Shrew responses to experimental manipulation of downed woody debris in boreal forests

Background/Question/Methods

Boreal forest management practices that significantly alter microclimates and microhabitats are likely to have important effects on small mammal species such as shrews, given their high moisture and metabolic requirements. With increasing interest in biofuel harvesting, supplies of downed woody debris (DWD) in boreal forest ecosystems have come under threat. DWD provides moist microclimatic conditions and serves as foraging substrate for shrews, and thus reductions in DWD may have significant effects on shrew communities. Further, because of their insectivorous habits, shrews are expected to strongly partition foraging microhabitats, and hence responses to variability in DWD quality and quantity may be species specific. Prior studies examining DWD-shrew associations have been largely correlative; it is unclear as to what extent these relationships reflect covariation with other habitat features rather than causative relationships with DWD per se.

We examined associations with DWD for three abundant shrew species in boreal mixedwood forest stands in which DWD supplies had been experimentally manipulated. DWD volumes were manipulated in 2006/2007 in 27 2.25-ha grids and shrews were sampled via drift fences from 2008 to 2012. A mixed-model design with repeated measures was used to test for the effects of early-decay, late-decay and total DWD volumes on shrew abundances.

Results/Conclusions

We found strong associations between shrew species and DWD quality and quantity. Abundances of common shrews (Sorex cinereus) showed a significant negative relationship with late-decay DWD volumes; abundances of smokey shrews (Sorex fumeus) were positively correlated with early-decay DWD volumes and total DWD volume; and pygmy shrews (Sorex hoyi) showed no significant relationships. Little evidence of threshold responses was obtained; instead, relationships appear to be linear. These results support differential microhabitat use among sympatric shrew species and niche differentiation according to foraging microhabitats. Reductions or changes in DWD quality and quantity have the potential to result in marked changes in shrew community composition. For example, reductions of early-decay DWD volumes from 40 to 20 m³/ha resulted in 25% reductions in smokey shrew abundances. Given the high biomass of shrews in boreal forest ecosystems, such changes have important implications for ecological processes and forest biodiversity.