Niche expansion after competitor extinction? A comparative assessment of habitat generalists and specialists in the tree floras of south-eastern North America and south-eastern Europe

Aim The aim of this study was to test R.H. MacArthur's hypothesis that realized niche breadth is constrained by species pool size - the greater the number of species in a region, the more competition restricts the distribution of each species with respect to environmental tolerances and habitat characteristics. Location The northern Balkan region in south-eastern Europe (Illyrian Floristic Province) and the southern Appalachian region of the USA. Methods We compared co-occurrence-based distributions of habitat specialization of tree species in two geographic regions that are ecologically similar but differ in species pool size. We applied two methods. First, we used a rank-ordering of species along a gradient of estimated niche breadth that is based solely on species co-occurrence information derived from vegetation databases from each region. To compare niche-breadth distributions of different datasets we developed a procedure that standardizes expected values of species co-occurrences independently of the size of the species pool. Second, we calculated species turnover along an elevational gradient for both regions, estimated as the rate of decay of compositional similarity with elevation distance. Results Despite a twofold larger species pool, and in contrast to our hypothesis, there was no greater specialization trend in the tree species of the southern Appalachian region, regardless of phylogenetic subgroupings or whether rare species were included. After correcting for differences in species pools, the similarity decay with elevation distance was marginally stronger in the southern Appalachian region. Main conclusions MacArthur's hypothesis was not supported by our analysis. While the compositional distance decay with elevation revealed only a slight trend towards narrower realized niches in the tree flora of the southern Appalachian region, the co-occurrence approach suggested the opposite. Our results indicate that species distributions are largely constrained by environmental tolerances, and that biotic pressure in the form of competition from ecologically similar species plays a relatively minor role in the ability of species to establish mature individuals in different habitat types.