Endangered Species Recovery Program

Population responses of Dipodomys ingens
to fluctuating precipitation during a 7.5-year period

Presented To:

75th Annual Meeting
American Society of Mammalogists
Washington, DC
20 June 1994

by

Daniel F. Williams
and
David J. Germano

Abstract

Populations of Dipodomys ingens were censused biannually between 1987 and 1994 on two plots in eastern San Luis Obispo Co., California. The study encompassed periods of above average rainfall and severe drought in 1986-87, 1988-89, 1989-90, and into late March 1991. Numbers of D. ingens increased in 1988 (53/ha) compared to 1987 (31/ha), a drought year, stayed at about that level in 1989 despite drought, declined to 21/ha in 1990 and 15/ha in August 1991, then increased to 110/ha in 1992 and 105/ha in 1993. Changes in density generally coincided with amount of rainfall, though numbers remained high in 1989 despite drought and low plant productivity. Large seed caches made in spring 1988 probably carried individuals through 1989. No successful reproduction was seen in 1990, a year with no seed production by annual plants; females were only about 85% of their normal winter weight and nonreproductive in February 1991. Drought was broken by heavy rainfall the last week of March 1991. Flooding and sheet flow of water were common, and probably caused a precipitous decline in numbers of kangaroo rats. Numbers of active burrow systems of D. ingens in April 1991 on four 8-ha plots declined to 7-58% of March numbers. Subsequently, densities increased and remained high through 1993. Adult Females were monestrous in 1987-1989 and mostly anestrous in 1990 and into April 1991, with no breeding by young-of-the- year. Following breaking of drought in 1991, both adult and young females became polyestrous, producing 1-3 or more litters between May and November 1991. Polyestry resumed in Dec. 1991, with a quiescent period between June and Nov. 1992. A similar pattern was noted in 1993. Providing food supplements during the final stage of drought in February and March 1991 on two 8-ha plots resulted in 2.5-3.2 times more active burrow systems of D. ingens by June, compared to two control plots. The populations on the experimental plots continued to increase in size compared to controls through the study period in 1991. These and other observations suggest that reproduction by giant kangaroo rats was affected both by population density and food availability.