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Marcel Holyoak - One of the best experts on this subject based on the ideXlab platform.
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Factors influencing detection of Density Dependence in British birds
Oecologia, 1996Co-Authors: Marcel Holyoak, Stephen R. BaillieAbstract: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). 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
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Identifying delayed Density Dependence in time-series data
Oikos, 1994Co-Authors: Marcel HolyoakAbstract:I investigated the ability of statistical tests to detect delayed Density Dependence in series of abundances per generation. I generated time series containing delayed Density Dependence using two simple population models (a host-parasitoid model and a version of the Ricker equation) and analysed these using the tests for delayed Density Dependence of Turchin (1990), the lag 2 partial autocorrelation coefficient (PACF) and a novel modification of Pollard et al's (1987) test. All tests of delayed Density Dependence are of low statistical power, and so any delayed Density Dependence that is present may frequently be overlooked, particularly with short (< 25 generation) time series. The modification of Pollard et al's test was the best test for detecting delayed Density Dependence
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New insights into testing for Density Dependence.
Oecologia, 1993Co-Authors: Marcel HolyoakAbstract:The reasons why tests for Density Dependence often differ in their results for a particular time-series were investigated using modelled time-series of 20 generations in lenght. The test of Pollard et al. (1987) is the most reliable; it had the greatest power with the three forms of Density dependent data investigated (mean detection rates of 50.8–61.1%) and was least influenced by the form of the Density Dependence in time-series. Bulmer's first test (Bulmer 1975) had slightly lower power (mean detection rates of 27.4–56.8%) and was more affected by the form of Density Dependence present in the data. The mean power of the other tests was lower and detection rates were more variable. Rates were 24.6–46.2% for regression of k-value on abundance, 6.4–32.6% for regression of k-value on logarithmic abundance and 0.2–13.7% for Bulmer's second test (Bulmer 1975). Bulmer's second test is not useful because of low power. For one method, regression of k-value on abundance. Density Dependence was detected in 19.9% of timeseries generated using a random-walk model. For regression of k-value on logarithmically-transformed abundance the equivalent figure was 18.3% of series. These rates of spurious detection were significantly (P
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The frequency of detection of Density Dependence in insect orders
Ecological Entomology, 1993Co-Authors: Marcel HolyoakAbstract:. 1 A priori, there are no obvious reasons why patterns should exist in the frequency of Density Dependence across insect orders. However, orders may reflect related factors which influence population regulation (e.g. life-history patterns and ecology) and are difficult to quantify. The frequency of occurrence of Density Dependence is compared in 171 time series (of ten or more generations) from Lepidoptera, Hemiptera, Diptera, Odonata, Hymenoptera and Coleoptera. A posteriori attempts are made to identify the cause of observed patterns. 2 Buhner's (1975) test found non-delayed Density Dependence more frequently in Odonata than Lepidoptera and Hymenoptera, which in turn showed non-delayed Density Dependence more frequently than Diptera, Hemiptera and Coleoptera. Similarly, detection was greater for Odonata than other orders using Dennis & Taper's (1993) test for Density Dependence and Crowley's (1992) test for attraction. Varley & Gradwell's (1960) test found Density Dependence less frequently in Hemiptera than other orders. These differences were independent of time series length, temporal trends and numbers of generations per year. 3 The reasons for observed patterns in detection of Density Dependence (and attraction) in insect orders are not clear; however, plausible explanations are differences in: (i) intrinsic growth rate, which is correlated with body size (although evidence to support this hypothesis is weak); (ii) the sampling method used; or (iii) whether individuals come from a single population or many populations. 4 Using Turchin's (1990) test, delayed (lag 2) Density Dependence was detected most frequently in Hymenoptera, which often show delayed diapause or are parasitoids.
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Comment arising from a paper by Wolda and Dennis: using and interpreting the results of tests for Density Dependence
Oecologia, 1993Co-Authors: Marcel Holyoak, John H. LawtonAbstract:We argue that tests for Density Dependence are useful in analyses of population dynamics and suggest guide lines for their use and interpretation of results which avoid many of the problems discussed by Wolda and Dennis (1993). Processes other than Density Dependence per se can cause statistical tests to indicate the presence of Density Dependence (Wolda and Dennis 1993 and unpublished simulations). Tests for Density Dependence cannot reveal the mechanism of regulation, but they do indicate the nature of long-term population dynamics. Tests for Density Dependence give misleading results if sampling is not at generation intervals; however, this problem is avoided if we only use tests on data collected in each generation (Holyoak 1993a). Similarly, species should be semelparous. Non-delayed Density Dependence should not be considered without looking for delayed Density Dependence, since the presence of delayed Density Dependence can lead to over-detection of non-delayed Density Dependence (Woiwod and Hanski 1992; Holyoak 1993b). The partial autocorrelation function and knowledge of life-history are more useful than tests for Density Dependence for indicating whether any Density Dependence is delayed or not (Royama 1992; Holyoak 1993b). Estimation error with a constant upper size limit causes tests for Density Dependence to overestimate the frequency of delayed Density Dependence; however we do not know whether estimation error is bounded in real populations. Work in progress suggests that 20–40 generations (depending on the nature of population dynamics) gives a moderate level of accuracy with tests for Density Dependence, and >40 generations are necessary for tests to be accurate in their assessment of the strength of Density Dependence. We conclude that tests are useful indicators of whether Density Dependence, or other feedback mechanisms are likely to be acting.
Andrew W. Steiner - One of the best experts on this subject based on the ideXlab platform.
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Constraints on the Density Dependence of the symmetry energy.
Physical review letters, 2009Co-Authors: M. B. Tsang, Yingxun Zhang, Pawel Danielewicz, Michael Famiano, W. G. Lynch, Andrew W. SteinerAbstract:Collisions involving {sup 112}Sn and {sup 124}Sn nuclei have been simulated with the improved quantum molecular dynamics transport model. The results of the calculations reproduce isospin diffusion data from two different observables and the ratios of neutron and proton spectra. By comparing these data to calculations performed over a range of symmetry energies at saturation Density and different representations of the Density Dependence of the symmetry energy, constraints on the Density Dependence of the symmetry energy at subnormal Density are obtained. The results from the present work are compared to constraints put forward in other recent analyses.
Theunis Piersma - One of the best experts on this subject based on the ideXlab platform.
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seasonal variation in Density Dependence in age specific survival of a long distance migrant
Ecology, 2013Co-Authors: Otto Overdijk, Joost M Tinbergen, Theunis PiersmaAbstract:Density Dependence in vital rates is key to population regulation. Rather than being constant, the strength of Density Dependence may vary throughout the year, but empirical evidence is limited. Based on 22 years of data of color-banded birds from a recovering population of Eurasian Spoonbills Platalea leucorodia leucorodia, we show, for the first time, seasonal variation in Density Dependence in survival of a long-distance migrating bird. Combining resightings and dead recoveries at breeding, stopover, and nonbreeding areas enabled us to (1) separate true survival from permanent emigration from the breeding area, and (2) estimate survival in three seasons: summer, early winter (including autumn migration), and late winter (including spring migration). Accompanying the rapid population growth, juvenile annual survival initially increased, manifested in early winter, but thereafter, at high population sizes, it strongly decreased through a combination of decreasing survival in all seasons. Annual survival of subadult (second- and third-year) and adult birds decreased more gradually with increasing population size, with Density Dependence occurring in early winter for subadults and late winter for adults. Thus, the shape and strength of Density Dependence in survival varied with age and season. Understanding the seasonal timing of Density Dependence, especially with reference to underlying mechanisms, is important for the design of effective conservation strategies.
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Seasonal variation in Density Dependence in age‐specific survival of a long‐distance migrant
Ecology, 2013Co-Authors: Otto Overdijk, Joost M Tinbergen, Theunis PiersmaAbstract:Density Dependence in vital rates is key to population regulation. Rather than being constant, the strength of Density Dependence may vary throughout the year, but empirical evidence is limited. Based on 22 years of data of color-banded birds from a recovering population of Eurasian Spoonbills Platalea leucorodia leucorodia, we show, for the first time, seasonal variation in Density Dependence in survival of a long-distance migrating bird. Combining resightings and dead recoveries at breeding, stopover, and nonbreeding areas enabled us to (1) separate true survival from permanent emigration from the breeding area, and (2) estimate survival in three seasons: summer, early winter (including autumn migration), and late winter (including spring migration). Accompanying the rapid population growth, juvenile annual survival initially increased, manifested in early winter, but thereafter, at high population sizes, it strongly decreased through a combination of decreasing survival in all seasons. Annual survival of subadult (second- and third-year) and adult birds decreased more gradually with increasing population size, with Density Dependence occurring in early winter for subadults and late winter for adults. Thus, the shape and strength of Density Dependence in survival varied with age and season. Understanding the seasonal timing of Density Dependence, especially with reference to underlying mechanisms, is important for the design of effective conservation strategies.
Takashi Saitoh - One of the best experts on this subject based on the ideXlab platform.
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Mechanisms of Density Dependence in fluctuating vole populations: deducing annual Density Dependence from seasonal processes
Population Ecology, 2003Co-Authors: Takashi Saitoh, Nils Chr. Stenseth, Hildegunn Viljugrein, Marte O. KittilsenAbstract:Based on recent advances in time-series analyses of ecological dynamics using statistical and mathematical models, we summarise our recent results on the seasonal processes in the annual population dynamics of the grey-sided vole Clethrionomys rufocanus (Sundevall, 1846) in Hokkaido, Japan, and report additional analyses on annual and seasonal Density Dependence. Annual direct Density Dependence was strong in almost all populations. In contrast, delayed Density Dependence was generally weak, although clear delayed Density Dependence was detected in some of the studied populations. Although seasonal Density Dependence was observed both in winter and summer, direct Density Dependence was much more profound during winter; thus, winter Density Dependence contributed most to the overall annual direct Density Dependence. We found no correlation between the seasonal components of annual direct Density Dependence; however, the corresponding seasonal components for annual delayed Density Dependence were positively correlated. We conclude that winter conditions influence the strength of annual direct Density Dependence most profoundly. Moreover, we conclude that direct Density Dependence during summer and winter may be generated by different mechanisms, whereas delayed Density Dependence seems to be generated by a common mechanism. Candidate mechanisms are discussed in relation to general knowledge of northern rodent populations and to specific insights provided by earlier studies of grey-sided voles in Hokkaido.
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Density Dependence IN VOLES AND MICE: A COMPARATIVE STUDY
Ecology, 1999Co-Authors: Takashi Saitoh, Ottar N. Bjørnstad, Nils Chr. StensethAbstract:The presence of direct and delayed Density Dependence in populations of three sympatric rodent species (Clethrionomys rufocanus, Apodemus speciosus, and A. argenteus) in Hokkaido, Japan, was evaluated using triannual census data (spring, summer, and fall) spanning 30 years (1963-1992) on 79 populations for each species. The average abundance and population variability (the s-index) generally increased from spring to fall in C. rufocanus but were typically highest in summer for the Apodemus populations. Based on a comprehensive and comparative review of the population and community biology of the species we made four explicit predictions about the pattern of Density Dependence: (1) the three species were expected to exhibit socially induced direct Density Dependence, but (2) this was expected to be weaker in the Apodemus species than C. rufocanus; (3) delayed Density Dependence caused by predation was only expected in C. rufocanus; thus (4) time series of C. rufocanus were expected to reflect a second-order dynamic process, and those of the Apodemus species were expected to reflect a first-order process. Dennis and Taper's method based on the Gompertz model was used to test for direct and delayed Density Dependence and thereby to test the predictions. Direct Density Dependence was detected in most series (81.0-97.5%) for all three seasons and for all three species. A significant proportion of the time series of C. rufocanus (11.8-18.5%) exhibited negative delayed Density Dependence, whereas detection rates in the two Apodemus species did not differ from that expected by chance alone. Autoregressive analyses corroborated this: a second- order process was commonly found to be the appropriate model for the time series of C. rufocanus, whereas a first-order process was preferred for most time series of the Apodemus species. The high incidence of direct Density Dependence in all three species and the con- trasting results on delayed Density Dependence between C. rufocanus and the Apodemus specieNare discussed with reference to social and trophic interactions. Territoriality, delayed maturation, and reduced pregnancy rates are probable causes for the high incidence of direct Density Dependence in all species. The more unpredictable variability in Apodemus food resource is argued to have a potential to disrupt social regulation and thus to lower the incidence of direct Density Dependence. A candidate mechanism for the incidence of delayed Density Dependence is differential vulnerability to predation: the demography of C. rufo- canus is much more affected by predators than Apodemus.
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Density Dependence in fluctuating grey-sided vole populations
The Journal of Animal Ecology, 1997Co-Authors: Takashi Saitoh, Nils Chr. Stenseth, Ottar N. BjørnstadAbstract:1. The presence of direct and delayed (1 year) Density Dependence in fluctuating grey-sided vole populations (Clethrionomys rufocanus) in Hokkaido, Japan, were tested using 90 time series spanning 31 years (1962 92). 2. The autocorrelation test of Bulmer, the randomization test of Pollard. Lakhani & Rothery, and the bootstrap test of Dennis & Taper using two different models (Ricker and Gompertz) detected direct Density Dependence in most of the time series (90.0-100%) at the 5% level (one-tailed test). 3. Plotting population growth rates as functions of abundance suggested that the Gompertz model is more appropriate than the Ricker model for the studied populations. The tests for direct Density Dependence using the Gompertz model also rejected the null hypothesis of Density-independent growth more frequently than the tests using the Ricker model. 4. We extended the randomization and the bootstrap methods to investigate delayed Density Dependence by using partial correlation and partial regression, respectively. The extended tests detected delayed Density Dependence in 8-15 time series (8.9%-16.7% at the 5% level (two-tailed test). 5. The high incidence of direct Density Dependence and the low, but significant, incidence of delayed Density Dependence in these vole populations are discussed. Delayed maturation at high densities by social interference and a rich generalist fauna are thought to represent plausible mechanisms generating direct Density Dependence. As a candidate mechanism for delayed Density Dependence, specialist predator and climate conditions (affecting the exposure to predators) are discussed.
Nils Chr. Stenseth - One of the best experts on this subject based on the ideXlab platform.
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Density Dependence IN NORTH AMERICAN DUCKS
Ecology, 2005Co-Authors: Hildegunn Viljugrein, Nils Chr. Stenseth, Graham W. Smith, Gunnhildur H. SteinbakkAbstract:Knowing whether, and to what extent, populations are regulated by Density-dependent factors is important both in its own right and when developing management strategies for wildlife species. However, available tests for Density Dependence are typically sensitive to sampling errors in the data. By using a state-space modeling approach, incorporating both an ecological process model and an observation model, it is possible to account for both measurement and process error. Here we focus on the detection and estimation of direct Density Dependence in two species of North American ducks: the Mallard (Anas platyrhynchos) and the Canvasback (Aythya valisineria). Yearly aerial counts on the major breeding grounds of ducks in North America provide estimates of abundances as well as standard errors of these estimates for both species. Including the number of ponds as a covariate, we demonstrate evidence for Density Dependence in prairie areas for both species. The appropriateness of the applied state-space method is validated through a simulation study.
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Mechanisms of Density Dependence in fluctuating vole populations: deducing annual Density Dependence from seasonal processes
Population Ecology, 2003Co-Authors: Takashi Saitoh, Nils Chr. Stenseth, Hildegunn Viljugrein, Marte O. KittilsenAbstract:Based on recent advances in time-series analyses of ecological dynamics using statistical and mathematical models, we summarise our recent results on the seasonal processes in the annual population dynamics of the grey-sided vole Clethrionomys rufocanus (Sundevall, 1846) in Hokkaido, Japan, and report additional analyses on annual and seasonal Density Dependence. Annual direct Density Dependence was strong in almost all populations. In contrast, delayed Density Dependence was generally weak, although clear delayed Density Dependence was detected in some of the studied populations. Although seasonal Density Dependence was observed both in winter and summer, direct Density Dependence was much more profound during winter; thus, winter Density Dependence contributed most to the overall annual direct Density Dependence. We found no correlation between the seasonal components of annual direct Density Dependence; however, the corresponding seasonal components for annual delayed Density Dependence were positively correlated. We conclude that winter conditions influence the strength of annual direct Density Dependence most profoundly. Moreover, we conclude that direct Density Dependence during summer and winter may be generated by different mechanisms, whereas delayed Density Dependence seems to be generated by a common mechanism. Candidate mechanisms are discussed in relation to general knowledge of northern rodent populations and to specific insights provided by earlier studies of grey-sided voles in Hokkaido.
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Density Dependence IN VOLES AND MICE: A COMPARATIVE STUDY
Ecology, 1999Co-Authors: Takashi Saitoh, Ottar N. Bjørnstad, Nils Chr. StensethAbstract:The presence of direct and delayed Density Dependence in populations of three sympatric rodent species (Clethrionomys rufocanus, Apodemus speciosus, and A. argenteus) in Hokkaido, Japan, was evaluated using triannual census data (spring, summer, and fall) spanning 30 years (1963-1992) on 79 populations for each species. The average abundance and population variability (the s-index) generally increased from spring to fall in C. rufocanus but were typically highest in summer for the Apodemus populations. Based on a comprehensive and comparative review of the population and community biology of the species we made four explicit predictions about the pattern of Density Dependence: (1) the three species were expected to exhibit socially induced direct Density Dependence, but (2) this was expected to be weaker in the Apodemus species than C. rufocanus; (3) delayed Density Dependence caused by predation was only expected in C. rufocanus; thus (4) time series of C. rufocanus were expected to reflect a second-order dynamic process, and those of the Apodemus species were expected to reflect a first-order process. Dennis and Taper's method based on the Gompertz model was used to test for direct and delayed Density Dependence and thereby to test the predictions. Direct Density Dependence was detected in most series (81.0-97.5%) for all three seasons and for all three species. A significant proportion of the time series of C. rufocanus (11.8-18.5%) exhibited negative delayed Density Dependence, whereas detection rates in the two Apodemus species did not differ from that expected by chance alone. Autoregressive analyses corroborated this: a second- order process was commonly found to be the appropriate model for the time series of C. rufocanus, whereas a first-order process was preferred for most time series of the Apodemus species. The high incidence of direct Density Dependence in all three species and the con- trasting results on delayed Density Dependence between C. rufocanus and the Apodemus specieNare discussed with reference to social and trophic interactions. Territoriality, delayed maturation, and reduced pregnancy rates are probable causes for the high incidence of direct Density Dependence in all species. The more unpredictable variability in Apodemus food resource is argued to have a potential to disrupt social regulation and thus to lower the incidence of direct Density Dependence. A candidate mechanism for the incidence of delayed Density Dependence is differential vulnerability to predation: the demography of C. rufo- canus is much more affected by predators than Apodemus.
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Density Dependence in fluctuating grey-sided vole populations
The Journal of Animal Ecology, 1997Co-Authors: Takashi Saitoh, Nils Chr. Stenseth, Ottar N. BjørnstadAbstract:1. The presence of direct and delayed (1 year) Density Dependence in fluctuating grey-sided vole populations (Clethrionomys rufocanus) in Hokkaido, Japan, were tested using 90 time series spanning 31 years (1962 92). 2. The autocorrelation test of Bulmer, the randomization test of Pollard. Lakhani & Rothery, and the bootstrap test of Dennis & Taper using two different models (Ricker and Gompertz) detected direct Density Dependence in most of the time series (90.0-100%) at the 5% level (one-tailed test). 3. Plotting population growth rates as functions of abundance suggested that the Gompertz model is more appropriate than the Ricker model for the studied populations. The tests for direct Density Dependence using the Gompertz model also rejected the null hypothesis of Density-independent growth more frequently than the tests using the Ricker model. 4. We extended the randomization and the bootstrap methods to investigate delayed Density Dependence by using partial correlation and partial regression, respectively. The extended tests detected delayed Density Dependence in 8-15 time series (8.9%-16.7% at the 5% level (two-tailed test). 5. The high incidence of direct Density Dependence and the low, but significant, incidence of delayed Density Dependence in these vole populations are discussed. Delayed maturation at high densities by social interference and a rich generalist fauna are thought to represent plausible mechanisms generating direct Density Dependence. As a candidate mechanism for delayed Density Dependence, specialist predator and climate conditions (affecting the exposure to predators) are discussed.