Fluctuations In Growth Rate Research Articles

Extinction conditions for isolated populations affected by environmental stochasticity

We determine the critical patch size below which extinction occurs for populations living in one-dimensional habitats surrounded by completely hostile environments in the presence of environmental fluctuations. The population dynamics is reformulated in terms of a stochastic reaction–diffusion equation and is reduced to a deterministic equation that incorporates the systematic contributions of the noise. We obtain bifurcation diagrams and relations for the mean population density at the stationary state, the critical patch size, and the mean number of individuals in the habitat. The effect of the noise differs, depending on whether it affects the net growth rate or the intraspecific competition term. Fluctuations in the net growth rate decrease the critical patch size, whereas fluctuations in the competition term do not change the critical patch size. We compare our analytical results with numerical solutions of the stochastic partial differential equations and show that our procedure proves useful in dealing with reaction–diffusion equations with multiplicative noise.

Dynamics of Microbial Growth and Coexistence on Variably Saturated Rough Surfaces

The high degree of microbial diversity found in soils is attributed to the highly heterogeneous pore space and the dynamic aqueous microenvironments. Previous studies have shown that spatial and temporal variations in aqueous diffusion pathways play an important role in shaping microbial habitats and biological activity in unsaturated porous media. A new modeling framework was developed for the quantitative description of diffusion-dominated microbial interactions focusing on competitive growth of two microbial species inhabiting partially saturated rough surfaces. Surface heterogeneity was represented by patches with different porosities and water retention properties, yielding heterogeneous distribution of water contents that varies with changes in relative humidity or soil matric potential. Nutrient diffusion and microbial growth on the variably hydrated and heterogeneous surface was modeled using a hybrid method that combines a reaction diffusion method for nutrient field with individual based model for microbial growth and expansion. The model elucidated the effects of hydration dynamics and heterogeneity on nutrient fluxes and mobility affecting microbial population growth, expansion, and coexistence at the microscale. In contrast with single species dominance under wet conditions, results demonstrated prolonged coexistence of two competing species under drier conditions where nutrient diffusion and microbial movement were both limited. The uneven distribution of resources and diffusion pathways in heterogeneous surfaces highlighted the importance of position in the landscape for survival that may compensate for competitive disadvantages conferred by physiological traits. Increased motility was beneficial for expansion and survival. Temporal variations in hydration conditions resulted in fluctuations in microbial growth rate and population size. Population growth dynamics of the dominant species under wet-dry cycles were similar to growth under average value of diffusion coefficients for dry and wet conditions, respectively, suggesting that the time-averaged diffusion coefficient could serve as a useful indicator for estimation of microbial activities in a highly dynamic system such as that found in soils.

Crystal growth rate dispersion modeling using morphological population balance

AbstractCrystal growth in solution is a surface‐controlled process. The variation of growth rates of different crystal faces is considered to be due to the molecular arrangement in the crystal unit cell as well as the crystal surface structures of different faces. As a result, for some crystals, the growth rate for a specific facet is not only a function of supersaturation, but also dependent on some other factors such as its size and the lattice spread angle. This phenomenon of growth rate dispersion (GRD) or fluctuation has been described in literature to have attributed to the formation of some interesting and sophisticated crystal structures observed in experimental studies. In this article, GRD is introduced to a recently proposed morphological population balance model to simulate the dynamic evolution of crystal size distribution in each face direction for the crystallization of potash alum, a chemical that has been reported to show GRD phenomenon and sophisticated crystal structures. The GRD is modeled as a function of the effective relative supersaturation, which is directly related to crystal size, lattice spread angle, relative supersaturation, and solution temperature. The predicted results clearly demonstrated the significant effect of GRD on the shape evolution of the crystals. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Fatigue Crack Growth Rate Analysis in a Titanium Alloy

Reliable prediction of fatigue crack growth rates in aerospace materials and components underpins the so-called defect-tolerant approach to lifing. In this methodology the presence or appearance of defects and cracks in components is accepted. However, safe operation is guaranteed by regular inspections and health monitoring, and ensuring (by means of reliable modelling) that no crack may grow far enough to reach the critical size in the interval between inspections. Under such circumstances it is clear that particular attention has to be paid to the development and validation of predictive modelling capabilities for fatigue crack propagation. The situation is complicated by the fact that it is often a challenge to represent correctly the in-service loading experienced by a cracked component. In practice, on top of the major cycles associated with each flight (LCF component), cycles of higher frequency and lower amplitude are also present (HCF component). Sensitivity to dwell at maximum load is also often observed. Furthermore, it is well established that complex load sequences involving overloads and underloads result in fluctuations of fatigue crack growth rates (retardation and acceleration) that must be accounted for in crack growth calculations. In the present study we consider the application of an approach due to Noroozi et al. [1] to the analysis of R-ratio effects in Ti-6Al-4V material, on the basis of the experimental crack growth rate data collected under the auspices of AGARD programme [2]. The approach shows promising results, and has the capacity to capture loading sequence effects.

Comparison of Thoroughbred Growth Data to Body Weights Predicted by the NRC

The National Research Council (NRC) recently published a method for calculating expected body weight (BW) in growing horses. The objective of this study was to compare foal BW data obtained from commercial Thoroughbred (TB) farms with BW estimates obtained using the 2007 NRC equation. A total of 9,589 observations on 885 growing TB horses were obtained. Data were sorted by age, and a subset was used for statistical analysis. Comparisons between actual BW and 2007 NRC-predicted BW at each month (0–18 months of age) were made using paired t-tests. The 2007 NRC closely predicted BW of growing TB horses at birth and between 6 and 9 months of age (within 3 kg). However, the equation underestimated BW between 1 and 5 months of age (by as much as 10 kg) and overestimated BW between 10 and 13 months of age (by as much as 9 kg). The 2007 NRC also underestimated BW by as much as 9 kg between 15 and 16 months of age. In the current study, the 2007 NRC equation did not precisely predict BW of horses at all ages. The 2007 NRC equation predicts that daily gain will decrease over time, whereas foals in the current study had an increase in daily gain between 12 and 14 months of age. Foals in the current study were raised in pasture-based management systems in which variations in forage availability and quality could have resulted in fluctuations in growth rate.

Establishment of shell growth analysis technique of juvenile Manila clam Ruditapes philippinarum: semidiurnal shell increment formation

The conventional acetate peel method was modified to analyze the shell growth pattern of juvenile Manila clam Ruditapes philippinarum as small as 2 mm in shell length (SL). In the outer shell layer along the axis of maximum growth, two types of growth increments were observed: distinct increments and indistinct increments, which, respectively, do and do not continue to the middle shell layer. The distinct increments were found to be formed every two days in intertidal and shallow subtidal zones by field enclosure experiments of juveniles with datum points marked with alizarin complexone. Growth patterns of juveniles (12 mm SL) collected from the Seaside Park of Yokohama in Tokyo Bay were analyzed to confirm the modified method. Mean daily shell growth rate from April to July 2005 ranged 120–142 μm/day, which was reasonable as compared with previous studies. It was impossible to backcalculate the growth to the settlement size (i.e. 0.2 mm SL) because of erosion of the outer shell surface, and the smallest backcalculated minimum shell length was 0.8 mm. Fluctuations in daily growth rate were high, ranging 29–315 μm/day, and did not show a clear two-weekly rhythm.

Effects of crystal rotation rate on the melt–crystal interface of a CZ-Si crystal growth in a transverse magnetic field

A series of computations were carried out to study the effect of crystal rotation rate on the melt–crystal interface shape and temperature gradient at the interface during CZ-Si crystal growth in a transverse magnetic field (TMCZ). A three-dimensional (3D) global model was used in this study. It was found that the interface deflection changes from non-uniformity in the azimuthal direction to an axisymmetric distribution with increasing crystal rotation rate. The mechanism of this effect is mainly attributed to the spatial fluctuations of local growth rate, which is derived as a function of crystal rotation rate and non-uniformity of interface deflection in the azimuthal direction. It contributes to the formation of the shape of the melt–crystal interface through the heat release of solidification at the melt–crystal interface. Even though the melt–crystal interface shape is nearly axisymmetric at a high crystal rotation rate, local growth rate fluctuations are still noticeable and play an important role in the characteristics of heat transfer and impurity segregation at the melt–crystal interface.

Growth rate of selected woody species in northern Maputaland, KwaZulu–Natal, South Africa

Annual stem circumference growth rates of 23 woody species of the Sand Forest and woodland in Maputaland are presented for the first time. The rare Sand Forest, has been identified as an endemic, diverse vegetation type that is under threat from land transformation and human utilisation outside conservation areas. The growth rate of the selected woody species was evaluated over a 71-month survey that encompassed climatic extremes, oscillating from heavy rainfall to drought in a short period of time. The mean annual stem circumference growth rate among the investigated species varied thirteen fold, from a low of 2.04 mm/yr for Brachylaena huillensis to 26.28 mm/yr for Garcinia livingstonei. When the data of all species were considered, there was a significant positive relationship between stem circumference and growth rate, but no significant relationship between wood density and growth rate. The present results suggest that there is no significant difference in terms of mean annual growth rate between the Sand Forest and woodland vegetation types. In general, woodland species showed larger fluctuations in growth rate and the temporal pattern of Sand Forest species seemed to lag behind that of the woodland species. These trends can in part be ascribed to the woodland species all being deciduous, whereas the Sand Forest suite included both deciduous and evergreen species. Annual growth rates measured from dry season to dry season were not related to the seasonal rainfall pattern, but appeared to be highly dependent on the water availability at the time of the enumeration. The larger size classes reacted sooner to decreased water availability and a reduction in growth of larger trees was accompanied by an increase in growth in the smaller size classes.

Desymmetrization of the polyhedral crystal shape of tetragonal lysozyme due to face-growth rate fluctuations

We have studied the desymmetrization of the polyhedral crystalline shape of tetragonal lysozyme crystals due to the growth rate differences of the equivalent {1 0 1} planes. Using atomic force microscopy, we have observed the evolution of the multifaceted structures composed of four equivalent {1 0 1} faces during growth. In our growth condition, lateral step flow, where a large density of dislocations acts as a source of steps, is the dominant growth mechanism. The measured step flow velocities are almost independent of the separation between the neighboring steps, revealing that the local face normal growth rate is determined by the local step density. By tracing the motion of the vertex surrounded by the {1 0 1} faces, we have found that the desymmetrization of the crystalline shape is due to the large fluctuation of the local face normal growth rate, which is comparable in magnitude to the average growth rate.

Optimal growth temperature hypothesis: Why do anchovy flourish and sardine collapse or vice versa under the same ocean regime?

The out-of-phase population oscillations between anchovy and sardine have been attributed to climate changes. However, the biological processes causing these species alternations have remained unresolved. Here we propose a simple "optimal growth temperature" hypothesis, in which anchovy and sardine regime shifts are caused by differential optimal temperatures for growth rates during the early life stages. Dome-shaped relationships between growth rate and sea temperature were detected for both Japanese anchovy (Engraulis japonicus) and Japanese sardine (Sardinops melanostictus) larvae based on otolith microstructure analysis. The optimal growth rate for anchovy larvae occurred at 22.0 °C, whereas that for sardine larvae occurred at 16.2 °C. Ambient temperatures have historically fluctuated between these optima, which could lead to contrasting fluctuations in larval growth rates between the two species. This simple mechanism could potentially cause the shifts between the warm anchovy regime and the cool sardine regime in the western North Pacific. Although retrospective analysis suggested synergistic effects of other factors (e.g., trophic interactions and fishing), the optimal growth temperature concept would provide a possible biological mechanism of anchovy and sardine regime shifts.

Modulation of the growth of auroral kilometric radiation by electromagnetic ion cyclotron waves

The role of electromagnetic ion cyclotron (EMIC) waves in the stimulation of auroral kilometric radiation is addressed in this study. Observations of the modulation of electron flux and energy due to EMIC/electrostatic ion cyclotron waves within the auroral kilometric radiation (AKR) source region have been reported in the past. These modulations produce in turn oscillations of the electron phase space density and therefore lead to fluctuations of AKR growth rate that may translate along the magnetic field line at the group velocity of the stimulating waves. For a model electron “horseshoe” distribution function, we use the theory of the cyclotron maser instability to investigate how this process might influence the AKR wave spectrum and fine structure.

Pre-natal exposures to cocaine and alcohol and physical growth patterns to age 8 years

Two hundred and two primarily African American/Caribbean children (classified by maternal report and infant meconium as 38 heavier, 74 lighter and 89 not cocaine-exposed) were measured repeatedly from birth to age 8 years to assess whether there is an independent effect of pre-natal cocaine exposure on physical growth patterns. Children with fetal alcohol syndrome identifiable at birth were excluded. At birth, cocaine and alcohol exposures were significantly and independently associated with lower weight, length and head circumference in cross-sectional multiple regression analyses. The relationship over time of pre-natal exposures to weight, height, and head circumference was then examined by multiple linear regression using mixed linear models including covariates: child's gestational age, gender, ethnicity, age at assessment, current caregiver, birth mother's use of alcohol, marijuana and tobacco during the pregnancy and pre-pregnancy weight (for child's weight) and height (for child's height and head circumference). The cocaine effects did not persist beyond infancy in piecewise linear mixed models, but a significant and independent negative effect of pre-natal alcohol exposure persisted for weight, height, and head circumference. Catch-up growth in cocaine-exposed infants occurred primarily by 6 months of age for all growth parameters, with some small fluctuations in growth rates in the preschool age range but no detectable differences between heavier versus unexposed nor lighter versus unexposed thereafter.

OS2-2-7 Application of statistical interferometry to monitor biological activity of plant under environmental stress

In this study, a novel optical interferometric technique called 'statistical interferometry'^<1-3> has been developed. In contrast to the conventional interferometry where the phase is determined in a completely deterministic way, we consider the interference of completely random wave fronts, i.e., speckle fields, and it has been proved that the complete randomness of the speckle field can play the role of a standard phase in a statistical sense. The advantage of the method is that since the phase of the object under testing can be derived in a statistical way, the accuracy of the measurement depends only on the number of data taken to calculate a probability density distribution of speckle phase. This feature permits a simple optical system to achieve measurements with an extremely high accuracy. According to a computer simulation, the accuracy of λ/1000 can be achieved using 40,000 data of the speckle intensity. Statistical interferometry was applied to monitor biological activity or growth rate of plant, aiming to investigate the influence of the environmental pollutions. In the experiments, the plants were exposed to ozone that is the main substance of photochemical oxidant, and the growth rates were measured before and after the exposure. It was clearly observed that the fluctuation of growth rate as well as its mean rate was dramatically affected by the exposure of ozone. By the observation of growth rate of plant with the accuracy of sub-nanometer scale and a time scale of second, it was newly revealed that the fluctuation of the growth rate reflects the biological activity of the plant.

Demographic consequences of population subdivision on the long-furred woolly mouse opossum (Micoureus paraguayanus) from the Atlantic Forest

Habitat destruction and fragmentation severely affected the Atlantic Forest. Formerly contiguous populations may become subdivided into a larger number of smaller populations, threatening their long-term persistence. The computer package VORTEX was used to simulate the consequences of habitat fragmentation and population subdivision on Micoureus paraguayanus, an endemic arboreal marsupial of the Atlantic Forest. Scenarios simulated hypothetical populations of 100 and 2000 animals being partitioned into 1–10 populations, linked by varying rates of inter-patch dispersal, and also evaluated male-biased dispersal. Results demonstrated that a single population was more stable than an ensemble of populations of equal size, irrespective of dispersal rate. Small populations (10–20 individuals) exhibited high instability due to demographic stochasticity, and were characterized by high rates of extinction, smaller values for metapopulation growth and larger fluctuations in population size and growth rate. Dispersal effects on metapopulation persistence were related to the size of the populations and to the sexes that were capable of dispersing. Male-biased dispersal had no noticeable effects on metapopulation extinction dynamics, whereas scenarios modelling dispersal by both sexes positively affected metapopulation dynamics through higher growth rates, smaller fluctuations in growth rate, larger final metapopulation sizes and lower probabilities of extinction. The present study highlights the complex relationships between metapopulation size, population subdivision, habitat fragmentation, rate of inter-patch dispersal and sex-biased dispersal and indicates the importance of gaining a better understanding of dispersal and its interactions with correlations between disturbance events.

Modeling ice growth on Canadian lakes using artificial neural networks

This paper presents artificial neural network (ANN) models designed to predict ice in Canadian lakes and reservoirs during the early winter ice thickness growth period. The models fit ice thickness measurements at one or more monitored lakes and predict ice thickness during the growth period either at the same locations for dates without measurements (local ANN models) or at any site in the region (regional ANN model), provided that the required meteorological input variables are available. The input variables were selected after preliminary assessments and were adapted from time series of daily mean air temperature, rainfall, cloud cover, solar radiation, and average snow depth. The results of the ANN models compared well with those of the deterministic physics‐driven Canadian Lake Ice Model (CLIMO) in terms of root‐mean‐square error and in terms of relative root‐mean‐square errors. The ANN models predictions were also marginally more precise than a revised version of Stefan's law (RSL), presented herein. They reproduced some intrawinter and interannual growth rate fluctuations that were not accounted for by RSL. The performance of the models results in good part from a careful choice of input variables, inspired from the work on deterministic models such as CLIMO. ANN models of ice thickness show good potential for the use in contexts where ad hoc adjustments are desirable because of the limited availability of measurements and where poor data nature, availability, and quality precludes using deterministic physics‐driven models.

Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane

During the past 15 years the annual growth rate of tropospheric methane (CH4) has shown striking changes over 2–3 year periods, varying from +1% yr−1 to slightly negative values (−0.2% yr−1). These fluctuations are superimposed on an overall slowdown of the CH4 growth rate since the 1980s. Here we use our complementary measurement of other compounds (ethane, tetrachloroethene) to confirm the influence of biomass burning on large global CH4 pulses in 1998 and 2002–2003. Methane growth rate fluctuations also track ENSO indices, most likely via the influence of ENSO activity on large‐scale biomass burning. We also report the seventh year of near‐zero growth of global CH4 levels (Dec. 1998–Dec. 2005). The global CH4 mixing ratio was 1772 ± 1 ppbv in 2005, and CH4 increases of 118–376 ppbv between 2000–2020 (ten scenarios in the 2001 IPCC report, to levels around 1900+ ppbv by 2020, now appear quite unlikely.

Population dynamics of the Mongolian gerbils: Seasonal patterns and interactions among density, reproduction and climate

The Mongolian gerbil ( Meriones unguiculatus) is one of key rodents widely distributed in semi-arid, typical steppes, and desert grasslands in Inner Mongolia, China. We studied population dynamics of Mongolian gerbils under semi-natural conditions using monthly live trapping from 2001 to 2004 in south-central Inner Mongolia. Mongolian gerbils displayed seasonal fluctuations of density and population growth rate. Reproduction and recruitment of gerbils occurred primarily from March–August with a breeding lull in autumn. Population growth rates of Mongolian gerbils were not related to population density but were negatively related to temperature and precipitation. Enhanced reproductive performance and success of females increased population growth of gerbils in our enclosure. We also found that increased temperature and precipitation during the plant growing season negatively affected recruitment and rate of pregnancy. Mongolian gerbils prefer habitats with short, sparse vegetation and dry, loose and sandy soil. Increases in temperature and rainfall enhance vegetation growth; consequently, tall, dense and moist vegetation might reduce the suitability of habitats and retard population growth of Mongolian gerbils. Pronounced seasonal climatic fluctuations in northern latitudes may be the main cause of seasonal population dynamics of Mongolian gerbils.

Spatiotemporal fluctuation-induced transition in a tumor model with immune surveillance

We report on a simple model of spatially extended antitumor system with a fluctuation in growth rate, which can undergo a nonequilibrium phase transition. Three states as excited, subexcited and nonexcited states of a tumor are defined to describe its growth. The multiplicative noise is found to have opposite effects: The positive effect on a nonexcited tumor and the negative effect on an excited tumor.

The Japanese Economy

The Japanese economic behavior is modeled. GDP evolution is represented as a sum two components: economic tend and fluctuations. The trend is an inverse function of GDP per capita with a constant numerator. The growth rate fluctuations are numerically equal to two thirds of the relative change in the number of eighteen-year-olds. Inflation is represented by a linear function of labor force change rate. The models provide an accurate description for the poor economic performance and deflation separately. Using the models, GDP per capita is predicted for the next ten years and recommendations are given to overcome deflation.

The Effects of Mortgage Prepayments on M2

Mortgage prepayments can contribute significantly to fluctuations in M2 growth rates. These mortgage prepayment effects are primarily driven by certain rules of mortgage-backed-security (MBS) insurers that require mortgage servicers to hold in M2-type deposits the prepayment proceeds due to MBS investors. This paper provides a methodology for estimating prepayment effects on M2. The effects are estimated separately for refinancing and home sales. The results indicate that excluding the mortgage prepayment effects from M2 produces smoother monthly growth rates. The stability of the relationship between money and GDP as measured by M2 velocity is also increased. Refinancing prepayments account for most of the prepayment effects on M2.