Movement patterns of riparian small mammals during predictable floodplain inundation

We monitored movements of small mammals resident on floodplains susceptible to spring floods to assess whether and how these animals respond to habita t inundation. The 2 floodplains were associated with 6th order river segmen ts in a semiarid landscape; each was predictably inundated each year as sno wmelt progressed in headwater areas of the Rocky Mountains. Data from live trapping, radiotelemetry, and microtopographic surveys indicated that Perom yscus maniculatus, Microtus montanus, and Dipodomys ordii showed different responses to inundation, but all reflected a common tendency to remain in t he original home range until "forced" to leave. The reluctance of Dipodomys ordii to abandon the home burrow often resulted in death in situ, whereas individual P. maniculatus and M, montanus moved to nearby higher ground but not necessarily toward upland. This behavior could lead to occupancy of an island that disappeared as floodwaters rose. Peromyscus maniculatus climbe d into sapling cottonwood, but the quality of such arboreal refuges was unc lear. We found only weak support for the hypothesis that displacement was t emporary; most floodplain residents, including P. maniculatus, disappeared over the flood period. No secondary effect from flooding on adjacent upland small-mammal assemblages was detected. Our data suggest populations of fac ultatively riparian, nonarboreal small mammals such as M. montanus and D. o rdii generally experience habitat inundation as a catastrophy, Terrestrial species capable of using an arboreal refuge, such as P. maniculatus, face a more variable risk, determined in part by timing and duration of the flood event. River regulation can affect both sets of risks.