Parasitoid-mediated effects: apparent competition and the persistence of host–parasitoid assemblages

Indirect effects such as apparent competition (in which two hosts that do not compete for resources interact via a shared natural enemy) are increasingly being shown to be prevalent in the structure and function of ecological assemblages. Here, we review the empirical and theoretical evidence for these enemy-mediated effects in host–parasitoid assemblages. We first address questions about the design of experiments to test for apparent competition. Second, we consider factors likely to affect the coexistence of host species that share a parasitoid and are involved in apparent competition. We show that parasitoid aggregation, and the switching effect that this can generate when hosts occur in separate patches, not only promotes persistence but is also strongly stabilizing. The broader consequences of these effects are discussed. Received: November 6, 1998 / Accepted: January 13, 1999     

Indirect interactions in aphid–parasitoid communities

Indirect interactions between populations of different species can be important in structuring natural communities. Indirect effects are either mediated by changes in population densities (trophic or density-mediated effects) or by changes in the behavior of species that are not trophically connected (behavioral or trait-mediated effects). We reviewed the literature on aphids and their parasitoids to explore the various possible indirect interactions that can occur in such communities. The review was motivated by our study of a particular aphid–parasitoid community in a natural (i.e., nonagricultural) habitat, and by the wealth of information that exists about aphid–parasitoid systems in agricultural settings. We focused our review on aphid–parasitoid interactions, but considered how these were influenced by the other aphid natural enemies and also by aphid mutualists and host plants. We conclude that indirect effects are likely to have a major effect in structuring aphid–parasitoid communities, and that the latter are a valuable model system for testing ideas about community interactions. Received: December 20, 1998 / Accepted: January 12, 1999     

Slow‐fast dynamics in a model of population and resource growth

Models of the interaction of population, the economy, and the environment often contain nonlinear functional relationships and variables that move at different speeds. These properties foster apparent unpredictabilities in system behaviour. Using a simple deterministic model of demographic, economic and environmental interactions we illustrate the usefulness of geometric singular perturbation theory in environmental population economics. In contrast to local stability analysis, the theory of slow‐fast dynamics helps to gain new insights into the global behaviour of the system. In particular, the knowledge of the basins of attraction of the stationary states enables one to determine the regions of sustainable future paths of resources and population.     

The dynamics of the population sex ratio in India, 1971–96

This paper reconstructs the trend in the population sex ratio in India between 1971 and 1996 from available information on changes in sex differentials in mortality in the country since the beginning of the century. It is estimated that, although the mortality of females relative to that of males in India has improved since 1968, the population sex ratio increased between 1971 and 1981, stayed constant between 1981 and 1991, and started to decrease only after 1991. This implies that the recorded decrease and increase in the periods 1971–81 and 1981–91 respectively were both spurious and were the results of undercounts of females in 1971 and 1991. Another implication of this finding is that, owing to the lagged effect of past mortality on current trends in the population sex ratio, this ratio is a bad proxy for use in the study of changes in differential mortality by sex.     

Population fluctuation and persistence of one-host–two-parasitoid systems depending on resource distribution: from parasitizing behavior to population dynamics

Population dynamics and variability were examined in one-host–two-parasitoid experimental systems with different resource distributions: resource-clumped and resource-sparse conditions. The system consists of a seed beetle host, Callosobruchus chinensis, and two parasitoid wasps, Anisopteromalus calandrae (Pteromalidae) and Heterospilus prosopidis (Braconidae). In the resource-clumped condition, suitable hosts for parasitism (the late fourth-instar larvae and pupae) were clumped in 1 large resource patch, but they were scattered evenly among 16 small patches in the resource-sparse condition. Population censuses were conducted at 10-day intervals in long-term cultures, renewing 10 g of azuki beans (Vigna angularis). In both resource conditions, the first period was a single-species system of C. chinensis only, and A. calandrae was added in the second period. The one-host–one-parasitoid system with C. chinensis and A. calandrae showed stable population dynamics with small fluctuations. After addition of H. prosopidis in the third period, two of three replicates persisted to day 800 in each resource condition, although one replicate in each went to extinction at an immediate outbreak of the H. prosopidis population after the introduction. Population variabilities of C. chinensis and H. prosopidis were significantly higher and the mean population size of A. calandrae was significantly smaller in the resource-sparse condition than that in the resource-clumped one. A short-term experiment on parasitism efficiencies revealed that H. prosopidis parasitized significantly more at a low host density in the resource-sparse condition than in the resource-clumped one. Mutual interference of H. prosopidis was weak enough at low parasitoid densities but became abruptly stronger with high densities. Providing fresh hosts in a mixture of already parasitized ones, host-searching behaviors of a parasitoid were recorded by video for 3 h and were compared between the two wasp species. H. prosopidis could parasitize fresh hosts more efficiently than A. calandrae through frequent long-distance walks (walking to distant beans at one bout or outside a clump of beans with hosts and back soon on a distant bean of the clump) after reencounters with parasitized hosts. Considering all the experimental results, populations were judged to be more fragile and more likely to go to extinction in the resource-sparse condition than in the resource-clumped one. A higher attacking efficiency of H. prosopidis destabilized population dynamics more in the resource-sparse condition. Received: December 23, 1998 / Accepted: January 20, 1999     

An age‐structured model of population dynamics with dominant ages,delayed behavior,and oscillations

An age‐structured model of population dynamics with age‐dominance is proposed and analyzed. Existence and uniqueness of solutions are established as well as the uniqueness and local asymptotic stability of steady‐states. Conditions for convergence to or oscillation about the steady‐state are specified in some cases.     

The effects of migrant remittances on population–environment dynamics in migrant origin areas: international migration, fertility, and consumption in highland Guatemala

International migration impacts origin regions in many ways. As examples, remittances from distant migrants may alter consumption patterns within sending communities, while exposure to different cultural norms may alter other behaviors. This paper combines these insights to offer a unique lens on migration’s environmental impact. From an environmental perspective, we ask the following question: is the likely rise in consumption brought about by remittances counterbalanced by a reduction in fertility in migrant households following exposure to lower fertility cultures? Based on ethnographic case studies in two western highland Guatemalan communities, we argue that the near-term rise in consumption due to remittances is not counterbalanced by rapid decline in migrant household fertility. However, over time, the environmental cost of consumption may be mitigated at the community level through diffusion of contraception and family planning norms yielding lower family size.     

A general host–pathogen model with free–living infective stages and differing rates of uptake of the infective stages by infected and susceptible hosts

In addition to their lethal effects, pathogens can cause a number of other debilitating effects on infected hosts. A population dynamical model of the interaction between an invertebrate host and a pathogen is constructed to examine the importance of one such debilitating effect on the host population dynamics. Specifically the feeding rate and therefore the uptake of pathogen free-living infective particles by infected individuals is reduced as a consequence of the pathogen infection. The pathogen is more likely to regulate the host and the equilibrium population density of the host is reduced. Less intuitively there is also an increased chance of the pathogen causing cyclic population dynamics in the host. Received: December 8, 1997 / Accepted: March 23, 1999     

Effects of migration on the educational levels of the black resident population at the origin and destination, 1955–1960 and 1965–1970

Using the 1960 and 1970 census data, this paper analyzes the net effects of the interregional migration of black males on the educational levels of the resident black male population at the regions of origin and destination. Significant variations are observed in the educational selectivity of out-migrants from each region, by region of destination. Comparing the educational levels of the return migrants to the South with those of the resident population in the nonsouthern regions provides no evidence that the return migrants are "failed" migrants. The net effect of interregional migration on the educational levels of the black male resident population at the regions of origin and destination is insignificant in most age groups, for both the 1955--1960 and 1965--1970 periods. In particular, in-migration from the South to nonsouthern regions has little effect on the educational levels of the resident population in most age groups. In fact, for nonsouthern regions, out-migration is more detrimental to the educational level of the resident black male population than is in-migration from the South. Furthermore, the net effect of interregional migration has declined from the 1955--1960 period to the 1965--1970 period.     

Stable growth in native‐dependent multistate population dynamics

Users of multistate life tables and projections have recognized that the Markovian assumptions underlying such models are unduly restrictive and should be relaxed whenever data permit. Efforts to include the influences of previous occupancies have included the incorporation of place‐of‐birth dependence. This paper addresses the stable growth properties of such generalized multistate models. It shows how place‐of‐birth‐specific stable growth measures can be calculated without projection simply by solving the characteristic equation. An example using Canadian data illustrates the argument.     

Reconstructing the size of the African American population by age and sex, 1930–1990

We estimate the size of the African American population in five-year age groups at census dates from 1930 to 1990 using a three-part strategy. For cohorts born after 1935, we follow the U.S. Census Bureau in using classical demographic analysis. To estimate the size of cohorts born before 1895, we use extinct-generation estimates. For remaining cohorts, we implement an age/period/cohort model of census counts. All approaches are applied to a data set in which the age distribution of deaths has been corrected for age misreporting. Results provide strong confirmation of the basic validity of Census Bureau estimates of census undercounts for African Americans while extending estimates to new cohorts and periods. Our estimates are less consistent with an historical series prepared by Coale and Rives (1973).     

The changing spatial distribution and concentration of Australias chinese and Vietnamese communities: An analysis of 1986–2006 Australian population census data

This paper describes the changing spatial distribution and concentration of Chinese and Vietnamese communities in Australia over the two past decades. The research analyses data of individuals (i) born in the People’s Republic of China or Vietnam, (ii) of Chinese or Vietnamese ancestry, and (iii) who spoke Chinese or Vietnamese languages at home. Basic demographic and economic factors at the SLA level are considered to provide an initial discussion of meso-level factors which may have contributed to changes in Chinese-and Vietnamese-Australian distributions and concentrations since 1986. As expected, from field observations, the data reveal a contrasting picture of the spatial distribution and concentration of the Chinese- and Vietnamese-Australian communities, and indicate that although a few areas of high Vietnamese spatial concentration have persisted into 2006, and a few areas of high’ Chinese spatial concentration have developed since 1986, overall there is an increasing dispersion of both communities into the broader Australian community.     

A note on the population of Iran, 1900–1966

The first national census of Iran was held in November 1956, and the second in November 1966. Prior to 1956 the only official population records that existed were figures for an urban headcount (held between June 1939 and August 1941) and the statistics of the Civil Registration Office (C.R.O.), which started operations in 1928.     

A demographic history of the Indo-Dutch population, 1930–2001

Indonesia was a Dutch colony until 1949. In the aftermath of World War II and the independence of the former Dutch East Indies, many people migrated from Indonesia to the Netherlands or other Western countries. This migrant population, known as the Indo-Dutch population, consists of Europeans, Asians, and persons of mixed European-Asian blood. These groups have all associated themselves with and experienced the colonial culture of the former Dutch Indies, and have carried this cultural experience elsewhere through migration. This paper provides a demographic history of the Indo-Dutch population, using a variety of data sources and methods. Starting from the population of ‘Europeans’ according to the 1930 census of the Dutch Indies, a demographic projection is made covering the period 1930–2001. By the beginning of 2001, the estimated number of Indo-Dutch persons is 582,000, including the second generation. Of these 582,000, an estimated 458,000 are living in the Netherlands and 124,000 elsewhere. The composition by age, sex and generation very clearly reflects the demographic history of the population.     

Evaluating multi-regional population projections with Taylor’s law of mean–variance scaling and its generalisation

Organisations that develop demographic projections usually propose several variants with different demographic assumptions. Existing criteria for selecting a preferred projection are mostly based on retrospective comparisons with observations, and a prospective approach is needed. In this work, we use the mean–variance scaling (spatial variance function) of human population densities to select among alternative demographic projections. We test against observed and projected Norwegian county population density using two spatial variance functions, Taylor’s law (TL) and its quadratic generalisation, and compare each function’s parameters between the historical data and six demographic projections, at two different time scales (long term: 1978–2010 vs. 2011–2040; and short term: 2006–2010 vs. 2011–2015). We find that short-term projections selected by TL agree more accurately than the other projections with the recent county density data and reflect the current high rate of international migration to and from Norway. The variance function method implemented here provides an empirical test of an ex ante approach to evaluating short-term human population projections.     

John Maynard Keynes's theories of population and the concept of “optimum”

This review of Keynes' scattered writings on population, which in view of his general importance as an economist might be of interest in itself, is used to highlight the marked shift in demographic theory during the past three decades. The revival of Malthusianism during the 1920's was followed by a contrary emphasis during the following depression decade, and this change in opinion was much more violent than the one in demographic trends that it in part reflected. The current dispute over the relevance of Malthus's theory is marked by a fundamental confusion between a Malthusian and a Keynesian frame of reference. Explicitly or implicitly, various policies are set in accord with what is taken to be the optimum population of a country, but this optimum is not determinable in terms of present theory.     

The colonialism of Canada’s Métis health population dynamics: caught between bad data and no data at all

In the last two decades, Canada’s brand of colonial recognition politics has fueled several social and cultural changes that have, in turn, produced startling growths in the “Métis population” in Canada. Demographers and policy makers alike have expressed dubiousness about the extent to which “non-demographic factors” (that is, factors other than fertility and mortality) are fueling this growth, a dubiousness expressed in the growing use of “ethnic mobility” to explain population growth. In this article I explore the historical contexts within which the idea of a single Métis population took hold as a statistical technology, the kinds of social and cultural juxtaposition that making use of a single population masks, and the impact single population estimates have on the ability of Métis nation policy actors to fashion evidence-based policy relevant to the concerns of the Métis nation. While the dynamics and the data perused in this article are specific to Canada, they possess broader resonances with other nation-states grappling with their colonial histories and longstanding Indigenous peoples as these dynamics relate to official data dynamics.     

Views and concepts relating to population problems in the Netherlands 1918–1939

The problem of the possibility of overpopulation in the Netherlands has never been a subject of debate. The tempo of decline of fertility in this densely populated country was, however, much slower than elsewhere. Discussion started only after 1919, when economic problems relating to a rapidly expanding population were studied. The object of this article is to trace the development of such discussion in the Netherlands between the two world wars. The article is intended to make a contribution to explaining the exceptional situation of the Netherlands during the last hundred years in respect of population.     

The population statistics of China,A.D. 2–1953

Chinese population statistics are collated from the time of the Western Han Dynasty (A.D. 2) to 19 5 3, including statistics of population and households for provinces as well as for China as a whole. Evidence bearing on the definitions of the statistics, methods of compilation, and reliability of the figures as measures of population size and change, is summarized. An "emended series " of population totals for China proper, A.D. 2 to 1953 is presented, excluding figures which are clearly defective.