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Results for “population dynamics”

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At least 613 records · Page 34Linked to original sources
PubMed2001

Two-sex population dynamics in space: effects of gestation time on persistence.

Most single-species population models assume either that one sex dominates the growth dynamics (usually the female), or that the life cycles of the two sexes are identical; however, sexual differences in ontogenetic features can render this assumption invalid. Further, the interaction between sexes is necessarily nonlinear, and the dependence of dynamic behavior on sexual interactions can be complicated. Here we examine a two-sex population model, related to the well-known logistic model, with explicit sexual interactions. The model is bistable and, by the addition of diffusion, admits traveling wave solutions. Dominance of states via this spatial dynamic are examined. A simple condition for neutral dominance is obtained; sexual interactions inhibit the dominance of the nonzero population, making persistence more difficult.

Animals↗
PubMed1981

Boundary-layer model for the population dynamics of single species.

We develop a new discrete-time model, called the boundary-layer model, to describe the dynamics of single species that have a capacity for fast growth at very low population densities. The model explicitly separates the dynamics of the population at very low densities (within the "boundary layer") and at high densities. The boundary-layer model provides a better fit than other models such as the logistic or the theta model to data from experimental populations of Drosophila willistoni and D. pseudoobscura.

Journal Article↗
PubMed1994

Seasonal and spatial population dynamics of the nitrogen-efficient guild in a desert bajada grassland.

This study examined the temporal and spatial variation in the populations of bacteria from a Chihuahuan desert bajada grassland that could grow on nearly nitrogen-free medium, a nitrogen-efficient guild (NEG). A Curtis similarity index of 0.876 and nearly identical diversity and equability indexes (H' = 1.22, J = 0.46 and H' = 1.27, J = 0.48 for the crown edge and interplant samples, respectively) indicated that there was no qualitative difference between the NEG assemblages isolated from samples taken at Bouteloua eriopoda plant crowns and in nonvegetated areas 45 cm from crowns. The difference in NEG populations between the crown and interplant samples was only 2-fold, despite a 9.5-fold difference in root biomass between the sites. These differences, while consistent, were statistically significant at only 50% of the sampling times. There was over an order of magnitude difference in NEG numbers in root-associated soil and in bulk soil from the crown or intercrown sites. The typical trend for temporal variation in NEG numbers was that they increased in the spring, fluctuated dramatically over the summer, and declined at the summer's end. The pattern of soil moisture change was the only abiotic variable which showed the same fluctuation pattern as NEG numbers.

Bacteria↗
PubMed2006

Robustness: predicting the effects of life history perturbations on stage-structured population dynamics.

Matrix-based models lie at the core of many applications across the physical, engineering and life sciences. In ecology, matrix models arise naturally via population projection matrices (PPM). The eigendata of PPMs provide detailed quantitative and qualitative information on the dynamic behaviour of model populations, especially their asymptotic rates of growth or decline. A fundamental task in modern ecology is to assess the effect that perturbations to life-cycle transition rates of individuals have on such eigendata. The prevailing assessment tools in ecological applications of PPMs are direct matrix simulations of eigendata and linearised extrapolations to the typically non-linear relationship between perturbation magnitude and the resulting matrix eigenvalues. In recent years, mathematical systems theory has developed an analytical framework, called 'Robustness Analysis and Robust Control', encompassing also algorithms and numerical tools. This framework provides a systematic and precise approach to studying perturbations and uncertainty in systems represented by matrices. Here we lay down the foundations and concepts for a 'robustness' inspired approach to predictive analyses in population ecology. We treat a number of application-specific perturbation problems and show how they can be formulated and analysed using these robustness methodologies.

Animals↗
PubMed1994

General practitioner distribution and population dynamics: Munich, 1950-1990.

The problem of variable geographic distribution of physicians appears to be universal and intransigent. There is growing evidence from several developed countries of primary physicians avoiding areas in large cities increasingly populated by poor and ethnic minorities. This paper extends the research to Germany. Here, the changing population structure of Munich from 1950 through 1990, characterized by a large influx of foreign guestworkers, is illustrated. Selected changes in the medical care system and the role of the general practitioner pertinent to the question of supply and distribution are also discussed. Within this context, the geographic distribution of general practitioners' offices is examined for possible response to the concentration of foreigners in Districts within the City. In the aggregate, the distribution of general practitioners follows the geographic trends of the general population during the study period. However, geographic distribution of general practitioners varies widely on a District by District assessment. Though the exception, in several Districts there are indications that general practitioners may be avoiding locating offices in areas characterized by high density of foreign populations. This paper also draws attention to the importance of investigative scale to conclusions relating general practitioner distribution to spatial-temporal population trends.

Avoidance Learning↗
PubMed1986

The population dynamics and epidemiology of intestinal nematode infections.

The paper reviews recent studies on the population biology, transmission dynamics and epidemiology of intestinal nematode infections. Particular attention is given to patterns of change in average intensity of infection with age, the possible role of acquired immunity assessed via reinfection studies after chemotherapy, evidence of predisposition to heavy (or light) infection by single and multiple species of parasites and control by mass, selective and targetted chemotherapy.

Adolescent↗
PubMed1980

The model for human population dynamics as a part of the global biosphere model: some aspects of modeling in a dialogue regime.

"A matrix model for an age structured population with 4 groups is presented.... The demography matrix is identified with data from global demographic statistics for the 1970s. When calibrating the matrix elements, a semi-formal procedure was used to calculate the dominant eigenvalue and corresponding eigenvector. This procedure was based essentially on the dialogue mode of computation provided by the programing language APL." The advantages of using APL are discussed

Demography↗
PubMed2005

The role of competition and clustering in population dynamics.

A simple argument based on the distribution of individuals amongst discrete resource sites is used to show how the form of single species population models depends on the type of competition between, and the spatial clustering of, the individuals. For scramble competition between individuals, we confirm earlier demonstrations that the Ricker model is a direct consequence of a uniform random distribution of individuals across resources. By introducing spatial clustering of individuals according to a negative binomial distribution, we are able to derive the Hassell model. Furthermore, the tent map model is seen to be a consequence of scramble competition and an ideal-free distribution of individuals. To model contest competition under different degrees of spatial clustering we derive a new three-parameter model, of which the Beverton-Holt and Skellam models are special cases, where one of the parameters relates directly to the clustering distribution. Other population models, such as the quadratic model and the theta-Ricker models, cannot be derived in our framework. Taken together our derivations of population models allows us to make a more rigorous prescription for model choice when fitting to particular datasets.

Animals↗
PubMed1994

Population dynamics of the copia, mdg1, mdg3, gypsy, and P transposable elements in a natural population of Drosophila melanogaster.

The insertion site polymorphism of the copia, mdg1, mdg3, gypsy, and P transposable elements was analysed by in situ hybridization to the polytene chromosomes in genomes of males from a natural population of Drosophila melanogaster. Parameters of various theoretical models of the population biology of transposable elements were estimated from our data, and different hypotheses explaining TE copy number containment were tested. The copia, mdg1 and gypsy elements show evidence for a deficiency of insertions on the X chromosomes, a result consistent with selection against the mutational effects of insertions. On the contrary, mdg3 and P copy numbers fit a neutral model with a balance between regulated transposition and excisions. There is no strong evidence of a systematic accumulation of elements in the distal and proximal regions of the chromosomes where crossing over and ectopic exchanges are reduced. For all chromosome arms but 3L, however, the TE site density increases from the proximal to the distal parts of the chromosomes (the centromeric regions were excluded in this analysis) with sometimes a sharp decrease in density at the extreme tip, following in part the exchange coefficient. The way the copy number of TEs is contained in genomes depends thus on the element considered, and on various forces acting simultaneously, indicating that models of TE dynamics should include details of each element.

Animals↗
PubMed1984

Population dynamics of killing parasites which reproduce in the host.

For a parasitic infection in human hosts a model is derived from basic assumptions on the population structure of the host, in particular mortality depending on age and parasite load, and on the reproduction and transmission of parasites. The model assumes the form of a system of partial differential equations. The paper contains proofs of local and global existence and existence and uniqueness of nontrivial stationary states, and a discussion of the relation to birth and death processes and other models for parasitic infections.

Age Factors↗