1,721,072 research outputs found
Food abundance, kittiwake life histories, and colony dynamics in the Northeastern Pacific:implications of climate change and regime shifts
No full textBlack-legged kittiwakes Rissa tridactyla in the Northeastern Pacific will increasingly experience climate-induced changes in the variability of forage fish, which will influence both the quantity and quality of food and may thus alter the population dynamics of kittiwake colonies. However, the relative roles of individual- and population-level traits in determining colony dynamics and risk of extinction are still unclear. We combined models of components of the Pacific kittiwake life history with empirical data linking physiological stress and food abundance to provide a unified treatment of kittiwake colony dynamics. We simulated the dynamics of colonies with high, medium and low responsiveness of productivity to variation in nutritional stress in breeding birds, using data from Alaskan colonies. We found that the risk of quasi-extinction strongly decreased with a moderate increase in the potential number of yearly immigrants. Pre-breeding mortality as a function of growth during development had only a marginal role in determining median number of breeding pairs over simulation time. We predict that temporal auto - correlation of colony-wide average productivity and high nutritional stress, particularly if consistent over time, will increase quasi-extinction risk. Our work shows that colonies with low productivity have little chance of persistence even when survival of pre-breeding and breeding birds is high, and that the nature of the temporal auto-correlation of food conditions and productivity is crucial to understand the effect of environmental fluctuations, regime shifts, and climate change on population dynamics of kittiwakes. We use the model to highlight the most valuable future empirical studies
The Benefits of Induced Defenses Against Herbivores
Full text linkPrevious explanations for the evolution of induced resistance of plants to herbivory emphasized arguments based on saving costs when allocations to defense were not needed; these models met with limited empirical support. We offer a novel explanation based on induced resistance providing increased variability in defense. As long as maximal levels of defense are constrained, variability will increase the effectiveness of a given level of investment in defense. We show that variability can decrease herbivore performance if herbivore performance is a concave function of the level of resistance. In particular, if herbivores can choose among different plants and plant tissues, then variability created by induced resistance may benefit plants under attack and hence may be favored by selection. The key assumptions of this model are broadly supported by empirical data from many plant–herbivore systems.This work was supported by USDA NRI 9602065
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Adapted to Environmental Change: Life History, Diet, and Habitat Choice of Krill in Winter
Full text linkHigh latitude oceans are strongly seasonal ecosystems where winter conditions are marked by periods of low primary productivity. These oceans tend to have shortened food webs with relatively few species linking primary production to upper trophic levels. In the case of the Southern Ocean, a single species, Euphausia superba, is thought to be this link between trophic levels. The polar regions in both hemispheres are also among the ecosystems heavily impacted by climate change. For example, the western Antarctic Peninsula is experiencing some of the most rapidly changing climate on the planet with changes in temperature, wind, and sea ice durations and extent (Vaughn et al 2003, Stammerjohn et al 2008). Understanding how climate change will affect these ecosystems requires knowledge of trophic structure, its key species, their life history, and their plasticity to environmental variability. In this thesis, I explore the seasonal life history strategies of Antarctic euphausiids. In chapter one, I introduce the high latitude marine ecosystem of the Southern Ocean, Antarctic euphausiids, and climate change and its impact on the ecosystem in this region. In chapter two, I fill in gaps in our knowledge of the life history strategy of the highly abundant, but relatively understudied, Thysanoessa macrura, by synthesizing distribution, maturity, and diet data from summer and winter surveys in the Antarctic Peninsula region. I find that krill show knife edge maturity, can spawn in their first year, are more dispersed and offshore in winter, and are more predatory with increasing size and in winter. In the third chapter I investigate E. superba from the perspective of optimal foraging theory to examine when krill generalize or specialize to unify seasonal and regional differences in the diet and feeding behavior of E. superba under the Trophic Wave Hypothesis. I predict that krill have a more specialized yet higher mortality risk diet in summer and more of a generalist diet in winter. In chapter four, I ask whether E. superba found in benthic and mesopelagic habitats are an aberration or reflect an important part of their life history strategy. In this chapter, I use a Stochastic Dynamic Programming model (VertiKrill) to explore how food, predation, and respiration drive vertical habitat selection across a range body conditions throughout the year. I find that for both juveniles and adults, deep water habitats provide important refuges for avoiding predation and starvation in winter as well as during the transition between seasons. In chapter five I summarize the main findings of each chapter, discuss their implications, and offer future directions for this research
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Unraveling Steelhead Life History Complexity Through Mathematical Modeling
Full text linkSteelhead trout exhibit highly variable population dynamics that are driven by the vast life history variability of individuals that results in a complex system with highly non-linear dynamics for both individuals and populations. In this dissertation, I consider three different projects with the aim of understanding both individual processes and population dynamics using mathematical and statistical tools. In each chapter I develop new mathematical and statistical methodology that incorporates the biological knowledge about steelhead life history in order to answer different questions about the individual and population dynamics observed in nature. The first project focuses on the development and application of methodology to better explain the observed adult returns in the Carmel River. To do this, I incorporate knowledge about conditional migration strategies into a life history model. In this work, I discover a decreasing trend in mean stream temperature that is coupled with a decreasing trend in mean length of individuals. One of the primary goals of this chapter is to test the scientific hypothesis that the inclusion of life history attributes would improve the predictions of adult returns. I demonstrate that the mechanistic inclusion of life history attributes into the mathematical model does significantly improve the ability to predict adult returns.In the second project, I develop a Physiologically Structured Population Model (PSPM) for steelhead trout. The PSPM framework allows me to account for the biological processes driving the life history of steelhead through their entire lifetime. A benefit of this framework is that the resultant population dynamics arise solely from the interactions between individuals and their environment. The model captures the wide variability of life histories that are observed in nature. With this model, I explore the effects that size-dependent mortality and competition have on the population dynamics. I also explore the effects of three different temperature regimes on the population dynamics, highlighting the non-linear nature of the system. The last project is the development of a Bayesian Semi-parametric model that describes the relationship between temperature and individual consumption that determines growth. Developing a state-space model that incorporates a Gaussian Process prior for the temperature-consumption relationship allows the data to determine the shape of the relationship and account for both measurement error and process stochasticity. I first test the model with simulated data with different levels of data availability, measurement error and process stochasticity. This application demonstrates that the total number of temporal measurements affects the performance of the model more than the number of individuals. I then apply the model to experimental data from a growth experiment of steelhead trout. The results demonstrate the ability of the model to describe the growth of individuals as well as to capture individual consumption. The model shows agreement between the shape of the temperature-consumption function that I predict and the relationship that is commonly used for steelhead
Within- and among-population variation in vital rates and population dynamics in a variable environment
No full textUnderstanding the causes of within- and among-population differences in vital rates, life histories, and population dynamics is a central topic in ecology. To understand how within- and among-population variation emerges, we need long-term studies that include episodic events and contrasting environmental conditions, data to characterize individual and shared variation, and statistical models that can tease apart shared and individual contribution to the observed variation. We used long-term tag-recapture data to investigate and estimate within- and among-population differences in vital rates, life histories, and population dynamics of marble trout Salmo marmoratus, an endemic freshwater salmonid with a narrow range. Only ten populations of pure marble trout persist in headwaters of Alpine rivers in western Slovenia. Marble trout populations are also threatened by floods and landslides, which have already caused the extinction of two populations in recent years. We estimated and determined causes of variation in growth, survival, and recruitment both within and among populations, and evaluated trade-offs between them. Specifically, we estimated the responses of these traits to variation in water temperature, density, sex, early life conditions, and extreme events. We found that the effects of population density on traits were mostly limited to the early stages of life and that growth trajectories were established early in life. We found no clear effects of water temperature on vital rates. Population density varied over time, with flash floods and debris flows causing massive mortalities (>55% decrease in survival with respect to years with no floods) and threatening population persistence. Apart from flood events, variation in population density within streams was largely determined by variation in recruitment, with survival of older fish being relatively constant over time within populations, but substantially different among populations. Marble trout show a fast to slow continuum of life histories, with slow growth associated with higher survival at the population level, possibly determined by food conditions and age at maturity. Our work provides unprecedented insight into the causes of variation in vital rates, life histories, and population dynamics in an endemic species that is teetering on the edge of extinction
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Trade-offs between accuracy and interpretability in von Bertalanffy random-effects models of growth
No full textBetter understanding of variation in growth will always be an important problem in ecology. Individual variation in growth can arise from a variety of processes; for example, individuals within a population vary in their intrinsic metabolic rates and behavioral traits, which may influence their foraging dynamics and access to resources. However, when adopting a growth model, we face trade-offs between model complexity, biological interpretability of parameters, and goodness of fit. We explore how different formulations of the von Bertalanffy growth function (vBGF) with individual random effects and environmental predictors affect these trade-offs. In the vBGF, the growth of an organism results from a dynamic balance between anabolic and catabolic processes. We start from a formulation of the vBGF that models the anabolic coefficient (q) as a function of the catabolic coefficient (k), a coefficient related to the properties of the environment (γ) and a parameter that determines the relative importance of behavior and environment in determining growth (ψ). We treat the vBGF parameters as a function of individual random effects and environmental variables. We use simulations to show how different functional forms and individual or group variability in the growth function's parameters provide a very flexible description of growth trajectories. We then consider a case study of two fish populations of Salmo marmoratus and Salmo trutta to test the goodness of fit and predictive power of the models, along with the biological interpretability of vBGF's parameters when using different model formulations. The best models, according to AIC, included individual variability in both k and γ and cohort as predictor of growth trajectories, and are consistent with the hypothesis that habitat selection is more important than behavioral and metabolic traits in determining lifetime growth trajectories of the two fish species. Model predictions of individual growth trajectories were largely more accurate than predictions based on mean size-at-age of fish. Our method shares information across individuals, and thus, for both fish populations investigated, allows using a single measurement early in the life of individual fish or cohort to obtain accurate predictions of lifetime individual or cohort size-at-age
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