During and following the 1994-1995 winter, biologists noted a decline in abundance of kangaroo rats (Dipodomys spp.) in the southern San Joaquin Valley. On 5 widely separated study areas, total captures and trap response were substantially reduced in comparison with 1994 and earlier results. Declines were noted in short-nosed (Dipodomys nitratoides brevinasus), Tipton (D. n. nitratoides), and Heermann's (D. heermanni) kangaroo rats, although only modest reductions were noted for giant kangaroo rat (D. ingens) populations on the valley floor. The 1994-95 winter was cooler and wetter than typical. Causes of the observed population decline in the San Joaquin Valley are speculative. Widespread flooding and short-term habitat degradation were not observed on the study sites, and are probably not factors. Thermal stress, reduced caloric value of seeds, disease, and mycotoxic factors could be involved. Interactions among several factors are likely. Conditions similar to the 1994-1995 winter have a historic probability of occurrence of 11%, and are widely dispersed over the last 45 years. Anthropogenic impacts to kangaroo rat populations could be more significant during cool wet winters; changes in conservation and protection strategies during these winters may be warranted. Consideration of the effects of catastrophic winter population declines, in addition to the effects of drought on kangaroo rat numbers, is merited in the long-term conservation planning for kangaroo rats in the San Joaquin Valley.
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