Diets and coexistence in Neomys and Sorex shrews in Bialowieza forest, eastern Poland

Prey selection, food niche overlap and resource partitioning were investigated in semi-aquatic Neomys fodiens and Neomys anomalus and terrestrial Sorex araneus and Sorex minutus coexisting in marshland in Bialowieza Forest, eastern Poland. Evidence of prey selectivity was found but high levels of overlap, particularly in prey size, reflected the abundance of invertebrates in field samples. Despite similarities in diets between all four species, evidence of niche differentiation was found in terms of foraging mode and prey composition. Neomys ate predominantly terrestrial prey but 20% of prey of N. fodiens was aquatic (compared with 11% in N. anomalus), with Asellus being the dominant aquatic prey. Sorex shrews were exclusively terrestrial in foraging mode. All species ate predominantly small prey (<= 5 mm) and these were most abundant in field samples, but small prey were most important for S. minutus. Pairwise comparisons suggested that the most important promoter of resource partitioning was body size, indicating different foraging modes. Food niche overlap was least between species most dissimilar in size. The tiny S. minutus was predominantly an epigeal forager on small Araneae, Opiliones and Coleoptera; the medium-sized S. araneus fed extensively on Lumbricidae and Coleoptera; and the large, semi-aquatic Neomys fed on different amounts of freshwater prey in addition to terrestrial prey. Our results support the prediction that microhabitat selection among these species indicates differentiation in foraging mode.