Reduction of free-roaming cat population requires high-intensity neutering in spatial contiguity to mitigate compensatory effects

Abstract

When free-roaming in natural areas, the domestic cat (Felis silvestris catus) is ranked high among the most destructive alien species. Near human vacancy, it might risk humans, impair sanitation, and suffer from poor welfare. Cats’ popularity as companion animals complicates their population control. Thus, culling is often replaced by a fertility control method called “Trap-Neuter-Return/Release (TNR),” which is considered more humane. Despite the extensive application of TNR, a long-term controlled study was never performed to test its effectiveness. We present a uniquely designed controlled field experiment for examining TNR effectiveness. The study was performed over a twelve-year period, divided into pre-intervention, mixed- and full-intervention phases, and spanned a 20Km2 urban area. Trends of cat, queen, and kitten counts, cat reproduction, and carcass reports were compared among study phases and areas with different neutering intensities. The cat population increased during the first two study phases and did not decline in highly neutered populations, presumably due to cat immigration. Expansion of high-intensity neutering to the entire city in the full-intervention phase (>70% neutering percentage) reversed cat population growth, reaching an annual ca. 7% reduction. This population reduction was limited by a rebound increase in cat reproduction and longevity. We conclude that cat population management by TNR should be performed in high-intensity, continuously, and in geographic contiguity to enable population reduction. To enhance management effectiveness and mitigate compensatory effects, we recommend further evaluating an integrated strategy that combines TNR with complementary methods (e.g., vital resource regulation, ill cat euthanasia, and adoption).Significance StatementThough popular companion animals, domestic cats pose numerous problems when free-roaming, including predation of wildlife, hazards to humans, impaired sanitation, and a decrease in their welfare. Thus, managing their populations is essential. The Trap-Neuter-Return method (TNR, capturing, sterilizing, returning/releasing) is widely employed for managing cat populations. However, its use is under continuous debate due to the lack of long-term controlled evidence. We examined the outcomes of high-intensity TNR by performing a twelve-year controlled field experiment, the largest to date. Neutering over 70% of the cats caused population decline when applied over contiguous areas. However, it was limited by a rebound increase in reproduction and survival. These findings provide a robust quantification of the limitations and the long-term effectiveness of TNR.

Rethinking the Animal Shelter's Role in Free-Roaming Cat Management

“…On a larger scale, the critical threshold for cat population control through removal has been estimated at 50% or more in multiple modeling studies (13,14,16,17). While lower than the estimates of 57% to >90% for TNR to reach a threshold of control (13,14,17,44,45), this level is still substantially out of reach for shelter-based removal programs. For instance, even at the low end of the estimated range of unowned cat populations in the US, 50% removal would require admission of 15 million cats to shelters (over 13 million more than the ∼1.34 million free-roaming/stray cats estimated to enter shelters in 2019) (46).…”

The animal welfare, environmental impact, pest control functions, and disease effects of free‐ranging cats can be generalized and all are grounds for humanely reducing their numbers

“…Furthermore, provisioning them increases the likelihood of pest infestations (Ayyad et al, 2018;Tamayo-Uria et al, 2014;Theimer et al, 2015), while irresponsible owners may abandon animals near TNR clowders (Castillo & Clarke, 2003;Natoli et al, 2006;Kilgour et al, 2017). Long-term population reduction at scale using TNR is either unconvincing (excluding adoptions, census numbers at the end of TNR studies frequently reflect those at the start; Crawford et al, 2020), or expensive and requiring high effort (Gunther et al, 2022;Kreisler et al, 2019). Consequently, implementing TNR often means accepting cat clowders, with associated problems, indefinitely (see Natoli et al, 2019 where urban clowders persisted for 30+ years under TNR management).…”