Population dynamics determine whether adrenocortical or adrenomedullary factors predominate in psychosocially stressed rats.
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Adrenal Glands↗
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Long-term dynamics of Ligula intestinalis and roach Rutilus rutilus: a study of three epizootic cycles over thirty-one years.
Data are presented on 2 full epizootic cycles and the start of a third of Ligula intestinalis in roach Rutilus rutilus in a small lake, and the relationships of these cycles to the densities of rudd, Scardinius erythrophthalmus, and Great Crested Grebes, Podiceps cristatus, over 31 years. The parasite was introduced to the lake by P. cristatus in 1973 at a time when the roach population had increased in response to eutrophication to a level at which individual fish growth was stunted and the hithero dominant rudd population had declined in numbers as a consequence of inter-specific competition with roach. Ligula prevalence peaked at 28% in only 2 years: thereafter parasite-induced host mortality caused a decline in the roach population, releasing fish from stunting and allowing the rudd population to recover. The consequent improved growth of roach individuals and their short life-span reduced Ligula transmission rates and prevalence levels declined to approximately 1% although Ligula nevertheless persisted for a further 10 years. Following a massive winter-kill of the fish populations in 1984-1985, fish and Ligula numbers declined to barely detectable levels and the parasite disappeared from samples. Rudd recovered first, then roach and interspecific competition again led to a decline in rudd numbers. This increase in roach numbers led to a decrease in roach growth rates, which coincided with the re-colonization of the lake by Ligula. This second epizootic of Ligula peaked within 2 years in 1991-1992, when up to 78% of roach were infected with a maximum abundance of 2.2 parasites and intensity of 21 parasites. Heavy parasite-induced mortality of roach led to a decline in numbers, an improvement in individual growth rate and a reduction of Ligula transmission rates such that the epizootic died out in 1996. Similar conditions of roach numbers and growth prevailed at the start of a third cycle in 1998. The course of events over the second cycle was so similar to that of the first that it confirms the interpretations of that cycle. Comparison with other localities shows that epizootics of Ligula always coincide with rapid increases in roach numbers, for whatever cause, and stunted growth, which together attract piscivorous birds. At the start of a cycle Ligula is a major determinant of the population dynamics of the roach, but at the end of the cycle the fish population dynamics determine those of the parasite. The cycles are not regulated and the roach-Ligula system is inherently unstable.
Animals↗
[The epidemiology of yellow fever in Western Africa].
Observations made during the epidemics in Côte d'Ivoire (1982), Burkina Faso (1983), Nigeria (1986 and 1987) and Mali (1987), together with studies conducted in the last 10 years, particularly in Côte d'Ivoire, now make it possible, without calling into question the dynamics of yellow fever virus circulation in space and time, to redefine some features of the pattern suggested in 1977 and refined on a number of occasions up to 1983. The endemicity area is still the region of epizootic and enzootic sylvatic circulation, and contains the natural focus and the endemic emergence zone. --The natural focus is no longer confined to the forest alone, now that transovarial transmission has been demonstrated. --The endemic emergence zone is tending to become conterminous with the endemicity area on account of increasing deforestation. Emergence in forest regions, due to Aedes africanus, is still few and isolated, unlike that observed in savanna regions where A. furcifer is the major vector. The different behaviour of these two vectors and their population dynamics determine the quality of the man-vector contact and are responsible for these two patterns of emergence. --The emergence front limits the endemicity area in the north. Its position varies and depends on annual rainfall patterns. The epidemicity area, where the virus does not circulate before an epidemic and where the immunity status of unvaccinated populations is low, is geographically heterogeneous. It consists of regions to the north of the emergence front and of towns anywhere. It is characterized by high potentials for the development of A. aegypti populations. Only man can introduce the virus into this area. Three types of epidemic are distinguished, depending on the vectors: --Urban epidemics resulting from transmission by a domestic vector. These epidemics always occur within the epidemicity area, either in dry savanna (rural subtype) or in towns (urban subtype). The virus is introduced into the ecosystem by man. Transmission is always strictly interhuman. --Intermediate epidemics consist of two successive phases: first of all there is a series of endemic emergences, followed by interhuman transmission involving A. aegypti. These epidemics can only occur in the endemicity area. --Sylvatic epidemics occur in villages, but only involve the sylvatic vectors. They result from a conjunction of a very large number of emergences for which A. furcifer is almost always mainly responsible, and occur in the endemicity area, usually close to the emergence front. Transmission is never strictly interhuman, as the same vector populations are responsible for epizootic and epidemic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)
Aedes↗
Aboriginal population prospects.
The authors examine data from the 1986 and 1991 Australian censuses to assess discrepancies between the census data and past projections of the size and structure of the Aboriginal population. They also "comment on ways in which determinants of Aboriginal population change are diverging from the parameters used for previous projections. We pay particular attention to mortality prospects.... We note the evidence for under-enumeration of the Aboriginal population in particular age groups in the 1991 Census as in previous censuses, and estimate the size of adjustments necessary to correct for some, but not all, of these deficiencies. The analysis shows that Aboriginal fertility increased in the second half of the 1980s."
Age Distribution↗
Niche breadth in parasites: an evolutionarily stable strategy model, with special reference to the protozoan parasite Leishmania.
A parasite's host range essentially defines its niche breadth, which, as foraging theory predicts, is influenced by resource availability. For parasites, the interaction of infection and transmission characteristics with host population dynamics determines host availability. An epidemiological model, involving two host types and describing competition between a "generalist" parasite strain and a related "specialist" strain, is used to examine the interplay among host range, relative host availabilities, and adaptational compromises engendered by increased host range. Results show that the generalist can predominate even when it cannot maintain itself in either host alone, but that the specialist can persist if its reproductive rate attains some threshold relative to either of the generalist's respective rates in its two hosts. The model is in rough, qualitative agreement with observed dynamics of two Leishmania parasite-host systems, and overall results suggest that infection of two species with a common parasite can lead to complex, indirect coevolutionary dynamics.
Adaptation, Biological↗
A neural mass model for MEG/EEG: coupling and neuronal dynamics.
Although MEG/EEG signals are highly variable, systematic changes in distinct frequency bands are commonly encountered. These frequency-specific changes represent robust neural correlates of cognitive or perceptual processes (for example, alpha rhythms emerge on closing the eyes). However, their functional significance remains a matter of debate. Some of the mechanisms that generate these signals are known at the cellular level and rest on a balance of excitatory and inhibitory interactions within and between populations of neurons. The kinetics of the ensuing population dynamics determine the frequency of oscillations. In this work we extended the classical nonlinear lumped-parameter model of alpha rhythms, initially developed by Lopes da Silva and colleagues [Kybernetik 15 (1974) 27], to generate more complex dynamics. We show that the whole spectrum of MEG/EEG signals can be reproduced within the oscillatory regime of this model by simply changing the population kinetics. We used the model to examine the influence of coupling strength and propagation delay on the rhythms generated by coupled cortical areas. The main findings were that (1) coupling induces phase-locked activity, with a phase shift of 0 or pi when the coupling is bidirectional, and (2) both coupling and propagation delay are critical determinants of the MEG/EEG spectrum. In forthcoming articles, we will use this model to (1) estimate how neuronal interactions are expressed in MEG/EEG oscillations and establish the construct validity of various indices of nonlinear coupling, and (2) generate event-related transients to derive physiologically informed basis functions for statistical modelling of average evoked responses.
Algorithms↗
Conditions determining the local stability of populations.
The conditions that determine the local stability classification of an equilibrium population configuration are analyzed. The population investigated is age-structured and density-dependent, where density is determined by an age-weighted population size. 2 demographic parameters are introduced: the marginal birth rate and the marginal death rate, which describe the marginal density-dependence of the birth and death rates of the equilibrium population. Certain necessary and/or sufficient conditions determining stability are developed, most of them involving the net reproduction rate of the population, and examples illustrating these conditions are presented.
Age Factors↗
[The singularity of population policies in Latin America and the Caribbean in the late 20th century].
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Americas↗
[361 million problems].
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Conservation of Natural Resources↗
A century of demographic and socio-economic dynamics in Egypt.
"The purpose of the present paper is to help elucidate the mechanisms and reasons behind demographic changes in Egypt during the century from 1882 to 1981 using the Box-Jenkins time series technique...." Projections of demographic and socioeconomic trends up to the year 2000 are also made.
Africa↗
Differential plague-transmission dynamics determine Yersinia pestis population genetic structure on local, regional, and global scales.
Plague, the disease caused by the bacterium Yersinia pestis, has greatly impacted human civilization. Y. pestis is a successful global pathogen, with active foci on all continents except Australia and Antarctica. Because the Y. pestis genome is highly monomorphic, previous attempts to characterize the population genetic structure within a single focus have been largely unsuccessful. Here we report that highly mutable marker loci allow determination of Y. pestis population genetic structure and tracking of transmission patterns at two spatial scales within a single focus. In addition, we found that in vitro mutation rates for these loci are similar to those observed in vivo, which allowed us to develop a mutation-rate-based model to examine transmission mechanisms. Our model suggests there are two primary components of plague ecology: a rapid expansion phase for population growth and dispersal followed by a slower persistence phase. This pattern seems consistent across local, regional, and even global scales.
Animals↗
Intercensal population change in the Belfast urban area 1971-91: the correlates of population increase and decrease in a divided society.
"This paper analyses population change in the Belfast Urban Area (BUA) [of Northern Ireland] between 1971 and 1991 in its socio-economic context to understand the nature of urban social change. Although considering religious factors as a cause of population change, the research compares the BUA with other urban areas and relates developments in Belfast to some literature on sociospatial polarisation and the ¿underclass'.... The results suggest that population change is not driven by explicitly religious factors, the relationship between religion and socio-economic well-being has changed, and that changes in religious mixing and segregation occur in many contexts. The BUA is like other urban areas, albeit with unique features arising from the influence of religion."
Demography↗
The rapid fertility decline in Guam natives.
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Adolescent↗
The Easterlin effect.
"The Easterlin effect posits cyclical changes in demographic and social behavior as the result of fluctuations in birth rates and cohort size during the post-World War II period.... The Easterlin effect has generated a large literature in the several decades since it was first proposed. Our review of the empirical studies notes the diversity of support across behaviors, time periods, and nations.... Our review emphasizes both the contingent nature of the Easterlin effect and the way in which conditions have changed in recent decades to reduce the salience of cohort size for social and demographic behavior."
Behavior↗
The demography of inequality: considerations on black people in Brazil.
The author examines the demographic characteristics of the self-declared black population in Brazil. "A...general conclusion is that there are three demographic profiles: that of Whites, of Blacks, and of mixed blood.... The analysis has shown that [blacks and mixed bloods] are subjected to changes in mortality, marriage, and fertility, though at differing times. Mixed bloods are increasingly coming closer to whites, certainly as a result of the different rhythms of social, economic and cultural changes."
Americas↗
Unifying the epidemiological and evolutionary dynamics of pathogens.
A key priority for infectious disease research is to clarify how pathogen genetic variation, modulated by host immunity, transmission bottlenecks, and epidemic dynamics, determines the wide variety of pathogen phylogenies observed at scales that range from individual host to population. We call the melding of immunodynamics, epidemiology, and evolutionary biology required to achieve this synthesis pathogen "phylodynamics." We introduce a phylodynamic framework for the dissection of dynamic forces that determine the diversity of epidemiological and phylogenetic patterns observed in RNA viruses of vertebrates. A central pillar of this model is the Evolutionary Infectivity Profile, which captures the relationship between immune selection and pathogen transmission.
Adaptation, Physiological↗
Telomere dynamics determine episodes of anticancer drug resistance in rat hepatoma cells.
Clinical and experimental observations indicate that resistance to anticancer drugs may be spontaneously reversible over time, but the mechanisms of this reversal are unknown. The resistance of cultured hepatoma cells to methotrexate (MTX) and cisplatin was followed for 9 months. Cells were exposed to three treatments: MTX 200 nM for 24 h or 15 nM continuously and cisplatin 50 microM for 2 h. We investigated the relation between the temporal pattern of cell resistance and the previously reported fluctuations in cell proliferation rate, telomere length and telomerase activity. Spontaneous major peaks in resistance to each drug fell in time windows of 2-3 months (60-70 population doublings) and were at different times for each drug. The frequency of the fluctuations in drug resistance was the same as that of variations in cell growth rate, but amplitudes were unrelated. By contrast, resistance was directly related to telomere length dynamics in the same cells. MTX resistance occurred when telomeres shortened and cisplatin resistance when they were elongated. Furthermore, peaks of resistance to the continuous treatment with MTX were observed at 350-bp intervals of mean telomere length (9.06, 9.41, and 9.76 kbp) during the two 2-month phases of telomere shortening. Statistical analysis demonstrates the sinusoidal relationship between intermittent MTX resistance and telomere length. Possibly, erosion of telomeres encroaches on periodically spaced nucleosomal proteins, defining the onset of resistance phases. This evidence that resistance of tumoral cells to anticancer drugs may be intermittent and that onset of resistance is dictated by telomere length has major implications for clinical practice.
Animals↗
An evaluation of strategic and threshold control measures against the Karoo paralysis tick, Ixodes rubicundus (Acari: Ixodidae) in South Africa.
Paralysis caused by feeding female Ixodes rubicundus ticks is a major problem in large areas of South Africa. As the life cycle of the tick extends over a period of 2 years, it was hypothesized that strategic treatment of sheep with an acaricide over a 2 year period, timed to kill most engorging females, should markedly lower the biotic potential of the tick. Two flocks of sheep grazing in separate paddocks known to be infested with I. rubicundus were treated either strategically or on a threshold basis (i.e. only when tick challenge exceeded a predetermined critical level in terms of paralysis) for a 2 year period. The tick burdens of untreated control sheep running with the two flocks were monitored over a 4 year period and their seasonal dynamics determined. The times at which peak infestations occurred were similar for both flocks of sheep, but significant differences in mean tick burdens between the two flocks were recorded. Tick numbers on sheep in the strategically treated flock did not decrease during the third and fourth years of the trial as was expected. Possible reasons for this were low stocking densities, especially during times of peak abundance of adults and the presence of wild hosts which maintained tick populations.
Animals↗