The effects of food and density on the nocturnal behaviour ofArion ater andAriolimax columbianus (Pulmonata: Stylommatophora)

Summary The effects of food supply and population density on the nocturnal behaviour ofArion ater andAriolimax columbianus were investigated. Density did not significantly affectA. ater's level of activity or short-term movement, resting, or feeding.A. columbianus was more active and moved, rested, and fed more frequently when slug density was high.A. ater foraged and rested more often, but fed less when good food was unavailable.Ariolimax's only response to the food regime was to feed more when good food was available. Seasonal changes in the level of activity and behaviour ofArion were evident, whereasAriolimax's activity and behavioural repertoire were not similarly affected.Arion ater's nightly activity appeared to be mainly food oriented, whileAriolimax columbianus seemed most responsive to slug density during its nocturnal activity periods.     

Longer Life and Population Growth

Enthusiasm about the prospect of large increases in human life expectancy is often dampened by fears that lower mortality will increase population size, hence population pressure. A simple mathematical model of life-cycle stretching demonstrates that if increased longevity is accompanied by later childbearing, a trend that is already underway, future declines in mortality will not increase population size.     

Demography of an island population of the lizard,Eumeces okadae, on Miyake-jima, Izu Islands

Summary The demography of the lizardEumeces okadae was studied for 4 years on Miyake-jima, in the Izu Islands, central Japan. Most males matured at 3 years of age with a mean snout-vent length of 77.5 mm. Females matured at 3–4 years with a mean SVL of 77.8 mm, and reproduced biennially thereafter. Growth continued after maturity with a decreasing rate. In females, annual growth was negligible in reproductive years, but resumed markedly in non-reproductive years. Clutch size increased with SVL and was used to construct a fecundity schedule. Population density (exclusive of hatchlings) was estimated to be ca. 4000 individuals/ha in August, an exceptionally high density for lizards. Before introduction of the weasel onto Miyake-jima, low predation pressure had allowedE. okadae to maintain a high population density. Estimated annual survival rates were 36% for hatchlings, 56% for yearlings, 80% for 2-year-olds, 63% for adult males and 76% for adult females. The cohort generation time (6.95 years) was so long that theE. okadae population can be characterized by a combination of low natality and slow turnover.     

Comparative analysis of population characteristics of the brown planthopper,Nilaparvata lugens st?l, between wet and dry rice cropping seasons in West Java, Indonesia

Summary Population dynamics of the brown planthopper (BPH),Nilaparvata lugens St?l, were investigated in paddy fields in the coastal lowland of West Java, Indonesia, where rice is cultivated twice a year, in the wet and dry cropping seasons. Distinct differences in the basic features of population dynamics were detected between the two rice cropping seasons: (1) In the wet season, BPH populations multiplied rapidly in the period from initial to peak generation, reaching quite often the destructive level despite the low density of initial immigrants. However, in the dry season, the population growth rate and the peak population density were much lower than those in the wet season. The abundance of natural enemies such as arthropod predators played a major role in determining such a difference in seasonal population development. (2) The density at the peak generation or the occurrence of outbreaks in each field was predictable in the wet season with fairly high accuracy on the basis of the density at the initial or previous seasonal generations. In the dry season, however, the rate of population growth and the peak population density widely varied among the fields depending on the water status in each field. (3) Density-dependent processes to regulate the population density were detected in both cropping seasons. In the wet season, the regulatory processes were only detected in such high densities as cause the considerable deterioration of host plants, which suggested that the processes were largely attributable to intra-specific competition. In the dry season, however, the regulatory processes operated at a much lower density in the earlier stages of the crops. The results of an analysis of adult longevity or residence period suggested that the density-dependent dispersal of macropterous adults played an important role in stabilizing the population fluctuation among the fields in the early dry season.     

Population dynamics of the green leafhopper,Nephotettix virescens Distant (Hemiptera: Cicadellidae) in synchronized and staggered transplanting areas of paddy fileds in Indonesia

Summary The population dynamics ofNephotettix virescens, a vector of rice tungro virus disease was investigated in a synchronized transplanting area at Jatisari (1984–1986), West Java and in a staggered transplanting area at Sidan (1986–1988), Bali, Indonesia. The FARMCOP suction sampler was employed for population censuses ofN. virescens and its natural enemies. The population growth pattern was affected by transplanting pattern: In the staggered transplanting area, the population density increased from the immigrant generation to the first generation, and sharply decrease thereafter, while in the synchronized transplanting area the population density often reached the highest peak in the second generation. The degree of contageousness in the spatial distribution ofN. virescens was negatively correlated with population density of the immigrant generation. Contribution from Indonesia-Japan Joint Program on Food Crop Protection     

The Council of Economic Advisers on Work and Retirement Among the Elderly

The 1999 Annual Report of the US council of Economic Advisers (a document of some 450 pages, formally an Annex to the Economic Report of the President Transmitted to the Congress February 1999, Washington, DC: US Government Printing Office) addresses some of the questions an aging population raises for the American economy. (Aspects of this topic were also discussed in earlier Council reports; see the Documents section of the March 1996 and June 1997 issues of PDR.) Chapter 4 of the 1999 report is titled “Work, Retirement, and the Economic Well-Being of the Elderly.” The excerpts from this chapter reproduced below provide a summary of the expected future changes in the age distribution of the US population—a result of improving life expectancy and low fertility—and the concomitant marked changes that will characterize the racial and ethnic composition of the elderly. The report notes recent declines in the rate of disability among persons aged 65 and older and discusses changing patterns of retirement and their causes. Despite long-term improvements in health and longevity, the retirement age has fallen rather than risen during the twentieth century. The report suggests, however, that this trend has now run its course, as influenced by the increasing dominance of physically less demanding service-sector jobs, more flexible work schedules (including availability of part-time employment), and changes in persion arrangements.     

Competition between laboratory populations of green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

Summary Intra- and interspecific competition between laboratory populations of four green leafhoppers,Nephotettix spp. was studied in the laboratory under three different temperature regimes of 24°C, 27°C and 30°C. For the single-species population of the three tropical species, the equilibrium density increased as the temperature increased. On the other hand, for the temperature speciesN. cincticeps, the highest equilibrium density was at the intermediate temperature and the lowest at high temperature. Interspecific interactions between two tropical (N. virescens vs.N. nigropictus), a tropical and a temperature (N. virescens vs.N. cincticeps) and a rice-feeding and a grass-inhabiting (N. virescens vs.N. malayanus) Nephotettix species were also studied in the laboratory at the three temperature regimes. Temperature differentially affected the outcome of competition between twoNephotettix species. BetweenN. virescens andN. nigropictus, the latter was more successful over the former at low and intermediate temperatures, while the former was more successful at high temperature. BetweenN. virescens andN. cincticeps, the temperate species inhibited the growth of the tropical species at low temperature while the tropical species inhibited the growth of the temperate species at high temperature. At intermediate temperature, the population ofN. virescens persisted at a slightly higher density over the population ofN. cincticeps. Between the rice-feedingN. virescens and the grass-inhabitingN. malayanus, regardless of temperature the population density of the latter was greatly reduced and later became extinct while the population of the former continued its growth. These consequences of competition between twoNephotettix species conformed fairly well to those predicted by theLotka-Volterra model using demographic parameters specified for each species.     

Gradients of metropolitan dominance in turkey: Alternative models

The dominance or influence of metropolitan centers in the developing country of Turkey, as measured by gradients of population characteristics (population density, percent urban, sex ratio, percent over five years of age, and percent literate) by distance from the nearest metropolitan center for 1960 and 1965, is found to decrease sharply from the central zone to the next zone (25–49 kilometers from the center), and then change very little with increasing distance from the center, for both urban and rural populations. None of several alternative mathematical models describes the form of the influence of metropolitan centers on the population characteristics of their hinterlands very well, but the best fitting models suggest that population density declines by distance according to an exponential function to the base e, percent urban declines according to a function to the one-fourth power, sex ratio declines for about 100 kilometers and then increases according to a quadratic function, and percent under six years increases and percent literate declines according to a linear function. These results suggest that dominance of a metropolitan center over its hinterland beyond 25 or 50 kilometers in Turkey is not very strong, possibly because of the relatively poorly developed transportation and communication systems. Although metropolitan dominance in Turkey in 1960 and 1965 seems to diminish more sharply with distance and to extend less far from the metropolitan center than in developed countries or than it may as Turkey develops economically and technologically in the future, additional comparative data over time in Turkey and in developed countries at the same time are needed for a rigorous test of this apparent finding.     

Population dynamics of a phytophagous lady-beetle,Epilachna vigintioctopunctata (Fabricius), living in spatio-temporally heterogeneous habitats. III. Effects of habitat structure on population dynamics

Temporal changes in the population size of a phytophagous lady-beetle were analyzed to identify mechanisms affecting lady-beetle population dynamics at different spatial scales. The study area (15 ha) included 18 habitat patches. The major host plants were potato for first generation larvae and eggplant for second generation larvae. The habitat patches were classified into three groups according to the major host plants in each patch: P-E patches (both host plants available), P patches (potato only), and E patches (eggplant only). The winter disappearance of adults in the whole study area, and larval mortality in E patches were apparently the most important factors disturbing the overall population density. Density-dependent movement of females appeared to have the greatest stabilizing effect on the yearly fluctuation of population density. Rate of increase of female adults from the first to the second generation,R, was generally higher on eggplants in E patches than in P-E patches because the adult density of the first generation was much higher in P-E patches. The yearly fluctuation of adult density in each generation tended to be less in patches with all habitat components necessary for the full life cycle (P-E patches). However, such patches were not favorable for first generation females, as indicated by the lower rate of increase from the first to the second generation. The density and stability of lady-beetle populations is discussed in relation to habitat structure.     

The outbreak mechanisms of the green rice leafhopper,Nephotettix cincticeps Uhler, in northern Japan

Intraspecific regulatory processes keep the population ofNephotettix cincticeps stable at a low density in southern Japan. In northern Japan, however, the yearly population density of the insect fluctuates violently, and large outbreaks occasionally take place. To clarify the difference in the population dynamics between the two regions, we analyzed light-trap and sweep-net sampling records from prefectural and national agricultural experimental stations. The survival rate of the overwintering population decreased with increases in the period of continuous snow cover (PCSC) in the north, and initial population densities in the years of long PCSC were too low for populations to reach equilibrium density by the end of the active breeding season. This made yearly population fluctuations in the north much larger than in the south. The equilibrium density in the north was higher than in the south. The higher equilibrium density presumably permits the higher population density and larger yearly population fluctuations in the north. A major factor responsible for the difference in equilibrium densities between the two regions is the difference in heading dates of the host plant (rice). Qualitative differences among rice plant varieties, and among biotypes ofN. cincticeps, may also be important.     

Temporal and spatial relationships between an herbivorous lady beetleEpilachna niponica and its predator, the earwigAnechura harmandi

Summary The relationships between egg predation of an herbivorous lady beetleEpilachna niponica (Lewis) and its predator, the earwigAnechura harmandi (Burr), were examined in both time and space. In spite of little annual, changes in egg densities, egg mortality due to predation varied considerably. There was no, clear relationship between the earwig density and the proportionate predation over the five years. The seasonal occurrence of earwig nymphs on thistle plants, however, was closely synchronized with that of egg predation. Predator attacks on the beetle occurred in a time-restricted manner. Thus, later cohorts mostly escape from heavy predatory pressure. No spatially density-depent egg predation was detected at the level of either thistle plants or thistle patches. Furthermore, there was no indication of aggregative behaviour of the earwig in response to local egg density. The earwig density was more likely to be associated with particular localities with sandy deposits available for its nest site.     

Environmental change and out-migration: evidence from Nepal

Scholars and activists have hypothesized a connection between environmental change and out-migration. In this paper, we test this hypothesis using data from Nepal. We operationalize environmental change in terms of declining land cover, rising times required to gather organic inputs, increasing population density, and perceived declines in agricultural productivity. In general, environmental change is more strongly related to short- than long-distance moves. Holding constant the effects of other social and economic variables, we find that local moves are predicted by perceived declines in productivity, declining land cover, and increasing time required to gather firewood. Long-distance moves are predicted by perceived declines in productivity, but the effect is weaker than in the model of short-distance mobility. We also show that effects of environmental change vary by gender and ethnicity, with women being more affected by changes in the time required to gather fodder and men by changes in the time to gather firewood, and high-caste Hindus generally being less affect than others by environmental change.     

Environmental Change and Out-Migration: Evidence from Nepal

Scholars and activists have hypothesized a connection between environmental change and out-migration. In this paper we test this hypothesis using data from Nepal. We operationalize environmental change in terms of declining land cover, rising times required to gather organic inputs, increasing population density, and perceived declines in agricultural productivity. In general, environmental change is more strongly related to short- than long-distance moves. Holding constant the effects of other social and economic variables, we find that local moves are predicted by perceived declines in productivity, declining land cover, and increasing time required to gather firewood. Long-distance moves are predicted by perceived declines in productivity, but the effect is weaker than in the model of short-distance mobility. We also show that effects of environmental change vary by gender and ethnicity, with women being more affected by changes in the time required to gather fodder and men by changes in the time to gather firewood, and high caste Hindus generally being less affect than others by environmental change.     

Natural mortality at different population densities of a pine shoot moth,Evetria cristata

Summary A field population ofEvetria cristata was studied in 10 plots in 1962 and in 6 plots in 1963. These plots were divided into 2 or 3 groups of different population levels of the shoot moth in respective years. The survival of the insect was then analysed in these different groups of plots. The survival rate ofE. cristata from eggs to adults in the first generation was found always higher in the group with low population density, which indicates the existence of some factors that affect the population more severely when the insect is more abundant.Lissonota evetriae andPediobius sp. seemed to have killed more proportion of the hosts where the shoot moth density was high. However, the total effect of the all natural enemies was not always great in the plots with high density of the moth. The survival of the second generation of the moth in 1963 was observed to be much higher at any population level than in the other generations.     

Food limitation of population density in the orb-wed spider,Nephila clavata

Summary Field studies were conducted to clarify whether variation in food availability among habitats influences population density, and whether population density has a negative effect on foraging success in the orb-web spider,Nephila clavata. Lifetime food consumption per individual (i.e., foraging success) strongly correlated with mean body size of adult females and mean fecundity in populations. Also, there was a positive correlation between foraging success and population density. Since foraging success reflected potential prey availability in the habitat, food resource appeared to be a limiting factor for populations in this spider. Mean fecundity per individual correlated with population density of the following year, suggesting that decreased reproduction is a major component of food limitation on population density. Consistent defferences in mean body size between particular sites were observed over years, while such difference was less obvious in density. Thus, ranking of food abundance among habitats seems to be predictable between years. A field experiment revealed that an artificial increase in population density had no negative effect on the feeding rate of individuals, suggesting that intraspecific competition for food is not important in this species.     

Influence of parental density on the distribution pattern of eggs in the common cabbage butterfly,Pieris rapae crucivora

Summary To investigate the relation between the distribution pattern of eggs and the parental density in the common cabbage butterfly,Pieris rapae crucivora, the countings of egg number per plant were made on both cabbage plants cultivated in the farm and planted in the net house in which the female butterflies were released at various densities. The frequency distribution of eggs fits well to the negative binomial excepting the cases where they agree withPoisson series, and the degree of aggregation expressed as the reciprocal of the parameter, 1/k, tends to decrease as the egg or parental density increases. At the same parental density, however, the distribution of eggs can be described by the negative binomial with a common parameter,k _c, regardless of the difference in the density of laid eggs. In the case where a single female butterfly lays eggs, the spatial pattern of egg distribution is always lean, while its frequencies conform toPoisson or the negative binomial series. This lean changes toward patchy with increasing the parental density. From these results, it is concluded that the degree of aggregation in the distribution of eggs decreases with the increase of the parental density.     

Land use and first birth timing in an agricultural setting

The dramatic changes in the earth’s landscape have prompted increased interest in the links between population, land use, and land cover. Previous research emphasized the notion of population pressure (population pressure increases demands on natural resources causing changes in land use), overlooking the potentially important effects of changes in land use on humans. Using multiple data sets from the Chitwan Valley Family Study in Nepal, we test competing hypotheses about the impact of land use on first birth timing. We argue that while agricultural land should encourage early childbearing, land area devoted to public infrastructure should discourage it. The results show that individuals from neighborhoods with larger proportions of land under agriculture experienced first birth at rates higher than those from neighborhoods with smaller proportions. On the other hand, individuals from neighborhoods with larger proportions of land under public infrastructure experienced first birth at rates lower than those from neighborhoods with smaller proportions. However, the effects of public infrastructure are not as strong as the land area devoted to agriculture.

The global and regional impact of mortality and fertility transitions, 1950-2000

During the second half of the twentieth century, world population grew at a record pace, both in absolute and relative terms, from 2.5 billion to 6 billion (or 1.75 percent annually). Demographers have long identified rapid mortality declines as the main explanation. This article finds that one-fourth of today's world population is alive because of mortality improvements since mid-century. Very rapid growth is unlikely to continue as substantial fertility declines also occurred in recent decades. This article finds that already by the year 2000, these fertility declines have almost exactly compensated for the impact of mortality declines from mid-century levels. This result may suggest homeostasis, but analyses of underlying trends contradict this impression. First, the impact of fertility declines will soon and significantly exceed that of mortality declines. Second, that mortality and fertility declines jointly affect the size of the world population by less than one percent conceals a significant impact on the population's age composition as well as on regional population sizes.     

Leisure: Definitions,Trends, and Policy Implications

Economic theories predict that with modernity and with the increase in standards of living, individuals will aspire for more leisure. However, the results of empirical studies which examined period trends in leisure time across developed countries do not confirm this presumption. The current study asks: If changes in leisure stem from ideational changes among different generations, will trends in leisure look different if examined across cohorts, or if measured differently? By integrating theoretical definitions of leisure based on literatures in economics, sociology, and psychology, this research derives three main macro-level empirical measures of leisure from various sources. These measures are used to analyze the contribution of population turnover to changes in the quantity of leisure, in developed countries, using linear regression decomposition method. Our results show an almost unequivocal increase in leisure across cohorts, across 159 country-periods, suggesting that new policies supporting domestic consumption are warranted.

     

Population density,urban centrality,and agricultural intensification in Jordan

This paper explores the relationship between population growth, agricultural production, and urban development. Ongoing debate in the literature regarding the relationship between population and economic development is restricted by the limited availability of time series data and the difficulty of evaluating causality using cross-sectional data. This analysis uses the special case of Jordan with its massive refugee flows to evaluate the influence of a sudden and exogenous change in population in urban areas on the intensification of agricultural production. Spearman Rank Order correlations are calculated from time series data to show that the districts which experienced the most rapid population growth and increases in population density also exhibited the largest increases in agricultural intensity. Cross-sectional analysis in which measures of agricultural intensification were predicted by population density and urbanization factors reveals a significant interaction between density and urban centrality, where centrality is an indicator of the accessibility of urban goods to rural communities. The results suggest that population density has a strongly positive effect on agricultural intensity in areas with few urban goods and services available, whereas the influence of population density on agricultural intensity is substantially reduced in areas with a greater diversity of urban goods and services.This article is based on a paper presented at the 1992 Meeting of the Southern Demographics Association, Charleston, SC, 15–17 October 1992.