Small-mammal Populations on Hog Island 1990-1994: Effects of the Halloween Storm and Seasonal Differences

John Porter1 and Raymond Dueser2

1Dept. Environmental Sciences, University of Virginia, Charlottesville, VA 22903, 2Dept. of Fisheries and Wildlife, Utah State Univ., Logan, UT 84322.

Understanding of the biogeographic, trophic and community relationships between populations of small mammals and external forces (such as storms and flooding) requires data over a long-time scale. Populations of the Rice Rat (Oryzomys palustris), House Mouse (Mus musculus) and Norway Rat (Rattus norvegicus) have been monitored along four transects on Hog Island, VA since the summer of 1988. Beginning in 1990 spring trapping was instituted and starting in 1991 Autumn trapping was instituted.

In examining the number of individuals encountered on each transect (note: transects have between 22 and 83 traps at 15-m intervals [12.5 m spacing along the transect + 2.5 m off to alternating sides of the transect], with the number of traps being dictated by the width of the island), several clear patterns emerge. First, the number of individuals encountered on a transect is consistently higher in the spring. Note that the number of individuals captured are not adjusted to correct for different numbers of traps between transects and T2 was not trapped during the autumn of 1991 or 1992. Second, during the spring Oryzomys tend to be numerically dominant on the northern transects (T1 & T2), while Mus is numerically dominant on the southern two transects (T4 & T5). During the Oryzomys numbers were reduced relative to other years only on T1 and possibly on T4. Mus numbers were even less effected, with among the highest autumn densities observed immediately after the storm. This observation suggests that for flooding to seriously impact populations it must be either complete or of longer duration than was experienced during the Halloween Storm. For neither Oryzomys nor Mus were spring numbers particulary low in 1992, the spring following the storm.