Critical appraisals allow the analytical review of existing knowledge on current topics of significance in ecological entomology. They should assess the worth or quality of the work in the field and suggest areas for investigation.

Summary. Previous studies have synthesized life-table data from herbivore species to identify general trends in the demography of herbivorous insects. Frequency-based analyses were used to ascertain which of five mortality sources (enemies, plant factors, competition, weather, intrinsic developmental failure) and which of five ecological characteristics of herbivores (feeding biology, invasion status of the herbivore, latitude, cultivation, and successional status of the habitat) had important influences on mortality patterns. Here these results are reinforced with a quantitative analysis that relies on actual numbers of herbivores killed at different developmental stages by each of the five mortality sources in different ecological settings. We also examine the relationship between taxonomic category (Coleoptera, Diptera, Lepidoptera, and Hymenoptera) and mortality. The analysis identified developmental changes of herbivores as having an important influence on sources of mortality; feeding biology, latitude, and cultivation status also influenced the distribution of mortality sources. Other aspects of the herbivores’ ecology and taxonomy had limited effects. Natural enemies were identified as the most important mortality source overall, and their importance increased from the early larval stages to the pupal stages. They also kill more exophytic insects than endophytic insects, and kill a higher proportion of insects in cultivated habitats than in natural habitats. Weather kills more temperate-zone immatures than tropical/subtropical immatures. The results of the quantitative analysis generally confirm the earlier frequency-based tests. Several predictions that can serve as the foundation of an empirically-based theory of herbivore demography are offered: (1) natural enemies are the dominant cause of mortality in exophytic herbivore populations and may compete more intensely than on endophytics; (2) plant factors and enemies play a more balanced role in endophytic populations; (3) exophytic species should be particularly susceptible to top-down effects, especially in agroecosystems; (4) plant defences will often have sublethal effects, but when they are lethal they will be most important as the hatchling larva is just getting established on the plant. These predictions should be viewed as a challenge to engage in a broader way of thinking about herbivore demography.     

Evolution in stage-structured populations

For many organisms, stage is a better predictor of demographic rates than age. Yet no general theoretical framework exists for understanding or predicting evolution in stage-structured populations. Here, we provide a general modeling approach that can be used to predict evolution and demography of stage-structured populations. This advances our ability to understand evolution in stage-structured populations to a level previously available only for populations structured by age. We use this framework to provide the first rigorous proof that Lande's theorem, which relates adaptive evolution to population growth, applies to stage-classified populations, assuming only normality and that evolution is slow relative to population dynamics. We extend this theorem to allow for different means or variances among stages. Our next major result is the formulation of Price's theorem, a fundamental law of evolution, for stage-structured populations. In addition, we use data from Trillium grandiflorum to demonstrate how our models can be applied to a real-world population and thereby show their practical potential to generate accurate projections of evolutionary and population dynamics. Finally, we use our framework to compare rates of evolution in age- versus stage-structured populations, which shows how our methods can yield biological insights about evolution in stage-structured populations.     

Archaeology,demography and life history theory together can help us explain past and present population patterns

Population matters. Demographic patterns are both a cause and a consequence of human behaviour in other important domains, such as subsistence, cooperation, politics and culture. Demographers interested in contemporary and recent historical populations have rich data at their fingertips; the importance of demography means many interested parties have gathered demographic data, much of which is now readily available for all to explore. Those interested in the demography of the distant past are not so fortunate, given the lack of written records. Nevertheless, the emergence in recent years of a new interest in the demography of ancient populations has seen the development of a range of new methods for piecing together archaeological, skeletal and DNA evidence to reconstruct past population patterns. These efforts have found evidence in support of the view that the relatively low long-term population growth rates of prehistoric human populations, albeit ultimately conditioned by carrying capacities, may have been owing to ‘boom–bust’ cycles at the regional level; rapid population growth, followed by population decline. In fact, this archaeological research may have come to the same conclusion as some contemporary demographers: that demography can be remarkably hard to predict, at least in the short term. It also fits with evidence from biology that primates, and particularly humans, may be adapted to environmental variability, leading to associated demographic stochasticity. This evidence of the fluctuating nature of human demographic patterns may be of considerable significance in understanding our species'' evolution, and of understanding what our species future demographic trajectories might be.This article is part of the theme issue ‘Cross-disciplinary approaches to prehistoric demography’.     

The use of the coefficient of agreement and likelihood ratio test to examine the development of the tympanic plate using a known-age sample of fetal and infant skeletons

Previous attempts to use the development of the tympanic plate as an aging standard for fetal and infant cranial remains have either produced only a developmental sequence (Anderson, 1960), or a sequence for which age has been estimated using another independent aging standard (Weaver, 1979). Following an evaluation of the first three stages of development of the tympanic plate using a known-age sample of fetal and infant skeletal remains, a simple test of inter-observer agreement, Cohen's coefficient of agreement (Cohen, 1960), is demonstrated using the developmental stages.     

Levels and differentials in childhood mortality in South Africa, 1977-1998

This study uses the 1996 Census and the 1998 Demographic and Health Survey (DHS) to investigate the level of and trend in infant and child mortality and their covariates in South Africa. Census estimates of childhood mortality are higher than those from the DHS. Analysis suggests that the former overestimate mortality while the latter are probably slightly too low. Both inquiries document a reversal of the trend toward lower mortality in the 1990s. Under-five mortality increased by about a third during the five years up to early 1998. By then the infant mortality rate was about 55 per 1000 and under-five mortality 72 per 1000. Other factors may explain the tapering off of the decline in mortality after the late 1980s but AIDS deaths account for its increase. Inequalities in childhood mortality between population groups, rooted in past discriminatory apartheid policies, shrank between the late-1970s and mid-1990s. However, they remain substantial and are largely unaccounted for by province, metropolitan residence and inter-group differences in mothers' education. The HIV/AIDS epidemic is likely to offset the beneficial impact of post-apartheid pro-poor policies and may exacerbate racial differences in childhood mortality in South Africa. There is an urgent need to improve the routine collection of statistics to monitor child mortality so as to assess progress towards the Millennium Development Goals and track inequalities.     

A dynamic framework for the study of optimal birth intervals reveals the importance of sibling competition and mortality risks

Human reproductive patterns have been well studied, but the mechanisms by which physiology, ecology and existing kin interact to affect the life history need quantification. Here, we create a model to investigate how age‐specific interbirth intervals adapt to environmental and intrinsic mortality, and how birth patterns can be shaped by competition and help between siblings. The model provides a flexible framework for studying the processes underlying human reproductive scheduling. We developed a state‐based optimality model to determine age‐dependent and family‐dependent sets of reproductive strategies, including the state of the mother and her offspring. We parameterized the model with realistic mortality curves derived from five human populations. Overall, optimal birth intervals increase until the age of 30 after which they remain relatively constant until the end of the reproductive lifespan. Offspring helping each other does not have much effect on birth intervals. Increasing infant and senescent mortality in different populations decreases interbirth intervals. We show that sibling competition and infant mortality interact to lengthen interbirth intervals. In lower‐mortality populations, intense sibling competition pushes births further apart. Varying the adult risk of mortality alone has no effect on birth intervals between populations; competition between offspring drives the differences in birth intervals only when infant mortality is low. These results are relevant to understanding the demographic transition, because our model predicts that sibling competition becomes an important determinant of optimal interbirth intervals only when mortality is low, as in post‐transition societies. We do not predict that these effects alone can select for menopause.     

Developmental specialization and geographic structure of host plant use in a polyphagous grasshopper, Schistocerca emarginata (=lineata) (Orthoptera: Acrididae)

Host plant use and availability were determined in early nymphal and adult-stage Schistocerca emarginata (=lineata) (Orthoptera: Acrididae) populations at six localities in Texas, USA. Early instar nymphal populations were feeding almost exclusively on either Ptelea trifoliata (Rutaceae) or Rubus trivialis (Rosaceae). This study represents the first demonstration of a geographic structure of host plant specificity in a polyphagous grasshopper. Recognizing this geographic structure required investigations of both developmental and geographical variation in host plant use. Nymphal diet breadths were significantly less than adult diet breadths at four of six localities and smaller overall when pooled nymphal and adult diet breadths were compared among sites. Neither restricted nymphal mobility nor host plant availability accounted for the observed differences in host plant use between developmental stages and among localities. Evidence suggests that the differences in host use among populations are due to host-plant-associated genetic differentiation. Received: 4 June 1998 / Accepted: 10 March 1999     

AMBUSH: A Computer Program for Stochastic Microsimulation of Small Human Populations

A new generation of computer simulation programs for exploring problems in demography, kinship ties, descent groups, and other kinds of connectivity in small human populations is now available for use. AMBUSH is a simulation package which may be useful for simulating effects of fertility and mortality schedules, rules of marriage and kinship, density-dependent infanticide, and other features of closed breeding groups, and their stochastic variability. The program, described here, is available from the authors . [microsimulation, demography, kinship, connectivity]     

Demography and dental needs,an international perspective

The population of the world is changing and the science dealing with the study of these changes in human populations, in terms of disease, health and mortality has been called demography. It can be understood to include the evaluation of the size, density, and distribution of populations including the influence of a variety of factors such as age and economics. This paper will review the distribution of the world's population with regard to age and utilization of dental care with particular emphasis on the older cohorts.     

Limited inbreeding depression in a bottlenecked population is age but not environment dependent

Inbreeding depression is known to vary greatly between populations and among species. Some of this variation is due to differences in genetic load between populations, while some is due to differences in the environment (e.g. local weather conditions) or demography of the population (e.g. age structure and breeding experience) in which inbreeding is expressed. Although the effects of these factors in isolation are well understood, there is still relatively little known about the interface between inbreeding on one hand, and environment and demography on the other in wild populations. We examined how environmental and demographic factors mediated the effects of inbreeding in a threatened species of bird. The Stewart Island robin, Petroica australis rakiura, has been subjected to a prolonged bottleneck for over 150 years. A complete pedigree of a reintroduced island population, extending back seven seasons to its founding, was available for analysis along with survival data (at the level of the brood) obtained from intensive monitoring over two breeding seasons. We found no strong support that the degree to which a brood was inbred affected its survival at either the hatching, fledging or recruitment stages. The inbreeding coefficient of the mother did have an effect on brood survival when analysed over all three life history stages, but only as a result of an interaction with female age, with broods of one‐year‐old inbred females suffering greater mortality than those of older inbred females. Although habitat type, temperature, rainfall and year were the best predictors of brood survival for most life history stages, their effects were weak and there were no interactions with inbreeding. Furthermore, there was no strong evidence of inbreeding depression associated with two periods of severe weather. This population is atypical in that inbreeding depression appears to be weak even under severe environmental conditions, and may be indicative that this bottlenecked population has either reduced genetic load or has fixed deleterious alleles.     

Collagen fibril morphology in developing chick metatarsal tendon: 2. Electron microscope studies

Transverse section of embryonic chick metatarsal tendons ranging in age from 11 days to 18 days fetal were examined by electron microscopy to determine both the diameters and the lateral arrangements of the cylindrical collagen fibrils. In early developmental stages, from 11 days to 14 days fetal, sharp unimodal distributions of diameters centred near 32 or 40 nm were observed, but increasingly heterogeneous diameters were seen with increasing age. The heterogeneous diameter distributions were not uniform, but showed discrete populations of preferred diameters. The centre-to-centre distance separating the fibrils in the early developmental stages was about twice the fibril diameter and constant with age. Comparison of X-ray diffraction results with these observations indicated that the saptial relationships of the structures are preserved during the preparative procedures for electron microscopy, but that a transverse shrinkage of 25–30% had occurred relative to the wet dimension.     

Extrinsic Mortality Effects on Reproductive Strategies in a Caribbean Community

Extrinsic mortality is a key influence on organisms’ life history strategies, especially on age at maturity. This historical longitudinal study of 125 women in rural Domenica examines effects of extrinsic mortality on human age at maturity and pace of reproduction. Extrinsic mortality is indicated by local population infant mortality rates during infancy and at maturity between the years 1925 and 2000. Extrinsic mortality shows effects on age at first birth and pace of reproduction among these women. Parish death records show huge historical variation in age-specific mortality rates. The infant mortality rate (IMR) in the early 1920s was low, increased dramatically beginning in 1929, and reached a maximum in the 1950s, at which point IMR declined steadily to its present low rate. The mortality rate early in life showed a quadratic association with age at first birth. Women who experienced conditions of low IMR early in life reproduced relatively late in life. Those born into moderately high levels of infant mortality tended to reproduce earlier than those born at low levels. At very high infant mortality levels early in life, women went on to delay reproduction until relatively late, possibly as a result of somatic depletion and energetic stress associated with the conditions that lead to high IMR. Population mortality rates at age of maturity also showed a quadratic association with age at first birth. The pace of reproduction, estimated as number of surviving offspring controlled for maternal age, showed a similar quadratic effect. There were complex interactions between population mortality rates in infancy and at maturity. When extrinsic mortality was high during infancy, extrinsic mortality later in life had little effect on timing of first birth. When extrinsic mortality was low to moderate in infancy, extrinsic mortality later in life had significant effects on adult reproduction. I speculate that these effects are mediated through development of personality facets associated with reproduction.     

Prenatal diagnosis by amniocentesis in 800 pregnancies

Prenatal diagnoses by amniocentesis have been made in 800 pregnancies at the Genetics Division of the Los Angeles County-University of Southern California Medical Center. This experience has indicated that the amniocentesis procedure is safe and highly accurate. Amniocentesis was not associated with significant morbidity or mortality for either infant or mother. The observed complications are assumed to be related to the high-risk nature of these pregnancies, the patients having been selected primarily on the basis of advanced maternal age or a previous abnormal child. The needle puncture marks, which occurred in 2.4 percent of the live births, resulted in no serious developmental or cosmetic effects to the infants. No errors in cytogenetic diagnosis are known to have occurred in this series.     

Nutritional ecology and diachronic trends in Paleolithic diet and health

Modern nutritional studies have found that diverse diets are linked to lower infant mortality rates and longer life expectancies in humans. This is primarily because humans require more than fifty essential nutrients for growth and cell maintenance and repair; most of these essential nutrients must come from outside food sources rather than being manufactured by the body itself; and a diversity of food types is required to consume the full suite of essential nutrients necessary for optimal human health. These principles and their related affects on human adaptations and demography are the hallmarks of a theoretical paradigm defined as nutritional ecology. This essay applies concepts derived from nutritional ecology to the study of human evolution. Principles of nutritional ecology are applied to the study of the Middle‐to‐Upper Paleolithic transition in order to broadly illustrate the interpretive ramifications of this approach. At any stage in human evolution, those hominid populations that chose to diversify their subsistence base may have had a selective advantage over competitors who restricted their diet principally to one food type, such as terrestrial mammals.     

Demography and the Age of Rare Variants

Large whole-genome sequencing projects have provided access to much rare variation in human populations, which is highly informative about population structure and recent demography. Here, we show how the age of rare variants can be estimated from patterns of haplotype sharing and how these ages can be related to historical relationships between populations. We investigate the distribution of the age of variants occurring exactly twice ( variants) in a worldwide sample sequenced by the 1000 Genomes Project, revealing enormous variation across populations. The median age of haplotypes carrying variants is 50 to 160 generations across populations within Europe or Asia, and 170 to 320 generations within Africa. Haplotypes shared between continents are much older with median ages for haplotypes shared between Europe and Asia ranging from 320 to 670 generations. The distribution of the ages of haplotypes is informative about their demography, revealing recent bottlenecks, ancient splits, and more modern connections between populations. We see the effect of selection in the observation that functional variants are significantly younger than nonfunctional variants of the same frequency. This approach is relatively insensitive to mutation rate and complements other nonparametric methods for demographic inference.     

Acquired Antibiotic Resistance: Are We Born with It?

The rapid emergence of antibiotic resistance (AR) is a major public health concern. Recent findings on the prevalence of food-borne antibiotic-resistant (ART) commensal bacteria in ready-to-consume food products suggested that daily food consumption likely serves as a major avenue for dissemination of ART bacteria from the food chain to human hosts. To properly assess the impact of various factors, including the food chain, on AR development in hosts, it is important to determine the baseline of ART bacteria in the human gastrointestinal (GI) tract. We thus examined the gut microbiota of 16 infant subjects, from the newborn stage to 1 year of age, who fed on breast milk and/or infant formula during the early stages of development and had no prior exposure to antibiotics. Predominant bacterial populations resistant to several antibiotics and multiple resistance genes were found in the infant GI tracts within the first week of age. Several ART population transitions were also observed in the absence of antibiotic exposure and dietary changes. Representative AR gene pools including tet(M), ermB, sul2, and bla(TEM) were detected in infant subjects. Enterococcus spp., Staphylococcus spp., Klebsiella spp., Streptococcus spp., and Escherichia coli/Shigella spp. were among the identified AR gene carriers. ART bacteria were not detected in the infant formula and infant foods examined, but small numbers of skin-associated ART bacteria were found in certain breast milk samples. The data suggest that the early development of AR in the human gut microbiota is independent of infants' exposure to antibiotics but is likely impacted by exposure to maternal and environmental microbes during and after delivery and that the ART population is significantly amplified within the host even in the absence of antibiotic selective pressure.     

Genotypic structure in clonal Rhododendron ferrugineum L. (Ericaceae) populations: origin and maintenance

Genotypic structure of a closed population of the clonal ericaceous shrub Rhododendron ferrugineum is examined in the light of two independent studies previously conducted on this species. In the first study, spatial distribution of genotypes in the closed population was inferred from the amplified fragment length polymorphism (AFLP) procedure. Age of clones was estimated using their spatial extent and the annual growth rate of shoots. In the second study, ramet demography was studied in the three most representative stages of shrub invasion on two different sites, including the site where the population investigated by AFLP grew. The demographic data recorded were the area occupied by ramets and ramet age, and from this information the developmental pattern of Rhododendron populations was determined. Additional data such as genet density and distance between genets were calculated.These two sources of information allow us to propose that all or most of the clones detected in the closed population established at the early successional stage, and that the present genotypic structure was established several hundred years ago, long before the population reached total closure. Hypotheses concerning the future development of this genotypic structure are discussed.     

A stage-based study of drought response in Cryptantha flava (Boraginaceae): gas exchange, water use efficiency, and whole plant performance

Models of global climate change predict an increase in the frequency of major droughts, yet we know little about the consequences of drought for the demography of natural populations. This study examined a population of the semi-desert perennial Cryptantha flava (Boraginaceae) to determine how plants of different developmental stages respond to drought through changes in leaf gas exchange, leaf water potential, water use efficiency, growth, and reproduction. In two of the four years, drought was applied using rainout shelters, and a severe natural drought occurred in another. Small, presumably younger, plants sometimes had lower rates of maximum photosynthesis, lower leaf water potentials, and lower instantaneous or integrated water-use efficiency than large plants. Small plants also had higher relative growth rates and lower reproductive effort. Large plants with evidence of shrinkage from a previously larger size often produced less growth and reproduction than large healthy plants, suggesting a decline in plant vigor with age. Drought depressed gas exchange and leaf water potentials equally in all plant stages. Thus, leaf-level physiological attributes provide no clues for why drought reduces growth more strongly in large plants. The results point to several additional avenues of research relevant to understanding stage-dependent or age-dependent plant performance under drought conditions.     

Developmental trap or demographic bonanza? Opposing consequences of earlier phenology in a changing climate for a multivoltine butterfly

A rapidly changing climate has the potential to interfere with the timing of environmental cues that ectothermic organisms rely on to initiate and regulate life history events. Short‐lived ectotherms that exhibit plasticity in their life history could increase the number of generations per year under warming climate. If many individuals successfully complete an additional generation, the population experiences an additional opportunity to grow, and a warming climate could lead to a demographic bonanza. However, these plastic responses could become maladaptive in temperate regions, where a warmer climate could trigger a developmental pathway that cannot be completed within the growing season, referred to as a developmental trap. Here we incorporated detailed demography into commonly used photothermal models to evaluate these demographic consequences of phenological shifts due to a warming climate on the formerly widespread, multivoltine butterfly (Pieris oleracea). Using species‐specific temperature‐ and photoperiod‐sensitive vital rates, we estimated the number of generations per year and population growth rate over the set of climate conditions experienced during the past 38 years. We predicted that populations in the southern portion of its range have added a fourth generation in recent years, resulting in higher annual population growth rates (demographic bonanzas). We predicted that populations in the Northeast United States have experienced developmental traps, where increases in the thermal window initially caused mortality of the final generation and reduced growth rates. These populations may recover if more growing degree days are added to the year. Our framework for incorporating detailed demography into commonly used photothermal models demonstrates the importance of using both demography and phenology to predict consequences of phenological shifts.