1,721,164 research outputs found
Interactions between structure and stochasticity in demogaphic models
Full text linkDemography is the study of population dynamics. Populations can be considered as
groups of individuals living within a given region. These simple statements
encompass highly disparate systems, which respond to demographic and
environmental stochasticity in predictable and unpredictable ways. The responses
depend on the structure of the population, since individuals can have vastly different
survival and recruitment, which, with dispersal, determine population abundance.
Whilst some variation is inter-st(age) – increases in reproductive performance with
age, for example – substantial intra-st(age) variation is not uncommon. Using longterm
individual-based data on two disparate vertebrate populations, the focus of this
thesis is the interaction between structure and stochasticity in demographic models,
and consequences on resultant aspects of population growth.
Structured models that incorporated variation in demographic rates detected marked
differences within, between and across diverse habitats for different age-classes in
both populations. These results were consistent for a wide range of scaling and
definition to account for mathematical dependence. Spatial structure was more
influential than age-structure in responses to stochastic predation. Despite significant
changes in performance and phenotype with age, individual heterogeneity within ageclasses
was vast.
These results are of importance for conservation and management action, as well as
predictors of evolutionary change. The population is a fundamental force in ecology
and evolution. This work adds weight to the argument that characteristics of
individual performance in response to variability in their environments are pivotal to
increased understanding of changes in population abundance. These individual
responses are dependent upon the opportunity generated by population structure. A
failure to incorporate either structure or stochasticity neglects crucial aspects in
population regulation, and therefore ecological and evolutionary change
How sensitive are elasticities of long-run stochastic growth to how environmental variability is modelled?
No full textA variable environment leaves a signature in a population's dynamics. Deriving statistical and mathematical models of how environmental variability affects population projections has – in the wake of reports of substantial climatic fluctuations – received much recent attention. If the model changes, then so too does the population projection. This is because a different model of environmental variability changes estimates of long-run stochastic growth, which is a function of demographic rates and their temporal sequence. Decomposing elasticities of long-run stochastic growth into constituent parts can assess the relative influence of different components. Here, we investigate the consequences of changing the environmental state definition, and therefore altering the shape of demographic rate distributions and their temporal sequence, by using age-structured matrix models to project vertebrate populations into the future under a range of environmental scenarios. The identity of the most influential demographic rate was consistent among all approaches that perturbed only the mean, but was not when only the variance was perturbed. Furthermore, the influence of each demographic rate fluctuated among projections by up to factors of six and two for changes to the variance and mean, respectively. These changes in influence depend in part upon how environmental variability – in particular, the color of environmental noise – is incorporated. In the light of predictions of increasing climatic variability in the future, these results suggest caution when drawing quantitative conclusions from stochastic population projections
Estimating the functional form for the density? Dependence from life history data
No full textCoulson, Tim ... et al.Two contrasting approaches to the analysis of population dynamics are currently popular: demographic approaches where the associations between demographic rates and statistics summarizing the population dynamics are identified; and time series approaches where the associations between population dynamics, population density, and environmental covariates are investigated. In this paper, we develop an approach to combine these methods and apply it to detailed data from Soay sheep (Ovis aries). We examine how density dependence and climate contribute to fluctuations in population size via age- and sex-specific demographic rates, and how fluctuations in demographic structure influence population dynamics. Density dependence contributes most, followed by climatic variation, age structure fluctuations and interactions between density and climate. We then simplify the density-dependent, stochastic, age-structured demographic model and derive a new phenomenological time series which captures the dynamics better than previously selected functions. The simple method we develop has potential to provide substantial insight into the relative contributions of population and individual-level processes to the dynamics of populations in stochastic environments. © 2008 by the Ecological Society of America.Peer Reviewe
Correlations between age, phenotype, and individual contribution to population growth in common terns
No full textThere have been numerous reports of changes in phenology, which are frequently attributed to environmental change. Age-dependent change in phenotypic traits, fledgling production, and the timing of events in the life cycle is also widespread. This means that changes in the age structure of a population could generate changes in phenology, which may be incorrectly attributed to environmental change or microevolution. Here, estimates of selection for arrival date, arrival mass, and laying date are compared when age is and is not corrected for. This is achieved using long-term individual-based data collected from a breeding colony of Common Terns (Sterna hirundo) and a novel fitness measure: individual contributions to population growth. The failure to correct for age generated deceptive estimates of selection in eight out of nine comparisons. In six out of nine comparisons, the direction of selection differed between age-corrected and uncorrected estimates. Persistent individual differences were detected: individuals remained within the same part of the phenotype distribution throughout life. The age-corrected estimates of selection were weak and explained little variation in fitness, suggesting that arrival date, arrival mass, and laying date are not under intense selection in this population. These results also demonstrate the importance of correcting for age when identifying factors associated with changes in seabird phenology
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
Modelling the Dynamics of Translocated Populations
Full text linkClimate change is widely accepted as one of the worst threat to the world’s biodiversity. A
recently proposed solution to help species survive climate change is “assisted colonisation”,
i.e. translocations of species to more suitable sites. Although assisted colonisation could be
a powerful conservation tool, translocations are known to have a low success rate despite
being commonly-used. Before promoting assisted colonisation as a solution, we need to
make two major advances: first, understand how translocated species will respond to a
changing climate and if their population viability will be compromised and, second, develop
a quantitative framework to improve the success rates of conservation translocation under
global environmental change. Those were the two aims of my thesis. The hihi (stitchbird;
Notiomystis cincta ) was used as a study species. It has been the subject of several
translocations in the past few decades and every translocated population is intensely
monitored, yielding long-term demographic datasets.
Climate was found to be a key factor in the dynamics of translocated population, and thus
viability. However, it may also interact with other intrinsic factors like density and age to
influence species’ long-term persistence. A stochastic population model built to quantify the
potential impacts of future climate change on translocated populations’ long-term
persistence showed the importance of taking into account the impact of longer and more
frequent extreme weather on translocated population’s viability. Moreover, MaxEnt was
found to be a good habitat suitability model for translocated populations and used to
identify future translocation sites for the hihi under climate change. Finally, quantitative
guidelines for the planning and implementation of assisted colonisations were developed.
They address the known issues impeding translocation success, and, if used, will maximise
the success of assisted colonisations
A demographic perspective on trait heritability and sex differences in life history
Full text linkBiologists have long used demographic approaches to answer questions in ecology and evolution. The utility of these approaches has meant a constant development and refinement of methods. A key milestone has been the development of phenotype structured population models that link ecology and evolution. Moreover, biostatistical research steadily improves methods to coax demographic information from scarce data. In this thesis, I build upon some of the recent advances in the field. My first three studies focus on the consequences of sex differences in life history for population dynamics. Firstly, I test whether males matter for the dynamics of African lion (Panthera leo) populations via a previously unquantified mechanism: the inheritance of phenotype from father to offspring. Secondly, I develop a method to estimate age-specific mortality rates for both sexes in species where one of the sexes disperses around the age of maturity. Thirdly, I apply this method to study variation in mortality between the sexes and between two populations of African lions. After these three chapters, which make contributions to the field of sex-structured population dynamics, I focus on the integration of phenotype structured modelling and quantitative genetics. I illustrate how heritability of a quantitative character that develops with age depends on (i) viability selection, (ii) fertility selection, (iii) the development of the phenotype with age, and (iv) phenotype inheritance from parents to offspring. Our results question the adequacy of quantitative genetics methods to obtain unbiased estimates of heritability for wild populations. This thesis advances our understanding of population development over ecological time scales. This knowledge has applications in conservation and population management, but also contributes to untangling evolutionary processes in wild animals
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