Can we use the young : female ratio to infer ungulate population dynamics? An empirical test using red deer Cervus elaphus as a model

Abstract

Summary Management 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 yeart+ 1 and the young : female ratio in yeartto obtain juvenile survival. However, the reliability of such estimates has yet to be demonstrated. We 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. The 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. The 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). The temporal pattern in the probability of ob…