully conducted without blow-up of calculation even in the extreme jet flow condition.

scholarly journals On exact sampling of stochastic perpetuities

2011 ◽  
Vol 48 (A) ◽  
pp. 165-182 ◽  
Author(s):  
Jose H. Blanchet ◽  
Karl Sigman
Keyword(s):  
Fixed Point ◽  
Net Present Value ◽  
Positive Density ◽  
Simulation Algorithm ◽  
Present Value ◽  
The Past ◽  
Coupling From The Past ◽  
Fixed Point Equation ◽  
Point Equation ◽  
The Right

A stochastic perpetuity takes the formD∞=∑n=0∞exp(Y1+⋯+Yn)Bn, whereYn:n≥0) and (Bn:n≥0) are two independent sequences of independent and identically distributed random variables (RVs). This is an expression for the stationary distribution of the Markov chain defined recursively byDn+1=AnDn+Bn,n≥0, whereAn=eYn;D∞then satisfies the stochastic fixed-point equationD∞D̳AD∞+B, whereAandBare independent copies of theAnandBn(and independent ofD∞on the right-hand side). In our framework, the quantityBn, which represents a random reward at timen, is assumed to be positive, unbounded with EBnp<∞ for somep>0, and have a suitably regular continuous positive density. The quantityYnis assumed to be light tailed and represents a discount rate from timenton-1. The RVD∞then represents the net present value, in a stochastic economic environment, of an infinite stream of stochastic rewards. We provide an exact simulation algorithm for generating samples ofD∞. Our method is a variation ofdominated coupling from the pastand it involves constructing a sequence of dominating processes.

Sign in / Sign up

Export Citation Format

Share Document