Variable age structure and apparent density dependence in survival of adult ungulates
Article first published online: 3 JUL 2003
Journal of Animal Ecology
Volume 72, Issue 4, pages 640–649, July 2003
How to Cite
Festa-Bianchet, M., Gaillard, J.-M. and Côté, S. D. (2003), Variable age structure and apparent density dependence in survival of adult ungulates. Journal of Animal Ecology, 72: 640–649. doi: 10.1046/j.1365-2656.2003.00735.x
- Issue published online: 3 JUL 2003
- Article first published online: 3 JUL 2003
- Received 22 January 2003;accepted 7 April 2003
- age structure;
- Capreolus capreolus;
- individual differences;
- life-history theory;
- Oreamnos americanus;
- Ovis canadensis;
- 1Large herbivores have strongly age-structured populations. Because recruitment often decreases as population density increases, in unexploited populations the proportion of older adults may increase with density. Because survival senescence is typical of ungulates, ignoring density-dependent changes in age structure could lead to apparent density-dependence in adult survival.
- 2To test for density dependence in adult survival, we used data from three populations that underwent considerable changes in density. Bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta, ranged from 94 to 232, mountain goats (Oreamnos americanus) on Caw Ridge, Alberta, varied from 81 to 147, and estimates of roe deer (Capreolus capreolus) older than 1 year at Chizé, France, ranged from 157 to 569.
- 3We used recent developments of capture–mark–recapture modelling to assess the response of adult survival to changes in density when age structure was and was not taken into account.
- 4Survival rates were 10–15% higher during the prime-age stage than during the senescent stage for all sex-species combinations. When adults were pooled into a single age class there was an apparent negative effect of density on female survival in bighorns and roe deer, and negative trends for female mountain goats, male roe deer and male bighorn sheep. When age class was taken into account, there were no significant effects of density on adult survival. Except for male mountain goats, the strength of density dependence was lower when age was taken into account.
- 5In ungulate populations, age structure is an important determinant of adult survival. Most reports of density dependence in adult survival may have been confounded by changes in age structure.