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PubMed2006

Natural selection and population dynamics.

To what extent, and under which circumstances, are population dynamics influenced by concurrent natural selection? Density dependence and environmental stochasticity are generally expected to subsume any selective modulation of population growth rate, but theoretical considerations point to conditions under which selection can have an appreciable impact on population dynamics. By contrast, empirical research has barely scratched the surface of this fundamental question in population biology. Here, we present a diverse body of mostly empirical evidence that demonstrates how selection can influence population dynamics, including studies of small populations, metapopulations, cyclical populations and host-pathogen interactions. We also discuss the utility, in this context, of inferences from molecular genetic data, placing them within the broader framework of quantitative genetics and life-history evolution.

Adaptation, Biological↗
PubMed2003

The molecular mechanisms of life history alterations in a rotifer: a novel approach in population dynamics.

The rotifer Brachionus plicatilis is a widely-used model for population dynamics studies. During the population growth of B. plicatilis, life history parameters such as reproduction and lifespan change widely, and determine the balance between birth and death rates that regulates the population fluctuations. The lifespan of B. plicatilis was extended 30% by inhibiting a phosphatidylinositol-3-OH kinase involved in an insulin/insulin-like growth factor (IGF) signal transduction pathway that regulates the reproduction and lifespan in nematodes. Subsequently, we cloned a cDNA encoding Mn-superoxide dismutase (SOD), which may function downstream of the IGF pathway. Real-time reverse-transcription polymerase chain reaction analysis revealed that the expression level of Mn-SOD mRNA was higher in B. plicatilis with longer lifespans than those with shorter lifespans. In addition, stress proteins may also influence population dynamics as molecules regulating lifespan and molecular chaperones to maintain the cellular integrity. Accordingly, we cloned two stress protein genes encoding HSP70 and GRP94, and found that their expression changed during the population growth of rotifers. Thus, this novel approach of integrating population ecology and molecular biology has potential use in investigation the detailed mechanisms of rotifer population dynamics.

Animals↗
PubMed2001

Wave of chaos: new mechanism of pattern formation in spatio-temporal population dynamics.

The dynamics of a simple prey-predator system is described by a system of two reaction- diffusion equations with biologically reasonable non-linearities (logistic growth of the prey, Holling type II functional response of the predator). We show that, when the local kinetics of the system is oscillatory, for a wide class of initial conditions the evolution of the system leads to the formation of a non-stationary irregular pattern corresponding to spatio-temporal chaos. The chaotic pattern first appears inside a sub-domain of the system. This sub-domain then steadily grows with time and, finally, the chaotic pattern invades the whole space, displacing the regular pattern.

Animals↗
PubMed2003

Climate and density shape population dynamics of a marine top predator.

Long-term studies have documented that climate fluctuations affect the dynamics of populations, but the relative influence of stochastic and density-dependent processes is still poorly understood and debated. Most studies have been conducted on terrestrial systems, and lacking are studies on marine systems explicitly integrating the fact that most populations live in seasonal environments and respond to regular or systematic environmental changes. We separated winter from summer mortality in a seabird population, the blue petrel Halobaena caerulea, in the southern Indian Ocean where the El Niño/Southern Oscillation effects occur with a 3-4-year lag. Seventy per cent of the mortality occurred in winter and was linked to climatic factors, being lower during anomalous warm events. The strength of density dependence was affected by climate, with population crashes occurring when poor conditions occurred at high densities. We found that an exceptionally long-lasting warming caused a ca. 40% decline of the population, suggesting that chronic climate change will strongly affect this top predator. These findings demonstrate that populations in marine systems are particularly susceptible to climate variation through complex interactions between seasonal mortality and density-dependent effects.

Animals↗
PubMed2004

Role of evolution by natural selection in population dynamics.

Using a Monte Carlo approach we study the role of inheritance and natural selection in the dynamics of populations. We show that a population subject to inheritance has a much better chance of survival in a given condition than a population where new generations do not inherit genomes of their parents. The dependence of the survival chance on such factors as selection pressure, fecundity, or carrying capacity of the system is much stronger when selection and inheritance are present. We demonstrate, in accordance with biological observations, that in certain conditions evolution can save a population which would perish without it.

Animals↗
PubMed2003

Stochastic population dynamics: the Poisson approximation.

We introduce an approximation to stochastic population dynamics based on almost independent Poisson processes whose parameters obey a set of coupled ordinary differential equations. The approximation applies to systems that evolve in terms of events such as death, birth, contagion, emission, absorption, etc., and we assume that the event-rates satisfy a generalized mass-action law. The dynamics of the populations is then the result of the projection from the space of events into the space of populations that determine the state of the system (phase space). The properties of the Poisson approximation are studied in detail. Especially, error bounds for the moment generating function and the generating function receive particular attention. The deterministic approximation for the population fractions and the Langevin-type approximation for the fluctuations around the mean value are recovered within the framework of the Poisson approximation as particular limit cases. However, the proposed framework allows to treat other limit cases and general situations with small populations that lie outside the scope of the standard approaches. The Poisson approximation can be viewed as a general (numerical) integration scheme for this family of problems in population dynamics.

Monte Carlo Method↗
PubMed2002

Density-dependent birth rate, birth pulses and their population dynamic consequences.

In most models of population dynamics, increases in population due to birth are assumed to be time-independent, but many species reproduce only during a single period of the year. We propose a single-species model with stage structure for the dynamics in a wild animal population for which births occur in a single pulse once per time period. Using the discrete dynamical system determined by the stroboscopic map, we obtain an exact periodic solution of systems which are with Ricker functions or Beverton-Holt functions, and obtain the threshold conditions for their stability. Above this threshold, there is a characteristic sequence of bifurcations, leading to chaotic dynamics, which implies that the dynamical behaviors of the single species model with birth pulses are very complex, including small-amplitude annual oscillations, large-amplitude multi-annual cycles, and chaos. This suggests that birth pulse, in effect, provides a natural period or cyclicity that allows for a period-doubling route to chaos.

Animals↗
PubMed2006

Complex population dynamics and complex causation: devils, details and demography.

Population dynamics result from the interplay of density-independent and density-dependent processes. Understanding this interplay is important, especially for being able to predict near-term population trajectories for management. In recent years, the study of model systems-experimental, observational and theoretical-has shed considerable light on the way that the both density-dependent and -independent aspects of the environment affect population dynamics via impacting on the organism's life history and therefore demography. These model-based approaches suggest that (i) individuals in different states differ in their demographic performance, (ii) these differences generate structure that can fluctuate independently of current total population size and so can influence the dynamics in important ways, (iii) individuals are strongly affected by both current and past environments, even when the past environments may be in previous generations and (iv) dynamics are typically complex and transient due to environmental noise perturbing complex population structures. For understanding population dynamics of any given system, we suggest that 'the devil is in the detail'. Experimental dissection of empirical systems is providing important insights into the details of the drivers of demographic responses and therefore dynamics and should also stimulate theory that incorporates relevant biological mechanism.

Adaptation, Physiological↗
PubMed2003

Climate variation and regional gradients in population dynamics of two hole-nesting passerines.

Latitudinal gradients in population dynamics can arise through regional variation in the deterministic components of the population dynamics and the stochastic factors. Here, we demonstrate an increase with latitude in the contribution of a large-scale climate pattern, the North Atlantic Oscillation (NAO), to the fluctuations in size of populations of two European hole-nesting passerine species. However, this influence of climate induced different latitudinal gradients in the population dynamics of the two species. In the great tit the proportion of the variability in the population fluctuations explained by the NAO increased with latitude, showing a larger impact of climate on the population fluctuations of this species at higher latitudes. In contrast, no latitudinal gradient was found in the relative contribution of climate to the variability of the pied flycatcher populations because the total environmental stochasticity increased with latitude. This shows that the population ecological consequences of an expected climate change will depend on how climate affects the environmental stochasticity in the population process. In both species, the effects will be larger in those parts of Europe where large changes in climate are expected.

Animals↗
PubMed2005

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.

Aging↗
PubMed1988

Age-specific growth rates: the legacy of past population dynamics.

Recent developments in population mathematics have focused attention on a function that is widely available but rarely examined: the set of age-specific growth rates in a population. In particular, this set of rates is sufficient for translating the current birth rate and age-specific mortality rates into the current age distribution. This growth-rate function contains all of the pertinent features of a population's demographic history that are required to relate major demographic functions for a particular period to one another. This article presents an expression for the age-specific growth rate and uses it to derive an equation for age distribution. We show how the value of the age-specific growth rate is determined by a population's demographic past and present various sets of growth rates corresponding to stylized demographic scenarios. Several noteworthy sets of growth rates observed in human populations are discussed. Finally, we explain why age-specific growth rates make it possible to determine the age distribution solely from information on current demographic conditions.

Adolescent↗
PubMed2000

Spatial synchronization of vole population dynamics by predatory birds.

Northern vole populations exhibit large-scale, spatially synchronous population dynamics. Such cases of population synchrony provide excellent opportunities for distinguishing between local intrinsic and regional extrinsic mechanisms of population regulation. Analyses of large-scale survey data and theoretical modelling have indicated several plausible synchronizing mechanisms. It is difficult, however, to determine the most important one without detailed data on local demographic processes. Here we combine results from two field studies in southeastern Norway--one identifies local demographic mechanisms and landscape-level annual synchrony among 28 enclosed experimental populations and the other examines region-level multi-annual synchrony in open natural populations. Despite fences eliminating predatory mammals and vole dispersal, the growth rates of the experimental populations were synchronized and moreover, perfectly linked with vole abundance in the region. The fates of 481 radio-marked voles showed that bird predation was the synchronizing mechanism. A higher frequency of risky dispersal movements in slowly growing populations appeared to accelerate predation rate. Thus, dispersal may induce a feedback-loop between predation and population growth that enhances synchrony.

Animals↗
PubMed2006

Fuzzy subset approach in coupled population dynamics of blowflies.

This paper is a study on the population dynamics of blowflies employing a density-dependent, non-linear mathematical model and a coupled population formalism. In this study, we investigated the coupled population dynamics applying fuzzy subsets to model the population trajectory, analyzing demographic parameters such as fecundity, survival, and migration. The main results suggest different possibilities in terms of dynamic behavior produced by migration in coupled populations between distinct environments and the rescue effect generated by the connection between populations. It was possible to conclude that environmental heterogeneity can play an important role in blowfly metapopulation systems. The implications of these results for population dynamics of blowflies are discussed.

Animals↗
PubMed2001

Modelling viral and CD4 cellular population dynamics in HIV: approaches to evaluate intervention strategies.

Computational models, such as in epidemiology, provide a powerful tool that can be used to systematically examine an array of dynamic interactions among populations as well as to evaluate altemate disease intervention strategies. The specific objectives in this study were to: a/ examine the interaction of cellular (CD4) and HIV population dynamics and evaluate the impact of the use of combination chemotherapies on viral and CD4 populations (Experiment #1), b/ demonstrate how modelling can be used to evaluate the impact of an intervention (condom use) on reducing the rate of HIV/AIDS (Experiment #2). In this study, we used state transition models and conducted simulation experiments to evaluate various alternatives for the control and/or prevention of HIV/AIDS. The result indicated that combination therapy (double or triple drug therapies) was very effective. The HIV viral population decreased rapidly and remained suppressed for years. On the other hand, the CD4 cell population increased above 400 cells per ml and was maintained above that level for many years. Mono-therapy was not as effective; although the viral load decreased rapidly, it increased to its original levels within a few months. Since condom use is one of the key interventions of HIV/AIDS, we evaluated its use in 25%, 50% and 75% of an adult, sexually active population. Increasing condom use by 50% and 75% above an estimated baseline of 25% reduced the incidence of AIDS by 53% in Blacks, 49% in Hispanics and 43% in Whites. The study shows how a cellular/molecular level model can be incorporated within a macro-epidemiologic systems dynamics model to evaluate a variety of scientific questions such as to see if cellular/molecular level interventions reduce morbidity and mortality rates in HIV.

Anti-HIV Agents↗
PubMed2007

Sensitivity analysis of transient population dynamics.

Short-term, transient population dynamics can differ in important ways from long-term asymptotic dynamics. Just as perturbation analysis (sensitivity and elasticity) of the asymptotic growth rate reveals the effects of the vital rates on long-term growth, the perturbation analysis of transient dynamics can reveal the determinants of short-term patterns. In this article, I present a completely new approach to transient sensitivity and elasticity analysis, using methods from matrix calculus. Unlike previous methods, this approach applies not only to linear time-invariant models but also to time-varying, subsidized, stochastic, nonlinear and spatial models. It is computationally simple, and does not require calculation of eigenvalues or eigenvectors. The method is presented along with applications to plant and animal populations.

Animals↗
PubMed2004

Realistic population dynamics in epidemiological models: the impact of population decline on the dynamics of childhood infectious diseases. Measles in Italy as an example.

Most contributions in the field of mathematical modelling of childhood infectious diseases transmission dynamics have focused on stationary or exponentially growing populations. In this paper an epidemiological model with realistic demography is used to investigate the impact of the non-equilibrium conditions typical of the transition to sustained below replacement fertility (BRF) recently observed in a number of western countries, upon the transmission dynamics of measles. The results depend on the manner we model the relation between the (changing) age distribution of the population and contacts. Under some circumstances the transitional ageing phase typical of BRF populations might complexly interact with epidemiological variables leading to (i) a substantial reduction in the amount of vaccination effort required for eliminating the disease; (ii) a significant magnification of the perverse impact of vaccination in terms of the burden of severe age related morbidity.

Adolescent↗
PubMed2005

Herbivory and abiotic factors affect population dynamics of Arabidopsis thaliana in a sand dune area.

Population dynamics of the annual plant Arabidopsis thaliana (L.) Heynh. were studied in a natural habitat of this species on the coastal dunes of the Netherlands. The main objective was to elucidate factors controlling population dynamics and the relative importance of factors affecting final population density. Permanent plots were established and plants were mapped to obtain data on survival and reproductive performance of each individual, with special attention to herbivore damage. In experimental plots we studied how watering, addition of nutrients, artificial disturbance, and natural herbivores affected survival and growth. Mortality was low during autumn and early winter and high at the time of stem elongation, between February and April. A key factor analysis showed a high correlation between mortality from February to April and total mortality. The specialist weevils Ceutorhyncus atomus and C. contractus (Curculionidae) were identified as the major insect herbivores on A. thaliana, reducing seed production by more than 40 %. These herbivores acted in a plant size-dependent manner, attacking a greater fraction of the fruits on large plants. While mortality rates were not affected by density, fecundity decreased with density, although the effect was small. Adding water reduced mortality in rosette and flowering plant stages. Soil disturbance did not increase seed germination, but did have a significant positive effect on survival of rosette and flowering plants. Seed production of A. thaliana populations varied greatly between years, leading to population fluctuations, with a small role for density-dependent fecundity and plant size-dependent herbivory.

Animals↗