# Graphs of age structure allow predictions of future

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Graphs of Age Structure Allow Predictions of Future Population Growth Another technique for predicting how a population’s size may change begins with deter- mining information about individuals in each of the population’s age classes —groupings based on particular age intervals,such as 0 to 4 years.First,the number of individuals in each age class alive at the beginning of some period is determined.Then the number surviving throughout the period is calculated from age-specific birth and death rates.The informa- tion on all the age classes is tabulated in a life table , thus giving a picture of the makeup of the population.There is an example of a life table on the BioInquiry web site. The next step in predicting changes in a population is to arrange the life table data into an age-structure graph (Figure 15-14).The numbers of individuals in each age class are arranged in rows, starting with the youngest at the bottom. Notice that, within each age class, numbers have been separated by sex.Also notice that, in general, the length of each row—that is, the numbers of individuals in each age class—decrease with age. Figure 15-13 Different species have different patterns of population growth. K -selected species, such as these sperm whales (a) , maintain populations close to the carrying capacity. Typically, their population numbers are stable and change slowly. Species that are r -selected, including these deer mice (b) , maximize “little r,” the rate of population increase.Their population numbers are anything but stable and often change rapidly. (a) (b)

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Exploration 15-3 How Do Populations Grow? 505 2 Can you think of a situation in which this might not be true, that is, a situation in which the numbers of individuals in a particular age class could be greater than the age class immediately younger? Hint: There are two possible explanations. Age class Males Postreproductive individuals Reproductive individuals Prereproductive individuals High numbers of young Females Males Females (a) Few numbers of young (b) Individuals are born into the population only once, into the youngest age class. Mortality, on the other hand, exacts its toll on all age classes. 2 Notice that, in any given age class, the numbers of females may not be exactly equal to the numbers of males.There are various reasons that this might be so. Some are due to chance. (Flip a coin 50 times.Would you expect to get exactly 25 heads?) Other rea- sons have a more biological basis. In some human populations, infant mortality is high- er for females than for males. Or mortality may differ between the sexes during adulthood. During the breeding season, mature male moose gather, defend, and mate with numerous females grouped into harems. Much time is spent intimidating other males, and little time is spent feeding. Such activities increase the mortality rate for reproductive-age males.
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