Estimating effects of adult male mortality on grizzly bear populationgrowth and persistence using matrix models

摘要

We radio monitored a hunted, sexually segregated grizzly bear (Ursus arctos) population and an unhunted, unsegregated population for demographics and constructed a stage- and age-classified matrix model to test for the effects of adult male mortality and resulting sexual segregation on population growth and persistence. Population parameters in the model were adult female survival, subadult female survival, offspring survival, probability of litter sizes, and probability of unsuccessful pregnancy. The last three parameters were affected by adult male mortality and segregation, the others were not. We compared population growth with and without effects of hunting by holding adult female and subadult female survival constant and by using hunted and unhunted values for offspring survival, litter size, and pregnancy. Population growth (Lambda) showed the greatest elasticity for adult survival, subadult survival, offspring survival, litter size, and unsuccessful pregnancy, in that order. This corresponds with observed anti-infanticide tactics (sexual segregation) by adult females to maximize their fitness. The hunted population decreased at a rate of 0.99 whereas the simulated, unhunted population increased at a rate of 1.05. The hunted population was much more susceptible to population extinction. Under demographic stochasticity mean time to extinction was 32 years in the hunted population and 110 years in the unhunted population. Under environmental stochasicity mean time to extinction was 21 years in the hunted population and 43 years in the unhunted population. We suggest that sexual segregation caused by hunting resident adult males can result in population decline and can even contribute to rapid population extinctions when numbers are small.