Census timing alters stage duration distributions in matrix population models

Matrix population models are widely used to study the dynamics of stage‐structured populations. A census in these models is an event monitoring the number of individuals in each stage and occurs at discrete time intervals. The two most common methods used in building matrix population models are the prebreeding census and postbreeding census. Models using the prebreeding and postbreeding censuses assume that breeding occurs immediately before or immediately after the censuses, respectively. In some models such as age‐structured models, the results are identical regardless of the method used, rendering the choice of method a matter of preference. However, in stage‐structured models, where the duration of the first stage of life varies among newborns, a choice between the prebreeding and postbreeding censuses may result in different conclusions. This is attributed to the different first‐stage duration distributions assumed by the two methods. This study investigated the difference emerging in the structures of these models and its consequence on conclusions of eigenvalue and elasticity analyses using two‐stage models. Considerations required in choosing a modeling method are also discussed.     

A Rapid Method to Estimate Population Variables for African Elephants

Abstract: We developed a noninvasive method to estimate reproductive and survival parameters for free-ranging African savannah elephants (Loxodonta africana africana) and used these to estimate finite population growth rates. We used published data from 2 populations with known growth rates and birth and survival histories to validate our technique. Based on body measurements, our method yielded estimates of age at first and last calving, calving interval, and age-specific survival rates that were similar to those determined during long-term studies at both Addo Elephant National Park and Amboseli National Park. Our technique generated population data required to estimate population growth rates. The method may be particularly useful where censuses yield imprecise estimates or where long-term population data are unavailable. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):822–829; 2008)     

The South African australopithecines: A palaeodemographic analysis

Anthropologists, demographers and historians alike are continually seeking information about demographic profiles of prehistoric and ancient human populations. There are many different approaches relevant to the problem, yet direct evidence of the demographic structure of any archaeological population is primarily provided by analysis of human skeletal and dental remains. This offers a possibility of extending demographic inferences back to Pliocene — Early Pleistocene times, which, in turn, would enhance our understanding of the principles of human survival, adaptation, social interaction and demographic evolution of man. Data on the age distribution of South African australopithecines has been analysed using life-table analysis, based on a stationary population model. The estimated demographic profile is the evaluated and interpreted within a framework of biological, cultural and ecological circumstances. It is concluded that palaeodemography, if carefully undertaken, can play a real and pragmatic role in understanding the demographic history of man.     

An anthropological genetic perspective on creolization in the anglophone caribbean

Variable socio‐cultural influences developed in the colonial Caribbean as a result of competing European hegemonic rule. In this study, we examine how colonial regulations regarding social hierarchies and mate choice worked to influence the genetic landscape of contemporary African Caribbean populations. To this end, 420 individuals from Dominica, Grenada, St. Kitts, St. Lucia, St. Thomas, St. Vincent, Jamaica, and Trinidad were genotyped for 105 autosomal ancestry informative markers. Based on these data, population substructure and admixture were assessed using an exact test, a model‐based clustering method, and principal components analysis. On average, individual admixture estimates of the pooled African Caribbean sample were 77% (SD ± 18%) West African, 15% (SD ± 15%) European, and 7.7% (SD ± 8%) Native American. In general, ancestry estimates were significantly different between Dominica and all other islands. Genetic structure analyses indicated subdivision into two subpopulations on most islands. Finally, unlike all of the other Caribbean populations that clustered adjacent to African populations, the Dominican population was more intermediate between the three parental groups in the principal components plot. As a result of the significant French influence throughout Dominican history, Dominica did not have the same cultural influences that typified other Anglophone colonies. Consequently, there were different social hierarchies and resulting mate choices on Dominica compared with the other considered islands. This study highlights the complex socio‐cultural history of a broad region of the Caribbean and attests to the interplay between social and biological factors in shaping the genetic diversity present in present‐day communities. Am J Phys Anthropol 151:135–143, 2013. © 2013 Wiley Periodicals, Inc.     

An integrated population model for a long‐lived ungulate: more efficient data use with Bayesian methods

We develop an integrated population model for Svalbard reindeer Rangifer tarandus platyrhynchus, and demonstrate how this type of model can be used to extract more information from the data and separate different sources of variability in population estimates. Our model combines individual mark–recapture data with population counts and harvesting data within a Bayesian model framework, and accounts for observation error, environmental and demographic stochasticity, and age structure. From this model we obtain annual estimates of age‐specific population size, survival and fecundity. The model provides estimates of age structure at a finer scale than that found in the census data, and enables us to estimate survival for the period before calves are first caught and marked, i.e. before they enter the individual mark–recapture data. The modeling framework provides an improved approach to studying age‐structured populations that are imperfectly censused and where the demography of only a sample of individuals is known. We use data from independent censuses of the same population to evaluate population estimates obtained from the model, and show that it is successful at correcting for different types of observation error. Based on our model results, we suggest that allocating resources to the collection of supplementary mark–recapture data could improve the reliability of population projections more than making regular population censuses as exhaustive as possible. Our work demonstrates how integrated Bayesian population modeling can be used to increase the amount of information extracted from collections of data, identifying and disentangling sources of variation in individual performance and population size. This represents an important step towards increasing the predictive ability of population growth models for long‐lived species experiencing changes in environmental conditions and harvesting regimes.     

Computing a population budget from sequentially sacrificed,replicated cultures

When a population budget must be obtained from censuses based on replicated, sacrificed cultures, it is difficult to obtain estimates of transition probabilities and of the errors of such estimates, because there is no logical basis for pairing successive census counts. In a study of this nature estimating a population budget of immature stages of the housefly, the problem was solved by a randomization treatment of the original census results obtained at two densities. One hundred randomly generated census matrices over all census times for each density were smoothed to remove the effects of sampling error and a population budget constructed according to defined rules. Transition probabilities computed from the population budget were plotted on triangular coordinate paper and mean probabilities, 95% confidence regions for these means, and 95% equal frequency ellipses computed. All computations and the graphing of the results were carried out on a digital computer. The computer program, available from the authors, is written in FORTRAN IV and could be easily modified for similar studies.     

Simulation of the population dynamics and social structure of the Virunga mountain gorillas

An agent-based model was developed to simulate the growth rate, age structure, and social system of the endangered mountain gorillas (Gorilla beringei beringei) in the Virunga Volcanoes region. The model was used to compare two types of data: 1) estimates of the overall population size, age structure, and social structure, as measured by six censuses of the entire region that were conducted in 1971-2000; and 2) information about birth rates, mortality rates, dispersal patterns, and other life history events, as measured from three to five habituated research groups since 1967. On the basis of the research-group data, the "base simulation" predicted a higher growth rate than that observed from the census data (3% vs. 1%). This was as expected, because the research groups have indeed grown faster than the overall population. Additional simulations suggested that the research groups primarily have a lower mortality rate, rather than higher birth rates, compared to the overall population. Predictions from the base simulation generally fell within the range of census values for the average group size, the percentage of multimale groups, and the distribution of females among groups. However, other discrepancies predicted from the research-group data were a higher percentage of adult males than observed, an overestimation of the number of multimale groups with more than two silverbacks, and an overestimated number of groups with only two or three members. Possible causes for such discrepancies include inaccuracies in the census techniques used, and/or limitations with the long-term demographic data set obtained from only a few research groups of a long-lived species. In particular, estimates of mortality and male dispersal obtained from the research groups may not be representative of the entire population. Our final simulation addressed these discrepancies, and provided a better basis for further studies on the complex relationships among individual life history events, group composition, population age structure, and growth rate patterns.     

AERIAL CENSUS OF PACIFIC WALRUSES IN THE CHUKCHI SEA, 1985

The U.S. Fish and Wildlife Service conducted a survey of the walruses in the pack ice of the Chukchi Sea between 16 September and 2 October 1985, as part of a joint effort with the Soviet Union to estimate the size of the Pacific walrus population. American observers conducted censuses from two aircraft along randomly selected north–south lines over the pack ice. The observers counted walruses within a constant viewing angle that corresponded to a total strip width of 1.38 km at an altitude of 152 m.
In nine days of flying, 15,312 walruses were observed, of which 10,140 were on 5,764 km² of census strips. Few walruses were observed east of 161°W longitude or west of 170°W longitude, hence the census effort was stratified. Walrus concentrations between 161° and 170° shifted slightly westward during the 2-wk duration of the censuses. The differences among days in observed walrus density were due to changes in the numbers of walruses on the ice within 37 km of the ice edge. The number of observable walruses in pack ice of the eastern Chukchi Sea was estimated to be 62,177 (SD = 19,480), based on censuses conducted on 29 and 30 September and 1 October. At that time there were also at least 15,238 in Bristol Bay, Bering Sea. The Soviets counted 39,572 on the shores of the western Chukchi and Bering seas and estimated 115,531 in pack ice of the western Chukchi Sea. Summing U.S. and Soviet estimates, the total population of Pacific walruses in 1985 was 232,518. This number was comparable with earlier estimates from censuses conducted jointly by the U.S. and the Soviets. However, information on fraction hauled out, segregation, and movements is needed for more precise estimates.     

Can population growth rates vary with the spatial scale at which they are measured?

1.  The ratio of successive population censuses is often assumed to reflect population growth rates. We identify three simple potential sources of bias in the estimation of population growth rates that relate to either the total number of censused individuals or the spatial areas over which censuses are conducted.
2.  The commonly used method of adding a constant to time series data to avoid problems caused by division by zero can lead to underestimation of growth rates at low densities in increasing populations.
3.  Variances associated with density estimates can lead to positive bias in estimation of growth rates when populations are distributed in ephemeral patches. The spatial variance and spatio-temporal covariance in bank vole census data suggest that this bias could be severe when small trapping grids are used. Use of logged estimators of growth rate avoids this problem.
4.  Using census data from non-randomly placed trapping grids that are smaller than twice the maximum range of natal dispersal to estimate population growth rates can lead to negatively biased estimates, particularly at low population densities.
5.  These three sources of bias are evaluated as explanations for scale-dependent changes in the estimates of growth rates identified in populations of snowshoe hare ( Lepus americanus ), bank voles ( Clethrionomys glareolus ) and lemmings ( Lemmus lemmus ).     

Used for ill; used for good: a century of collecting data on race in South Africa

Abstract

Data on race have been collected in South African censuses for a century. We examine the role played by the census in solidifying race as a social statistic and show that, in contrast to the majority of situations, operational and legislative factors rendered the census largely unimportant as a vehicle for doing this. Since 1994, race has been entirely self-reported and not subject to state reinterpretation. We examine the implications of this for future data collection exercises and caution against reifying race as a predictor of social outcomes in post-apartheid South Africa, and argue for its gradual phasing out.     

Core values and cultural identity

This article examines racial classifications on United States population census schedules between 1890 and 1990 to provide insights on the changing meanings of race in US society. The analysis uses a sociology of knowledge perspective which assumes that race is an ideological concept that can be interpreted most productively by relating its definition and measurement to the larger social and political context. Four themes are identified and discussed: (i) the historical and continuing importance of skin colour, usually dichotomized into white and non‐white, in defining race and counting racial groups; (ii) a belief in ‘pure’ races that is reflected in a preoccupation with categorizing people into a single or ‘pure’ race; (iii) the role of census categories in creating pan‐ethnic racial groups; and, (iv) the confusing of race and ethnicity in census racial classifications. Each theme demonstrates the potential or actual role of official statistics, exemplified by census racial data, in reflecting and guiding changes to the meaning and social perceptions of race. A detailed examination of racial classifications from the 1980 and 1990 Censuses shows that the influence of political interests on racial statistics is particularly important. The article concludes with a discussion of whether official statistical recorders such as population censuses should categorize and measure race, given the political motivations and non‐scientific character of the classifications used.     

Maintaining or Abandoning African Rice: Lessons for Understanding Processes of Seed Innovation

Rice breeding and crop research predominantly emphasize adaptation to ecological conditions. Based on qualitative and quantitative research conducted between 2000 and 2012 we show how ecological factors, combined with socioeconomic variables, cultural norms and values, shape the use and development of local technologies related to the cultivation of African rice (Oryza glaberrima Steud.) in seven West African countries (Ghana, Guinea, Guinea-Bissau, Senegal, Sierra Leone, The Gambia and Togo). In this region the role of African rice is diverse across ethnic groups. Findings suggest that farmers, through various pathways, are active in the development of promising new varieties based on genetic resources of Asian rice, African rice, or both, as well as in the adoption of modern varieties. These findings require further research into interactions among ecological, genetic, socioeconomic and cultural factors within farmers?? innovation systems and recognition of emergent knowledge and technologies resulting from such interactions.     

When and where to count? Implications of migratory connectivity and nonbreeding distribution to population censuses in a migratory bird population

Migratory connectivity is a metric of the co-occurrence of migratory animals originating from different breeding sites, and like their spatial dispersion, can vary substantially during the annual cycle. Together, both these properties affect the optimal times and sites of population censusing. We tracked taiga bean geese (Anser fabalis fabalis) during 2014–2021 to study their migratory connectivity and nonbreeding movements and determine optimal periods to assess the size of their main flyway population. We also compared available census data with tracking data, to examine how well two existing censuses covered the population. Daily Mantel's correlation between breeding and nonbreeding sites lay between 0 and 0.5 during most of the nonbreeding season, implying birds from different breeding areas were not strongly separated at other times in the annual cycle. However, the connectivity was higher among birds from the westernmost breeding areas compared to the birds breeding elsewhere. Daily Minimum Convex Polygons showed tracked birds were highly aggregated at census times, confirming their utility. The number of tracked birds absent at count sites during the censuses however exceeded numbers double-counted at several sites, indicating that censuses might have underestimated the true population size. Our results show that connectivity can vary in different times during the nonbreeding period, and should be studied throughout the annual cycle. Our results also confirm previous studies, which have found that estimates using marked individuals usually produce higher population size estimates than total counts. This should be considered when using total counts to assess population sizes in the future.     

Recruitment rates in forest plots: Gf estimates using growth rates and size distributions

1 In censuses of tree populations in permanent plots, short census intervals and small population size lead to uncertainty in the observed recruitment rate of a minimum size. Increasing the census interval, however, underestimates the rate because of unrecorded 'recruit and die' events.
2 We propose a new Gf procedure for estimation of recruitment rates. Recruitment rate per area is obtained by multiplication of the density in the smallest size class (f) and the average size growth rate in that class (G) divided by the width of the class. This procedure is valid when the size distribution of the population examined is continuous with size.
3 When tree size structure is negative‐exponentially distributed, as is often the case in natural rain forest populations, the Gf estimate of the recruitment rate for a given size class was least biased close to the midpoint size of this class.
4 Gf estimates agreed well with census estimates of recruitment rate from permanent plots in rain forests. A tendency for Gf estimates to be larger than census estimates disappeared when census estimates were corrected for mortality after recruitment.
5 The effects of plot size, census interval and variation in growth rate on estimates of recruitment rate were simulated using model populations. Small plot size caused substantially more among‐plot deviation for the census count of recruitment events than for the Gf estimate. The census recruitment rate also showed larger variation among plots for shorter intervals than the Gf estimate, which was independent of census interval. The Gf estimates were therefore more accurate than census counts in many situations. More than several tens of trees were needed in a size class to allow a reliable Gf estimates.     

Stages of Change and Weight Loss Among Rural African American Women

Objective: Obesity is a prevalent public health problem in the United States, especially for rural African American women, and causes increased morbidity and mortality. The purpose of this analysis was to determine whether the transtheoretical stages of change model was generalizable to weight loss intention among overweight and obese rural African American women and to identify important predictors of the stages of change. Research Methods and Procedures: The study was conducted in two rural counties in central Virginia. A population‐based sample of 200 women under the age of 40 completed questionnaires concerning weight loss behavior and beliefs about weight. Ordinal logistic regression was used to predict stage of change. Results: A total of 142 of the 200 women (71%) were overweight or obese (body mass index of ≥25) and were classified into a stage of change. Overall, 30% of respondents were in the precontemplation stage, 15% in the contemplation stage, 48% in the preparation stage, 4% in the action stage, and 3% in the maintenance stage. Education, what friends think about weight, body mass index, and a scale of the positive aspects of weight loss were significant predictors of the stage of change (p < 0.05). Conclusions: Several predictors of stage were the same as those found in studies of other health behaviors, and this research provides support for applying a stages of change model for weight loss intention among rural African American women. Two predictors in particular, significance of what friends think about weight and a scale of the positive aspects of weight loss, have implications for health education initiatives and social support in weight loss interventions.     

ESTIMATING POPULATION SIZE IN ASYNCHRONOUS AGGREGATIONS: A BAYESIAN APPROACH AND TEST WITH ELEPHANT SEAL CENSUSES

Many organisms reproduce in temporary aggregations where estimates of colony size can be made by direct counts. When individuals are not synchronous, however, early breeders depart before the last arrive, so counts underestimate the total breeding population. We present a model describing a colony's census as a function of arrival, breeding tenure, and the correlation between them, and we use it to illustrate how variance in arrival and tenure affect the census. Counts of breeding female northern elephant seals ( Mirounga angustirostris ) from 1975 to 2007 were used to test the model. Four of the model's parameters—population size, mean and variance of arrival date, and the correlation between arrival date and breeding tenure—could be estimated from census data using a Bayesian approach; prior estimates of two other parameters—mean tenure and its variance—had to be used to avoid overparameterization. The model's predictions fit observed censuses well and produced reliable estimates of population size and arrival behavior, showing that the maximum census was 8%–16% below the total number of breeding females. This method could be used for estimating abundance in any asynchronous aggregation, given independent information on one of the defining distributions: arrival, tenure, or departure.     

BOOTSTRAP CORRECTION FOR DIURNAL ACTIVITY CYCLE IN CENSUS DATA FOR ANTARCTIC SEALS1

Antarctic phocid seals and particularly the crabeater (Lobodon carcinophagus) have been observed to display a diurnal cycle in their propensity to haul out on pack ice where they are visible for census. The fact that they are not visible for much of the 24-h period means that density estimates made over broad geographic areas at various times of the day statistically confound this cycle with geographic variability. Limitation of census observations to times of peak haulout results in extreme logistical difficulties and/or considerable reduction in sample size upon which to base population estimates. Reduced sample size results in high variability in population estimates and broad confidence bands. To develop a model with which to correct density estimates for variability due to diurnal cycle, a series of stationary censuses at fixed locations in the Antarctic continental ice pack was made over significant fractions of several days. A unimodal polynomial model for the observed density variation in any one location was statistically significant; a similar model combining multiple locations with densities standardized to peak daily values was also significant. The latter model was used to make corrections for time of day to density estimates in three test data sets taken over broad geographic areas of the Antarctic. Statistical simulation (bootstrap) methods were used to determine if variances of corrected density estimates were lower than those based on uncorrected observations taken only during the peak haulout times of the day. Results were that 95% interval estimates for corrected densities were narrowed to between 40% and 61% of the uncorrected estimates. While there are additional possible sources of variation in haulout tendency, pending further data collection and analyses, the model developed represents a considerably more precise methodology than either averaging over haulout variability or limiting observations to peak daily periods.     

Knowledge and Uses of African Pangolins as a Source of Traditional Medicine in Ghana

Traditional medicine has been practised in Ghana for centuries with the majority of Ghanaians still patronising the services of traditional healers. Throughout Africa a large number of people use pangolins as a source of traditional medicine, however, there is a dearth of information on the use of animals in folk medicine in Ghana, in particular the use of pangolins. The aim of this study was to determine the prevalent use of pangolins and the level of knowledge of pangolin use among traditional healers in Ghana for the treatment of human ailments. Data was gathered from 48 traditional healers using semi-structured interviews on the traditional medicinal use of pangolin body parts in the Kumasi metropolis of Ghana. The cultural importance index, relative frequency of citation, informant agreement ratio and use agreement values were calculated to ascertain the most culturally important pangolin body part as well as the level of knowledge dissemination among traditional healers with regards pangolin body parts. Our study revealed that 13 body parts of pangolins are used to treat various medicinal ailments. Pangolin scales and bones were the most prevalent prescribed body parts and indicated the highest cultural significance among traditional healing practices primarily for the treatment of spiritual protection, rheumatism, financial rituals and convulsions. Despite being classified under Schedule 1 of Ghana’s Wildlife Conservation Act of 1971 (LI 685), that prohibits anyone from hunting or being in possession of a pangolin, our results indicated that the use of pangolins for traditional medicinal purposes is widespread among traditional healers in Ghana. A study on the population status and ecology of the three species of African pangolins occurring in Ghana is urgently required in order to determine the impact this harvest for traditional medical purposes has on their respective populations as current levels appear to be unmonitored and unsustainable.     

A census of the Manx Shearwater Puffinus puffinus on Skomer,Skokholm and Middleholm,west Wales

In 1997/98 a census was made of the Manx Shearwater populations on three Welsh Islands: Skomer, Skokholm and Middleholm. The census was made by counting all the burrows in the winter, playing a tape-recording of a male call (to which only males respond) down a sample of the burrows during the incubation period and applying a correction for the response rate (the proportion of males which respond). The response rate was determined by playing the tape-recordings down a set of burrows in which eggs had been laid and where the incubating birds could be examined. Knowing the total number of burrows, the number of burrows sampled, the number of responses and the response rate enabled us to make an estimate of the population size on each island. Skomer, with an estimated population of some 102000 breeding pairs is probably the largest colony of Manx Shearwaters in the world; the estimates for Skokholm and Middleholm were 46000 and 3000 respectively. These figures are compared with earlier censuses undertaken using traditional methods.     

Measurement error and estimates of population extinction risk

It is common to estimate the extinction probability for a vulnerable population using methods that are based on the mean and variance of the long‐term population growth rate. The numerical values of these two parameters are estimated from time series of population censuses. However, the proportion of a population that is registered at each census is typically not constant but will vary among years because of stochastic factors such as weather conditions at the time of sampling. Here, we analyse how such sampling errors influence estimates of extinction risk and find sampling errors to produce two opposite effects. Measurement errors lead to an exaggerated overall variance, but also introduce negative autocorrelations in the time series (which means that estimates of annual growth rates tend to alternate in size). If time series data are treated properly these two effects exactly counter balance. We advocate routinely incorporating a measure of among year correlations in estimating population extinction risk.