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Mortality Rates
In this next section of input, you enter the age-sex specific mortalities. In the language of matrix
life-table analysis (e.g., Caughley 1977; Caswell 2001), VORTEX defines mortality as the
mortality rate q x , or the percentage of animals alive at age x that die before reaching age x + 1.
Enter mortality rates as a percent (between 0 and 100) for each age-sex class. Once reproductive
age is reached, the annual probability of mortality remains constant over the life of the individual
and is entered only once (but see the section on Functions for information on how to relax this
assumption).
Mortality of Females (Males) as %: In these tables, enter the mean mortality rates for each age
class, and enter also a standard deviation (SD) for each mean to describe the environmental
variation (EV) in each rate.
Be aware that if you enter a standard deviation for each mean mortality rate that is at least half of the
survival rate (100% - mortality rate), in occasional years the mortality rate will be set at 100% and the
population will immediately go extinct. For example, if all age-specific mortality rates are 50% and the
standard deviations are set at 25%, then in about 1 in 40 years the mortality rate after adjustment for EV will be 100%
(since the rate will exceed the mean by 2 standard deviations about 2.5% of the time).
A substantial literature exists on the many methods by which one can estimate age-sex specific
mortality rates in wild populations; Caughley (1977) is a good text from which to start an
exploration of this body of information.
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