Browsing by Subject "Population ecology"

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.

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.

This dissertation investigates three areas where religious leaders have confronted changes in the modern world. In Chapter 2, I investigate trends in congregational worship towards more enthusiastic and contemporary styles to determine the extent to which this trend is driven by congregations adapting or congregations dying and being replaced. I find that, in contrast with the expectations of the strongest versions of population ecology theories, the overwhelming majority of change is the result of congregations themselves changing, though this varies by religious tradition. In Chapter 3, I examine which congregations were prepared to shift their worship to an online format on the eve of the COVID-19 pandemic. I find that, in addition to technological know-how and financial resources, contemporary worship practices and religious tradition were strong, independent predictors of streaming and recording capabilities. This suggests that cultural factors, and not just financial or technological resources, were important in enabling an effective response to the pandemic. In Chapter 4, I study the rhetoric of evangelical opinion leaders on immigration and Islam before and after Donald Trump rose to political prominence. I find that evangelical leaders were already discussing immigration and especially Islam with frames of foreign threat even before Trump became politically relevant, and this did not meaningfully shift in the Trump era. Taken together, these studies advance knowledge of how religious leaders and institutions are continuing to interact with an ever-changing modern world.