Dependence Of Total Density Research Articles

Density dependence in an age-structured population of great tits: identifying the critical age classes.

Classical approaches for the analyses of density dependence assume that all the individuals in a population equally respond and equally contribute to density dependence. However, in age-structured populations, individuals of different ages may differ in their responses to changes in population size and how they contribute to density dependence affecting the growth rate of the whole population. Here we apply the concept of critical age classes, i.e., a specific scalar function that describes how one or a combination of several age classes affect the demographic rates negatively, in order to examine how total density dependence acting on the population growth rate depends on the age-specific population sizes. In a 38-yr dataset of an age-structured great tit (Parus major) population, we find that the age classes, including the youngest breeding females, were the critical age classes for density regulation. These age classes correspond to new breeders that attempt to take a territory and that have the strongest competitive effect on other breeding females. They strongly affected population growth rate and reduced recruitment and survival rates of all breeding females. We also show that depending on their age class, females may differently respond to varying density. In particular, the negative effect of the number of breeding females was stronger on recruitment rate of the youngest breeding females. These findings question the classical assumptions that all the individuals of a population can be treated as having an equal contribution to density regulation and that the effect of the number of individuals is age independent. Our results improve our understanding of density regulation in natural populations.

Electronic transport across a layered structure of Fe/ $$\upbeta $$ β -poly vinylidene fluoride/Fe using DFT calculations

Quantum electronic transport across a $$\upbeta $$ β -poly(vinylidene fluoride) ( $$\upbeta $$ β -PVDF) ferroelectric barrier structured between two ferromagnetic Fe layers is explored using DFT calculations. The multifunctional junction is organized in capacitor like structure, as FM (ferromagnetic metal)/FE (ferroelectric)/FM to understand the mechanism of electron transfer by controlling the spin polarization of the electrodes and also the ferroelectric polarization of the barrier. These studies are carried on a single bcc layer of Fe atoms in both the electrodes and two monomers of PVDF is utilized as a barrier. We investigated the dependence of total density of states (DOS), projected DOS, transmission coefficient and I---V characteristics on applied bias voltage using SIESTA & TRANSIESTA package.

Tests of density dependence using indices of relative abundance in a deer population

A major question in animal ecology is explaining the causes of population fluctuations. Consensus about the most reliable method to detect density dependence (DD) or environmental effects from time‐series data, however, has not yet been achieved. Times series analyses have been used with indices of relative abundance in numerous studies, although these indices are rarely validated. Here, we used three different time series of relative abundance (number of deer seen per hunter per day, hunting success and proportion of males in the harvest) to explore direct and delayed DD in a white‐tailed deer Odocoileus virginianus population on Anticosti Island, Québec, Canada. Three mathematical approaches were tested: linear models, autoregressive (AR) models, and total DD in life history. Tests of DD using different indices of abundance on the same population should lead to similar results if all indices exhibit similar behaviour. Indices of relative abundance correlated with each other, although sometimes weakly, such that we obtained similar DD estimates with each index using detrended non‐stationary series. In most time series, linear regression of Nt−1 and Nt and AR models did not detect DD, while we obtained strong evidence for DD from the life‐history approach. This meant that contrasting conclusions about the role of density dependence within this population were reached depending on which method was used. We conclude that the method that incorporates most biological realism, the life‐history approach, provided a different result than classical interpretation of autoregressive coefficients. Only the life‐history interpretation supported our a priori belief that density dependence operating through competition for food regulates the Anticosti deer population. Phenomenological analysis aiming to investigate changes in abundance should be carefully conducted as the use of inappropriate indices or methods could lead to inappropriate conclusions or management strategies. Preferably, the method used should match the time scale of the population sampling regime and species life history.

Generation time and temporal scaling of bird population dynamics

Theoretical studies have shown that variation in density regulation strongly influences population dynamics, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds translate into distinct population dynamical patterns that are apparent only on a scale of generations.

Estimating density dependence in time–series of age–structured populations

For a life history with age at maturity alpha, and stochasticity and density dependence in adult recruitment and mortality, we derive a linearized autoregressive equation with time-lags of from 1 to alpha years. Contrary to current interpretations, the coefficients for different time-lags in the autoregressive dynamics do not simply measure delayed density dependence, but also depend on life-history parameters. We define a new measure of total density dependence in a life history, D, as the negative elasticity of population growth rate per generation with respect to change in population size, D = - partial differential lnlambda(T)/partial differential lnN, where lambda is the asymptotic multiplicative growth rate per year, T is the generation time and N is adult population size. We show that D can be estimated from the sum of the autoregression coefficients. We estimated D in populations of six avian species for which life-history data and unusually long time-series of complete population censuses were available. Estimates of D were in the order of 1 or higher, indicating strong, statistically significant density dependence in four of the six species.

Estimating Density Dependence from Population Time Series Using Demographic Theory and Life‐History Data

For populations with a density-dependent life history reproducing at discrete annual intervals, we analyze small or moderate fluctuations in population size around a stable equilibrium, which is applicable to many vertebrate populations. Using a life history having age at maturity alpha, with stochasticity and density dependence in adult recruitment and mortality, we derive a linearized autoregressive equation with time lags from 1 to alpha yr. Contrary to current interpretations, the coefficients corresponding to different time lags in the autoregressive dynamics are not simply measures of delayed density dependence but also depend on life-history parameters. The theory indicates that the total density dependence in a life history, D, should be defined as the negative elasticity of population growth rate per generation with respect to change in population size, [Formula: see text], where lambda is the asymptotic multiplicative growth rate per year, T is the generation time, and N is adult population size. The total density dependence in the life history, D, can be estimated from the sum of the autoregression coefficients. We estimate D in populations of seven vertebrate species for which life-history studies and unusually long time series of complete population censuses are available. Estimates of D were statistically significant and large, on the order of 1 or higher, indicating strong density dependence in five of the seven species. We also show that life history can explain the qualitative features of population autocorrelation functions and power spectra and observations of increasing empirical variance in population size with increasing length of time series.

Estimating Density Dependence from Population Time Series Using Demographic Theory and Life-History Data

For populations with a density‐dependent life history reproducing at discrete annual intervals, we analyze small or moderate fluctuations in population size around a stable equilibrium, which is applicable to many vertebrate populations. Using a life history having age at maturity α, with stochasticity and density dependence in adult recruitment and mortality, we derive a linearized autoregressive equation with time lags from 1 to α yr. Contrary to current interpretations, the coefficients corresponding to different time lags in the autoregressive dynamics are not simply measures of delayed density dependence but also depend on life‐history parameters. The theory indicates that the total density dependence in a life history, D, should be defined as the negative elasticity of population growth rate per generation with respect to change in population size, $$D=-\partial \mathrm{ln}\,\lambda ^{T}/ \partial \mathrm{ln}\,N$$, where λ is the asymptotic multiplicative growth rate per year, T is the generation time, and N is adult population size. The total density dependence in the life history, D, can be estimated from the sum of the autoregression coefficients. We estimate D in populations of seven vertebrate species for which life‐history studies and unusually long time series of complete population censuses are available. Estimates of D were statistically significant and large, on the order of 1 or higher, indicating strong density dependence in five of the seven species. We also show that life history can explain the qualitative features of population autocorrelation functions and power spectra and observations of increasing empirical variance in population size with increasing length of time series.

Direct measurements of the rate coefficients for the reactions of some hydrocarbons with chlorine atoms at high temperatures

The reactions of methane, ethylene, ethane, and propane with chlorine atoms have been studied at hightemperatures by using a laser photolysis-shock tube apparatus coupled with atomic resonance absorption spectroscopy. Chlorine atoms were produced by the laser photolysis of silicon tetrachloride. The overall rate coefficients were experimentally determined from the decay of chlorine atoms as: k(CH4+Cl)=10−9.26±0.26 exp[−(35.7±6.6) kJ mol−1/RT] cm3 molecule−1 s−1 (1100–1550 K) k(C2H4+Cl)=10−9.67±0.16exp[−(35.8±3.9) kJ mol−1/RT] cm3 molecule−1 s−1 (1100–1550 K) k(C2H6+Cl)=10−9.96±0.05 cm3 molecule−1 s−1 (1100–1400 K) k(C3H8+Cl)=10−9.77±0.06 cm3 molecule−1 s−1 (1100–1400 K) where the error limits are given at the two standard deviation level. With systematic errors also considered, the reliabilities, Δ[logk(RH+Cl)], were evaluated to be ±0.2 for reactions CH4+Cl and C2H4+Cl and to be ±0.15 for reactions C2H6+Cl and C3H8+Cl. These results are in good agreement with the extrapolations of the kinetic data measured up to 800 K by Pilgrim et al. To examine the product channels of these reactions, an ab initio molecular orbital calculation was performed at the QCISD(T)/6-311G (d,p)//MP2(full)/6–31G(d) level. The calculation shows that the energetically favorable channels of CH4_Cl, C2H6+Cl, and C3H8+Cl are H abstraction by Cl atoms. In the C2H4+Cl system, the measured rate coefficient has no total density dependence so that the Cl addition to ethylene is negligible in this experimental temperature range, although the addition channel has no energy barrier. Based on transition state theory, the rate coefficients were also calculated and discussed. The reactions of hydrocarbons with Cl atoms are suggested to follow the Evans-Polanyi law.

The replacement series

Summary 1 The replacement series has been used widely to assess interference, niche differentiation, resource utilization, and productivity in simple mixtures of species. Correctly used, the approach can lead to some valid interpretations. In order to avoid criticisms, however, researchers should appreciate the assumptions and limitations of this methodology. 2 A replacement series contains confounded species density treatments. Replacement series experiments therefore provide collective results and cannot distinguish separate contributions to interference by the constituents of a mixture. 3 In a replacement diagram, trends in observed yields per species can be due to a multitude of possible levels of intra‐ and interspecific interference. Similarly, trends in expected yields per species do not represent specific levels of intra‐ and interspecific interference. A replacement series is therefore unsuitable for the quantitative evaluation of interference or niche differentiation. 4 A standard replacement series, but not a proportional replacement series, may be used to detect an imbalance between intra‐ and interspecific interference for a component of a mixture. If competition for resources is assumed or known to be the sole cause of interference, then such experiments yield a qualitative evaluation of complementary resource utilization by the mixture components. 5 The possibility of total density dependence impedes generalization from a replacement series result. Other biases, due to initial size and time of observation, may occur with replacement series and related experimental structures. 6 Results from a replacement series are of questionable value in predicting the long‐term outcome of an association between species. 7 Replacement series are a valid setting, but not the only possible setting, for some kinds of yield comparisons. These include comparing productivity of monocultures with that of simple species mixtures. 8 In some cases, interpretations obtained from replacement series have not been confirmed using other methods but, on the whole, conclusions from replacement series do not seem to be characteristically different from those obtained using other approaches.

A stochastic theory for clustering of quenched-in vacancies-V. Temperature dependence of cluster density

It is known that the density of vacancy loops in quenched aluminium and stacking fault tetrahedra in quenched gold show a saturation for low ageing temperatures. The physical mechanism leading to this effect is not well understood. In this paper we consider a simple model which allows us to obtain the temperature dependence of total density. The analysis shows that the plateau region arises due to the fact that the number of absorption sites of a cluster is larger than the number of emission sites. The temperature dependence of the average number of vacancies in a cluster and the single vacancy concentration in equilibrium with the clusters are discussed.