Cats Are Not Fish: A Ricker Model Fails to Account for Key Aspects of Trap–Neuter–Return Programs

SIMPLE SUMMARY: Various population models have been used to predict the sterilization effort necessary to reduce free-roaming cat numbers through trap–neuter–return (TNR) programs. Among these is a Ricker model, first developed for application in the management of fisheries. We tested this model by using data from two long-term (i.e., >20 years) TNR programs with well-documented population reductions. Doing so revealed that the model cannot account for some key aspects of typical TNR programs. This model is therefore inappropriate for use in assessing the effectiveness of TNR programs. A more recently developed model that accounts for the movement of cats in and out of a given area is better suited for modeling TNR programs. ABSTRACT: In a frequently cited 2005 paper, a Ricker model was used to assess the effectiveness of trap–neuter–return (TNR) programs for managing free-roaming domestic cat populations. The model (which was originally developed for application in the management of fisheries) used data obtained from two countywide programs, and the results indicated that any population reductions, if they existed, were at best modest. In the present study, we applied the same analysis methods to data from two long-term (i.e., >20 years) TNR programs for which significant population reductions have been documented. Our results revealed that the model cannot account for some key aspects of typical TNR programs, and the wild population swings it predicts do not correspond to the relative stability of free-roaming cat populations. A Ricker model is therefore inappropriate for use in assessing the effectiveness of TNR programs. A more recently developed, stochastic model, which accounts for the movement of cats in and out of a given area, is better suited for predicting the sterilization effort necessary to reduce free-roaming cat numbers through TNR programs.