Directions for future study
We propose future research directions to improve our understanding of the impact of the longitudinal environmental gradient across the Palearctic region on organisms, populations, species, communities and ecosystems: We hypothesize that biogeographic and macro-ecological patterns that were found over latitudinal gradients, for example, species diversity (Mittelbach et al. 2007; Condamine et al. 2012) and body size (Bergmann’s rule; Olson et al. 2009), may also hold true across the Palearctic longitudinal gradient. Consequently, we call for broad examination of longitudinal gradients in a variety of taxonomic groups, communities, food-webs and ecosystems, Specifically, we hypothesize that this environmental gradient also affects various properties of bird migration (Briedis et al.2020), including migration distance, direction, timing and over-wintering areas of different migratory populations and species across the Palearctic.