We studied the effect of group size on survival of black-tailed prairie dogs (Cynomys ludovicianus) relocated to 12 experimental plots in Colorado to determine success of reintroductions. Six groups of 10, 3 groups of 30, and 3 groups of 60 prairie dogs were randomly assigned and released from July to October 1990 in 6 1-, 3 3-, and 3 6-ha plots, respectively, in former prairie dog colonies that were extirpated by sylvatic plague (Yersinia pestis) during fall 1988. We recaptured prairie dogs in plots during September-October 1990, February-March 1991, June 1991, and September-October 1991 to estimate survival. We used capture-recapture models to estimate survival and recapture rates. Daily survival rates between release and first recapture were different for 10- (0.9767, SE = 0.0044), 30- (0.9854, SE = 0.0023), and 60-animal groups (0;9914, SE = 0.0016) (P < 0.001) and were directly related to group size (P < 0.001). The number of progeny captured per animal released was greater in the 60- (($) over bar x = 0.79, SE = 0.06) than in the 10- (($) over bar x = 0.28, SE = 0.21) animal plots (P = 0.028), but did not vary between the 60- and 30-animal plots (($) over bar x = 0.62, SE = 0.09) (P = 0,333)or between the 30- and 10-animal plots (P = 0.156). The 60-animal groups attracted more immigrants than the 10- or 30-animal groups. Ratios of the number of prairie dogs 1 year after release (survivors and progeny) to those released varied (P = 0.018) among the 10- (($) over bar x = 0.46, SE = 0.21), 30- (($) over bar x = 0.81, SE = 0;09), and 60- (($) over bar x = 1.17, SE = 0.08) animal groups and were related linearly (P = 0.006) but not quadratically (P = 0.750) to the number of animals released. One year after release, only the 60-animal groups had more survivors and progeny than the number of prairie dogs released. We recommend releasing a minimum of 60 animals/site in areas with potential immigrants.
