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Results for “population density fluctuations”

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PubMed2004

Population response to climate change: linear vs. non-linear modeling approaches.

BACKGROUND: Research on the ecological consequences of global climate change has elicited a growing interest in the use of time series analysis to investigate population dynamics in a changing climate. Here, we compare linear and non-linear models describing the contribution of climate to the density fluctuations of the population of wolves on Isle Royale, Michigan from 1959 to 1999. RESULTS: The non-linear self excitatory threshold autoregressive (SETAR) model revealed that, due to differences in the strength and nature of density dependence, relatively small and large populations may be differentially affected by future changes in climate. Both linear and non-linear models predict a decrease in the population of wolves with predicted changes in climate. CONCLUSIONS: Because specific predictions differed between linear and non-linear models, our study highlights the importance of using non-linear methods that allow the detection of non-linearity in the strength and nature of density dependence. Failure to adopt a non-linear approach to modelling population response to climate change, either exclusively or in addition to linear approaches, may compromise efforts to quantify ecological consequences of future warming.

Age Factors↗
PubMed1998

Density-dependent prenatal androgen exposure as an endogenous mechanism for the generation of cycles in small mammal populations

Small mammal populations exhibit cyclic fluctuations in their population densities. Several hypotheses regarding the mechanisms underlying these population cycles have been advanced, but none has yet gained general approval. We propose here an endogenous mechanism based on the masculinization of female offspring in response to increased population levels. High population levels trigger the non-specific stress response resulting in high levels of circulating androgens in individuals of the population, including pregnant females. These androgens masculinize female offspring in utero, thereby reducing the reproductive capacity of the next generation and subsequently the population size. We have developed and analysed a mathematical model to investigate the possible role of prenatal androgen exposure in the generation of limit cycles. We find the locus of Hopf bifurcations for this model and show that limit cycles depend on three parameters: (1) the delay between birth and sexual maturation; (2) the slope of the function that relates average prenatal androgen exposure to total population density; and (3) the difference between the maximum birth rates of the low- and high-androgen exposed females. We derive the analytical form relating these parameters at the Hopf-bifurcation locus and discuss its biological ramifications. In brief, in each of these three parameters is sufficiently large, population cycles will results from the endogenous mechanism proposed.Copyright 1998 Academic Press Limited Copyright 1998 Academic Press Limited

Journal Article↗
PubMed1988

Extinction dynamics of age-structured populations in a fluctuating environment.

We model density-independent growth of an age- (or stage-) structured population, assuming that mortality and reproductive rates fluctuate as stationary time series. Analytical formulas are derived for the distribution of time to extinction and the cumulative probability of extinction before a certain time, which are determined by the initial age distribution, and by the infinitesimal mean and variance, mu and sigma 2, of a diffusion approximation for the logarithm of total population size. These parameters can be estimated from the average life history and the pattern of environmental fluctuations in the vital rates. We also show that the distribution of time to extinction (conditional on the event) depends on the magnitude but not the sign of mu. When the environmental fluctuations in vital rates are small or moderate, the diffusion approximation gives accurate estimates of cumulative extinction probabilities obtained from computer simulations.

Aging↗
PubMed1988

[Spatial-temporal organization of the deep cortex of the lymph node].

In various areas of the deep cortex: central and peripheral parts, as well as T-territory (adjoining the fundus of the germinative center) various cells (small and middle lymphocytes, immunoblasts, plasmoblasts, immature and mature plasma cells and mitotically dividing cells) have been counted. The population density of various immunocompetent cells differs significantly in various areas of the deep cortex. Immunoblasts predominate in the center, cells of the plasmic line--in the periphery of the deep cortex and in the T-territory. Fluctuations of the population density are time-dependent. Spectral composition and parameters of rhythmicity for each cellular type and general course of the process are estimated. In the spectral composition ultradian components predominate. The circadian components are more specific for T- than for B-dependent zone. For immunoblasts in all the zones a common period of fluctuations, nearly 10 h, is revealed, and for the cells of the plasmic line 7 hours' period is common; therefore, it is possible to suppose presence of synchronizing pace-makers of the rhythm for T-lymphocytes, on the one hand, and for antibody-forming cells, on the other hand. Spectral composition of rhythmicity of the cells in the T-territory and in other part of the deep cortex has a number of similar components, that demonstrates functional unity of these zones. A higher part of the cells of the plasmic line in the T-territory and in the periphery of the deep cortex is, perhaps, connected with their migration along these areas towards medullar cords. The general process is of a complex character, rises and drops are registered at various time of the day.

Animals↗
PubMed1962

The transmission of Schistosoma haematobium in an area of Lake Province, Tanganyika.

This paper records the results of detailed studies carried out over a two-year period on the transmission of Schistosoma haematobium in an area of Lake Province, Tanganyika. The ecology of a variety of snail habitats is described; and data on the biology of the principal molluscan host (Bulinus (Physopsis) nasutus productus), on seasonal fluctuations in its population density and associated cercarial infection rates, on its response to desiccation and on other aspects of population dynamics are presented and discussed.Taking into consideration the seasonal fluctuation in snail numbers which occurs in the area and the snails' capacity to survive desiccation, it is considered that a substantial reduction in transmission of S. haematobium might be effected by application of a molluscicide timed so as to lower the population density before aestivation begins, followed by a second treatment when the habitats have been refilled by rainfall to reduce yet further the population that has survived the first treatment and subsequent desiccation. It is also suggested that a combination of methods directed against two stages in the schistosome life-cycle-the snail and the miracidium-by application of molluscicides and treatment of infected persons might be more efficacious than an attempt merely to reduce snail density and alter the population structure.

Animals↗
PubMed1972

The biology of Biomphalaria choanomphala and B. sudanica in relation to their role in the transmission of Schistosoma mansoni in Lake Victoria at Mwanza, Tanzania.

A study of the intermediate snail hosts of Schistosoma mansoni in Lake Victoria at Mwanza, Tanzania, was begun in October 1969, the main aims being to investigate the distribution and seasonal variations in population densities of Biomphalaria choanomphala and B. sudanica in relation to the nature of the lake bottom and the biological features of the lake shore, the factors influencing variations in the intensity of S. mansoni transmission along the Mwanza shoreline, and the age structure of populations of B. choanomphala. Field surveys were made at 70 sites near Mwanza and in nearby bays, B. choanomphala being collected from the lake bottom by means of a wire-mesh dredge. Variations in the distribution and population density of B. choanomphala were correlated with the nature of the bottom and its depth profiles at depths of 0.5-6.0 m. Approximately 1-20 snails/m(2) were found on mixed sand and mud but only about 1 snail/m(2) on the predominantly muddy bottom farther out from the shore. Seasonal variations in the age structure and fluctuations in the population densities of B. choanomphala of as much as 10-13-fold were observed. A large and a small form of B. choanomphala, possibly ecophenotypes, were found. S. mansoni infection rates in B. choanomphala ranged from 0.2% to 3.3%, suggesting a tendency to higher infection rates in mature snails.

Animals↗
PubMed2002

Evolution of density- and patch-size-dependent dispersal rates.

Based on a marginal value approach, we derive a nonlinear expression for evolutionarily stable (ES) dispersal rates in a metapopulation with global dispersal. For the general case of density-dependent population growth, our analysis shows that individual dispersal rates should decrease with patch capacity and-beyond a certain threshold-increase with population density. We performed a number of spatially explicit, individual-based simulation experiments to test these predictions and to explore further the relevance of variation in the rate of population increase, density dependence, environmental fluctuations and dispersal mortality on the evolution of dispersal rates. They confirm the predictions of our analytical approach. In addition, they show that dispersal rates in metapopulations mostly depend on dispersal mortality and inter-patch variation in population density. The latter is dominantly driven by environmental fluctuations and the rate of population increase. These conclusions are not altered by the introduction of neighbourhood dispersal. With patch capacities in the order of 100 individuals, kin competition seems to be of negligible importance for ES dispersal rates except when overall dispersal rates are low.

Adaptation, Biological↗
PubMed1976

Temperature limitation of methanogenesis in aquatic sediments.

Microbial methanogenesis was examined in sediments collected from Lake Mendota, Wisconsin, at water depths of 5, 10, and 18 m. The rate of sediment methanogenesis was shown to vary with respect to sediment site and depth, sampling date, in situ temperature, and number of methanogens. Increased numbers of methanogenic bacteria and rates of methanogenesis correlated with increased sediment temperature during seasonal change. The greatest methanogenic activity was observed for 18-m sediments throughout the sampling year. As compared with shallower sediments, 18-m sediment was removed from oxygenation effects and contained higher amounts of ammonia, carbonate, and methanogenic bacteria, and the population density of methanogens fluctuated less during seasonal change. Rates of methanogenesis in 18-m sediment cores decreased with increasing sediment depth. The optimum temperature, 35 to 42 C, for sediment methanogenesis was considerably higher than the maximum observed in situ temperature of 23 C. The conversion of H2 and [14C]carbonate to [14C]methane displayed the same temperature optimum when these substrates were added to sediments. The predominant methanogenic population had simple nutritional requirements and were metabolically active at 4 to 45 C. Hydrogen oxidizers were the major nutritional type of sediment methanogens; formate and methanol fermentors were present, but acetate fermentors were not observed. Methanobacterium species were most abundant in sediments although Methanosarcina, Methanococcus, and Methanospirillum species were observed in enrichment cultures. A chemolithotropic species of Methanosarcina and Methanobacterium was isolated in pure culture that displayed temperature optima above 30 C and had simple nutritional requirements.

Cell Count↗
PubMed2003

Prevalence of coccidia parasites (Protozoa) in red squirrels (Sciurus vulgaris): effects of host phenotype and environmental factors.

We investigated the relative importance of environmental factors versus host phenotype in determining parasite prevalence in Eurasian red squirrels (Sciurus vulgaris). One hundred and forty-three fecal samples of 116 different squirrels collected in 2000 and 2001 from five study areas in the Italian Alps, were examined for intestinal protozoans. Two species of Eimeria were present with a medium to high prevalence in both years and in all areas, while two other species were rare, occurring only in some areas and not in all years. Cryptosporidium parvum had a high prevalence in the two study areas of the Western Alps, while in the three areas of the Central Alps it was recorded only once. The prevalence of Eimeria sciurorum and C. parvum fluctuated in parallel with squirrel density, suggesting a possible correlation between the presence of these protozoans and host density. A gender effect on E. sciurorum prevalence at low density could be explained by different space use patterns and social organization of males and females. C. parvum occurred more frequently in young squirrels, suggesting an acquired immunity in adults, but age-related susceptibility was not found for eimerian species. The coccidian community was more similar within than between regions, and study area and year were key parameters in predicting coccidia infection. There was no evidence of competition between coccidian species, but one positive interaction between E. sciurorum and E. andrewsi was observed. Our results suggest that the effects of geographic region, area features, and year effects probably related to fluctuations in host population density, were more important than individual phenotypic host characteristics in structuring the coccidian assemblage and determining levels of parasite prevalence in red squirrel populations.

Animals↗
PubMed2001

Age, sex, density, winter weather, and population crashes in Soay sheep.

Quantifying the impact of density, extrinsic climatic fluctuations, and demography on population fluctuations is a persistent challenge in ecology. We analyzed the effect of these processes on the irregular pattern of population crashes of Soay sheep on the St. Kilda archipelago, United Kingdom. Because the age and sex structure of the population fluctuates independently of population size, and because animals of different age and sex respond in different ways to density and weather, identical weather conditions can result in different dynamics in populations of equal size. In addition, the strength of density-dependent processes is a function of the distribution of weather events. Incorporating demographic heterogeneities into population models can influence dynamics and their response to climate change.

Aging↗
PubMed2001

Data estimation and the colour of time series.

There has been a long debate on the source of temporal fluctuations in natural population densities. The difficulty is that unpredictable irregularities might be attributed either to external environmental factors or to chaotic dynamics of populations, or even to the interaction of these two factors. Some years ago Cohen (1995) pointed out that real time series follow redshifted Fourier power spectra, while the simplest chaotic population dynamical models are mostly blueshifted. Since then, the controversy has focused on comparisons of Fourier spectra originating from different models and data. Here, we show experimentally that estimation process by human observers shifts power spectra to the red. This result implies that because of estimation distortion, real population data must be less redshifted than many recorded time series suggest.

Animals↗
PubMed1994

Theoretical studies of the effects of heterogeneity in the parasite population on the transmission dynamics of malaria.

Periodicity in malaria transmission has generally been ascribed to seasonal fluctuations in mosquito population density or spatial heterogeneity with periodic migration. In this paper we demonstrate that simple models of strain heterogeneity can generate periodic behaviour as a consequence of the interaction between parasite strains and host immunological defences. As the degree of cross-immunity between strains increases, the system moves from a régime of independent strain transmission and coexistence, through increasingly coupled behaviour, to the displacement of the strain of lower transmissibility by the strain with a higher basic reproductive rate (R0). Cross-immunity thus serves both to bring the strains into competition, and also to couple the dynamics. We find analytical and numerical results on strain coexistence to show how the range of possible outcomes may be read as an effect of the tension between these two effects of cross-protection.

Animals↗
PubMed1963

A LARGE-SCALE EXPERIMENT IN THE CONTROL OF AQUATIC SNAILS BY THE USE OF MOLLUSCICIDES ON A SUGAR ESTATE IN THE NORTHERN REGION OF TANGANYIKA.

The author describes a large-scale experiment in which the molluscicide Bayer 73 (Bayluscide) was used in an attempt to eliminate Biomphalaria pfeifferi, the snail host of Schistosoma mansoni, from an irrigation system in Tanganyika. Applied at a concentration of 1 p.p.m., the molluscicide gave a very high kill of snails and much of the treated area remained completely free of vector snails for seven months after treatment. However, there were a few survivors in small pockets associated with a drainage area that became flooded during heavy rains coincident with the application of molluscicide. From these survivors a dramatic resurgence of snails occurred in some of the treated canals. This resurgence may be analogous to similar phenomena observed in some insecticide work. The suggestion is made that removal of parasites and predators, in particular trematode parasites, by a molluscicide might increase the snail's capacity for repopulation. Studies of the seasonal fluctuations of snail population density in an adjacent, but separate, irrigation system suggest that molluscicide applications would be more effective if timed to coincide with the end of the rainy seasons.

Animals↗
PubMed1986

Field observations on the population dynamics of Bulinus globosus, the intermediate host of Schistosoma haematobium in the Ifakara area, Tanzania.

The population dynamics of Bulinus globosus were studied in a stream and an adjacent pool near Ifakara, Tanzania. Experiments with caged snails were used to determine fecundity, hatching rates, growth and mortality. Growth and mortality rates were also estimated for the natural population from successive size frequency distributions. Oviposition began in snails as small as 6.6 mm and there was a positive correlation between the size of the snails at oviposition and the number of embryos per egg mass. Fecundity was season-dependent. In the pool, snail growth was constant throughout the year, while in the stream, growth was markedly reduced during the rainy season. Mortality rates correlated with fluctuations in snail population density and were highest during the rainy season and among young snails. The high intrinsic rate of natural increase observed for B. globosus enables it to exist in habitats which alternate frequently between favorable and adverse conditions.

Animals↗
PubMed2003

Spatio-temporal dynamics of the grey-sided vole in Hokkaido: identifying coupling using state-based Markov-chain modelling.

Explaining synchronization of cyclical or fluctuating populations over geographical regions presents ecologists with novel analytical challenges. We have developed a method to measure synchrony within spatial-temporal datasets of population densities applicable to both periodic and irregularly fluctuating populations. The dynamics of each constituent population is represented by a discrete Markov model. The state of a population trajectory at each time-point is classified as one of 'increase', 'decrease', 'peak' or 'trough'. The set of populations at any time-point is characterized by the frequency distribution of these different states, and the time-evolution of this frequency distribution used to test the hypothesis that the dynamics of each population proceeds independently of the others. The analysis identifies years in which population coupling results in synchronous states and onto which states the system converges, and identifies those years in which synchrony remains high but is accounted for by coupling observed in previous years. It also enables identification of which pairs of sites show the highest levels of coupling. Applying these methods to populations of the grey-sided vole on Hokkaido reveals them to be fluctuating in greater synchrony than would be expected from independent dynamics, and that this level of synchrony is maintained through intermittent coupling acting in ca. 1 year in four or five. High synchrony occurs between sites with similar vegetation and of similar altitude indicating that coupling may be mediated through shared environmental stimuli. When coupling is indicated, convergence is equally likely to occur on a peak state as a trough, indicating that synchronization may be brought about by the response of populations to a combination of different stimuli rather than by the action of any single process.

Animals↗
PubMed2000

A Reevaluation of Density-Dependent Population Cycles in Open Systems.

Studies motivated by consideration of barnacle populations have led to the prediction of two different dynamic states for space-limited open populations subject to density-dependent mortality. Population densities may cycle or fluctuate stochastically around a mean value. Despite the potential generality of the associated theory, there are few examples of population cycling in open systems that have been shown to be driven by density-dependent effects. This may be because settlement and growth processes are generally too slow or too variable to generate consistent cycles. An alternative explanation is examined in this article using spatially explicit simulations. Even under conditions of consistent settlement and growth, the cycles predicted in at least one previous study are shown to represent a special case. Clear population cycles are only observed when the density-dependent disturbances are constrained to reoccur in exactly the same location. In the more general case, where density-dependent disturbances respond to local variations in population density, the cycling predicted from simple models is difficult to detect. Hence, a failure to detect cycling in population density does not refute a role for density dependence. Density-dependent disturbances can create a characteristic spatial structure consisting of a mosaic of cohorts.

barnacles↗
PubMed2006

Density, climate and varying return points: an analysis of long-term population fluctuations in the threatened European tree frog.

Experimental research has identified many putative agents of amphibian decline, yet the population-level consequences of these agents remain unknown, owing to lack of information on compensatory density dependence in natural populations. Here, we investigate the relative importance of intrinsic (density-dependent) and extrinsic (climatic) factors impacting the dynamics of a tree frog (Hyla arborea) population over 22 years. A combination of log-linear density dependence and rainfall (with a 2-year time lag corresponding to development time) explain 75% of the variance in the rate of increase. Such fluctuations around a variable return point might be responsible for the seemingly erratic demography and disequilibrium dynamics of many amphibian populations.

Animals↗
PubMed1986

Frequency-dependent selection in logistic growth models.

This paper describes the dynamics of a continuously reproducing diploid population with two alleles at one locus. The dependent variables are allele frequency and population density. We modify the basic density-dependent logistic growth model by inserting three possible types of frequency dependence in the fitness functions. These models are analyzed and contrasted with the purely density-dependent situation. Examples are given of periodic fluctuations in allele frequency and population density, which would be impossible for purely density-dependent fitness functions.

Animals↗