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The Mediterranean Basin is recognized for its high levels of species richness, rarity and endemicity. Our main aim was to evaluate the relative effects of environmental and spatial variables and their scale-specific importance on beta diversity patterns along a gradient of mediterraneity, using spiders as a model group. This study was carried out in 18 coastal dune sites along the Portuguese Atlantic coast. This area encompasses 445 km and comprises two distinct biogeographic regions, Eurosiberian (northern coast) and Mediterranean (centre and south). A forward selection procedure was carried out to select environmental and spatial variables responsible for determining beta diversity patterns. Variation partitioning and principal coordinates of neighbour matrices (PCNM) were used to estimate the contribution of pure environmental and pure spatial effects and their shared influence on beta diversity patterns and to estimate the relative importance of environmental structured variation and pure spatial variation at multiple spatial scales. Climate, ground vegetation dune cover and area were selected by a forward selection procedure. The same procedure identified three PCNM variables, all corresponding to large and medium spatial scales. Variation partitioning revealed that 46.1% of the variation of beta diversity patterns was explained by a combination of environmental and PCNM variables. Most of this variation (42.5%) corresponded to spatial variation (environmental spatially structured and pure spatial). Climate and vegetation structure influences were predominant at the PCNM1 and PCNM3 scales, while area was more important at the intermediate PCNM2 scale. Our study revealed that beta diversity of spiders was primarily controlled by a broad-scale gradient of mediterraneity. The relative importance of environmental variables on the spider assemblage composition varied with spatial scale. This study highlights the need of considering the scale-specific influence of niche and neutral processes on beta diversity patterns. [ABSTRACT FROM AUTHOR]