Linking multiple hypotheses to a unifying framework of range-size variation: A case study with American oaks (Quercus spp.)

AimClosely related species vary greatly in range size. To understand what drives range-size variation, we established an integrative framework focusing on two causal pathways: the amount of currently suitable habitat (ASH) and range filling (i.e., the occupancy of ASH). Instead of testing different hypotheses in isolation, we linked four hypotheses to this unifying framework: we tested the niche breadth and niche position hypotheses, which explain range-size variation via effects on ASH; we also tested the colonization ability and post-glacial migration lag hypotheses, which explain range-size variation via range filling. LocationThe Americas. Time periodLast Glacial Maximum (LGM) to present. Major taxa studiedOne hundred and eighty-three oak species. MethodsWe extracted locality records and measured range size as the area of occupancy (AOO) and extent of occurrence (EOO). We used Maxent models and current environmental layers to quantify ASH, niche breadth and niche position. We used seven phenotypic traits as proxies of colonization ability. We calculated post-glacial accessibility to currently suitable habitat, in addition to the proportion of currently suitable habitat covered by LGM ice sheets. Using structural equation models and regression models, we tested the four hypotheses. Finally, we examined how ASH and range filling contributed interactively to range-size variation by quantifying variance and covariance. ResultsOur results supported all four hypotheses. Additionally, we found that variance in ASH and range filling contributed to a similar extent to variance in AOO, with almost zero covariance between ASH and range filling. In contrast, range filling accounted for almost all the variance in EOO, and we detected a significant negative covariance between ASH and range filling. Main conclusionsIn oaks, factors associated with range filling are at least as important as niche properties in determining range size. Our framework is generalizable to other study systems for testing the contributions of two pathways, ASH and range filling, to range-size variation.