Secular movements in sex ratios of adults and of births in populations during the past half-century.
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There is some evidence for a small overall negative correlation across populations between sex ratio (proportion male) at birth and adult sex ratio. There seems to be no systematic correlation within populations across time of sex ratio at birth with adult sex ratio during the past 50 years. So even if adult sex ratios play some part in determining the overall level of the sex ratio at birth, they apparently have played little role in the recent widespread secular changes in sex ratio at birth. It is shown here that there is a strong cohort effect in adult sex ratios: if a woman is in a marriage squeeze (i.e. in a cohort with a relative abundance of women) at age 15 years, she will remain in such a squeeze for the rest of her reproductive life. In England and Wales, the maternal age-specific sex ratios at birth moved roughly in parallel across time during the years 1950-1995. This suggests that sex ratio at birth is not a cohort phenomenon (as it would be if it were affected by adult sex ratio) but is subject to some agents which change with time and affect women (parents) of all ages roughly equally. (+info)
Estimating the time to extinction in an island population of song sparrows.
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We estimated and modelled how uncertainties in stochastic population dynamics and biases in parameter estimates affect the accuracy of the projections of a small island population of song sparrows which was enumerated every spring for 24 years. The estimate of the density regulation in a theta-logistic model (theta = 1.09 suggests that the dynamics are nearly logistic, with specific growth rate r1 = 0.99 and carrying capacity K = 41.54. The song sparrow population was strongly influenced by demographic (sigma2(d) = 0.66) and environmental (sigma2(d) = 0.41) stochasticity. Bootstrap replicates of the different parameters revealed that the uncertainties in the estimates of the specific growth rate r1 and the density regulation theta were larger than the uncertainties in the environmental variance sigma2(e) and the carrying capacity K. We introduce the concept of the population prediction interval (PPI), which is a stochastic interval which includes the unknown population size with probability (1 - alpha). The width of the PPI increased rapidly with time because of uncertainties in the estimates of density regulation as well as demographic and environmental variance in the stochastic population dynamics. Accepting a 10% probability of extinction within 100 years, neglecting uncertainties in the parameters will lead to a 33% overestimation of the time it takes for the extinction barrier (population size X = 1) to be included into the PPI. This study shows that ignoring uncertainties in population dynamics produces a substantial underestimation of the extinction risk. (+info)
Rapid transition in the structure of a coral reef community: the effects of coral bleaching and physical disturbance.
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Coral reef communities are in a state of change throughout their geographical range. Factors contributing to this change include bleaching (the loss of algal symbionts), storm damage, disease, and increasing abundance of macroalgae. An additional factor for Caribbean reefs is the aftereffects of the epizootic that reduced the abundance of the herbivorous sea urchin, Diadema antillarum. Although coral reef communities have undergone phase shifts, there are few studies that document the details of such transitions. We report the results of a 40-month study that documents changes in a Caribbean reef community affected by bleaching, hurricane damage, and an increasing abundance of macroalgae. The study site was in a relatively pristine area of the reef surrounding the island of San Salvador in the Bahamas. Ten transects were sampled every 3-9 months from November 1994 to February 1998. During this period, the corals experienced a massive bleaching event resulting in a significant decline in coral abundance. Algae, especially macroalgae, increased in abundance until they effectively dominated the substrate. The direct impact of Hurricane Lili in October 1996 did not alter the developing community structure and may have facilitated increasing algal abundance. The results of this study document the rapid transition of this reef community from one in which corals and algae were codominant to a community dominated by macroalgae. The relatively brief time period required for this transition illustrates the dynamic nature of reef communities. (+info)
Umbrellas and flagships: efficient conservation surrogates or expensive mistakes?
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The use of umbrella and flagship species as surrogates for regional biota whose spatial distributions are poorly known is a popular conservation strategy. Yet many assumptions underlying the choice of surrogate species remain untested. By using biodiversity databases containing spatial incidence data for species of concern for (i) the southern California coastal sage scrub habitat, (ii) the Columbia Plateau ecoregion, and (iii) the continental United States, we evaluate the potential effectiveness of a range of conservation surrogate schemes (e.g., big carnivores, charismatic species, keystone species, wide-ranging species), asking how many species potentially are protected by each scheme and at what cost in each habitat area. For all three databases, we find that none of the surrogate schemes we evaluated performs significantly better than do a comparable number of species randomly selected from the database. Although some surrogate species may have considerable publicity value, based on the databases we analyzed, representing diverse taxa on three different geographic scales, we find that the utility of umbrella and flagship species as surrogates for regional biodiversity may be limited. (+info)
Population aging: a comparison among industrialized countries.
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Increasing longevity and declining fertility rates are shifting the age distribution of populations in industrialized countries toward older age groups. Some countries will experience this demographic shift before others will. In this DataWatch we compare the effects of population aging on health spending, retirement policies, use of long-term care services, workforce composition, and income across eight countries: Australia, Canada, France, Germany, Japan, New Zealand, the United Kingdom, and the United States. International comparisons suggest that the United States is generally well positioned to cope with population aging; however, three areas should be carefully monitored: heavy reliance on private-sector funding of retirement, coverage of pharmaceuticals for the elderly, and a high proportion of private long-term care financing. (+info)
Population aging in developing countries.
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Issues related to population aging--Social Security and pension reform, health care financing and provision, and long-term care--have long been the subject of public debate in the industrialized countries of Europe and North America. Economically less developed regions have been slower to adopt aging as a major public policy concern, despite the fact that older populations in many developing countries are growing more rapidly than are those of industrialized nations. Awareness of issues concerning older populations remains low in many nations, even as the absolute numbers of elderly persons double and even triple. This DataWatch considers trends in the basic demography of aging and mortality/health considerations for developing countries. (+info)
Coping with aging: international challenges.
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This DataWatch discusses trends in functional health status among the elderly in several countries. Life expectancy at an advanced age has increased, and there is some evidence of lower prevalence of severe disability and a trend toward deinstitutionalization. These changes may result in fewer severely disabled persons, but the impact is less clear from the perspective of public finances, because of the increased need for formal home care and the relative subsidization of institutions. The implications for health care need further research but also may point to increased expenditures. (+info)
Environmental colour affects aspects of single-species population dynamics.
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Single-species populations of ciliates (Colpidium and Paramecium) experienced constant temperature or white or reddened temperature fluctuations in aquatic microcosms in order to test three hypotheses about how environmental colour influences population dynamics. (i) Models predict that the colour of population dynamics is tinged by the colour of the environmental variability. However, environmental colour had no effect on the colour of population dynamics. All population dynamics in this experiment were reddened, regardless of environmental colour. (ii) Models predict that populations will track reddened environmental variability more closely than white environmental variability and that populations with a higher intrinsic growth rate (r) will track environmental variability more closely than populations with a low r. The experimental populations behaved as predicted. (iii) Models predict that population variability is determined by interaction between r and the environmental variability. The experimental populations behaved as predicted. These results show that (i) reddened population dynamics may need no special explanation, such as reddened environments, spatial subdivision or interspecific interactions, and (ii) and (iii) that population dynamics are sensitive to environmental colour, in agreement with population models. Correct specification of the colour of the environmental variability in models is required for accurate predictions. Further work is needed to study the effects of environmental colour on communities and ecosystems. (+info)