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mortality rate  =   number of individuals dying number of individuals surviving   ×   1000

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 give the mortality rate per thousand.

mortality rate  =   12 776   ×   1000     15.5

As can be seen from the mortality rate data (column D), a high death rate occurred when the sheep were between six months and a year 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.

This life table of Ovis dalli shows the number of deaths, number of survivors, mortality rate, and life expectancy at each age interval for Dall mountain sheep.
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.
A B C D E
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

Survivorship curves

Another tool used by population ecologists is a survivorship curve    , which is a graph of the number of individuals surviving at each age interval versus time. These curves allow us to compare the life histories of different populations ( [link] ). There are three types of survivorship curves. In a type I curve, mortality is low in the early and middle years and occurs mostly in older individuals. Organisms exhibiting a type I survivorship typically produce few offspring and provide good care to the offspring increasing the likelihood of their survival. Humans and most mammals exhibit a type I survivorship curve. In type II curves, mortality is relatively constant throughout the entire life span, and mortality is equally likely to occur at any point in the life span. Many bird populations provide examples of an intermediate or type II survivorship curve. In type III survivorship curves, early ages experience the highest mortality with much lower mortality rates for organisms that make it to advanced years. Type III organisms typically produce large numbers of offspring, but provide very little or no care for them. Trees and marine invertebrates exhibit a type III survivorship curve because very few of these organisms survive their younger years, but those that do make it to an old age are more likely to survive for a relatively long period of time.

 Graph plots the log of number of individuals surviving versus time. Three curves are shown, representing type I, type II, and type III survivorship patterns. Birds exhibit a type II survivorship curve, which decreases linearly with time. Humans show a type I survivorship curve, which starts with a gentle slope that becomes increasingly steep with time. Trees show a type III survivorship pattern, which starts with a steep slope that becomes less steep with time.
Survivorship curves show the distribution of individuals in a population according to age. Humans and most mammals have a Type I survivorship curve, because death primarily occurs in the older years. Birds have a Type II survivorship curve, as death at any age is equally probable. Trees have a Type III survivorship curve because very few survive the younger years, but after a certain age, individuals are much more likely to survive.

Section summary

Populations are individuals of a species that live in a particular habitat. Ecologists measure characteristics of populations: size, density, and distribution pattern. 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.

Practice Key Terms 9

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Source:  OpenStax, Environmental biology. OpenStax CNX. Aug 10, 2015 Download for free at https://legacy.cnx.org/content/col11863/1.1
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