Demise of the Sepaade Tradition: Cultural and Biological Explanations

In an attempt to find common theoretical and methodological ground between anthropological demography and human evolutionary ecology, this article analyzes and interprets one recent event, the termination of the sepaade tradition among Rendille pastoralists of northern Kenya. Long described in the anthropological literature as an example of cultural population regulation, the sepaade tradition ended in 1998. Previously collected qualitative and quantitative data were analyzed to assess the various rationales for the demise of the tradition. Qualitative data link its termination to changing group morals, while quantitative data point to the loss of individual-level benefits as the underlying rationale. Combining such seemingly disparate interpretations highlights common research interests for anthropological demography and human evolutionary ecology. [ anthropological demography, human evolutionary ecology, culture change ]     

Sustainability of culture-driven population dynamics

We consider models of the interactions between human population dynamics and cultural evolution, asking whether they predict sustainable or unsustainable patterns of growth. Phenomenological models predict either unsustainable population growth or stabilization in the near future. The latter prediction, however, is based on extrapolation of current demographic trends and does not take into account causal processes of demographic and cultural dynamics. Most existing causal models assume (or derive from simplified models of the economy) a positive feedback between cultural evolution and demographic growth, and predict unlimited growth in both culture and population. We augment these models taking into account that: (1) cultural transmission is not perfect, i.e., culture can be lost; (2) culture does not always promote population growth. We show that taking these factors into account can cause radically different model behavior, such as population extinction rather than stability, and extinction rather than growth. We conclude that all models agree that a population capable of maintaining a large amount of culture, including a powerful technology, runs a high risk of being unsustainable. We suggest that future work must address more explicitly both the dynamics of resource consumption and the cultural evolution of beliefs implicated in reproductive behavior (e.g., ideas about the preferred family size) and in resource use (e.g., environmentalist stances).     

Microsatellite mutations and inferences about human demography

Microsatellites have been widely used as tools for population studies. However, inference about population processes relies on the specification of mutation parameters that are largely unknown and likely to differ across loci. Here, we use data on somatic mutations to investigate the mutation process at 14 tetranucleotide repeats and carry out an advanced multilocus analysis of different demographic scenarios on worldwide population samples. We use a method based on less restrictive assumptions about the mutation process, which is more powerful to detect departures from the null hypothesis of constant population size than other methods previously applied to similar data sets. We detect a signal of population expansion in all samples examined, except for one African sample. As part of this analysis, we identify an "anomalous" locus whose extreme pattern of variation cannot be explained by variability in mutation size. Exaggerated mutation rate is proposed as a possible cause for its unusual variation pattern. We evaluate the effect of using it to infer population histories and show that inferences about demographic histories are markedly affected by its inclusion. In fact, exclusion of the anomalous locus reduces interlocus variability of statistics summarizing population variation and strengthens the evidence in favor of demographic growth.     

Modeling rendille household herd composition

Previous analysis of Rendille household herd composition revealed a transition from camel to cattle ownership for sedentary impoverished Rendille pastoralists of northern Kenya. In an attempt to delineate determinants of livestock holdings, logistic regression analysis of 112 household herds from the Rendille settlement of Korr, Marsabit District, Kenya was undertaken. Results indicated that household wealth, measured in present livestock holdings, past drought losses, and livestock sales, formed better predictors of cattle ownership than did household characteristics pertaining to labor supply, wage earners, age-set membership, and birth order of household head. These results are discussed in light of pastoral strategies designed to minimize risk.     

When did the human population size start increasing?

We analyze the frequency spectra of all available human nuclear sequence data sets by using a model of constant population size followed by exponential growth. Parameters of growth (more extreme than or) comparable to what has been suggested from mtDNA data can be rejected for 6 out of the 10 largest data sets. When the data are separated into African and non-African samples, a constant size no-growth model can be rejected for 4 out of 8 non-African samples. Long-term growth (i.e., starting 50-100 kya) can be rejected for 2 out of 8 African samples and 5 out of 8 non-African ones. Under more complex demographic models, including a bottleneck or population subdivision, more of the data are compatible with long-term growth. One problem with the data used here is that a subset of loci may reflect the action of natural selection as well as of demography. It remains possible that the correct demographic model is one of constant population size followed by long-term growth but that at several loci the demographic signature has been obscured by balancing or diversifying selection. However, it is not clear that the data at these loci are consistent with a simple model of balancing selection; more complicated selective alternatives cannot be tested unless they are made explicit. An alternative explanation is that population size growth is more recent (e.g., upper Paleolithic) and that some of the loci have experienced recent directional selection. Given the available data, the latter hypothesis seems more likely.     

Comparative demographics of a Hawaiian forest bird community

Estimates of demographic parameters such as survival and reproductive success are critical for guiding management efforts focused on species of conservation concern. Unfortunately, reliable demographic parameters are difficult to obtain for any species, but especially for rare or endangered species. Here we derived estimates of adult survival and recruitment in a community of Hawaiian forest birds, including eight native species (of which three are endangered) and two introduced species at Hakalau Forest National Wildlife Refuge, Hawai?i. Integrated population models (IPM) were used to link mark–recapture data (1994–1999) with long‐term population surveys (1987–2008). To our knowledge, this is the first time that IPM have been used to characterize demographic parameters of a whole avian community, and provides important insights into the life history strategies of the community. The demographic data were used to test two hypotheses: 1) arthropod specialists, such as the ‘Akiapōlā‘au Hemignathus munroi, are ‘slower’ species characterized by a greater relative contribution of adult survival to population growth, i.e. lower fecundity and increased adult survival; and 2) a species’ susceptibility to environmental change, as reflected by its conservation status, can be predicted by its life history traits. We found that all species were characterized by a similar population growth rate around one, independently of conservation status, origin (native vs non‐native), feeding guild, or life history strategy (as measured by ‘slowness’), which suggested that the community had reached an equilibrium. However, such stable dynamics were achieved differently across feeding guilds, as demonstrated by a significant increase of adult survival and a significant decrease of recruitment along a gradient of increased insectivory, in support of hypothesis 1. Supporting our second hypothesis, we found that slower species were more vulnerable species at the global scale than faster ones. The possible causes and conservation implications of these patterns are discussed.     

Demographic stochasticity and temporal autocorrelation in the dynamics of structured populations

Populations can show temporal autocorrelation in the dynamics arising from different mechanisms, including fluctuations in the demographic structure. This autocorrelation is often treated as a complicating factor in the analyses of stochastic population growth and extinction risk. However, it also reflects important information about the demographic structure. Here, we consider how temporal autocorrelation is related to demographic stochasticity in structured populations. Demographic stochasticity arises from inherent randomness in the demographic processes of individuals, like survival and reproduction, and the resulting impact on population growth is measured by the demographic variance. Earlier studies have shown that population structure have positive or negative effects on the demographic variance compared to a model where the structure is ignored. Here, we derive a new expression for the demographic variance of a structured population, using the temporal autocorrelation function of the population growth rate. We show that the relative difference in demographic variance when the structure is included or ignored (the effect of structure on demographic variance) is approximately twice the sum of the autocorrelations. We demonstrate the result for a simple hypothetical example, as well as a set of empirical examples using age‐structured models of 24 mammals from the demographic database COMADRE. In the empirical examples, the sum of the autocorrelation function was negative in all cases, indicating that age structure generally has a negative effect on the demographic variance (i.e. the demographic variance is lower compared to that of a model where the structure is ignored). Other kinds of structure, such as spatial heterogeneity affecting fecundity, can have positive effects on the demographic variance, and the sum of the autocorrelations will then be positive. These results yield new insights into the complex interplay between population structure, demographic variance, and temporal autocorrelation, that shapes the population dynamics and extinction risk of populations.     

The analysis of plant growth in ecological and evolutionary studies

Plant growth rate is often assumed to be an ecologically important life history trait. However, conventional plant growth analysis, while providing a useful accounting of rates of weight gain and its components, is ill-suited for testing relationships between growth and fitness, particularly in natural populations. Two new approaches that are suitable for testing such relationships have evolved over the past several years. The first - the population biology of plant parts, or 'modular demography' - permits non-destructive measures of growth rate in natural field populations. When modular demography is performed using matrix population models, controls over growth rate can be examined, as well as consequences of growth variation for reproduction. The second - demographic growth analysis - provides growth parameters analogous to those of conventional growth analysis, but can be performed in natural field populations. Demographic growth analysis allows measures of individual growth-rate variation, which, in turn, can be related to plant performance.     

Genetic diversity and relationships of indigenous Kenyan camel (Camelus dromedarius) populations: implications for their classification

The genetic diversity and relationships amongst the dromedary (Camelus dromedarius) populations are poorly documented. Four recognized Kenyan dromedary breeds (Somali, Turkana, Rendille, Gabbra) and dromedary from Pakistan and the Arabian Peninsula (Saudi Arabia, United Arab Emirates) were studied using 14 microsatellite loci. Phylogenetic analysis showed that Kenyan dromedaries are distinct from Arabian and Pakistani populations. Expected heterozygosity and allelic diversity values indicate that Kenyan dromedaries are less diverse than non-Kenyan populations. With the exception of the Somali population, the Kenyan dromedaries are poorly differentiated (average FST=0.009), with only one to two loci separating the Gabbra, Rendille and Turkana populations studied (P < 0.05). Individual assignments were performed using the maximum likelihood method. A correct breed assignment of only 39-48% was observed for the Kenyan dromedaries, using an allocation stringency of a log of the odds ratio >2. Our results do not support the present classification of the indigenous Kenyan dromedary into four distinct breeds based on socio-geographical criteria. Instead, our results point to just two separate genetic entities, the Somali and a group including the Gabbra, Rendille and Turkana populations.     

Dynamics of a northern fulmar (Fulmarus glacialis) population at the southern limit of its range in Europe

In the context of global changes, defining the source–sink dynamics of populations of emblematic species, such as seabirds, within the limits of their distribution range is often crucial to optimize the priorities of surveys and conservation management, especially in protected areas. However, ringing is often not possible and only simple survey methods, such as the ‘apparently occupied sites’ method, can be utilized by managers of protected areas and threatened species. Using data collected between 1997 and 2005, we investigated the population dynamics of the northern fulmar (Fulmarus glacialis) at the southern limit of its range on the western French coast, which hosts increasing populations. Using a robust design spatial occupancy model, we estimated the proportion of occupied nests, the rates of nest colonization, nest extinction and the population growth rates of four colonies of the largest population (Ouessant Island). The estimated annual growth rate was high (average 1.049). A deterministic population dynamics model indicated that the observed rapid increase of Ouessant populations cannot be explained by their intrinsic dynamics, which suggested an important role for immigration. Different demographic scenarios provide several lines of evidence that the large northern fulmar population in this Man and Biosphere Reserve is a sink population. The strong increase in a population located at the limits of the species distributional range implies that it functions as a population sink. Inexpensive effective survey methods could allow investigation of the demographic status of seabird populations and provide relevant information for the hierarchization of conservation priorities.     

Nestboxes and immigration drive the growth of an urban Peregrine Falcon Falco peregrinus population

Drivers of wildlife population dynamics are generally numerous and interacting. Some of these drivers may impact demographic processes that are difficult to estimate, such as immigration into the focal population. Populations may furthermore be small and subject to demographic stochasticity. All of these factors contribute to blur the causal relationship between past management action and current population trends. The urban Peregrine Falcon Falco peregrinus population in Cape Town, South Africa, increased from three pairs in 1997 to 18 pairs in 2010. Nestboxes were installed over this period to manage the interface between new urban pairs of Falcons and the human users of colonized buildings, and incidentally to improve breeding success. We used integrated population models (IPMs) formally to combine information from a capture–mark–recapture study, monitoring of reproductive success and counts of population size. As all local demographic processes were directly observed, the IPM approach also allowed us to estimate immigration by difference. The provision of nestboxes, as a possible stimulant of population growth, improved breeding success and accounted for an estimated 3–26% of the population increase. The most important driver of growth, however, was immigration. Despite low sample sizes, the IPM approach allowed us to obtain relatively precise estimates of the population‐level impact of nestbox deployment. The goal of conservation interventions is often to increase population size, so the effectiveness of such interventions should ideally be assessed at the population level. IPMs are powerful tools in this context for combining demographic information that may be limited due to small population size or practical constraints on monitoring. Our study quantitatively documented both the immigration process that led to growth of a small population and the effect of a management action that helped the process.     

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.     

On the number of independent cultural traits carried by individuals and populations

In species subject to individual and social learning, each individual is likely to express a certain number of different cultural traits acquired during its lifetime. If the process of trait innovation and transmission reaches a steady state in the population, the number of different cultural traits carried by an individual converges to some stationary distribution. We call this the trait-number distribution. In this paper, we derive the trait-number distributions for both individuals and populations when cultural traits are independent of each other. Our results suggest that as the number of cultural traits becomes large, the trait-number distributions approach Poisson distributions so that their means characterize cultural diversity in the population. We then analyse how the mean trait number varies at both the individual and population levels as a function of various demographic features, such as population size and subdivision, and social learning rules, such as conformism and anti-conformism. Diversity at the individual and population levels, as well as at the level of cultural homogeneity within groups, depends critically on the details of population demography and the individual and social learning rules.     

Which future for the French Pyrenean brown bear (Ursus arctos) population? An approach using stage-structured deterministic and stochastic models

The Pyrenean brown bear (Ursus arctos) population is considered as one of the most seriously threatened with extinction in Western Europe. To assess its viability and possible needs of augmentation, we develop deterministic and stochastic stage-structured demographic models. The deterministic model reveals that a bear population cannot have a high annual growth rate and is particularly sensitive to breeder survival. High demographic parameters appear to be crucial to population persistence, especially for a small population that remains vulnerable to demographic and environmental stochasticities. The Pyrenean population cannot therefore be considered as viable. Successful conservation strategies for this population would require releasing more bears in both sub-populations in the near future.     

Proximate and Distal Variables in the Demography of Rendille Pastoralists

Following the widespread application and success of Bongaarts' proximate fertility framework in the 1980s, anthropologists and demographers have shown increased interest in the delineation of distal fertility variables, alternatively called higher-order by cultural ecologists or ultimate variables by evolutionary ecologists. This shift in focus raises at least four immediate issues: (1) confusion over the role and effect of culture on individual members' behavior, (2) whether the individual or the group forms the basic unit of analysis, (3) discordance between external and internal perspectives of demographic regimes, and (4) difficulty comparing and evaluating quantitative survey-based data with qualitative information derived from focus groups or key informants. This paper presents one approach to dealing with these problems, featuring the assessment of anthropological and demographic data collected for Rendille pastoralists of northern Kenya, a group long cited in both anthropological and demographic literature as regulating their fertility in relation to ecological factors.     

Demographic variance in heterogeneous populations: matrix models and sensitivity analysis

The demographic consequences of stochasticity in processes such as survival and reproduction are modulated by the heterogeneity within the population. Therefore, to study effects of stochasticity on population growth and extinction risk, it is critical to use structured population models in which the most important sources of heterogeneity (e.g. age, size, developmental stage) are incorporated as i‐state variables. Demographic stochasticity in heterogeneous populations has often been studied using one of two approaches: multitype branching processes and diffusion approximations. Here, we link these approaches, through the demographic stochasticity in age‐ or stage‐structured matrix population models. We derive the demographic variance, σ²_d, which measures the per capita contribution to the variance in population growth increment, and we show how it can be decomposed into contributions from transition probabilities and fertility across ages or stages. Furthermore, using matrix calculus we derive the sensitivity of σ²_d to age‐ or stage‐specific mortality and fertility. We apply the methods to an extensive set of data from age‐classified human populations (long‐term time‐series for Sweden, Japan and the Netherlands; two hunter–gatherer populations, and the high‐fertility Hutterites), and to a size‐classified population of the herbaceous plant Calathea ovandensis. For the human populations our analysis reveals substantial temporal changes in the demographic variance as well as its main components across age. These new methods provide a powerful approach for calculating the demographic variance for any structured model, and for analyzing its main components and sensitivities. This will make possible new analyses of demographic variance across different kinds of heterogeneity in different life cycles, which will in turn improve our understanding of mechanisms underpinning extinction risk and other important biological outcomes.     

An integrated population model sheds light on the complex population dynamics of a unique colonial breeder

Climate models forecast increasing climatic variation and more extreme events, which could increase the variability in animal demographic rates. More variable demographic rates generally lead to lower population growth and can be detrimental to wild populations, especially if the particular demographic rates affected are those to which population growth is most sensitive. We investigated the population dynamics of a metapopulation of 25 colonies of a semi-arid bird species, the sociable weaver Philetairus socius, and how it was influenced by seasonal weather during 1993–2014. We constructed an integrated population model which estimated population sizes similar to observed population counts, and allowed us to estimate annual fecundity and recruitment. Variance in fecundity contributed most to variance in population growth, which showed no trend over time. No weather variables explained overall demographic variation at the population level. However, a separate analysis of the largest colony showed a clear decline with a high extinction probability (0.05 to 0.33) within 5 years after the study period. In this colony, juvenile survival was lower when summers were hot, and adult survival was lower when winters were cold. Rainfall was also negatively correlated with adult survival. These weather effects could be due to increased physiological demands of thermoregulation and rainfall-induced breeding activity. Our results suggest that the dynamics of the population on the whole are buffered against current weather variation, as individual colonies apparently react in different ways. However, if more and increasingly extreme weather events synchronize colony dynamics, they are likely to have negative effects.     

The influence of time since introduction on the population growth of introduced species and the consequences for management

Several processes likely act to change the demographic rates of introduced species over time, and this changing demography could influence the optimal management of invasive populations. Optimal management strategies should be derived based on the demography. However, we have a poor understanding of the degree to which the demography of introduced species changes following initial introduction. We used published matrix population models of introduced plant populations to test how population growth rate and elasticity change with time since introduction. We did not find a significant relationship between population growth rate and time since introduction. However, elasticity to stasis increased while elasticity to growth decreased with time since introduction. Broadly, as time since introduction progressed the elasticities of the introduced plant populations became more similar to those that have been reported for native species. These results suggest that the optimal management strategy should be derived incorporating elasticity through time, especially when the time scope of management is long or the available demographic data were obtained in the past.     

Unexpected Demography in the Recovery of an Endangered Primate Population

Assessments of the status of endangered species have focused on population sizes, often without knowledge of demographic and behavioral processes underlying population recovery. We analyzed demographic data from a 28-year study of a critically endangered primate, the northern muriqui, to investigate possible changes in demographic rates as this population recovered from near extirpation. As the population increased from 60 to nearly 300 individuals, its growth rate declined due to increased mortality and male-biased birth sex ratios; the increased mortality was not uniform across ages and sexes, and there has been a recent increase in mortality of prime-aged males. If not for a concurrent increase in fertility rates, the population would have stabilized at 200 individuals instead of continuing to grow. The unexpected increase in fertility rates and in adult male mortality can be attributed to the muriquis’ expansion of their habitat by spending more time on the ground. The demographic consequences of this behavioral shift must be incorporated into management tactics for this population and emphasize the importance of understanding demographic rates in the recovery of endangered species.     

Modeling the Pre-Industrial Roots of Modern Super-Exponential Population Growth

To Malthus, rapid human population growth—so evident in 18th Century Europe—was obviously unsustainable. In his Essay on the Principle of Population, Malthus cogently argued that environmental and socioeconomic constraints on population rise were inevitable. Yet, he penned his essay on the eve of the global census size reaching one billion, as nearly two centuries of super-exponential increase were taking off. Introducing a novel extension of J. E. Cohen''s hallmark coupled difference equation model of human population dynamics and carrying capacity, this article examines just how elastic population growth limits may be in response to demographic change. The revised model involves a simple formalization of how consumption costs influence carrying capacity elasticity over time. Recognizing that complex social resource-extraction networks support ongoing consumption-based investment in family formation and intergenerational resource transfers, it is important to consider how consumption has impacted the human environment and demography—especially as global population has become very large. Sensitivity analysis of the consumption-cost model''s fit to historical population estimates, modern census data, and 21^st Century demographic projections supports a critical conclusion. The recent population explosion was systemically determined by long-term, distinctly pre-industrial cultural evolution. It is suggested that modern globalizing transitions in technology, susceptibility to infectious disease, information flows and accumulation, and economic complexity were endogenous products of much earlier biocultural evolution of family formation''s embeddedness in larger, hierarchically self-organizing cultural systems, which could potentially support high population elasticity of carrying capacity. Modern super-exponential population growth cannot be considered separately from long-term change in the multi-scalar political economy that connects family formation and intergenerational resource transfers to wider institutions and social networks.