Suchergebnisse

Plant-soil feedback mechanisms influence the abundance and rarity of plant species and can favour invasive processes, including those of native species. To explore these mechanisms, we analysed correlations between spatial distributions of plant biomass and soil properties in two neighbouring grasslands at different phases of expansion of the native Eurasian tall-grass Brachypodium rupestre (Host) Roem & Schult (B. rupestre cover: >75 and 25–50%). For this, we applied spatially explicit sampling, geostatistical analysis and structural equation models (SEM) to probe causal relationships among measured variables involved in nutrient accumulation. We hypothesized that if litter accumulates as a result of reduced grazing, 'fertility islands' (spots of high SOM and nutrient contents) will form under B. rupestre clumps because the increase in resource inputs from litter will trigger SOM build-up and promote microbial growth. Our results show that 'fertility islands' of P and amino acids occurred under the patchy clumps of B. rupestre in the less invaded grassland. In addition, the SEMs indicated that nutrient accumulation was partially due to mineralization of the SOM and modulated by the soil microbial biomass. However, there was no correlation between spatial patterns of B. rupestre biomass, SOM and microbial biomass. Moreover, the SEMs explained small amounts of variance in them (SOM r2 = 0.22 and microbial biomass r2 = 0.08), suggesting that factors other than B. rupestre biomass were responsible for the high fertility below the patches. Our spatially explicit approach demonstrated that litter inputs in dense temperate grassland communities can generate 'fertility islands' that may favour the stability and expansion of a tall-grass invader and suggest that herbivory may enhance or inhibit this phenomenon. ; The study was funded by 'la Caixa' Foundation, Spain and CAN foundation, Spain (LCF/PR/PR13/51080004), the Ministry of Science and Innovation of the Spanish Government (project refs. CGL2010-21963, CGL2011-29746 and CGL2017-85490-R), and Interreg Sudoe Programme, European Regional Development Fund, European-Union, Open2preserve Project (SOE2/P5/E0804). L. Múgica and M. Durán were funded through a UPNA Research Staff Training Grant.

This article explores marital fertility on the Aegean island of Paros based on family-reconstitution data from one main town and one village on the island, namely Naoussa and Kostos. By probing the reproductive behaviour of couples who married between 1894 and 1953 it was found that fertility was still 'natural' on the island at the beginning of the twentieth century, while a substantial fertility decline made itself visible only in the late 1920s and early 1930s. The way the population switched from natural to controlled fertility is also explored, as well as the contribution of different socio-economic groups to fertility transition. In the end, an effort is made to place the examined population in a wider European and national context.

Rapid and continuous decline in the size of the human family has been widespread during the last half-century. The main facts are widely known. A brief summary will serve as an introduction to what follows.In most European countries a period of continuous decline in the birth-rate began about 1870. In France the decline began much earlier, in eastern European countries somewhat later. We find the same picture, varying only in time of origin and tempo, in the United States of America, the British Dominions, Russia, and probably in Japan. A precise measure of the trend in terms of gross and net reproduction rates cannot be given over a long period, but the changes in these measures are available for several countries from 1911 to 1931. The drop in the gross reproduction rate in these twenty years was 50 per cent in Sweden, 38 per cent in Denmark, 37 per cent in England and Wales. In Australia it was 41 per cent between 1912 and 1933. Studies of smaller population units reveal the same trend. Among the counties of England and Wales, the smallest percentage decrease between 1911 and 1931 was 18 per cent. In Scotland a few counties showed smaller percentage decreases, the smallest being 11 per cent.

The total fertility rate is lower in island nations than non-island nations, and in small land-area nations than large land-area nations. Because islands tend to be much smaller than non-islands, these phenomena raise the question of whether there are two distinct effects or two manifestations of a single effect. This paper uses 1990 cross-sectional data on 198 countries to estimate the independent effects of islands and land-area on fertility. We find evidence that there are two separate effects. One possible explanation for the island and small-country fertility effects can be drawn from the theory of the commons, but another possible explanation is offered by the new theory of economic growth. We explore each of these possible explanations and conclude that the small-country effect is consistent with new theories of economic growth while the island effect is likely the result of a 'commons effect'.

SummaryAtoll-dwelling people of Tokelau in the central Pacific have been studied in their home environment and after migration to the industrial culture of New Zealand (NZ). The NZ migrants reflect the predominantly male and young composition of the migrating group. Recent migration has eased the lot of the surplus young women from Tokelau, resulting in earlier marriage and less long-term spinsterhood. The apparently higher fertility of women residing in NZ is largely explained by selection of female migrants who already have two or more children. Earlier menarche and earlier marriage in NZ probably contribute little. Future trends will be monitored, and a decline in migrant fertility is expected, if Tokelauans follow the pattern of other Polynesian migrants to urban environments.

SummaryThe Tokelau Island Migrant Study is outlined. It is a multi-disciplinary study of health and social change in the Tokelau islanders, following up emigrants to New Zealand (NZ). In this paper fertility and related aspects are examined, comparing pre-migrants (persons subsequently known to have emigrated to NZ) with non-migrants. Pre-migrant females tended to be younger, to marry later, to be less often childless and to have suffered more miscarriages, than non-migrants. Tokelau parity lagged behind other Polynesian populations due to later marriages, and reached a level below that of the more westernized Rarotongans and NZ Maoris.Tokelau life expectancy was estimated to be 62·2–62·6 years for males and 62·9–66·4 years for females.

With high pop densities, large proportions dependent on agri, & rates of natural increase generally in excess of 2% per yr, the Caribbean exemplifies all the modern demographic problems of the underdeveloped areas of the world. Fertility remains high, though there is some suggestion of a decline in Puerto Rico, & mortality has declined steeply in nearly all the islands. Recent emigration has reduced rates of growth in Puerto Rico & the British Islands, but the indications are that massive increments are to be expected unless definite policies of fertility control materialize. AA.