Significant Density Dependence Research Articles

Resource dynamic plays a key role in regional fluctuations of the spruce bark beetles Ips typographus

Abstract 1 We analysed time series of spruce bark beetles (Nt) caught in pheromone traps from 1979 to 2000 in approximately 100 localities throughout south‐east Norway.2 The analysis was performed at two spatial scales (whole area and 12 subregions), and included additional analyses of the time‐series sequences before and after a large windfelling (1987) that appeared to induce a shift in the dynamics.3 Regression of Rt (logarithmic per‐capita growth rate) on Nt − d indicated that the endogenous dynamics were dominated by lag 1 density dependence. This was particularly so in the sequence after the large windfelling (R2 = 0.79), probably due to intraspecific competition for breeding substrate.4 The sequence before the windfelling (1979–1987) was declining without significant density dependence. We suggest this to be a transition period, reflecting a drop in carrying capacity due to depletion of susceptible trees during the preceding outbreak period (1970s) and a drop in beetle number to below the density required to kill trees.5 Windfelling (Wt − 1) appears to be an important predictor of the dynamics; however, it was difficult to evaluate the statistical significance of this factor due to only one large windfall event.6 The weak influence of drought stress (Dt − 1) could be due to the absence of severe drought periods within the time series.

Development of a Regional Stock–Recruitment Model for Understanding Factors Affecting Walleye Recruitment in Northern Wisconsin Lakes

Abstract We used data from 162 lakes in northern Wisconsin during 1990–1999 to develop a stock–recruitment model for walleye Stizostedion vitreum and to identify important factors affecting recruitment. We used the linear version of the Ricker stock–recruit model and regression tree analysis to model age-0 walleye density, which ranged from 0.24 to 505 per km of shoreline (mean = 51.2). Adult walleye density ranged from .040 to 5.9 per surface hectare (mean = 1.5). The density of adult (sexually mature) walleyes, calendar year, and angling harvest rate of yellow perch Perca flavescens were significant descriptors of age-0 walleye density. Adult walleye density explained 10% of the variation in age-0 walleye abundance, which exhibited significant compensatory density dependence. Calendar year and yellow perch density explained nearly 32% of the residual variation in age-0 walleye density. The average age-0 walleye density in high-recruitment years (1991, 1994, and 1995) was 74.2/km, whereas the average age...

Damping and sound absorption properties of particle reinforced Al matrix composite foams

In this paper, the damping and sound absorption properties of A356/ xSiC p composite foams have been studied ( x expresses SiC vol.%). The results show that the loss factor β of A356/SiC p foams changes slightly with the increasing of temperature and frequency within the measured range. But there is some significant density dependence that the lower density foams have a higher β than the denser foams. When the cell structure is decided, the sound absorption factor α n is insensitive to the processes and increases with the raising SiC particle volume percent. A356/SiC foams have better damping and sound absorption properties than Al foams due to the existence of a large number of SiC p particles and a great amount of Al/SiC p interface in the composite foams which are beneficial to the absorption of the vibration energy.

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.

A reverse bias, tip-insulator-semiconductor tunnel diode model accounting for the delineation of a p/p+ junction using scanning tunneling microscopy

In this article, a tunneling-generation-avalanche model has been proposed to explain the reverse bias current–voltage behavior observed at a tip/air/p-type silicon junction. Based on this model, under conditions where the applied bias is more negative than the flat band voltage, the current will be dominated by generation processes, which has significant doping density dependence. Since mechanically cut tips, used in this work, can have complicated geometries, geometric effects, such as extended gates and concentration of the electrical field must be taken into account. By taking these factors into account, good agreement between theory and experiments can be achieved. Finally, in the presence of illumination, p/p+ junctions can be delineated successfully by taking advantage of the generation process. These results demonstrate that scanning tunneling microscopy can be used as a powerful tool for characterizing semiconductor devices.

Joint Effects of Density Dependence and Rainfall on Abundance of San Joaquin Kit Fox

We analyzed time-series abundances of San Joaquin kit fox estimated during 1983-95 on the Naval Petroleum Reserves in California (NPRC). For the analysis, we modified a model of density-dependent, stochastic population growth to include the lagged effects of a weather covariate (vegetation growing season rainfall). Without the covariate in the model, a statistical test failed to detect significant density dependence in fluctuating kit fox abundances. However, when the covariate was added, strong density dependence was detected. According to an information-theoretic model-selection index, the model with both density dependence and rainfall is far superior to the models that result from deleting one or more of these factors. The 2-year time lag in the response of kit fox abundances to changes in rainfall is consistent with biological expectations of how rainfall affects habitat carrying capacity for kit fox. An additional covariate, a coyote abundance index, failed to improve the model. A population viability analysis (PVA) performed with the combined density dependence-rainfall model suggests that the San Joaquin kit fox on NPRC could face a risk of up to 52% of falling to low levels within 20 years.

Plasma confinement studies in LHD

The initial experiments on the Large Helical Device (LHD) have extended confinement studies on currentless plasmas to a large scale (R = 3.9 m, a = 0.6 m). Heating by NBI of 3 MW produced plasmas with a fusion triple product of 8 × 1018m-3·keV·s at a magnetic field strength of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.1 keV were achieved simultaneously at a line averaged electron density of 1.5 × 1019 m-3. The maximum stored energy reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to ⟨β⟩ = 0.7%. Energy confinement times reached a maximum of 0.17 s. A favourable dependence of energy confinement time on density remains in the present power density (∼40 kW/m3) and electron density (3 × 1019 m-3) regimes, unlike the L mode in tokamaks. Although power degradation and significant density dependence are similar to the conditions on existing medium sized helical devices, the absolute value is enhanced by up to about 50% from the International Stellarator Scaling 95. Temperatures of both electrons and ions as high as 200 eV were observed at the outermost flux surface, which indicates a qualitative jump in performance compared with that of helical devices to date. Spontaneously generated toroidal currents indicate agreement with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to the finite β effect was well described by 3-D MHD equilibrium analysis. A density pump-out phenomenon was observed in hydrogen discharges, which was mitigated in helium discharges with high recycling.

Population dynamics of a tree‐dwelling aphid: regulation and density‐independent processes

1. A population of the Turkey‐oak aphid (Myzocallis boerneri Stroyan) was sampled at approximately weekly intervals on two Turkey‐oak trees for 19 years. 2. On one tree (A), the aphids exhibited a distinct seasonal pattern with a spring increase, summer decrease, early autumn increase, and late autumn decline. On the other tree (B) the aphids remained at low densities after the decrease in summer. 3. On tree A, significant undercompensating density dependence occurred during all periods of the seasonal population development, and their strength varied little during the course of the season. On tree B, significant density dependence compensated exactly for increase, but appeared only after the decrease in summer when the population remained at very low densities for the rest of the season. 4. Density‐independent weather variables affected the population dynamics very little. Their influence was marginally significant only at very low densities when the aphids were regulated exactly by compensating density‐dependent factors. 5. The results suggest a curvilinear density dependence, with strong regulation at low densities, and weak at high densities. That is, this aphid was most regulated not at the peak but at the trough densities.

Population dynamics of the Indian meal moth: demographic stochasticity and delayed regulatory mechanisms

1. Laboratory populations of the Indian meal moth [Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae)], undergo sustained periodic fluctuations in abundance. The period is just longer than the generation time. The fluctuations are accentuated in the presence of the P. interpunctella granulosis virus (PiGV). 2. Time series spanning 8–10 generations from three replicate populations of the virus‐free (VF) system and three from the virus‐infected (VI) system are investigated using nonparametric autoregressive time series models. 3. The dynamics are concluded to correspond to a third order process consistent with interactions in a three‐dimensional stage‐structured model for both systems. The functionally different interactive stages are believed to be the egg stage (preyed upon by larvae), small larvae (competing for resources and cannibalized by large larvae) and large larvae (competing for resources). 4. The virus is seen as a modulator of the host vital rates more than an independent agent in a trophic host–pathogen interaction. The virus increases developmental time and decreases fecundity of the moths. 5. A significantly nonlinear additive autoregressive model of order 3 appears to give a parsimonious description of the series. 6. The demographic (birth and death) nature of the stochasticity inherent in the system is explicitly incorporated in the statistical model for the time series by assuming an overdispersed Poisson process. The variability around the skeleton is found to conform closely to this assumption. The demographic nature of the stochasticity cannot be fully understood on the basis of Gaussian (least‐squares) models on transformed (variance‐stabilized) data. 7. Significant density dependencies are found at a 1‐week lag, a 2‐ to 3‐week lag and at a 6‐ to 7‐week lag. These are argued to be the signatures of within‐stage competition, between‐stage interactions and reproduction, respectively. Negative and statistically significant density dependence is apparent for the first two of these. No significant negative density dependence is apparent in the lag corresponding to reproduction. 8. The fluctuations in both the VF and VI system appear to represent limit cycles or weakly dampened cycles clothed by Poisson demographic stochasticity. 9. The enhanced cycles of the VI system are demonstrated to be consistent with a situation where the functional forms for the interactions are nearly the same as for the VF, but with delay structure shifted by just less than a week.

Weather‐induced changes in moth activity bias measurement of long‐term population dynamics from light trap samples

AbstractInterpretation of light trap catches of moths is complicated by daily variation in weather that alters flight activity and numbers caught. Light trap efficiency is also modified by wind and fog, and daily weather may effect absolute abundance (numbers actually present). However, actograph experiments and other sampling methods suggest that changes in daily activity are large by comparison to changes in absolute abundance. Daily variation in weather (other than wind and fog) is therefore a form of sampling error in absolute abundance estimates. We investigated the extent of this sampling bias in 26 years of population dynamics from 133 moth species. In a subset of 20 noctuid and geometrid species, daily numbers caught were positively correlated with temperature in 14 species, and negatively correlated with rainfall in 11 species. The strength of correlations varied between species, making it difficult to standardize catches to constant conditions. We overcame this by establishing how weather variation changed with time and duration of the flight period. Species flying later in the summer and for shorter periods experienced more variable temperatures, making sampling error greater for these species. Of the 133 moth species, those with shorter flight periods had greater population variability and more showed significant temporal density dependence. However, these effects were weak, which is encouraging because it suggests that population analyses of light trap data largely reflect factors other than sampling error.

Density dependence in cereal aphid populations

SummaryLong sequences of data on the incidence of cereal aphids from five European countries were analysed for evidence of density dependent processes occurring between years. Using a randomisation test (Pollard, Lakhani & Rotheray, 1987), density dependence was revealed in all (16) population censuses of Metopolo‐phium dirhodum, 60% of Rhopalosiphum padi censuses (10 of 17) but only 25% of Sitobion avenae population censuses (4 of 16). Correcting for density independent effects of weather revealed the existence of significant direct density dependence in some populations censuses where it was previously undetected. The implications of density dependence in cereal aphid populations are considered.

Factors influencing detection of density dependence in British birds : I. Population trends.

We question why density dependence has remained elusive in series of annual abundances of British birds. In particular, an earlier study reported that significant temporal trends in abundances occur in up to 74% of time series from the Common Birds Census. Several studies showed that such trends can hinder detection of density dependence. Temporal trends do not preclude the presence of density dependence and two published tests for density dependence include temporal trends in the null hypothesis model. We explore the extent to which detection of density dependence was hindered by temporal trends in bird abundance data. We used a conservative method to test for trends, which found significant (P<0.05) linear population trends in only 7 of 60 time series of abundances (of 17-31 years) compiled from the Common Birds Census data. However, both of the tests for density dependence that allow for trends and a third method gave P-values that were strongly influenced by the strength of trends, including trends that were not significant (P>0.05). This shows that density dependence may be falsely rejected or detected when trends are present, even when these trends are weak and not statistically significant. To circumvent this problem we detrended the time-series prior to testing for the presence of density dependence. To minimize subjectivity we used simulated time series to check that this procedure did not increase the level of type I error (false rejection of density independence). Additionally, we confirmed that the method gave acceptable levels of type II error, where the test fails to reject density independence in series generated using a density dependent model. This showed that the detrending method was acceptable and represents a major improvement in our ability to detect density dependence in time series that contain temporal trends. Detrending the bird time series increased the number of series in which significant (P<0.05) density dependence was found from 10 (17%), when trends are ignored, to 27 (45%) when series are detrended. However, this rate of 45% is still surprisingly low by comparison to other taxa, and we believe that other factors may contribute to this, which we explore in the second of this pair of papers.

Alternation of bottom-up and top-down regulation in a natural population of an agromyzid leafminer, Chromatomyia suikazurae.

The population dynamics and the relative importance of bottom-up and top-down effects in a plant-leafminer-multiparasitoid interaction was studied between 1981 and 1990 in a natural forest in Kyoto, Japan. The leafminer, Chromatomyia suikazurae (Agromyzidae, Diptera), passed two generations (G1 and G2) on Lonicera gracilipes (Caprifoliaceae). The G1 population in February was free from parasitoid attack, and the mortality in G1 was mainly caused by resource limitation. Intraspecific competition for resources occurred at the larval stage in G1, and the larval mortality was density-dependent. The G1 adult density was resource-limited (the number of newly opened leaves), and its variability was lower than that of G2. The G2 population in April was not resource-limited but subject to intense attack by a species-rich parasitoid complex, and thus total mortality was much larger than that in G1. Significant density dependence was detected not in larval but in pupal mortalities, which were mainly caused by parasitism by parasitoids that attacked the pupa. The host population alternately experienced "bottom-up" effects during the larval stage in G1 and "top-down" effects during the pupal stage in G2. Overall population fluctuation was non-cyclic and mainly due to climatically-induced fluctuation of available plant resources in G1.

Density dependence tests, are they?

A large number of time series of abundances of insects and birds from a variety of data sets were submitted to a new density dependence test. The results varied enormously between data sets, but the relation between the frequency of statistically significant density dependence (SSDD) and the length of the series was similar to that of the power curve of the test, making the results consistent with the hypothesis of the density-dependent model being universally applicable throughout the data used. Pest and non-pest species did not differ in the incidence of SSDD. The more sampling error present in the data, the higher the percentages of SSDD. This was expected given that the power of the test increases with increasing sampling error in the data. Many of the data used here, as well as in the literature, clearly violate the basic assumption of the test that the organism concerned should be univoltine and semelparous. Yet the incidence of SSDD was the same in univoltine as in bi/polyvoltine species and the same in semelparous organisms as in birds that are reproductively active in more than one year. The seasonal migrant Autographa gamma in Britain and Czechoslovakia and even rainfall data were found to have SSDD. Statistical significance, however, does not automatically lead to the conclusion of density-dependent regulation. Any series of random variables which are in a stochastic equilibrium, such as a series of independent, identically distributed, random variables, is typically described better by the alternative (density-dependent) model than by the null (density-independent) model. Significant test results were often obtained with sloppy data, with data that clearly violate the basic assumptions of the test and with other data where an interpretation of the results in terms of densitydependent regulation was absurd. Given the fact that other explanations have to be found for significant test results for all these cases, mechanisms other than regulation may very well be applicable too where the data are entirely appropriate for the test. The test is simply a data-based choice between a model without and one with a stochastic equilibrium. A time series as such does not contain any information about the causes of the fluctuation pattern, so that one cannot expect statistics to produce such information from that time series. A significant test result using suitable data is entirely consistent with the hypothesis of density-dependent regulation, but also with any other suitable hypotheses. Because the test results were generally consistent with the hypothesis of a universal applicability of the density-dependence model, a negative test result may only mean that the time series was not long enough for the density dependence that was present to become statistically significant. Positive results are difficult to interpret, but so are negative results. A final decision needs to be based not so much on the test result as on much detailed information about the population concerned. Because the "density-dependence test" does not test for the presence of the mechanism of density-dependent regulation and because of the loaded, multiple meanings of the term "density-dependence", calling the test a "test of statistical density dependence" may be preferable.

Mean-Related Stochasticity and Population Variability

Temporal variance in population density V is very generally related to mean density x by the power function, V = ax(b), with the value of b typically ranging from 1 to above 2 in different sets of data. We describe three simple Population dynamic models which are capable of generating variance-mean relationships with the observed range of b values. In the first model, the level of density dependence is correlated with mean density. This model is rejected because we find no relationship between the incidence of significant density dependence and mean density in data for moths and aphids. In the two other models, the level of environmental stochasticity in population change is correlated with mean density, affecting either the intrinsic (density-independent) growth rate or the equilibrium density (density dependence). Only the former model is capable of generating both positive and negative relationships between temporal variability in population size and density dependence. A positive relationship is predicted for species with high intrinsic growth rate, which may generate complex dynamics and thereby add to population variability. Aphids have higher intrinsic growth rates than moths, and as predicted, the relationship between population variability and density dependence is positive in aphids but negative in moths.

Spatial Density Dependence in Populations of a Cynipid Gall-Former Andricus Quercuscalicis

Summary 1. Patterns of spatial density dependence in mortality factors affecting the sexual generation of Andricus quercuscalicis (Burgsdorf 1783) on Quercus cerris (L.) were investigated as part of a long-term study of the population dynamics of the gall wasp. Experimental enhancement of gall density was used to increase the range of densities observed in the field. 2. We present a new application of logistic regression for detecting spatial density dependence in single-sample collections of galls or leaf-mines. The test, first described by Cox (1970) uses GLIM with binomial errors. The performance of the test and the frequency of Type I and Type II errors were assessed by computer simulation of a variety of different ecological circumstances including density independence, mild and severe, direct and inverse density dependence, and proved satisfactory in most respects. 3. In our field data, statistically significant spatial density dependence was detected in about 15% of cases. Out of 62 cases of significant density dependence, 66% were of inverse density dependence. In particular, the gall wasp data demonstrate a preponderance of inverse density dependence in rates of parasitism. Only predation of galls by birds exhibited a consistent pattern of direct density dependence. There was marked variation in the intensity of all mortality factors from tree to tree. 4. As spatial scale is increased from catkins, through buds, shoots and twigs, so the sample size declines and the range of densities increases. Since reducing the sample size reduces the probability of detecting significant density dependence, but increasing the range of densities increases the likelihood of detecting density dependence, the overall effect of changing spatial scale on the probability of detecting density dependence is difficult to predict. Even when density dependence was detected at the smallest scale, the frequency of detection at higher scales declined, and this may be due in part to spatial heterogeneity in the action of density dependence. Thus, it is not obvious whether changes in the rate of detection of density dependence at different spatial scales are statistical artefacts or reflect real changes in the ecology of the organisms involved. 5. There are two potential problems in applying logistic regression to such data sets. First, overdispersion may occur if p (the probability of mortality) varies significantly within the sample. One way of dealing with this is to rescale the model, according to the degree of aggregation. Second, apparent underdispersion may occur if both p and n are low because, with small sample sizes, variation in p may be undetectable. The most obvious solution in this case is to increase the sample size n, though this may be difficult at larger spatial scales. 6. Enhancing gall densities by caging agamic females directly onto the branches of the host tree led to an increase in mean gall density and in the range of gall densities. This, in turn, led to an increase in the detection of inverse density dependence (mainly through parasitoid attack) but had no significant impact on the detection of direct density dependence. The use of experimentally enhanced gall densities appears to hold considerable potential for demonstrating the importance of spatial density dependence, so long as care is taken to ensure that the highest densities are not unrealistic. 567

Patterns of Density Dependence in Moths and Aphids

1. The incidence of density dependence was examined in 5715 time-series of annual abundances of 447 species of moths and aphids in the UK using three techniques (Ricker equation, Bulmer's test and Pollard et al.'s test). 2. Delayed density dependence (lag 2 years) was not found more frequently than expected by chance. In the moth family Noctuidae, delayed density dependence was slightly but significantly more frequent in woodland populations than in non-woodland populations, consistent with a higher frequency of outbreak species among forest Lepidoptera. 3. The incidence of significant density dependence increased with the length of the time-series (from 10 to 24 years), and density dependence was detected more frequently in species which did not show a significant trend in abundance over time. 4. The incidence of density dependence was 79% and 88% in moth and aphid time-series which were longer than 20 years (Bulmer's test). 5. There were only slight differences between localities in the average degree of density dependence when the effects of time-series length and trend were removed. 6. Significant differences were found between species in the average degree of density dependence, especially in noctuid moths and aphids. In noctuids, the average degree of density dependence increased with body size.

The Nature of Population Stability in Eurosta Solidaginis, A Nonoutbreaking Herbivore of Goldenrod

Eurosta solidaginis (Diptera: Tepritidae) is a common, nonoutbreaking herbivore of goldenrod (Solidago altissima) whose populations fluctuate little through time. A large body of theory is based upon the assumption that nonoutbreaking phytophagous insects are regulated at "endemic equilibria" by the temporally density—dependent mortality caused by their natural enemies. This equilibrium viewpoint has been challenged recently by proponents of "nonequilibrium theory," even though the evidence against population regulation is largely observational and likely to be biased by the almost exclusive study of abundant, pest species. To understand the nature of population stability in Eurosta solidaginis, I altered the density of 20 subpopulations, and looked for compensatory changes in mortality that could allow the populations to return to their pre—experimental density. Mortality diminished in those subpopulations whose density was decreased. Moreover, the change in mortality depended significantly on the change in density; subpopulations in which density was most severely diminished enjoyed a greater decrease in mortality. Of four individual mortality agents, only early larval death, which may be partly caused by a defensive reaction by the host plant, showed significant density dependence. No within—generation (spatial) density dependence was detected in either the first or the second (experimental) generation. At the end of the density—manipulation experiment, all Eurosta were removed from the patches. Most patches were recolonized within one generation.

On the stabilization of animal numbers. Problems of testing : 2. Conforntation with data from the field.

When testing for regulation of population numbers, rather than using Bulmer's second test in cases where population numbers are estimated instead of measured, we prefer to correct Bulmer's first test for estimation error. A correction method is expounded, discussed, and applied to two series of census data: the pine looper of Klomp and the garden chafer of Milne. In neither case the tentative conclusion from using the uncorrected test was changed after correction. Therefore, in practice Bulmer's first test without correction can be used well as a first orientation. Twelve long series (more than 10 years) of census data of both univoltine and semelparous (a necessary condition) insects were tested for significant density dependence in the fluctuations of numbers with the randomization test of Pollard et al. None of the series, all we could find to meet the necessary condition as well as being longer than 10 years, showed significant density dependence at the 0.05 level, though the pine looper of Klomp did so at the 0.1 level. Next, the same series were tested for regulation in the sense of "keeping density within limits" with both the first test of Bulmer and the permutation test of Reddingius and Den Boer. Onky Klomp's pine looper population at "Hoge Veluwe" scored significantly. In a following paper this population will be considered more closely, in order to enable understanding of this test result.

Character displacement and niche shift analyzed using consumer-resource models of competition

This paper analyzes the adaptive responses to competition (both character displacement and niche shift) in a two consumer-two resource model. The model includes density dependence that is unrelated to the resources that are explicit in the model. This could be due to another resource dimension, parasites, or interference competition. Competitors adapt by changing their relative consumption rate constants on the two resource types. This model can result in mutually divergent, parallel, or mutually convergent displacement of competitors. Parallel displacement may entail net divergence, net convergence, or no net change. Parallel change with net convergence is most likely when the competitors have similar constraints on the possible values of consumption rate constants, unequal allopatric abundances, and significant intraspecific density dependence. Numerical calculations of displacements are presented for several models and the effect of a number of different possible alterations of the model are discussed. The evolution of resource handling and processing efficiency, and displacement in the presence of additional selective pressures on the character are considered in detail. The results have implications for questions about maximization of population size, the relationship of character displacement and the competition coefficient, and “null” models in the study of competition. Differences between this and previous theoretical works are discussed. It is argued that conditions allowing parallel or convergent displacement are not biologically unlikely, and possible examples are discussed. Data on resource partitioning seem to be more consistent with the results reached here than with previous theory.