1,721,149 research outputs found
Collective movements and phenotypic plasticity : a study of locust phase polyphenism at multiple spatio-temporal scales
No full textLe polyphénisme de phase est une forme extrême de plasticité phénotypique densité-dépendante exprimée par une vingtaine d’espèces de criquets de la famille des Acrididae, nommées locustes. Les locustes présentent deux phénotypes extrêmes : la "phase solitaire" et la "phase grégaire", possédant des caractéristiques très différentes. Les deux phases se distinguent notamment par leur mode de vie : les locustes solitaires vont adopter un comportement sédentaire et cryptique, tandis que les grégaires vont se rassembler et se déplacer de manière coordonnée sur de longues distances, sous forme de bandes de larves ou d’essaims d’imagos dévastateurs. Dans cette thèse, nous nous intéressons à cet aspect particulier du polyphénisme de phase : les mouvements collectifs des individus grégaires, que nous étudierons sous différentes échelles spatio-temporelles.Une première partie de ces travaux de thèse vise à explorer l'impact de la variabilité spatiale de la ressource sur la recherche collective de nourriture à une échelle spatio-temporelle courte, correspondant à la distance parcourue par une bande larvaire grégaire en quelques heures de marche (< 100 m). Pour cela, nous avons développé un modèle à base d'agents, permettant de représenter les interactions locales entre individus et avec la végétation, à une échelle temporelle de l'ordre de 10s. Sur une grande variété de paysages, nous avons étudié sous quelles conditions le groupe sera avantagé par rapport à des larves solitaires en terme de recherche de nourriture. Nos résultats soulignent l'importance de l'alignement au sein du groupe pour optimiser la recherche de nourriture, et montrent que certains paysages où la ressource est agrégée et peu abondante (occupant moins de 40% de l'espace) avantagent les grégaires.Une seconde partie explore l'hypothèse d'un effet attractif des fèces sur les locustes grégaires, qui pourrait notamment permettre aux larves grégaires s'étant perdues de retrouver facilement la trace du groupe. Un tel effet pourrait assurer une meilleure cohésion du groupe sur une échelle spatio-temporelle plus grande (de l'ordre de la journée, pour une distance supérieure à 100m). Pour cela, nous avons réalisé des tests olfactifs comportementaux en laboratoire sur des larves au stade L3 du criquet pèlerin, Schistocerca gregaria, en présence de fèces de plusieurs classes d'âge (1h ou 24h). Nous avons également effectué des analyses chimiques (GC-MS) afin d'explorer quels composés organiques volatils étaient émis par les fèces de larves de locustes. Nos résultats montrent un effet attractif des fèces de 1h et de 24h sur les larves, suggérant que l'effet pourrait durer au moins sur une journée et ainsi permettre à des individus retardataires de retrouver la trace du groupe.Nous discutons enfin des implications possibles de ces résultats dans le contexte de l'émergence du polyphénisme de phase des locustes et des perspectives pour de futures études sur le sujet. Au vu de nos résultats de modélisation, et sachant qu'un environnement variable favorise l'apparition de plasticité phénotypique, la variabilité de la ressource pourrait être un des facteurs favorisant l'émergence du polyphénisme de phase. Des modèles d'optimum évolutif pourraient démontrer l'intérêt du polyphénisme, permettant de vivre isolé ou en groupe en fonction de la variabilité de la ressources. Des modèles à base d'agent démogénétique permettraient d'étudier l'influence de la variabilité spatiale et temporelle de la ressource sur l'émergence et l'évolution du polyphénisme de phase, au travers du seuil de grégarisation des locustes. La prise en compte de l'effet attractif des fèces, assurant une meilleure cohésion du groupe, pourrait avoir des implications sur les résultats d'un tel modèle. Ces futurs travaux permettraient de vérifier si l'évolution du polyphénisme de phase résulte d'interactions à des échelles spatio-temporelles variables.Phase polyphenism is an extreme form of density-dependent phenotypic plasticity expressed by about 20 species of grasshoppers of the Acrididae family, named locusts. Locusts present two extreme phenotypes: the "solitarious phase" and the "gregarious phase", showing very different characteristics. The two phases are distinguished in particular by their way of life: solitarious locusts will adopt a sedentary and cryptic behaviour, while the gregarious ones will gather and move in a coordinated way over long distances, as devastating bands of nymph or swarms of imagos. In this thesis, we focus on this particular aspect of phase polyphenism: the collective movements of gregarious individuals, which we will study under different spatio-temporal scales.The first part of this thesis aims to explore the impact of spatial variability of resources on collective foraging at a short spatiotemporal scale, corresponding to the distance covered by a gregarious nymphs band in a few hours of walking (< 100 m). For this purpose, we developed an agent-based model, allowing to represent local interactions between individuals and with vegetation, at a temporal scale of 10s. On a large variety of landscapes, we studied under which conditions the group will have an advantage over solitarious nymphs in terms of foraging. Our results highlight the importance of alignment within the band to optimize foraging, and show that certain landscapes where the resource is aggregated and sparse (occupying less than 40% of space) favor gregarious individuals.A second part explores the hypothesis of an attractive effect of faeces on gregarious locusts, which could notably allow gregarious nymphs that got lost to easily find the group's trace. Such an effect could ensure a better cohesion of the group on a larger spatio-temporal scale (of the order of the day, for a distance higher than 100m). To this end, we performed behavioural olfactory assays in the laboratory on L3-stage nymphs of the desert locust, Schistocerca gregaria, in the presence of faeces from several age classes (1h or 24h). We also performed chemical analyses (GC-MS) to explore which volatile organic compounds were emitted from nymphs faeces. Our results show an attractive effect of the 1h and 24h faeces on the nymphs, suggesting that the effect could last for at least one day and thus allow latecomers individuals to find the group's trace.Finally, we discuss the possible implications of these results in the context of the emergence of locusts' phase polyphenism, and prospects for future studies on the subject at higher spatiotemporal scales. Given our modeling results, and knowing that a variable environment favors the emergence of phenotypic plasticity, resource variability could be one of the factors favoring the emergence of phase polyphenism. Evolutionary optimum models could demonstrate the benefits of polyphenism, allowing to live isolated or in groups according to the variability of the resources. Demogenetic agent-based models would allow to study the influence of spatial and temporal variability of the resource on the emergence and evolution of phase polyphenism, through the locust gregarization threshold. Taking into account the attractive effect of feces, ensuring better group cohesion, could have implications on the results of such a model. This future work would test whether the evolution of phase polyphenism results from interactions at varying spatiotemporal scales
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Information criteria and approximate Bayesian computing for agent-based modelling in ecology: New tools to infer on Individual-level processes
Full text linkAgent or individual based models (ABMs) are computer simulation tools to represent complex systems where the unique autonomous individuals constituting the system interact, adapt or evolve. In ecology, ABMs have been applied to analyse the influence of individual variability, spatial interactions, development, behaviour, learning or genetic variability on group, population or community dynamics [1]. During the last 20 years, the approach of pattern-oriented modelling (POM) has been helping in making the use of ABMs more robust and reliable [2]. Two main elements of POM are a) ``inverse modelling'' to reduce parameter uncertainty and b) ``strong inferences'' to test how well alternative model versions changing in certain individual-level processes are at explaining multiple patterns observed at the population and/or community level. The latter has many parallels to model selection procedures in statistical modelling ( ``mutlimodel inference'' [3]). The inverse modelling borrows also concept and tools from statistics to fit unknown or uncertain parameters to real-world data. With the development of approximate Bayesian computing (ABC) in ecology [4] some propositions were made to use ABC in ABMs [5]. Bridging the two main elements of POM, an information criterion was proposed [6] that borrows concept of ABC such as Markov Chains Monte Carlo simulations without likelihood functions and multimodel inference in Bayesian context. This criterion (POMIC for pattern-oriented modelling information criterion) was applied in the original proposition paper to forest growth simulations to infer on individual tree growth function. Recently, it was applied to locust to infer on individual behaviour at hatching [7]. These kinds of cryptic processes (tree individual growth or animals' behaviour during first hours) can typically be analysed following few isolated individuals in natural or experimental conditions. However, in the context of inter-individual interactions, experimental designs to infer on how the interactions modify individual responses are hard, if not impossible to elaborate. Using population or group level data to fit ABMs and infer on the potential individual process with different ABMs version is then a way of overpassing these problems. However, the question of ABMs complexity (essential in statistical model selection procedures) needs to be addressed. The POMIC proposition includes a complexity measure and makes it therefore an interesting tool to infer on cryptic individual-level processes of complex systems. (Texte intégral
Locust invasions are cyclical: African states shouldn’t be caught napping
No full textInternational audienceThe Food and Agriculture Organisation (FAO) has warned that the locust invasion in the Horn of Africa may soon spread to other countries. The locusts feeding on crops could leave millions of people without food. Cyril Piou shares his insights on where they might go and how countries should prepare
Patterns and individual-based modeling of spatial competition within two main components of Neotropical mangrove ecosystems.
No full textThe main focus of the thesis was to look at the role of competition in shaping the spatial organization of two main components of Neotropical mangrove ecosystems. The first component was the Caribbean mangrove tree community. The second was populations of an ecologically and economically important mangrove burrowing crab in North-Brazil: Ucides cordatus. These two components were studied in two respective parts with one field and two individual-based modeling studies each. The two individual-based-models were used or developed with the pattern-oriented modeling (POM) approach, to look at: (1) the role of interspecific competition in the dynamic and species zonation patterns of mangrove forests, (2) the role of intraspecific competition in the recovery and spatial distribution patterns of crab populations. To infer on the mechanisms driving these patterns, the POM lacked an information criterion such as the Akaike's information criterion used in model selection procedures. The present thesis proposed and used a pattern-oriented modeling information criterion (POMIC) based on estimation of likelihood of field data to be reproduced by simulation results
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