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Population studies of endangered kangaroo rats
and blunt-nosed leopard lizards in the
Carrizo Plain Natural Area, California

Published in:

California Department of Fish and Game,
Nongame Bird and Mammal Section Rep. 93-01, 114 pp., 1993


Daniel F. Williams
David J. Germano
Walter Tordoff III


From July 1987 through December 1991, we studied interactions between cattle, the plant community, giant kangaroo rats (Dipodomys ingens), and blunt-nosed leopard lizards (Gambelia sila) in the Carrizo Plain Natural Area, with lesser efforts on short-nosed kangaroo rats (D. nitratoides brevinasus) and San Joaquin antelope squirrels (Ammospermophilus nelsoni). The main study sites were on the Elkhorn Plain, San Luis Obispo County, with additional sites on the Carrizo Plain, San Luis Obispo County, and along Panoche Creek in Fresno County, California.

Drought prevailed during the precipitation years 1986-87, 1988-89, 1989-90, and into late March 1991, while 1987-88 had > average rainfall. Drought limited livestock grazing to a period from November 1987 to June 1989. Herbaceous plant productivity ranged from 12.8 kg/ha (11.5 lb/ac) during severest drought to 1,807 kg/ha (1,620 lb/ac) in 1991, following late spring rains. Productivity was slight in 1989 (60 kg/ha, 53.8 lb/ac) with little seed production. In 1990, the annual crop failed and there was no seed production. Cattle browsed heavily on shrubs between autumn 1988 and summer 1989. Herbaceous mulch was reduced to about 808 kg/ha (725 lb/ac) by grazing in 1989, and fell to 88.4 kg/ha (79.2 lb/ac) in 1990. Wind and rain storms in 1990 and 1991 caused considerable soil erosion in areas unprotected by plant and mulch cover. More topsoil (9.8 mm, 0.39 inch) eroded from the grazed pasture compared to the exclosure protected from grazing. Productivity of annual plants on precincts of giant kangaroo rats was 3.1 times higher than surrounding ground in 1988, and 1.9 times higher in 1991. Over the 5-year period, productivity on the Elkhorn Plain averaged 551.4 kg/ha (494 lb/ac), with a net gain of 171 kg/ha (153 lb/ac) due to the presence of giant kangaroo rats. Further, plants on precincts differed in species composition and had an average of 48.4% more protein than plants surrounding precincts. Cattle grazed mostly on precincts, consuming about 60% of the net production before drying of the annual range plants, but eating only 21% of the forage surrounding precincts. Giant kangaroo rats cached large numbers of seeds in 1988 when production was high. An average of 29 liters of seedheads were placed in piles on the surface of precincts to cure before being moved to underground larders. Cattle ate 57% of the seeds in surface caches. Caching behavior varied from year to year—despite high production in 1991, few precincts had pit caches and surface piles were not seen.

Impacts of cattle grazing on populations of kangaroo rats and blunt-nosed leopard lizards were not apparent statistically, but grazing only occurred during a 20-month period. Numbers of short-nosed kangaroo rats declined throughout the study, reaching a lowpoint in spring 1991 when none were captured. An increase in density of giant kangaroo rats in 1988 and 1989 may have contributed to the decline of short-nosed kangaroo rats. No recruitment of young was seen between summers 1988 and 1991. Numbers of giant kangaroo rats increased in 1988, remained high in 1989 despite drought, probably because of large seed stores made in 1988, declined to a lowpoint in spring 1991, then began to increase in summer 1991. Estimated densities varied from about 59/ha in springs of 1988 and 1989 to < 10 in 1991. The decrease in numbers coincided with the severest drought in 1990-91, but flooding from storms in 1991 also contributed to the decline. Reproduction by giant kangaroo rats appears to be affected both by population density and food availability. The population on the Elkhorn Plain was apparently monestrous with no breeding by young-of-the-year during high densities in 1988 and 1989. During drought and seed-crop failure in 1990, few reproductive individuals were seen and no young were recruited. Yet low-density populations on the Carrizo Plain reproduced and successfully recruited young in 1990. Breaking of the drought in 1991 was accompanied by polyestry and breeding by young females. Providing food supplements during the final stages of the drought in March 1991 resulted in from 2.5 to 3.2 times more active precincts compared to nonfed controls by June. Higher numbers appeared to be due to higher survival and recruitment from surrounding areas, but the population receiving supplements continued to increase in size through October compared to the nonfed control, also suggesting higher reproduction. The mean interval between first and last capture of giant kangaroo rats present on plots > 30 days was 376 days; 15 were present for > 800 days and one marked as an adult was present 1,478 days with an estimated minimum age of 5.5 years. The greatest duration for short-nosed kangaroo rats was 766 days by an adult with an estimated minimum age of 3.4 years. For giant and short-nosed kangaroo rats, respectively, 45.4% and 47.1% were not recaptured beyond a single census session (10 days).

A translocated colony of giant kangaroo rats (15 of each sex) in a fallow grain field failed within a year. Only one progeny of the founders was captured. Predation probably was the proximate reason for failure. Standing feral grain and lack of microrelief may have interfered with predator avoidance of the kangaroo rats, but chance events also may have led to high predation. Another colony (15 of each sex) translocated to formerly occupied habitat with intact natural vegetation and microrelief grew rapidly: 18 progeny were marked in 1990 and 95 in 1991. Through September 1991, 114 active precincts were located. This translocated colony exhibited a higher rate of reproduction than the natural populations, breeding in 1990 and for a longer period in 1991 and growing when the natural populations declined. Although availability of more food cannot be ruled out, physical vigor from genetic diversity in the colony (animals came from 16, 1-mi˛ sections) also is a possible explanation.

Studies of blunt-nosed leopard lizards began in 1988. Numbers of lizards were about two times greater on the nongrazed plot compared to the grazed plot throughout the study, reflecting a site difference unrelated to grazing treatment. During June censuses, numbers were greatest in 1989 following a year of high plant productivity, fell to their lowpoint in 1990 during drought, then increased over twofold in 1991. The highest density estimate of adults and subadults on the nongrazed reserve was 8.2/ha in June 1989; the lowest was 0.7/ha in June 1990. In 1990, adults, hatched in 1988 or earlier years, did not emerge, and no hatchlings were found. Absence of adults aboveground was the major reason for low density estimates in June. Surface activity was limited to subadults hatched in 1989 who did not breed in 1990. In 1991, adults that remained underground in 1990 emerged after about 21 months. In 1991, females produced a mean of 3.3 eggs/clutch. At least some females had more than one clutch, perhaps as many as three. Increased rainfall in 1991 was accompanied by 10-14 times more grasshoppers in June compared to 1990 and 2-4 more hatchling side-blotched lizards (Uta stansburiana) in August, prey for blunt-nosed leopard lizards. Adults and subadults first appeared above ground in March or April, but had the greatest surface activity from late May through mid or late June. In 1989 and 1990, almost all had disappeared from the surface by July. Mating occurred in late May and June, and hatchlings appeared around the first of August in 1989 and 1991. In 1991, adults remained active on the surface in July, but only a few females and one male were aboveground by late July--eight females remained active into August.

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