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VORTEX Version 9.50 User’s Manual
specified number of years. Easy access to these two particular functions are provided because they are
needed more frequently than are detailed functional dependencies of most other rates. Even for these two
rates, however, you can specify these functions to have almost any shape if you use the function editor to
specify the rates.
For most users and for most purposes there will be no need or desire to model demographic rates as
functions; it is usually fully adequate to specify fixed demographic rates rather than functions.
Specification of rates as functions can be difficult: the appropriate form of the function is rarely known,
the function parameters are usually very difficult to estimate, and it is not trivial to enter a function
correctly. If alternative functions need to be examined in sensitivity testing, the number of combinations
of input parameters to be explored can quickly become overwhelming. Consequently, we would not
recommend that novice users or students use the function option within VORTEX.
Specification of Demographic Rates as Functions
Dependencies of demographic rates on population and individual parameters are entered into VORTEX by
specifying the functional relationships. There are two ways that you can enter a function rather than a
constant for an input variable: you can type the function directly into the input box for specifying the rate,
or you can open a Function Editor to help you develop the function to describe the relationship. VORTEX
provides an option (and this option is the default for new Projects) to have the Function Editor open
automatically whenever you enter a “=” as the first character in an input box for a demographic rate. The
other way to open the Function Editor is to click on the Function Editor icon on the toolbar. If you type a
function directly into an input box, you must precede the function with an “=” sign (to distinguish the
specification of a rate as a function rather than as a constant). If you open the Function Editor by typing
an “=” in an input box, and then develop a function within the Function Editor, then the Function Editor
will insert the function back into the active input box when you accept the function. It is usually easier
(and safer) to build a function first within the Function Editor and then send it over to the input screen.
Figure 58. The Function Editor utility.
Chapter 6
Using Functions in VORTEX
97