# In estimating the species population do you multiply

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species finds all its specific needs. In estimating the species’ population, do you multiply by the total area of the range or the total area of the habitat? Obviously, the latter would be more useful, but determining the area of the habitat is often as difficult as counting animals. Pacific Ocean CANADA UNITED STATES Range of the Bighorn Sheep MEXICO Figure 15-7 Rocky Mountain bighorn sheep roam throughout the mountains of North America from western Mexico to southwestern Canada. (Map adapted from W. H. Burt and R. P. Grossenheider, 1980. Mammals , 3rd ed. Peterson Field Guide Series. Houghton-Mifflin.)

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498 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? Mortality is often expressed as the gross mortality rate , the number of deaths in a specified period, divided by the total number of individuals in the population at the start of the period, multiplied by 1000. (We multiply by 1000 to get whole numbers rather than decimals, which are difficult to deal with.) An example might be a gross mortality rate of 200 per year per 1000 individuals.A more accurate picture of what is happening to a population is obtained from age-specific mortality rates , in which each age class is considered as a separate population. (An age class is all the individuals of the same age.) From age-specific mortality rates, it becomes apparent that, for example, most red foxes die during their first year. Any individuals that survive this critical period have a good chance of living another four or five years. Almost no red foxes live beyond six years. From age-specific mortality rates, ecologists construct life tables , an example of which is found on the BioInquiry web site in Section 15-3.We will work with life tables later. Expression of birth rates is even more variable.Some studies take the number of new individuals born into the population and divide by the total population to get the crude birth rate . Other studies determine crude birth rates by dividing by the number of fe- males in the population, since the females bear the new individuals. Still others divide by “number of females of reproductive age,” excluding, in human populations, prepu- bescent and postmenopausal females.Then there are various age-specific birth rates . Confusing, isn’t it? How population characteristics are expressed is the scientist’s or writer’s choice and the reader’s bane.We will be consistent in this book, but do not ex- pect such consistency elsewhere. Reader beware.Whenever you read a population num- ber, ask, “What does this number mean? What units are being used? How was the number determined?” Things get particularly difficult when you compare one study with another.Are numbers cited in different places comparable? In spite of these diffi- culties and ambiguities, population studies can provide meaningful insights into the past, present, and future of populations.
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