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.

A long-standing hypothesis in biogeography is that a species' abundance is highest at the centre of its geographical or environmental space and decreases toward the edges. Several studies tested this hypothesis and provided mixed results and overall weak support to the theory. Most studies, however, are affected by several limitations related to the sample size, the comparability among abundance measures, the definition of species geographic range and corresponding environmental space, and the proxy variables used to represent centrality/marginality gradients.Here we test the abundant-centre hypothesis on 108 bird and mammal species and embrace the plural nature of the hypothesis by considering 9 geographic and ecological centrality/marginality measures. We analyse the species-specific effect sizes using a meta-analytical approach, and test whether the support for the hypothesis is mediated by species dispersal abilities, and the geographic and environmental coverage of the data.The summary effect sizes estimated for the 9 measures are largely inconsistent with the theoretical expectations and show a significant amount of residual heterogeneity. Variables such as dispersal distance, geographic and environmental coverage of the data, appear important in explaining the variation observed between different species, but the results are contrary to those originally hypothesized, and inconsistent across centrality/marginality measures and the datasets used.We show that addressing common pitfalls in previous studies does not provide more support to the abundant-centre hypothesis, with support being very dependent on the centrality/marginality measure tested, the geographic extent considered for the test, and geographic and environmental coverage of the data. The abundant-centre hypothesis so far remains an appealing speculation with little and variable empirical support.

In 2012, an unusual outbreak of urban malaria was reported from Djibouti City in the Horn of Africa and increasingly severe outbreaks have been reported annually ever since. Subsequent investigations discovered the presence of an Asian mosquito species; Anopheles stephensi, a species known to thrive in urban environments. Since that first report, An. stephensi has been identified in Ethiopia and Sudan, and this worrying development has prompted the World Health Organization (WHO) to publish a vector alert calling for active mosquito surveillance in the region. Using an up-to-date database of published locational records for An. stephensi across its full range (Asia, Arabian Peninsula, Horn of Africa) and a set of spatial models that identify the environmental conditions that characterize a species’ preferred habitat, we provide evidence-based maps predicting the possible locations across Africa where An. stephensi could establish if allowed to spread unchecked. Unsurprisingly, due to this species’ close association with man-made habitats, our maps predict a high probability of presence within many urban cities across Africa where our estimates suggest that over 126 million people reside. Our results strongly support the WHO’s call for surveillance and targeted vector control and provide a basis for the prioritization of surveillance.

AimThe 'centre-periphery hypothesis' (CPH) predicts that species performance (genetics, physiology, morphology, demography) will decline gradually from the centre towards the periphery of the geographic range. This hypothesis has been subjected to continuous debate since the 1980s, essentially because empirical studies have shown contrasting patterns. Moreover, it has been proposed that species performance might not be higher at the geographic range centre but rather at the environmental optimum or at sites presenting greater environmental stability in time. In this paper we re-evaluate the CPH by disentangling the effects of geographic, climatic and historical centrality/marginality on the demography of three widely distributed plant species and the genetic diversity of one of them. Location Europe and North America.Methods Based on a species distribution modelling approach, we test whether demographic parameters (vital rates, stochastic population growth rates, density) of three plant species of contrasting life-forms, and the genetic diversity of one of them, are higher at their geographic range centres, climatic optima or projected glacial refugia. ResultsWhile geographic, climatic and historical centre-periphery gradients are often not concordant, overall, none of them explain well the distribution of species demographic performance, whereas genetic diversity responds positively only to a historical centrality, related to post-glacial range dynamics.Main conclusions To our knowledge, this is the first assessment of the response of species performance to three centrality gradients, considering all the components of different species life cycles and genetic diversity information across continental distributions. Our results are inconsistent with the idea that geographically, climatically or historically marginal populations generally perform worse than central ones. We particularly emphasize the importance of adopting an interdisciplinary approach in order to understand the relative effects of contemporary versus historical and geographic versus ecological factors on the distribution of species performance.

To meet the ambitious objectives of biodiversity and climate conventions, countries and the international community require clarity on how these objectives can be operationalized spatially, and multiple targets be pursued concurrently 1 . To support governments and political conventions, spatial guidance is needed to identify which areas should be managed for conservation to generate the greatest synergies between biodiversity and nature's contribution to people (NCP). Here we present results from a joint optimization that maximizes improvements in species conservation status, carbon retention and water provisioning and rank terrestrial conservation priorities globally. We found that, selecting the top-ranked 30% (respectively 50%) of areas would conserve 62.4% (86.8%) of the estimated total carbon stock and 67.8% (90.7%) of all clean water provisioning, in addition to improving the conservation status for 69.7% (83.8%) of all species considered. If priority was given to biodiversity only, managing 30% of optimally located land area for conservation may be sufficient to improve the conservation status of 86.3% of plant and vertebrate species on Earth. Our results provide a global baseline on where land could be managed for conservation. We discuss how such a spatial prioritisation framework can support the implementation of the biodiversity and climate conventions.

Species distribution models (SDMs) are widely used in ecology. In theory, SDMs capture (at least part of ) species' ecological niches and can be used to make inferences about the distribution of suitable habitat for species of interest. Because habitat suitability is expected to influence population demography, SDMs have been used to estimate a variety of population parameters, from occurrence to genetic diversity. However, a critical look at the ability of SDMs to predict independent data across different aspects of population biology is lacking. Here, we systematically reviewed the literature, retrieving 201 studies that tested predictions from SDMs against independent assessments of occurrence, abundance, population performance, and genetic diversity. Although there is some support for the ability of SDMs to predict occurrence (~53% of studies depending on how support was assessed), the predictive performance of these models declines progressively from occurrence to abundance, to population mean fitness, to genetic diversity. At the same time, we observed higher success among studies that evaluated performance for single versus multiple species, pointing to a possible publication bias. Thus, the limited accuracy of SDMs reported here may reflect the best-case scenario. We discuss the limitations of these models and provide specific recommendations for their use for different applications going forward. However, we emphasize that predictions from SDMs, especially when used to inform conservation decisions, should be treated as hypotheses to be tested with independent data rather than as stand-ins for the population parameters we seek to know.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Climate change is expected to severely impact cultivated plants and consequently human livelihoods 1-3 , especially in Sub-Saharan Africa (SSA) 4-6. Increasing agricultural plant diversity (agrobiodiversity) could overcome this global challenge 7-9 given more information on the climatic tolerance of crops and their wild relatives. Using >200,000 worldwide occurrence records for 29 major crops and 778 of their wild relative species, we assess for each crop how future climatic conditions are expected to change in SSA and whether populations of the same 2 crop from other continents, wild relatives around the world, or other crops from SSA are better adapted to expected future climatic conditions in the region. We show that climate conditions not currently experienced by the 29 crops in SSA are predicted to become widespread, increasing production insecurity, especially for yams. However, crops such as potato, squash and finger millet may be maintained by using wild relatives or non-African crop populations with climatic niches more suited to future conditions. Crop insecurity increases over time and rising greenhouse gas emissions, but the potential for using agrobiodiversity for resilience is less altered. Climate change will therefore affect Sub-Saharan agriculture but agrobiodiversity can provide resilient solutions in the short-and medium-term. Main Text: Global climate has changed rapidly over recent decades, and temperature and precipitation regimes are predicted to shift significantly in the near future 10. Future impacts on both biodiversity and human livelihoods are significant and primarily negative 2,4,11. By affecting plant productivity, and thus industrial and food crop yield, climate change is expected to impact global human economy and subsistence 1,2. Its tropical location, socioeconomic, demographic, policy, and farming characteristics place sub-Saharan Africa (SSA) at major risk 5,6. Assessing which sub-Saharan crops, regions and populations will be most affected, as well as potential future adaptations is therefore essential. Agrobiodiversity and breeding programs represent an important adaptive strategy for agriculture in a changing world 8,12. Currently cultivated crops may exhibit reduced genetic variation compared to that found in wild relative populations, which may limit their resilience and adaptation to future environmental conditions 13. Crop improvement through selection for