A theoretical and empirical investigation of the invasion dynamics of colicinogeny.
(9/4953)
A mathematical model describing the dynamics of a colicinogenic and a colicin-sensitive population propagated under serial transfer culture conditions was formulated. In addition, a series of in vitro invasion experiments using six representatives of the E colicin group was undertaken, together with the estimation of the growth rates and colicinogenic characteristics of the strains. Growth rates among the strains varied by up to 44%. There were 14-fold differences among strains in their lysis rates and there were up to 10-fold differences in the amount of colicin produced per lysed cell. The in vitro serial transfer invasion experiments revealed that regardless of initial frequency all colicinogenic strains succeeded in displacing the sensitive cell populations. The amount of time required for the colicin-sensitive cell population to be displaced declined as the initial frequency of the colicinogenic population increased and strains producing higher titres of colicin tended to displace the sensitive strain more rapidly. Overall, the observed dynamics of the invasion of colicinogenic strains was adequately described by the theoretical model. However, despite there being substantial differences among the strains in their growth rates and colicinogenic characteristics there were relatively few differences, observed or predicted, in the invasion dynamics of the six colicinogenic strains. These results suggest that the characteristics of different colicinogenic strains cannot be used to explain the extensive variation in the relative abundance of different colicins in natural populations of bacteria. (+info)
Cycles and trends in cod populations.
(10/4953)
Year-to-year fluctuations in fish stocks are usually attributed to variability in recruitment, competition, predation, and changes in catchability. Trends in abundance, in contrast, are usually ascribed to human exploitation and large-scale environmental changes. In this study, we demonstrate, through statistical modeling of survey data (1921-1994) of cod from the Norwegian Skagerrak coast, that both short- and long-term variability may arise from the same set of age-structured interactions. Asymmetric competition and cannibalism between cohorts generate alternating years of high and low abundance. Intercohort interactions also resonate the recruitment variability so that long-term trends are induced. The coupling of age-structure and variable recruitment should, therefore, be considered when explaining both the short- and long-term fluctuations displayed by the coastal cod populations. Resonant effects may occur in many marine populations that exhibit this combination of traits. (+info)
An economic evaluation of "Health for All".
(11/4953)
The World Health Organization's 'Global Strategy' is an ambitious vision, but to achieve its goals it must first be implemented. Implementation will require careful and detailed planning. This paper evaluates the possibilities of transforming the Global Strategy from a laudable policy initiative into an actual 'Plan for Health', from the point of view of a health economist. This economic evaluation assesses the probable costs of implementing various activities of the Strategy, and the likelihood that developing countries will be able to afford these costs, either on their own, or with the assistance of the developed countries. A final section considers the current global situation and presents trends over the last two decades. The numbers of countries that have already achieved the goals of the Strategy, that can be expected to achieve the goals of the Strategy by the year 2000, and that are unlikely to achieve these goals (on the basis of current trends) are shown. The WHO 'success indicator' based on numbers of countries is compared to a more epidemiological one based on deciles of the world's population. It is argued that, even several years after the initiation of the Global Strategy, insufficient information exists on the next logical step of transforming the Policy into a Plan. Unless adequate attention is paid to this vital step, implementation of the Strategy will inevitably be ad hoc and patchy. Further research on the costs of the activities proposed by the Global Strategy, and the probable effects on health of those activities, is desperately needed. (+info)
Ecological approaches and the development of "truly integrated" pest management.
(12/4953)
Recent predictions of growth in human populations and food supply suggest that there will be a need to substantially increase food production in the near future. One possible approach to meeting this demand, at least in part, is the control of pests and diseases, which currently cause a 30-40% loss in available crop production. In recent years, strategies for controlling pests and diseases have tended to focus on short-term, single-technology interventions, particularly chemical pesticides. This model frequently applies even where so-called integrated pest management strategies are used because in reality, these often are dominated by single technologies (e.g., biocontrol, host plant resistance, or biopesticides) that are used as replacements for chemicals. Very little attention is given to the interaction or compatibility of the different technologies used. Unfortunately, evidence suggests that such approaches rarely yield satisfactory results and are unlikely to provide sustainable pest control solutions for the future. Drawing on two case histories, this paper demonstrates that by increasing our basic understanding of how individual pest control technologies act and interact, new opportunities for improving pest control can be revealed. This approach stresses the need to break away from the existing single-technology, pesticide-dominated paradigm and to adopt a more ecological approach built around a fundamental understanding of population biology at the local farm level and the true integration of renewable technologies such as host plant resistance and natural biological control, which are available to even the most resource-poor farmers. (+info)
The transition to agricultural sustainability.
(13/4953)
The transition to sustainable growth in agricultural production during the 21st century will take place within the context of a transition to a stable population and a possible transition to a stable level of material consumption. If the world fails to successfully navigate a transition to sustainable growth in agricultural production, the failure will be due more to a failure in the area of institutional innovation than to resource and environmental constraints. (+info)
Phlebotomine sand fly (Diptera: Psychodidae) seasonal distribution and infection rates in a defined focus of Leishmania tropica.
(14/4953)
A two-year study was conducted of phlebotomine sand fly fauna in a defined focus of Leishmania tropica. A total of 17,947 sand flies representing 10 species were collected from the location. Phlebotomus guggisbergi, a vector of L. tropica in Kenya, was the most prevalent species through the entire period, representing about 80% of the total catch. There was marked seasonal fluctuation in the populations of the three most common species, with highest population levels reached in December and lowest levels reached in July and August. Leishmania-like infections were encountered in 489 P. guggisbergi. No flagellate infections were observed in any other species of sand fly. Although infected P. guggisbergi were collected during each month of the year, the percent parous infected flies was highest (27.5%) during the November through January time period. These data show that the greatest risk of transmission to humans at this focus occurs during December, when the vector is prevalent and infections are common. (+info)
Longitudinal study of infection with Borrelia burgdorferi in a population of Peromyscus leucopus at a Lyme disease-enzootic site in Maryland.
(15/4953)
The maintenance of Borrelia burgdorferi in a population of Peromyscus leucopus was investigated from 202 mark and recapture mice and 61 mice that were removed from a site in Baltimore County, Maryland. Borrelia burgdorferi infection was detected by culture and polymerase chain reaction (PCR) of ear tissue, and exposure to the spirochete was quantified by serology. Overall prevalence of B. burgdorferi, as determined by culture and PCR of ear tissue at first capture, was 25% in the longitudinal sample and 42% in the cross-sectional sample. Significantly more juvenile mice were captured in the longitudinal sample (18%) than in the cross-sectional sample (0%). Among 36 captured juvenile mice, only one was infected with B. burgdorferi; this contributed to a significant trend for infection with B. burgdorferi with age. Recovery from infection with B. burgdorferi was not detected among 77 mice followed for an average of 160 days. The incidence rate of infection with B. burgdorferi was 10 times greater in mice captured during two periods of high risk of exposure to nymphal Ixodes scapularis ticks compared with a period of low risk. Maintenance of B. burgdorferi in this population was dependent on indirect transmission of the organism from infected ticks to susceptible mice and development of chronic infection with the spirochete, which had no measurable effect on the survival of infected mice. (+info)
Using maximum likelihood to estimate population size from temporal changes in allele frequencies.
(16/4953)
We develop a maximum-likelihood framework for using temporal changes in allele frequencies to estimate the number of breeding individuals in a population. We use simulations to compare the performance of this estimator to an F-statistic estimator of variance effective population size. The maximum-likelihood estimator had a lower variance and smaller bias. Taking advantage of the likelihood framework, we extend the model to include exponential growth and show that temporal allele frequency data from three or more sampling events can be used to test for population growth. (+info)