Can we use the young : female ratio to infer ungulate population dynamics? An empirical test using red deer Cervus elaphus as a model
Christophe Bonenfant, Center for Ecology and Evolutionary Synthesis, Department of Biology, University of Oslo, PO Box 1050, N-0316 Oslo, Norway (fax + 47 22 85 72 94; firstname.lastname@example.org).
- 1Management decisions for exploited species often rely on count ratios, such as the young to adult female ratio as a proxy of birth rate, and the decrease in the ratio between yearling and females in year t+ 1 and the young : female ratio in year t to obtain juvenile survival. However, the reliability of such estimates has yet to be demonstrated.
- 2We investigated the consistency of the young to female ratio method by using observations on individually marked female red deer. These females were known to have raised their calves to weaning successfully. We compared temporal variation in the proportion of females with a calf at heel from count ratios with the probability of observing a calf with its mother from marked animals.
- 3The proportion of females observed with a calf at heel decreased over time (from 0·43 early May to 0·15 early March). This measure was influenced by individual variation and by interactive effects of year and habitat type, although there was no direct relationship with the density of vegetation cover.
- 4The proportion of females seen with a calf at heel assessed from counts did not correspond with the probability of observing a female with a calf at heel, given that she successfully weaned a calf in autumn (estimated from direct observation). The probability of observing a female with a calf was highest in September (0·4) prior to the rut and lowest just after the birth season (0·08) and during the rut (0·18).
- 5The temporal pattern in the probability of observing a calf with its mother, given that she successfully weaned a calf, reflected changes in the mother–calf bond from birth to weaning, rather than changes in offspring mortality or changes in pregnancy rates. These variations in the probability of observing the mother–calf pair in the field may lead to strong biases in count ratio-based methodologies.
- 6Synthesis and applications. This study has important implications for the management of ungulate populations when decisions are based on count ratios. Our results indicate that count-based estimates of vital rates are misleading, especially in closed habitats. Pregnancy rate and juvenile survival rate are both key parameters in detecting density-dependence responses. Reliable estimates of survival and pregnancy rates obtained from marked animals and foetus sampling are more accurate. Alternatively, yearly monitoring of young body mass or bone length coupled with an index of grazing pressure would improve the assessment of population dynamics.