title: Geographic variation in genetic and demographic performance: new insights from an old biogeographical paradigm.
abstract: The 'centre-periphery hypothesis' (CPH) is a long-standing postulate in ecology that states that genetic variation and demographic performance of a species decrease from the centre to the edge of its geographic range. This hypothesis is based on an assumed concordance between geographical peripherality and ecological marginality such that environmental conditions become harsher towards the limits of a species range. In this way, the CPH sets the stage for understanding the causes of distribution limits. To date, no study has examined conjointly the consistency of these postulates. In an extensive literature review we discuss the birth and development of the CPH and provide an assessment of the CPH by reviewing 248 empirical studies in the context of three main themes. First, a decrease in species occurrence towards their range limits was observed in 81% of studies, while only 51% demonstrated reduced abundance of individuals. A decline in genetic variation, increased differentiation among populations and higher rates of inbreeding were demonstrated by roughly one in two studies (47, 45 and 48%, respectively). However, demographic rates, size and population performance less often followed CPH expectations (20-30% of studies). We highlight the impact of important methodological, taxonomic, and biogeographical biases on such validation rates. Second, we found that geographic and ecological marginality gradients are not systematically concordant, which casts doubt on the reliability of a main assumption of the CPH. Finally, we attempt to disentangle the relative contribution of geographical, ecological and historical processes on the spatial distribution of genetic and demographic parameters. While ecological marginality gradients explain variation in species' demographic performance better than geographic gradients, contemporary and historical factors may contribute interactively to spatial patterns of genetic variation. We thereby propose a framework that integrates species' ecological niche characteristics together with current and past range structure to investigate spatial patterns of genetic and demographic variation across species ranges.
authors.affiliation: Instituto Pirenaico de Ecología (IPE-CSIC), Box 1005, avenida, Montañana, 50059, Zaragoza, Spain.
authors.forename: Samuel
authors.initials: S
authors.lastname: Pironon
authors.affiliation: UMR 5175 Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, Box 1019, route de Mende, 34090, Montpellier, France.|Dipartimento di Scienze della Natura e del Territorio, Università degli Studi di Sassari, Box 21, Piazza Universitá, 07100, Sassari, Italy.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Papuga
authors.affiliation: Department of Biology, Duke University, Box 90338, Durham, NC, 27708-0338, U.S.A.
authors.forename: Jesús
authors.initials: J
authors.lastname: Villellas
authors.affiliation: Departments of Botany and Zoology, University of British Columbia, Box 4200-6270, University Boulevard, Vancouver, V6T 1Z4, Canada.
authors.forename: Amy L
authors.initials: AL
authors.lastname: Angert
authors.affiliation: Instituto Pirenaico de Ecología (IPE-CSIC), Box 1005, avenida, Montañana, 50059, Zaragoza, Spain.
authors.forename: María B
authors.initials: MB
authors.lastname: García
authors.affiliation: UMR 5175 Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, Box 1019, route de Mende, 34090, Montpellier, France.
authors.forename: John D
authors.initials: JD
authors.lastname: Thompson
mesh_terms: D063147:Animal Distribution / Q000502:physiology*; D000818:Animals; D017753:Ecosystem; D051456:Gene Flow; D014644:Genetic Variation; D008957:Models, Genetic
keywords: abundant-center model; centre-periphery hypothesis; demography; ecological marginality; genetic diversity; meta-analysis; range limits; rear-leading edge; species distribution