Modeling the decline of the Porcupine Caribou Herd, 1989-1998: the importance of survival vs. recruitment

Abstract: The Porcupine caribou (Rangifer tarandus granti) herd increased from approximately 100 000 animals during the 1970s to 178 000 in 1989, then declined to 129 000 by 1998. Our objective was to model the dynamics of this herd and investigate the potential that lower calf recruitment, as was observed during 1991-1993, produced the observed popula-tion changes. A deterministic model was prepared using estimates of birth and survival rates that reproduced the pattern of population growth from 1971-1989. Then, parameters were changed to simulate effects of lower calf recruitment and adult survival. Reducing recruitment for 3 years caused an immediate reduction in population size, but the population began to recover in 5-6 years. Even a dramatic temporary reduction in recruitment did not explain the continuing decline after 1995. In contrast, a slight but persistent reduction in adult survival caused a decline that closely followed the observed pattern. This suggests that survival of adults, and perhaps calves, has declined since the late 1980s. Key words: Alaska, population model, Rangifer tarandus. Rangifer, Special Issue No. 14: 123-130 ntroduction The Porcupine caribou herd (PCH) has been the sub-ject of a variety of research and monitoring efforts that began during the early 1970s, increased greatly during the 1980s, and has continued at a reduced level since 1994 (Griffith et al., 2002). Throughout this period, a series of surveys were conducted to estimate population size by counting caribou on aer¬ ial photographs taken during the post-calving peri¬ od. In addition, research during the 1980s provided estimates of parturition, survival, and sources of mortality of calves and adults (Fancy et al., 1989