Paul Sabatier University

About: Paul Sabatier University is a education organization based out in Toulouse, France. It is known for research contribution in the topics: Population & Adipose tissue. The organization has 15431 authors who have published 23386 publications receiving 858364 citations.

The problem of pattern and scale in ecology: what have we learned in 20 years?

Abstract: Over the past 20 years, major advances have clarified how ecological patterns inform theory, and how in turn theory informs applied ecology. Also, there has been an increased recognition that the problem of scale at which ecological processes should be considered is critical if we are to produce general predictions. Ecological dynamics is always stochastic at small scales, but variability is conditional on the scale of description. The radical changes in the scope and aims of ecology over the past decades reflect in part the need to address pressing societal issues of environmental change. Technological advances in molecular biology, global positioning, sensing instrumentation and computational power should not be overlooked as an explanation for these radical changes. However, I argue that conceptual unification across ecology, genetics, evolution and physiology has fostered even more fertile questions. We are moving away from the view that evolution is played in a fixed ecological theatre: the theatre is being rapidly and relentlessly redesigned by the players themselves. The maintenance of ecosystem functions depends on shifts in species assemblages and on cellular metabolism, not only on flows of energy and matter. These findings have far reaching implications for our understanding of how ecosystem function and biodiversity will withstand (or not) environmental changes in the 21st century.

Neutral theory and community ecology

Abstract: I review the mathematical and biological aspects of Hubbell’s (2001) neutral theory of species abundance for ecological communities, and clarify its historical connections with closely related approaches in population genetics. A selective overview of the empirical evidence for and against this theory is provided, with a special emphasis on tropical plant communities. The neutral theory predicts many of the basic patterns of biodiversity, confirming its heuristic power. The strict assumption of equivalence that defines neutrality, equivalence among individuals, finds little empirical support in general. However, a weaker assumption holds for stable communities, the equivalence of average fitness among species. One reason for the surprising success of the neutral theory is that all the theories of species coexistence satisfying the fitness equivalence assumption, including many theories of niche differentiation, generate exactly the same patterns as the neutral theory. Hubbell’s neutral theory represents an important synthesis and a much needed demonstration of the pivotal role of intraspecific variability in ecology. Further improvements should lead to an explicit linking to niche-based processes. This research programme will depend crucially on forthcoming theoretical and empirical achievements.

Representation and combination of uncertainty with belief functions and possibility measures

Abstract: The theory of evidence proposed by G Shafer is gaining more and more acceptance in the field of artificial intelligence, for the purpose of managing uncertainty in knowledge bases One of the crucial problems is combining uncertain pieces of evidence stemming from several sources, whether rules or physical sensors This paper examines the framework of belief functions in terms of expressive power for knowledge representation It is recalled that probability theory and Zadeh's theory of possibility are mathematically encompassed by the theory of evidence, as far as the evaluation of belief is concerned Empirical and axiomatic foundations of belief functions and possibility measures are investigated Then the general problem of combining uncertain evidence is addressed, with focus on Dempster rule of combination It is pointed out that this rule is not very well adapted to the pooling of conflicting information Alternative rules are proposed to cope with this problem and deal with specific cases such as nonreliable sources, nonexhaustive sources, inconsistent sources, and dependent sources It is also indicated that combination rules issued from fuzzy set and possibility theory look more flexible than Dempster rule because many variants exist, and their numerical stability seems to be better La theorie des fonctions de credibilite proposee par G Shafer est de plus en plus reconnue, dans le domaine de l'intelligence artificielle, pour le traitement de l'incertitude dans les bases de connaissances Un des problemes cruciaux est la combinaison d'informations issues de plusieurs sources (regles ou capteurs) Cet article etudie le cadre propose par les fonctions de credibilite quant a son pouvoir expressif pour la representation de connaissances On rappelle que d'un point de vue mathematique la theorie des probabilites et la theorie des possibilites sont des cas particuliers de la theorie des fonctions de credibilite pour ce qui est de l'evaluation de l'incertitude Les fondements empiriques et axiomatiques des fonctions de credibilite et de possibilite sont discutes Puis le probleme general de la combinaison est aborde en mettant l'accent sur la regle de Dempster On met en evidence le fait que cette regle est peu adaptee a la combinaison d'informations conflictuelles D'autres regles sont proposees pour traiter ce probleme dans diverses situations telles que sources inegalement fiables, incompletes, incoherentes, dependantes On indique aussi que les regles de combinaison proposees par la theorie des ensembles flous et des possibilites semblent plus flexibles que la regle de Dempster, car de nombreuses variantes existent; de plus leur stabilite numerique semble meilleure