Niche modification and stability of competitive systems. III. Simulation model analysis

Summary Simulation model for niche shift in ecological time scale was constructed on the basis of the optimal foraging theory. In accordance with the previous experimental study (Shimada andFujii, 1985), the model competitive system consisted of 2 parasitoid wasp species utilizing 4 host stages. Wasps were assumed to choose host stages in the manner that they realized the maximal gain/cost values, where gain was represented by body weight of a wasp progeny emerging from each host stage and cost was expressed by time required to search for and detect an unparasitized host. The number of parasitized hosts in each host stage was calculated numerically by usingArditt's (1983) model for avoidance of superparasitism. The model simulated well the experimental results ofShimada andFujii (1985) andShimada (1985). Sensitivity analysis of the model showed that the experimentally derived criterion for competitive coexistence (different second-best host stages between competing species even with the common best) was not necessarily the indispensable condition for niche shift and separation, but that if the criterion was not satisfied, stable competitive coexistence occurred only in the narrow range of the parametric values. Further, niche shift in ecological time scale made the competitive coexistence more stable than fixed niche on which the current niche theory stands.     

Niche modification and stability of competitive systems. II. Persistence of interspecific competitive systems with parasitoid wasps

Summary Effects of niche shift in ecological time scale on the population dynamics of competing species were studied in the experimental populations of two parasitoid wasp species,Anisopteromalus calandrae andHeterospilus prosopidis (both are solitary parasites), on a host, the azuki bean weevil,Callosobruchus chinensis. Four resource conditions were set up with combination of kind of bean (azuki or black eye), and host distribution (uniform or clumped). In each resource condition, four developmental stages of hosts were provided as a resource spectrum for parasitoid wasps. Population dynamics of the two wasp populations were investigated in each resource condition in Multi-Generation Competitive Systems (MGCS), in which fresh hosts of four developmental stages were periodically introduced and were parasitized competitively by the two wasp species. Competitive coexistence of both wasps occurred in the azuki-clumped condition, where the peaks of the resource utilization curves separated in the two species; pupae inA. calandrae and the early or late fourth instar inH. prosopidis, A. calandrae was eliminated in the azuki-uniform condition andH. prosopidis went extinct in two black eye conditions irrespective of host distributions. The degrees of overlap of the resource utilization patterns of the two wasp species during population dynamics were not significantly different among resource conditions irrespective of the results of coexistence or extinction. Even in the azuki-clumped condition, however, extinction ofA. calandrae was observed when resource partitioning could not be realized with only the late fourth instar larvae available to wasps. Further analytical experiments showed that parasitizing ability ofA. calandrae increased with host density per bean with azuki beans, butA. calandrae could express higher parasitizing ability with black eye beans thanH. prosopidis irrespective of host density per bean. The flexibility in parasitizing ability byA. calandrae for various host stages under different resource conditions was thought to be the major factor in determining the competitive coexistence or the extinction of either species under different resource conditions. The present experiments also suggested that different second-best host stages between competitors could be a major contributing factor to competitive coexistence.     

Interspecific relations in forest floor coleopteron assemblages: Niche overlap and guild structure

Summary In six different environments of the Towada-Hachimantai National Park, the forest floor coleopterons were sampled by means of pitfall traps with five different baits. Their niche breadths and niche overlaps were assessed for the electivity of habitat, bait, and habitat × bait, and compared with those expected from neutral models constructed randomly. The niche breadths in the observed assemblages were smaller than those expected from the neutral model, while the niche overlaps were more widely distributed than those expected from the neutral model. Mean overlaps between species neighbors in niche space, cluster analyses, and ordinations of species revealed a guild for habitat, bait and habitat × bait. Lawlor’s (1979) gamma matrices concerning the habitat electivity show that the rates of mutualism resulted from indirect interspecific competitions are significantly smaller in the guild than in the whole assemblage. This suggests that the interspecific competition among the guild members is mitigated or even converted into mutualism by joining of the species which do not belong to the guild. The rates of mutualism were also significantly larger in the resource state of two dimensions (habitat × bait) than in that of either one. Therefore, if niche overlaps are measured on the basis of resource state of multi-dimensions, the rates of mutualism may increase further.     

A modification of a continuous time model for first conception

The interval between marriage and the first conception leading to a live birth plays an important role in the determination of fertility components. Several probability models (Potter and Parker, 1964; Singh, 1961, 1964, 1967) based on varying sets of assumptions relating to this interval have been propounded in the recent past. All of them are based on the assumption that the females under study are susceptible to conception at the time of marriage. However, in certain situations, where some of the females already pregnant at the time of marriage report to have conceived within a short interval following marriage, this condition is not satisfied and these models become unsuitable. A probability distribution which is an inflated form of the continuous model proposed by Singh, for the time of the first conception leading to a live birth, is presented in this paper. It describes reasonably well the data on first conception times in the context of premarital conceptions. Simple expressions for the maximum likelihood estimators of the parameters involved in the model are obtained and a method for finding the asymptotic covariance matrix of the estimators is outlined. The model is applied to four sets of data.     

A test and modification of zipf’s hypothesis for predicting interstate migration

The purpose of this paper is to evaluate the accuracy of Zipf’s hypothesis in estimating interstate migration streams in the United States, and then to determine whether the predictive power of Zipf’s hypothesis can be improved by adding additional variables. The three independent variables in Zipf’s hypothesis accounted for 57 percent of the variation in interstate migration streams in 1935–1940, 61 percent in 1949–1950, and 68 percent in 1955–1960. The addition of per capita personal income of the states of origin and of destination increased the explained variance by only four percentage points in 1935–1940 and by less than one percentage point in 1949–1950 and 1955–1960. Then, the Petersen-Greenwood hypothesis that interstate migration streams in any period are a function of previous flows was tested by adding cumulative lifetime migration as an independent variable. Over 80 percent of the variation in interstate migration streams was explained by cumulative lifetime mobility, as reflected by state of birth data. The conclusion of the study is that the Petersen-Greenwood hypothesis provides a better estimate of interstate migration streams than does Zipf’s hypothesis. Nevertheless, the Petersen Greenwood hypothesis requires further evaluation to determine whether cumulative lifetime mobility is simply a proxy for some other underlying variable and whether it provides accurate estimates of migration streams for other geographic areas.     

A modification for use in destabilized populations of brass's technique for estimating completeness of death registration

The analysis of censuses for pre-industrial Europe has caused doubts about the supposed dominance of stem and joint family organization in earlier times. Using a hypothetical example of a nuclear family organization where extended composition is only found when widowed persons find shelter in the household of one of their children, we show that both the expected value and the variance of the estimated frequency of extended composition are high under demographic circumstances typical of pre-industrial Europe. This makes inferences about family organization based on data concerning family composition hazardous     

A modification for use in destabilized populations of brass's technique for estimating completeness of death registration

Summary Brass has developed a method of estimating completeness of death registration using only data on deaths and population by age and sex. In this paper, his method is briefly outlined and the assumptions upon which it is based are discussed. In particular, the implications of the failure of the assumption of stability of the population are investigated. It is found that in populations where mortality has been declining, use of the technique leads to underestimation of completeness. A modification of the technique based on knowledge of the duration and rate of mortality change is proposed for use in such populations. Using simulated destabilized populations, the modification is tested and found to yield more accurate estimates of completeness of death registration than the unmodified technique. The usefulness of the modified technique is further illustrated by applying it to data for Costa Rican females in 1963.     

A simple method to estimate insect mortality from field census data: A modification of the Kiritani-Nakasuji-Manly method

Various methods have been proposed to estimate demographic parameters such as mortality from field census data. Simple methods proposed earlier are applicable only for limited situations. For example, the Kiritani-Nakasuji-Manly method is applicable only if individuals are observable until their death. Improved methods proposed later are not subject to such limitations, but are not so widely used in the field of applied entomology, probably because of the complexity of the calculations involved. In this paper, I propose an intermediate method that requires only a pocket calculator, considering the practical convenience for field scientists. This method, which is a modification of the Kiritani-Nakasuji-Manly method, gives an estimate of the number of individuals entering a stage from the frequency of two stages when the stage duration is known.     

Ancient practice . . . current conflict. (Abortion)

A brief overview of abortion as a legal and social issue with emphasis on controversial factors is presented. The author notes that abortion involves moral codes and medical developments which are obscured by polarized rhetoric. This editorial is an introduction to a Population Bulletin issue examining abortion from several viewpoints.     

An econometric analysis of the U.S. postwar G.N.P.

Whilst most developed countries have experienced stable economic conditions during the postwar period, the acceptance of the unit root null hypothesis implies wild fluctuations in the major economic variables over time. This paper investigates the sensitivity of the decisions to accept the unit root hypothesis to the specification of the trends underlying the U.S. postwar GNP and other macro-variables. In particular, the relationship between the nominal GNP and the resident population is found to be a non-linear one. The unit root null hypothesis can be firmly rejected when the conditional mean of the nominal GNP series is represented by a quadratic trend variable and the assumptions that changes in the price level and the resident population lead to equiproportionate changes in the GNP are not enforced on the data. The case of quarterly observations is also investigated for seasonally unadjusted and adjusted data.The author is indebted to two anonymous referees and the Managing Editor for helpful comments.     

Correlates of divorce in the U.S.S.R.

The theoretical rationale of this study is that conditions associated with divorce reside outside the family within a broader social system where the family finds itself located. The absence of major differences in divorce law from one place to another within the Soviet Union makes it possible to explore this hypothesis by examining areal differentials in divorce rates. Crude divorce rates and crude marriage rates for 1960 have been published in Vestnik Statistiki for 109 political-administrative areas in the Soviet Union. Several indicators of modernization are available for the same areas from the 1959 U.S.S.R. Census of Population. About 80 per cent of the variation among areas with respect to the crude divorce rate is accounted for by six variables: the crude marriage rate, the percentage of urban population, and the employee-worker ratio in the labor force, each of which is positively associated with the divorce rate; and the proportion of poorly educated women, the ratio of children to adult males, and the mean household-family size, each of which is negatively associated with the divorce rate.     

A rejoinder to S. K. Datta and J. B. Nugent

Extract

The question of old-age security as a motivation for fertility in less-developed rural areas can be put in clearer perspective by pausing to consider the changing roles of land and offspring under the influence of fundamental demographic upheaval. Under the pre-transition regime, one generation approximately replaced the preceding one, particularly once unused but usable land became scarce and the possibility of expanding farm operations became remote. Judging from the settlement patterns and the history of the Maharashtrian study area, such a circumstance probably obtained long before the secular drop in mortality began. During this period, a single son, typically, would survive to adulthood, gradually assuming control of the father's land (or the father's trade, among non-agriculturalists) and, if the father lived long enough, would eventually be a source of security in the father's old age. It is not inappropriate to mention that this generational cycle no doubt fostered a strong urge to leave the family land to a son, so that a sonless farmer would keenly feel a lack of fulfilment. In fact, responses to certain survey questions suggest that ancestral land and male progeny are still somehow connected, according to the way village men think, to their sense of immortality. It would be hard, consequently, to separate old-age security, the idea of ‘continuing a lineage’, and the sense of immortality conferred by owning land into distinct motives for conceiving children.     

A rejoinder to S. K. Datta and J. B. Nugent

Abstract Extract The question of old-age security as a motivation for fertility in less-developed rural areas can be put in clearer perspective by pausing to consider the changing roles of land and offspring under the influence of fundamental demographic upheaval. Under the pre-transition regime, one generation approximately replaced the preceding one, particularly once unused but usable land became scarce and the possibility of expanding farm operations became remote. Judging from the settlement patterns and the history of the Maharashtrian study area, such a circumstance probably obtained long before the secular drop in mortality began. During this period, a single son, typically, would survive to adulthood, gradually assuming control of the father's land (or the father's trade, among non-agriculturalists) and, if the father lived long enough, would eventually be a source of security in the father's old age. It is not inappropriate to mention that this generational cycle no doubt fostered a strong urge to leave the family land to a son, so that a sonless farmer would keenly feel a lack of fulfilment. In fact, responses to certain survey questions suggest that ancestral land and male progeny are still somehow connected, according to the way village men think, to their sense of immortality. It would be hard, consequently, to separate old-age security, the idea of 'continuing a lineage', and the sense of immortality conferred by owning land into distinct motives for conceiving children.     

The status of women in the rural U.S.S.R.

Degree of sex equality in rural areas of the U.S.S.R. is assessed with reference to a multi-variable model which specifies demographic, technological, social, and ideological factors associated with sex stratification. Such analysis reveals that the emphasis in the U.S.S.R. on women's participation in production as the key to sex equality ignores other dimensions of sex stratification which are not changed using this tactic. In particular, rural traditions of higher birth rates, more authoritarian families, greater religious emphasis, and male scorn for women have kept the status of rural women even lower than that of their urban counterparts. Additionally, the agrarian techno-economic base and lack of institutional supports for childcare and housework help perpetuate sex stratification. Increasing sex differentiation is probably in store for the Soviet Union, because official pronatalist policies are likely to be facilitated by expanding the service sector, which will further increase the division of labor in the market.