“A Long-Term Lens: Cumulative Impacts of Free-Roaming Cat Management Strategy and Intensity on Preventable Cat Mortalities,” published in Frontiers in Veterinary Science, 2019. Complete article available (open access) online here.
Building on earlier research , a diverse team of researchers from the fields of animal welfare, wildlife conservation, and veterinary medicine used sophisticated population modeling techniques to examine various lethal and non-lethal methods for managing free-roaming cats over a 10-year period.
Results show that intensity (i.e., the number of cats sterilized in a given area) is critical to the success of all management methods designed to reduce free-roaming cat populations, and to minimize mortalities of both kittens and adult cats 
This analysis compared management methods in terms of the number of preventable deaths — defined as cats killed and kittens born that die before one year of age — as well as cat population abundance over this period. Computer modeling shows that, over 10 years, taking no action to manage free-roaming cats results in more deaths — especially of kittens — than any other scenario examined, including high-intensity lethal removal efforts.
Management through trap-neuter-return (TNR) can significantly reduce free-roaming cat populations as well as preventable deaths; the best results are achieved with higher-intensity efforts (i.e., sterilizing at least 75% of fertile cats at a given location every six months).
"More than any of the other methods examined, high-intensity TNR achieves a significant reduction in population and maximum reduction in preventable deaths, without resorting to the active removal of cats."
Active and regular removal of cats, especially at higher intensities, produces the most rapid population decline along with moderate reduction in preventable deaths. However, unless all of the cats removed can be adopted, it requires the killing of healthy animals.
Periodic culling (e.g., lethal removal when a population of cats becomes large enough to be considered a nuisance or a threat to wildlife) was shown to be relatively ineffective at reducing free-roaming cat populations and preventing deaths of both kittens and adult cats.
Because TNR can successfully reduce preventable deaths and population abundance at higher intensities — but may perform more poorly than removal at lower intensities — the authors note that “lessons for the animal welfare community are both encouraging and cautionary.”
The model examined management scenarios based on available reproductive biology data; however, the authors note that a number of other factors (e.g., cost effectiveness, feasibility, ethics, etc.) should also be considered in the development of an effective strategy for managing free-roaming cats .
Miller, P.S.; Boone, J.D.; Briggs, J.R.; Lawler, D.F.; Levy, J.K.; Nutter, F.B.; Slater, M.; Zawistowski, S. Simulating Free-Roaming Cat Population Management Options in Open Demographic Environments. PLoS ONE 2014, 9, e113553.
Boone, J.D.; Miller, P.S.; Briggs, J.R.; Benka, V.A.W.; Lawler, D.F.; Slater, M.; Levy, J.K.; Zawistowski, S. A Long-Term Lens: Cumulative Impacts of Free-Roaming Cat Management Strategy and Intensity on Preventable Cat Mortalities. Frontiers in Veterinary Science 2019, 6.