ong>Answer:

Age structure can be somewhat variable and inconsistently distributed among juniper stands, whereas pinyon tends to show more consistent, unimodal age distributions, suggesting sporadic juniper recruitment alongside more consistent pinyon recruitment (Huffman et al., 2012;

1 Macrolevel Determinants

In the field of demography, age structure is an imbedded concept. Nonetheless, it is surprising to learn that in the eighteenth and nineteenth century there was virtually nothing written about age per se or age structure, although considerable attention was devoted to population size and the determinants of population change—fertility, mortality, and migration. In fact, it is not until the beginning of the twentieth century that the Swedish statistician, Gustav Sundbarg (1900), introduced a CLASSIFICATION of countries based on the proportions of population under age 15, 15 to 49, and 50 and over. He observed that the proportions in the working AGES for a number of European countries appeared to remain constant over time (roughly 50 percent), while the relative share of young and older persons shifted in magnitude from the former to the latter. Thus, he proposed that countries undergo transitions from youthful population structures (that he termed progressive) to stationary and eventually to old structures (regressive), developments largely determined by declining fertility and mortality. This remarkable insight, although subsequently found somewhat inadequate, nonetheless gave important impetus to studies of the determinants of population change, especially mortality and fertility, the possibility of transitions in these vital rates and their systematic impact on population structure, and time-series cross-national research.

Attention to the determinants of age structure benefited from the original mathematical contributions of Lotka in the early twentieth century and later at midcentury in the development of STABLE population and demographic accounting models by Coale, Bourgeois-Pichat, and others (Myers 1996b). The important role played by the succession of cohorts (usually determined by year/s of birth) over time has been recognized in transforming age structures. This has BROUGHT attention to the notion of disordered cohorts, in which catastrophic events (e.g., wars, famines, etc.) have produced large deficits in the numbers of persons at subsequent ages.

Effects of Male Age Structure

Age structure of males also can influence breeding dynamics of age-structured populations. In polygynous species, populations with greater numbers of older-age-class males have been shown to have shorter, earlier, and less socially disruptive breeding periods. Conversely, where fewer older-age-class males are present, breeding periods tend to be longer and females are frequently bred later in the season. Because later breeding may lead to later birth dates, and later birth dates to lower juvenile survival, the age structure of the male population may potentially influence both pregnancy rates and survival of juveniles, thus affecting population rate of increase. However, this cascade of effects has not been conclusively demonstrated in free-ranging populations. Few studies indicate that later-bred females have significantly later parturition dates, while much evidence indicates that female NUTRITIONAL condition can override potential effects of male age structure and breeding date by allowing females to shorten the length of gestation. Further, ages of males tending harems may not be dominated by prime-aged males until male/female ratios are very high even in polygynous species, indicating that younger males may breed a significant proportion of females regardless of male age structure. Consequently, observed recruitment of juveniles has been shown to be independent of adult sex ratios and male age structure in several polygynous ungulate species. Thus, whereas theory and modeling frequently suggest that male age and adult sex ratios can potentially have a strong influence on population productivity, actual management has driven male age structure and male/female ratios well below thresholds theorized to affect population-level productivity, without any significant decreases in population productivity being documented in free-ranging populations.