# A large harvest was thought to be not only possible

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quite large, indicating that the herd’s numbers would probably increase.A large harvest was thought to be not only possible, but desirable to keep the population from growing too large. For a time, hunter success was consistently high. But, in the early 1980s, when sport hunters took to the field, few animals were found.What had happened? Another factor important to population dynamics is movements of individuals be- tween populations.There are two types: Immigrations bring new individuals into a pop- ulation, while emigrations remove individuals. Some such movements are regular and easy to predict.At the start of the breeding season, nearly all members of certain song- bird populations leave winter homes in Central and South America and migrate to nest- ing grounds in North America. In other cases, predictions are not so easy. Apparently, large segments of the Nelchina caribou herd emigrated to an adjacent herd—the Forty- mile. Mass movements from the Nelchina had not been observed before and have not occurred since,but have been observed in other herds of caribou.Such movements make predicting future changes in populations difficult at best. At least we can rewrite our mathematical formula relatively easily as: where the new terms are i , the population’s immigration rate, and e , its emigration rate. r = 1 b - d 2 + 1 i - e 2 , b 7 d ; b 6 d ; b = d

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15-2 How Do Ecologists Study Populations? 497 Population Characteristics Are Expressed in a Variety of Ways A word of caution: Beware when reading about populations. Relevant characteristics such as size and mortality each can be expressed in more than one way.The most straight- forward expression of population size is the absolute number . Count all individuals in the population and express the total.This is almost never done, because it is nearly im- possible to count each individual. A notable exception is the U.S. population census taken every 10 years—a massive effort that is fraught with errors and assumptions. More frequently, population size is expressed in terms of density , the number of individuals per unit area. Theoretically, determining population density is easy: Count all individuals in several representative areas and determine the average. (For example, “the population density of Monaco is over 18,000 people per square kilometer.”) Then, to estimate the total population,multiply the density by the number of unit areas the pop- ulation occupies. In practice, especially when dealing with organisms, this is not as easy as it sounds.What is a “representative area”? Where are the population’s boundaries? This leads to another consideration.According to maps in field guides, such as the one in Figure 15-7, Rocky Mountain bighorn sheep are found throughout all noncoastal, western United States, extending north into Canada and south into Mexico.This is the animal’s range . But bighorns are not evenly distributed within their range.Typically, they are restricted to mountainous areas in summer, descending into adjacent valleys in winter.The mountains and adjacent valleys are the bighorn’s habitat , areas in which the species finds all its specific needs. In estimating the species’ population, do you multiply
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