title: Population Biology of the Black-tailed Jackrabbit (Lepus californicus) in Northern Utah
Creators: name: Stoddart, L. Charles
Publisher: name: Utah State University
Year: 1972

description: Population biology of the black-tailed jackrabbit population on a 250-square-mile area in Curlew Valley, northern Utah, was studied from 1962-70. During this period the fall population density index increased from 40.0 in 1962, to 60.6 in 1963, decreased progressively to a low of 21.2 in 1967, then increased the following 3 years to a high of 185.0 in 1970.
Breeding was synchronous with four conception periods each year; in some years a fifth conception period was evident. The first conception period occurred about the last half of January; other periods followed at 40-day intervals indicating a 40-day gestation period and postpartum estrus with subsequent conception. Over the 9 years of study, the mean percentages of females breeding during the five conception periods were 88, 99, 100, 70, and 11 percent, respectively. The mean number of ova ovulated per breeding female for the five periods was 1.9, 5.1, 6.4, 4.9, and 3.6, respectively.
During the period of decreasing density, 1963-67, the yearly mean number of ova ovulated per female surviving the breeding season ranged from 13.2-19.3, but varied independently of density. During the 3 consecutive years of density increase, 1968-70, however, the number of ova ovulated per female decreased progressively from 19.8 in 1968 to 14.2 in 1970.
Mortality rates of the total population from October-March remained relatively constant (mean: 63 percent) during the years of population decline, but dropped to 33 percent during the first year of population increase (1968). March-October mortality of adults decreased to 9 percent during the first year of population increase from a previous mean of 73 percent, and juvenile mortality from parturition to fall census, decreased from a mean of 68 percent to 38 percent.
The effects of variations in mortality rates on population density have overshadowed the effects of the relatively less extreme variations in natality rates. As a result the pattern of density change was almost entirely a result of changes in mortality rates.
Changes in mortality rates of adults and juveniles were well correlated with the coyote/rabbit ratio on the study area. Exceptions occurred with juvenile mortality rates at the relatively high rabbit densities observed in 1969-70. During these two years, juvenile mortality rates from parturition to fall census (61 and 68 percent, respectively) were greater than could be accounted for by coyote predation. The factor or factors responsible for the increased juvenile mortality are not known.
Observed annual density changes were described with the mathematical model:
Nt+4 = Nt (1 - 37.8 - 988x1) (1 + 11.2 - 1130x2 - 0.0581x3 + 42000x22 + 0.00115x32)
where Nt is the number of animals at the end of October, Nt+1 is the number of animals at the end of the following October, x1 is the coyote/rabbit ratio from October-March, x2 is the coyote/rabbit ratio from March-October, and x3 is the mean number of rabbits per square mile from March-October. The model accounts for 99 percent of the observed change in rabbit density from 1968-70.