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For example, between ages three and four, 12 individuals die out of the 776 that were remaining from the original 1000 sheep. This number is then multiplied by 1000 to get the mortality rate per thousand.
As can be seen from the mortality rate data (column D), a high death rate occurred when the sheep were between 6 and 12 months old, and then increased even more from 8 to 12 years old, after which there were few survivors. The data indicate that if a sheep in this population were to survive to age one, it could be expected to live another 7.7 years on average, as shown by the life expectancy numbers in column E.
| Life Table of Dall Mountain Sheep Data Adapted from Edward S. Deevey, Jr., “Life Tables for Natural Populations of Animals,” The Quarterly Review of Biology 22, no. 4 (December 1947): 283-314. | ||||
|---|---|---|---|---|
| Age interval (years) | Number dying in age interval out of 1000 born | Number surviving at beginning of age interval out of 1000 born | Mortality rate per 1000 alive at beginning of age interval | Life expectancy or mean lifetime remaining to those attaining age interval |
| 0-0.5 | 54 | 1000 | 54.0 | 7.06 |
| 0.5-1 | 145 | 946 | 153.3 | -- |
| 1-2 | 12 | 801 | 15.0 | 7.7 |
| 2-3 | 13 | 789 | 16.5 | 6.8 |
| 3-4 | 12 | 776 | 15.5 | 5.9 |
| 4-5 | 30 | 764 | 39.3 | 5.0 |
| 5-6 | 46 | 734 | 62.7 | 4.2 |
| 6-7 | 48 | 688 | 69.8 | 3.4 |
| 7-8 | 69 | 640 | 107.8 | 2.6 |
| 8-9 | 132 | 571 | 231.2 | 1.9 |
| 9-10 | 187 | 439 | 426.0 | 1.3 |
| 10-11 | 156 | 252 | 619.0 | 0.9 |
| 11-12 | 90 | 96 | 937.5 | 0.6 |
| 12-13 | 3 | 6 | 500.0 | 1.2 |
| 13-14 | 3 | 3 | 1000 | 0.7 |
Another tool used by population ecologists is a survivorship curve , which is a graph of the number of individuals surviving at each age interval plotted versus time (usually with data compiled from a life table). These curves allow us to compare the life histories of different populations ( [link] ). Humans and most primates exhibit a Type I survivorship curve because a high percentage of offspring survive their early and middle years—death occurs predominantly in older individuals. These types of species usually have small numbers of offspring at one time, and they give a high amount of parental care to them to ensure their survival. Birds are an example of an intermediate or Type II survivorship curve because birds die more or less equally at each age interval. These organisms also may have relatively few offspring and provide significant parental care. Trees, marine invertebrates, and most fishes exhibit a Type III survivorship curve because very few of these organisms survive their younger years; however, those that make it to an old age are more likely to survive for a relatively long period of time. Organisms in this category usually have a very large number of offspring, but once they are born, little parental care is provided. Thus these offspring are “on their own” and vulnerable to predation, but their sheer numbers assure the survival of enough individuals to perpetuate the species.
Populations are individuals of a species that live in a particular habitat. Ecologists measure characteristics of populations: size, density, dispersion pattern, age structure, and sex ratio. Life tables are useful to calculate life expectancies of individual population members. Survivorship curves show the number of individuals surviving at each age interval plotted versus time.
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