An Integrative Assessment Of A Major Biogeographical Paradigm: The "Centre-Periphery" Hypothesis
The discipline of biogeography has long focused on understanding the patterns and processes associated with geographical variation in species performance. In this context, the centre-¬ periphery hypothesis (CPH) has been proposed as a general framework explaining the distribution of intra-¬specific variation across species geographical ranges. Based on the assumption that species ranges are spatial representations of the distribution of their environmental requirements (i.e. ecological niche), it predicts habitat suitability to decrease gradually from the centre to the periphery of a species range. Subsequently, species performance, measured as different characteristics (e.g. abundance, growth, reproduction, genetic diversity...), is expected to exhibit the same distributional pattern. The low performance in peripheral populations/individuals would ultimately be the main factor setting range margins, and confer them a particular interest for conservation. However, the empirical validity of this major biogeographical paradigm has been highly questioned over the last decades. This PhD dissertation provides an integrative and thorough assessment of the CPH, as well as new perspectives for the study of the distribution of intra-¬specific variation. First, it tackles the very basis of the CPH by investigating the demographic structure of the ecological niches of two herbaceous plant species (Plantago coronopus, Clarkia xantiana xantiana). This study shows that different species vital rates do not exhibit the same distribution pattern across environmental gradients and that environmental suitability may not systematically exhibit a centre-¬ periphery distribution in geographic space. Additionally, it highlights that a compensation process among different vital rates may play an important role in guaranteeing the stability of species geographic ranges. Second, it disentangles the effects of geographical, climatic, and historical centre-¬periphery gradients on the distribution of different components of the performance of three widespread plant species (Plantago coronopus, Polygonum viviparum, Silene acaulis) across their continental ranges. Overall, none of these gradients describe the variation in species demographic performance, whereas P. coronopus genetic diversity follows its post-¬glacial range dynamics (i.e. historical centre-¬ periphery gradient). Third, it analyzes occupancy, local abundance, and ecological patterns of the flora of an important European biogeographical crossroad, the Pyrenees Mountains. By comparing species found at their limit of distribution in this mountain range (peripheral species) to those occurring in more central positions (central species), this study shows that peripheral species exhibit lower abundances at large spatial scale, but not at a local one. It also discusses the particular conservation status of these species occurring at range margins. Finally, although it could not be included in this dissertation because of a previous inclusion in another PhD manuscript (Papuga 2016), we conducted another extensive assessment of the CPH (Pironon, Papuga et al. 2016). Based upon the analysis of the literature related to the hypothesis, this study evaluates the overall validity of the latter on different genetic, morphological, and demographic components of performance of hundreds of both plant and animal species, across different biomes of the world, at different spatial scales, and considering different centre-¬periphery gradients. Overall, across the 248 papers analyzed, this study highlights that the main assumption of the CPH (i.e. concordance between environmental and geographical centre-¬periphery gradients) is rarely verified, and that the hypothesis may be valid for occupancy, but not for the other demographic or genetic components of species performance. These different studies come to the main conclusion that the CPH cannot be considered a general rule explaining the geographic distribution of all the components of the performance of all species. They particularly highlight that the hypothesis is based on a main assumption that cannot be systematically accepted. Moreover, they show that the plurality of both the notions of centre/periphery and species performance is misleading in that different species features respond differently to different centre-¬periphery gradients. Further theoretical models helping to explain species range limits should therefore be developed, particularly those considering the interactions between and among the different components of species performance and geographical, ecological, and historical gradients. Finally, peripheral populations have an intrinsic value but cannot be systematically considered of high priority for conservation, especially at small spatial scales. Instead, their conservation status should be assessed after examining their overall biogeographical context.
Tesis Doctoral leída en la Universidad Rey Juan Carlos de Madrid en 2016. Directora de la Tesis: María Begoña García González
- Tesis Doctorales