Browsing by Subject "Demography"

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Alien species are considered one of the primary threats to native plant populations and their control is often prominent among proposed management actions. While negative alien effects are well documented, there are also many ways that alien species can have positive effects on native plant populations that may actually contribute to their persistence. Moreover, the effect of alien species on native plants can change in magnitude and direction over varying abiotic conditions. The success of native plant populations is determined by a mix of ecological and genetic factors. Alien (and native) species and abiotic conditions could also drive selection of plant traits. In order to understand the drivers of native plant population success in the face of changing climate and increasing prevalence of alien species, it is vital to understand the relationship between genotype, phenotype, and fitness of native plants. In chapter one, I quantified the effects of neighboring alien and native plants on all demographic rates in a population of the Hawaiian endemic plant Schiedea globosa, performing biannual censuses for 4 years to encompass relatively harsh and as well as benign seasons and years. The effects of alien neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien neighbors benefit S. globosa plants through multiple mechanisms, such as nurse plant effects and associational resistance. The effects of heterospecific native neighbors were less often positive, indicating fundamentally different effects of native and alien neighbors on the demography of the focal native. These mixed effects highlight the need to consider potential benefits of alien species in the management of threatened native plants and that those benefits may be altered by changing abiotic conditions. In chapter two, I constructed population models for multiple Schiedea species across populations and years, using demographic rate regressions driven by the effects of alien and native neighbors, integrating the mixed effects of alien and native species on demographic rates of populations to project the net effect on population growth of native populations. The effects of alien and heterospecific native plant neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien and native neighbors benefit native plants through multiple mechanisms, such as nurse plant effects and associational resistance. The effect of alien and heterospecific native neighbors on population growth was generally positive-- the mixed, but largely positive, net effects of alien and native neighbors on population growth highlight the need to consider potential benefits of alien, as well as native, species in the management of at-risk native plant populations, and that those benefits may be altered by changing abiotic conditions, as indicated by differing effects across (and within) years and populations. In chapter 3, I used paternal half-sibship pairs to measure the heritability of morphological traits under field conditions of the critically endangered Schiedea adamantis, which were found to be heritable in prior studies in greenhouse conditions, in reintroduced populations. I also performed a selection analysis, regressing fitness components against traits of outplants that I hypothesized might influence response to climate and alien and native neighbors to study the relationship between genotype, phenotype, and fitness of plants in restoration outplantings and assessing potential for evolutionary rescue. I found no significant heritability of any of the morphological traits. I did find evidence of selection, as leaf shape, area, and whole plant morphology had significant effects on fitness components (growth, survival, and reproduction), and significant interaction effects showing traits influenced fitness components differently at different levels of shade. Together, these results suggest that while variation in traits benefit individual plants in differing field conditions, these outplantings may not have the ability to respond to selection through evolution.

Malaria is hyper-endemic in Ghana, and remains one of the leading causes of morbidity and mortality, especially in pregnant women and children under the age of five. The use of insecticide-treated nets is an effective intervention for malaria control; however, despite known evidence of its efficacy, there is still a low rate of implementation and usage. Considering the significant public health impact to be gained from proper use of ITNs, it is worthwhile to study factors that determine ownership of ITNs in Ghana. Cross-sectional data from the Demographic and Health Surveys (DHS) for Ghana in 2008 was analyzed to determine which variables were predictors of ITN ownership. Additional semi-structured interviews were also conducted to explore these variables. Approximately 33% of Ghanaian households own at least one ITN. The strongest positive predictors of ITN ownership were number of children <5 years (52%) and residence in a rural area (47%). Marital status and birth in the last five years were also positive predictors, 48% and 56% respectively Electricity in the household was found to be a negative predictor (28%), while wealth was not a significant predictor. Consequently, there is the need for increased targeted government interventions towards groups that are less likely to own bed nets - urban areas, low level of education. It is also paramount that already existing interventions that target risk groups - young children and pregnant women are continued to ensure continued reduction in the global burden of malaria.

This dissertation consists of two original empirical studies on the determinants of teenage childbearing in the United States. The first study examines the impact of educational attainment on teenage childbearing, using school entry laws as an instrument for education and a highly detailed North Carolina administrative dataset that links birth certificate data to school administrative records. I show that being born after the school entry cutoff date affects educational success in offsetting ways, with a negative impact on years of education but positive impact on test scores. Using an IV regression strategy to distinguish the impacts of years of education and test scores, I show that both educational measures have negative impacts on teenage childbearing.

The second study examines potential causes of the decline in the U.S. teenage birth rate between 1991 and 2010. Using age-period-cohort models with Vital Statistics birth data and Census population counts, I show that the decline was driven by period changes in the early 1990s but by cohort changes between the mid-1990s and mid-2000s. I also use a difference-in-differences model to investigate the extent to which social policies in the 1970s-1980s can explain these cohort changes. The evidence suggests that while legalization of abortion for adult women and unilateral divorce laws had a significant impact on teenage birth rates in the 1990s-2000s, abortion legalization is unlikely to be a major explanation for the observed decline.

Under the increasing threat of climate change, it is imperative to understand the impact that environmental phenomena have on the demography and behavior of natural populations. In the last few decades an ever increasing body of research has documented dramatic changes in mortality rates and breeding phenology for a large number of species. A number of these have been attributed to the current trends in climate change, which have been particularly conspicuous in bird populations. However, datasets associated to these natural populations as well as to the environmental variables that affect their biology tend to be partial and incomplete. Thus, ecological research faces the urgent need to tackle these questions while at the same time develop inferential models that can handle the complex structure of these datasets and their associated uncertainty. Therefore, my dissertation research has focused on two main objectives: 1) to understand the relationship that demographic rates and breeding phenology of a colony of seabirds has with the environment in the context of climate change; and 2) to use and develop models that can encompass the complex structure of these natural systems, while also extending the process not only to inference but to building predictions. I divided this work in three research projects; for the first one I developed a hierarchical Bayesian model for age-specific survival for long lived species with capture-recapture data that allows the use of incomplete data (i.e. left-truncated and right-censored), and builds predictions of years of birth and death for all individuals while also drawing inference on the survivorship function. I compared this method to more traditional ones and address their limitations and advantages. My second research chapter makes use of this method to determine the age-specific survivorship of the Dry Tortugas sooty tern population, and explores the effect of changes in sea surface temperature on their cohort mortality rates. Finally, my third research chapter addresses the dramatic shift in breeding season experienced by the Dry Tortugas sooty tern colony, the most unprecedented shift reported for any bird species. I explore the role of climatic and weather variables as triggering mechanisms.

In this dissertation, I examine at the aggregate and individual levels, why contemporary fertility preferences diverge from actual fertility. I use three waves of cross-sectional survey data from the National Family Health Surveys from India (also known as the Indian Demographic and Health Surveys), fielded in 1992-3, 1998-9 and 2005-6 to investigate the disjuncture between preferences and behavior. The first empirical chapter outlines and operationalizes a new framework to decompose the incongruence between stated preferences and actual fertility into a set of parameters, such as unwanted births, gender preference and postponement of births to later ages, each of which varies in its level and intensity between societies and over time. By delineating the societal constraints that women do not incorporate in their childbearing intentions, this model provides a useful framework to explain variability in fertility in contemporary intermediate- and low-fertility populations. Equally important, the framework provides avenues to enquire into the fundamental structural and cultural correlates producing differences between family size preferences and actual fertility.

Subsequent empirical chapters explore various aspects of fertility preferences in detail. The second chapter probes a key socioeconomic correlate of individual-level fertility preferences, namely educational differences in preferences. In brief, I find that educational differences in family size preferences have considerably converged over time using two-way fixed effects models. However, there is still considerable heterogeneity in the implementation of preferences (as manifested by the use of contraception). Accordingly, in the third chapter, I analyze the multilevel sources of variation in the use of contraception by young women, given that they express a preference to space or stop childbearing. Using multilevel models, I find that community norms play a strong role in the use of contraception by young women to meet their fertility preferences to space or stop childbearing. I argue that community norms are an influential determinant of young women's ability to regulate their own fertility - serving to enhance or constrain their use of contraception to either space or stop childbearing.

Overall, findings from this dissertation highlight the macro-level factors that explain variation in contemporary fertility, of which fertility preferences emerge as a critical parameter. This dissertation also illuminates the growing convergence of fertility preferences across socioeconomic categories, while focusing attention on local community forces that influence fertility behavior even in the face of women's stated preferences.

Immigrants rely on social networks upon arrival to their country of destination to access resources, find a job, and begin the process of incorporation. However, the contours of how and under what circumstances networks support a job search or facilitate assimilation remain unexplored. In this dissertation, I look at the intersection of migration, social networks and employment to shed light on both the limitations and benefits of social networks for immigrant incorporation. In Chapter 1, I study whether return migrants use social networks to find a job when they return to their home country. In doing so, I contribute to the academic debate on whether immigrants lose or maintain their connections to friends and family when they leave. Using Colombia as a case study, I draw on data from two years of Colombian nationally representative household surveys conducted in 2016 and 2017. I use a Difference-in-Differences strategy and exploit a mass deportation event of Colombian migrants from Venezuela in 2015 which prompted a wave of return-migrants. This yields three main findings: (1) Return migrants are more likely to use networks in their search than never migrants; (2) social networks are a last resort in return migrants’ job search, and (3) jobs found through networks for return migrants may be lower quality than jobs found through other means. This paper contributes to the literature on return migrant integration, and speaks to an important question in the literature: Will friends and family still be there for you after you’ve left? In Chapter 2, co-authored with Giovanna Merli and Ted Mouw, we study how immigrants’ personal networks are related to their migration experience and key indicators of assimilation. We draw on novel data that includes network data for over 500 immigrants and use model-based clustering to understand the assimilation of a particular case of first-generation immigrants: Chinese immigrants in a sparsely dispersed in a mixed suburban/urban area (Raleigh-Durham). We identify four Chinese immigrant typologies, Chinese Friendship Networks, Socially Embedded, Undecided Newcomers, and Economically Integrated, which are distinguished simultaneously by their social networks and their demographic characteristics. In turn, we find different clusters show different patterns in assimilation indicators. These findings contribute to a growing literature that calls for more granular study of immigrant groups so we can better understand heterogeneity in their outcomes. In Chapter 3, I study the limits of social networks for the immigrant job search. The idea that migrants draw on their networks to obtain employment upon arrival at their destination is central to the immigrant integration literature. However, despite the wealth of evidence on migrants’ use of networks, little is known about when and why migrants are willing to help newcomers find work. To study this, I deploy an online vignette experiment among Latin American immigrants to the United States. I find that immigrants are more likely to provide job search support to other immigrants from their home country but are less likely to lend support to newcomers that pose a reputational risk. I also find that tie strength is important – respondents in our sample are more likely to help a close friend than a stranger, which can help immigrants overcome the difficulties associated with a competitive labor market.

Ecological factors and the policy environment are central constraints on population well-being. This dissertation emphasizes the role of shocks to help understand the nature of such constraints, and explores the relationship between population, environment, and development in greater detail than is typically possible.

Chapter 1 opens by contributing to a growing body of evidence around the impacts of old-age pensions on the well-being of pension recipients and their families. I draw from the unique disbursement structure of a popular, widely utilized benefits program in rural Brazil, and data from two nationally representative surveys conducted in 2013 by the Brazilian Institute of Geography and Statistics. I first employ regression discontinuity design to measure the direct effect of the program’s age threshold on pension take-up. Second, I compare differences in reports of health and well-being among age-eligible and age-ineligible adults in rural areas to the same differences among populations that generally do not qualify for the benefit (i.e., urban populations). This difference-in-differences shows robust evidence of a beneficial pension effect, though along somewhat different dimensions by gender. I then show evidence of two credible mechanisms for improved health and well-being: first, improved food security within households that have eligible pension recipients; second, the cohabitation of younger family members, potentially providing support to aging family members. Taken together, this chapter demonstrates that the rural benefits program in Brazil leads to tangible health benefits for its recipients, through channels that are likely to complement rather than crowd out other public services.

Chapter 2 moves on to explore how a massive natural disaster affected smoking behavior, a common coping mechanism. External stressors are commonly hypothesized to play a role in the adoption of certain health behaviors, but understanding the role of exposure is frequently hampered by research designs and data that are inadequate for tracing causality. I use this study to evaluate the relationship between unanticipated exposure to a natural disaster and smoking behaviors using longitudinal data collected from families in Aceh, Indonesia before and after the 2004 Indian Ocean tsunami. Exposure to the tsunami is measured as a community indicator of physical proximity to damage, and as individual indicators of personal experiences at the time of the tsunami. My analysis indicates that the effect of exposure on smoking uptake varies considerably by age, and that most forms of exposure increase smoking volume. These effects appear to be temporary, but even in the context of Indonesia’s extraordinarily high smoking rates an impact is perceptible even ten years after the tsunami.

Chapter 3 delves further into the effects of the tsunami, exploring the distribution of resources after the broad destruction of infrastructure and subsequent, fast-paced reconstruction. I evaluate multiple aspects of water access for roughly 6,000 families through 2014. Logit regression analysis show increasing disparities in access to basic amounts of water, and multinomial logit regression analysis indicates that shifts are driven by a massive increase in the market for privately distributed bottled water. This study disentangles key distributional processes to show how reconstruction influenced a central social determinant of health among an already vulnerable population.

The chapters to follow aim to relate the well-being of individuals to the influences that arise from interconnected policy choices and ecological factors. The first chapter emphasizes a policy shock, the second an ecological shock, and the second seeks to identify a combined effect of the two. This original research is intended to help illuminate the role that institutions might play in improving population well-being.

Over the past three decades, the phenomenon of children's health in China has been marked by a co-existence of overweight and underweight. As the primary institution for a child, family is an opportune place for child malnutrition intervention. By advancing a framework that addresses the contextual factors which shape the heterogeneity of socioeconomic gradients of child overweight/obesity, this dissertation has sought to understand the channels through which access to family resources influences child overweight/obesity in China. Based on these developed understandings, I identified the mechanisms by which having any younger siblings and three generation living together or proximately affect child malnutrition in China. Using data drawn from China Health and Nutrition Survey, this dissertation uncovered remarkable differences in multiple levels of contextual factors that shape a child's risk of overweight/obesity and underweight in China as compared to Western society. China's stage of economic development together with the drastically increasing income inequality has created an ever-increasing socioeconomic gap in child overweight/obesity, especially after 1997. This finding confirmed the position of the Ecological System framework that access to obesogenic environment is much more important than willpower based on knowledge in shaping one's obesity-related risk behavior. Although the fertility level has been lowered by One Child Policy, resource dilution effect still affects girls' nutrition status, especially for those exposed to poverty and food insecurity. Children in the care of grandparents are healthier, probably due to the generally low degree of access to obesogenic foods and a closer intergenerational relationship that facilitates effective communication and promotes healthy lifestyle formation.

Many older adults in the developing world rely on their adult children for financial, instrumental, and emotional support. The People's Republic of China (PRC), which will experience rapid population aging in the current century, is no exception. Many scholars and policy-makers are concerned that rapid economic, social, and demographic change in China is leading to a decline in traditional support for aging parents. This study examines the impact of family ties and economic resources on the receipt of support and the health of older adults across communities in China at different levels of economic development.

I analyze data from the 2002 and 2008 waves of the Chinese Longitudinal Healthy Longevity Survey (CLHLS) as well as the 2000 and 2005 1% Chinese Census. Initiated in 1998, the CLHLS interviewed older adults residing in a random sample of counties and cities in 22 provinces and municipalities of China. Additionally, in 2002 a subset of adult children of CLHLS respondents were also interviewed in a separate survey. Furthermore, the 2008-2009 wave collected additional data from middle-aged and older adults residing in specially designated "longevity areas" in China. In addition to the standard questionnaire and health exam, samples of blood and urine were also collected by medical personnel.

The first empirical chapter of this dissertation examines the association between filial piety/altruism and financial transfers to aging parents from adult children using factor analysis, binary logistic regression, and linear regression. The second paper looks at the socioeconomic-status health gradient using biomarker data among older adults residing in longevity areas using binary logistic regression analysis. The third paper examines both individual-level and community-level determinants of non-normative intergenerational coresidence - living with an adult daughter instead of an adult son-- through multilevel binary logistic models analyzing both survey and census data.

I find that (1) adult children's attitudes towards filial piety and family values are associated with both presence and amount of financial transfers to older parents, net of controls for adult child's socioeconomic status, parental need, parents' earlier life transfers to children, and whether elderly parents' provide instrumental support to adult children. (2) Similar to previous research in middle-income countries, many biomarkers were not associated with socioeconomic status but those that were demonstrated a reversed gradient - higher socioeconomic status was associated with worse health. (3) Greater numbers of daughters, higher levels of individual socioeconomic status, and residing in a more developed community was associated with greater likelihood of coresidence with adult daughters versus adult sons.