Distribution of breeding, wintering, and resident waterbirds in Europe: biotic regions and the macroclimate

We developed a new method of testing whether spatial change in species comp osition follows a gradual or discrete pattern. To do this, we first divided Europe into regions based on 55 river basins. We then tested similarity of waterbird species assemblages between river basins using presence/absence data, and classified the basins using a probabilistic analysis. Breeding, w intering and resident species were analyzed separately. Some hypotheses rel ated to macroclimate were tested to account for the distribution of waterbi rd species composition of river basins, using stepwise logistic regression and canonical variate analysis. We detected a strong biotic boundary dividi ng Europe into large northern and southern regions in both the breeding and wintering seasons. These large regions were subsequently divided, mainly b y longitudinal weak boundaries, into a total of six biotic regions for each season. A more fragmented pattern was detected for resident species: four large regions and a total of eight biotic regions. Variables related to tem perature characterized the majority of biotic boundaries. Low energy availa bility of the northern basins in the wintering season may limit the distrib ution of many wintering species, whereas the higher energy level of the sou thern basins in the breeding season might cause thermoregulatory stress for many breeding species.