Bees (Hymenoptera: Apoidea) as indicators of xerisation processes in the lower Vistula Valley. Acta zoologica cracoviensia

Abstract. In the lower Vistula Valley, between the towns of W³oc³awek and Che³mno, 244 species were found, which makes ca. 56% of the total number of species found in Poland and 87% of that found in the Kujawy-Pomerania region. Locally, 86 to 144 bee species were reported. An important factor affecting the species diversity of bee communities in the environments of the lower Vistula Valley was the habitat differentiation of their nutrient vegetation, which was affected by the xerisation gradient (increasing from the river towards the valley edge). In nearly all the plots with south-exposed slopes, the numbers of species associated with the marginal zone (1), the slope zone (2) and the valley floor zone (3) were in the proportion 1 : 2 : 1. Studying the occurrence of the particular species in those valley zones, it was found that all of them nested or were associated with nesting sites in the slope zone, and from there they spread to the valley floor environments (51% of slope species) and to the nearby environments on a plateau (43%). On the opposite slopes exposed to the north, and therefore cooler, the bees nested in the vast sandy fields of the valley floor