Study Finds High-Intensity TNR Saves More Lives and Reduces Populations Over Time More Effectively Than Culling or Removal

A ground-breaking study, published in 2019, used a simulation method to estimate the success (as defined by cat and kitten survival rates as well as decrease in overall population) of TNR in comparison with other population control methods over a 10-year period. Although many previous studies have evaluated the effectiveness of population control projects by tracking changes in population size over time, this is the first to use computer-generated modeling in order to be able to compare different population management methods on a single population of cats.

The seven different management methods explored in this study were (1) taking no action, (2) low-intensity removal*, (3) high-intensity removal, (4) low-intensity episodic culling, (5) high-intensity episodic culling, (6) low-intensity trap-neuter-return (TNR), and (7) high-intensity TNR. For each method, the computer program acco7unted for probable birth and death rates related to the age of the individual cats within the simulated group. Probable numbers of new cats joining the population from surrounding areas, as well as cats leaving the group, were factored into the model as well.

The results showed that high-intensity TNR results in the lowest number of “preventable deaths” over 10 years while taking no action (the control scenario) resulted in the most. The study’s authors define preventable deaths as “those that could likely be reduced or eliminated using an alternative population management approach, specifically the deaths of kittens under 6 months old that fail to reach adulthood, and the deaths of any cats due to lethal management.”

Both low-intensity and high-intensity culling methods, as well as low-intensity removal, resulted in what the study’s authors call “moderate” reduction in preventable deaths. Low-intensity TNR (sterilization of 25% of the population) and high-intensity removal resulted in a higher reduction of preventable deaths, and high-intensity TNR (sterilization of 50% of the population) resulted the largest reduction of preventable deaths. In fact, high-intensity TNR “resulted in 31 times fewer preventable deaths than the no action scenario.” This is mainly due to the large difference in kitten deaths between the two scenarios.

Another important conclusion to be drawn from this study is that, while both high-intensity and low-intensity TNR greatly reduce the number of preventable deaths in a group, high-intensity TNR is much more effective in reducing the total population over time.
 
*“Removal” refers to trapping and removing a percentage of the population every six months. It does not exclude the possibility of the removed cats being euthanized. “Culling,” in contrast, refers to euthanizing a percentage of the population, allowing it to recover to its carrying capacity, and then culling the same percentage in repetition over the 10 year time period.
 
Source: Boone JD, Miller PS, Briggs JR, Benka VAW, Lawler DF, Slater M, Levy JK and Zawistowski S (2019) A Long-Term Lens: Cumulative Impacts of Free-Roaming Cat Management Strategy and Intensity on Preventable Cat Mortalities. Front. Vet. Sci. 6:238. doi: 10.3389/fvets.2019.00238