Effects of Storms on Rice Rats Inhabiting Coastal Marshes

Abstract

During 1979, three storms with winds in excess of 73 km/hr and tides up to 1.5 m above normal struck a section of the Mississippi coast where a population study of the rice rat, Oryzomys palusrris, was in progress. There were no noticeable effects on population levels, and many individual animals survived the storms. INTRODUCTION The rice rat, Oryzomys palustris, is a semiaquatic mammal that occurs in wetland habitats throughout the southeastern United States. Throughout most of its range, 0. palustris is the most abundant small mammal in coastal marshes. It is also the most common mammal on many offshore islands in the northern Gulf of Mexico (Negus et al. 1961; personal observation). The tidal marsh and insular habitats of rice rats are occasionally subjected to devastating storms; and yet the rats appear either to survive these storms or to rapidly recolonize immediately following them. MATERIALS AND METHODS Since January 1979 I have been monitoring a rice rat population on the northern shore of St. Louis Bay in Harrison County, Mississippi. Two study grids about 1 km apart have been live-trapped quarterly. These grids have trapping stations placed at 15-m intervals. A 300-mm square floating platform that rises and falls with the tides is placed at each station. Each sampling session consists of one day of prebaiting the platforms with a mixture of oatflakes, peanut butter and sardines followed by four or five days of live-trapping. One Sherman live-trap is used on each platform. All animals captured are weighed, sexed, checked for reproductive condition, marked by toe clipping, and released. The westemmost grid is 12 stations long and 4 stations deep. Its long axis parallels shore. Water depth on its southern edge is about 400 mm at normal high tide. The northern edge parallels the marshforest ecotone and it is dry at normal high tide. The other grid consists of 100 stations in a 10 x 10 pattern. It extends from the forest-marsh ecotone for 135 m out into the marsh. Although the grid is about 0.5 km from shore, the southem half is flooded to a depth of about 300 mm daily by overflow from tidal creeks in the area. In the spring and summer of 1979 three storms had an impact on the area. A storm in mid-April produced a tide of 1 m above normal and winds gusting to about 40 knots. Manuscript received July 29, 1981; accepted September 30,1981. Hurricane Bob struck the area on July 1 1 with a storm tide of 1.5 m and winds up to 64 knots. On August 12, the western edge of Hurricane Frederick passed through with 74-knot winds but no storm tides, as the winds were from the north. The April storm and Hurricane Bob completely inundated both grids with a tidh surge that swept away all the trapping platforms and deposited them in the forest, 50-100 m to the north. The duration of these storm tides was 2-4 hrs.