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.

The ‘central‐peripheral’ hypothesis has provided a baseline for many studies of population dynamics and genetic variability at species distribution limits. Although peripheral populations are often assumed to occur in ecologically marginal conditions, little is known about whether they effectively occur in a distinct ecological niche. A cross‐taxa analysis of 11 Mediterranean vascular plants were studied. We quantified variation in the ecological niche between populations at the northern range limits of species in Mediterranean France and those in the central part of the distribution in continental Spain or Italy in 2013–2014. We analyzed both the macro‐ecological niche where populations occur in terms of broad habitat and altitudinal range and the micro‐ecological niche where individual plants grow in terms of soil and structural biotic and abiotic characteristics. Most species occur in a single broad habitat type common to central and peripheral populations and have a narrower altitudinal range in the latter. In contrast, for the micro‐ecological niche we detected marked variation in several niche parameters among central and peripheral populations. Although many differences are species‐specific some are common to several species. We found a trend towards narrower micro‐niche breadth in peripheral populations. Our results illustrate the importance of studying the precise ecological characteristics where plants grow and the pertinence of a multi‐species approach to correctly assess niche variation. The ecological originality of peripheral populations underlines their evolutionary potential and conservation significance.

The delimitation of bioregions helps to understand historical and ecological drivers of species distribution. In this work, we performed a network analysis of the spatial distribution patterns of plants in south of France (Languedoc‐Roussillon and Provence‐Alpes‐Côte d'Azur) to analyze the biogeographical structure of the French Mediterranean flora at different scales. We used a network approach to identify and characterize biogeographical regions, based on a large database containing 2.5 million of geolocalized plant records corresponding to more than 3,500 plant species. This methodology is performed following five steps, from the biogeographical bipartite network construction to the identification of biogeographical regions under the form of spatial network communities, the analysis of their interactions, and the identification of clusters of plant species based on the species contribution to the biogeographical regions. First, we identified two sub‐networks that distinguish Mediterranean and temperate biota. Then, we separated eight statistically significant bioregions that present a complex spatial structure. Some of them are spatially well delimited and match with particular geological entities. On the other hand, fuzzy transitions arise between adjacent bioregions that share a common geological setting, but are spread along a climatic gradient. The proposed network approach illustrates the biogeographical structure of the flora in southern France and provides precise insights into the relationships between bioregions. This approach sheds light on ecological drivers shaping the distribution of Mediterranean biota: The interplay between a climatic gradient and geological substrate shapes biodiversity patterns. Finally, this work exemplifies why fragmented distributions are common in the Mediterranean region, isolating groups of species that share a similar eco‐evolutionary history.

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Editor: Félix ForestPremise of research. Comparative studies of variation in the ecology and genetics of natural plant populations located at the limits and in the center of a species range provide fundamental insights into the historical formation of species distribution patterns.Methodology. In this study, we quantify variation in the ecological niche and the expression of a floral polymorphism across the range of the Mediterranean geophyte Narcissus dubius Gouan. An exhaustive data set of known locations was compiled to distinguish groups of geographically central and peripheral populations in both Spain and France. These occur across a double climatic gradient. First, there is a gradient from a hot and dry Mediterranean climatic regime in eastern Spain to a milder Mediterranean climatic regime in southern France (lower maximum temperature and shorter summer drought). Second, there is a shift to a more continental climate in peripheral populations in inland Spain. We also modeled the current climatic niche and produced historical projections of potential glacial refugia for this species.Pivotal results. Peripheral populations showed consistent patterns of ecological niche differences with a less rupicolous ecology, more bare soil, and a higher cover of annual species. The ecological niche was more variable among peripheral populations than among central populations. Peripheral populations showed a repeated pattern of loss of stigma-height polymorphism and floral traits indicative of a reproductive strategy based on within-morph mating (probably selfing to assure seed set). Climate models indicate that contemporary peripheral populations (particularly in Spain) occur in areas that were least likely to have served as glacial refugia.Conclusions. Historical isolation and recolonization may have shaped contemporary patterns of ecological niche and reproductive trait variation among central and peripheral populations.

Hybridisation plays a prominent role in plant evolution due to its influence on genetic diversity, fitness and adaptive potential. We identify a case of on-going hybrid evolution of floral phenotypes in disjunct populations of Cyclamen balearicum and C. repandum subsp. repandum on Corsica and Sardinia. Hybrid populations on the two islands contain similar patterns of variation in flower colour and size but are probably at different stages in the evolutionary process of hybridisation, and differences in the frequency of floral types and flower size suggest hybrid vigour that may contribute to the dynamics and maintenance of hybrid forms. In a review of cases of hybridisation in Mediterranean plants we found an equivalent number of cases for the contemporary occurrence of mixed hybrid populations, as there are cases of homoploid hybrid species differentiation. We argue for the development of a conservation strategy for Mediterranean plants that integrates the need to protect not just pure endemic species (some of hybrid origin) but also mixed populations where adaptive variation and new species are evolving due to contemporary hybridisation.

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