Long-term population fluctuations of the western harvest mouse (Reithrodontomys megalotis), a rare indicator species, in semi-arid habitats in the Okanagan Valley, British Columbia

We report a 25-year record of changes in abundance and demography of the western harvest mouse ( Reithrodontomys megalotis ) in semi-arid perennial grasslands (old fields) and sagebrush habitats in southern British Columbia, Canada from 1982 to 2006. We tested three hypotheses (H) that: (H ₁ ) R. megalotis populations will have higher abundance and demographic attributes in old fields than in sagebrush habitats; (H ₂ ) interspecific competition between western harvest mice and montane voles ( Microtus montanus ) and deer mice ( Peromyscus maniculatus ) will cause harvest mice to be lower in density when the other two species are higher; and (H ₃ ) harvest mice have the ability to disperse and colonize available habitat. R. megalotis had variable population fluctuations with mean abundance ranging from 0 in some years to 1.2 to 4.4 mice per ha in sagebrush and old field habitats. Annual peak numbers per ha ranged from 3.6 to 5 in sagebrush and 9 to 13 in old field habitats; results that partly supported (H ₁ ). There was a tendency for more successful pregnancies, recruits, and juvenile productivity in the old field than sagebrush habitats. H ₂ was partly supported, but in peak years of deer mice and montane voles there seemed to be little effect of these associate species on mean abundance of R. megalotis . Some old field sites may have substantial vegetative food and cover and be able to support all three rodent species at high densities. H ₃ was supported as harvest mice seemed able to disperse and colonize available sagebrush habitat. R. megalotis may be a good indicator to identify and conserve grassland-steppe ecosystems and possibly agroecological sites such as tree fruit orchards, hedgerows, and other linear habitats.