About Vortex


Vortex is an individual-based simulation of deterministic forces as well as demographic, environmental and genetic stochastic events on wildlife populations. It can model many of the extinction vortices that can threaten persistence of small populations. Vortex models population dynamics as discrete, sequential events that occur according to probabilities that are random variables following user-specified distributions. Vortex simulates a population by stepping through a series of events that describe an annual cycle of a typical sexually reproducing, diploid organism: mate selection, reproduction, mortality, increment of age by one year, dispersal among populations, removals, supplementation, and then truncation (if necessary) to the carrying capacity. The simulation of the population is iterated many times to generate the distribution of fates that the population might experience.

Vortex is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.

  • How to Cite

    Lacy, R.C., and J.P. Pollak. 2014. Vortex: A stochastic simulation of the extinction process. Version 10.0. Chicago Zoological Society, Brookfield, Illinois, USA.

    Please acknowledge SCTI in your reports and publications. Suggested wording:

    Vortex PVA software (Lacy & Pollak 2014) is provided under a CreativeCommons Attribution-NoDerivatives International License, courtesy of the Species Conservation Toolkit Initiative (https://scti.tools).

  • Key References

    Lacy, R.C. 1993. VORTEX: A computer simulation model for Population Viability Analysis. Wildlife Research 20:45-65.

    Lacy, R.C. 2000. Structure of the VORTEX simulation model for population viability analysis. Ecological Bulletins 48:191-203.


Vortex is constantly changing. We fix bugs when they are identified, although not always immediately if the bug is more of a nuisance (e.g., bad formatting on a screen or output file) rather than something that is causing misleading results. We also add new features and options when we believe that they will be useful to a significant number of users, and when we have the time to add the enhancements. We frequently make small changes to screen lay-outs, screen text, and file formats when we feel that the change will improve the program for users. In the Help menu of the program there is an option to check for any updates that are more recent than the version you are currently using.

Note that we do not guarantee that all changes to the program have been documented here. In particular, we don’t list here many very minor changes made to the interface.

We also cannot guarantee that all bugs in code or errors in algorithms have now been fixed. Reports of apparent bugs can be made at www.vortex10.org/Bug_Reports.aspx . We are very grateful to the many colleagues who have carefully documented and reported apparent bugs. Even when the issue turns out to be a misunderstanding by the user, and not a true bug, we still appreciate the report. (Indeed, we are happier when the problem turns out not to be a bug in the program!)

  • 20 June 2019 (version 10.3.7)

    The distribution and support site for Vortex is now www.scti.tools!

    A bug was fixed that would have incorrectly initialized a PSvar when there was a metapopulation and the PSvar initialization function included as parameters earlier PSvars that differed in initial value among the populations.

  • 3 April 2019

    DAMID and SIREID are now provided as synonyms for DAM and SIRE in functions. This can be useful when an ISvar is given the label “DAM” (to allow for cross-fostering!), but the biological dam is required in other functions. DAMID will always be the biological dam, even if an ISvar “DAM” is changed.

  • 20 Jan 2019

    A new operator was added to those available in functions. DISCRETE(a;b;c;d;e) selects a value (from 1 to 5, in this case) from the distribution with relative probabilities of a, b, c, d, and e. E.g., DISCRETE(1;1;2;2;4) will select 1 in 10%, 2 in 10%, 3 in 20%, 4 in 20%, and 5 in 40% of the time. SDISCRETE(x;a;b;c; …) selects a value using seed x.

    The option for offspring dependency on the dam now can be specified to tally the number of dependent offspring (in IS1) but not kill those offspring if the dam dies. Enter the number of years as a negative value to invoke this option. This can be useful for tracking the number of young individuals in a breeding group (e.g., helpers).

Accessory Tools

Accessory tools are apps that can be used in conjunction with Vortex. They are automatically included with Vortex installation. For more information about each of these tools, please see the Vortex manual and technical documentation.