Interpretation of Age Ratios

The extent to which age ratios reflect the dynamics of a population was examined by simulation. The results indicate that age ratios often provide ambiguous information and that their facile interpretation can lead to serious management blunders. A sudden rise or fall in mortality rate that affects all age classes equally has no effect on the age ratio. It remains constant at the level prevailing before the demographic change. Even when the ratio responds to a change in rate of increase there are circumstances in which its trend is the same for two populations, one of which is erupting and the other plunging to extinction. Age ratios cannot be interpreted, even in a general way, without additional demographic information, particularly on the population's rate of increase. But since the ratios are themselves usually collected to gauge rate of increase, the direct measurement of this rate renders them largely redundant. J. WILDL. MANAGE. 38(3):557-562 This paper explores the extent to which age ratios reflect a population's dynamics, and in particular whether they signal changes in rate of increase. Most animals can be classified as juveniles, subadults, or adults. These classes are defined here by Hanson's (1963) criteria: (1) juveniles are less than fully grown animals; (2) subadults are essentially fully grown, but the majority of their cohort have not completed their first breeding season; (3) adults are fully grown and the majority of their cohort have completed one or more breeding seasons. When adults and subadults are combined in one class they are called mature animals. Two age ratios will be considered. The primary age ratio is the ratio of juveniles to mature animals. The secondary age ratio is that of subadults to adults. Although most papers on wildlife populations provide age ratios, these are seldom interpreted. One's curiosity over why these data were collected or what they reveal about the population is usually left unsatisfied by the author. Occasionally he confides that the reported ratio indicates that the population is in good heart, or that the ratio gives cause for concern, but never have I seen spelled out the logical steps by which these judgments are reached. Age ratios unsupported by other information seem to be statistics in search of an application. My purpose in the following investigation is to determine what age ratios reveal about the dynamics, as opposed to the statics, of a population.