Overlap of temporal niches among four sympatric species of shrews

Hypotheses about the dependence of circadian activity from metabolic rate and the segregation of temporal niches among competing species were verified by the study of activity patterns in a shrew community of two semiaquatic species, Neomys anomalus Cabrera, 1907 and N. fodiens (Pennant, 1771), and two terrestrial species, Sorex araneus Linnaeus, 1758 and S. minutus Linnaeus, 1766, co-existing in wet habitats of Bialowieza Forest (E Poland). In ten trapping sessions, performed in early summer between 1991 and 2000, traps were open 24 hours continuously and patrolled at 1:00, 5:00, 10:00, 15:00, and 20:00. All the shrew species were most active between 20:00 and 1:00, and least active around mid-day (10:00-15:00). However, activity of the two Sorex species was lower than that of the two Neomys species in the period 20;00-1:00, but higher in the period 15:00-20:00. Both Neomys species displayed clearly nocturnal, unimodal patterns of activity. In contrast, activity of both Sorex species was relatively evenly distributed over 24 hours and they increased their activity earlier (ie after 15:00) than both Neomys species (after 20:00). These results confirm the idea that small shrew species with higher metabolic rate have more frequent and more equally distributed activity bouts than large species. Overlap of temporal niches was the highest within genera (99.29% between both Neomys species and 98.36% between both Sorex species), the lowest between N. fodiens and S. araneus (88.26%) and S. minutus (89.34%), and intermediate between N. anomalus and both Sorex species (91.78 and 93.34%, respectively). Such high interspecific overlaps in activity suggest a joint-action of other mechanisms that separate ecological niches of these species also in other dimensions (eg food, microhabitat).