Case Study: Marion Island proves that removing cats from an area is a futile effort that leads to decades of cruelty
The only documented instance of a population of cats being permanently, “successfully” removed from their habitat occurred in an abhorrently cruel program that proves just how impossible, impractical, and inhumane it is to attempt to kill off a target population of cats.
In 1975, scientists set out to kill all of the 2,500 feral cats living on Marion Island—a tiny, uninhabited sub-Antarctic island measuring just 115 square miles, where there was no chance that new cats could move in. In their first attempt, researchers aerially sprayed feline distemper virus over the island. Sixty-five percent of the cats suffered and died painfully, but the other 35% developed immunity and the population quickly rebounded. Compounding this failed effort with further cruelty, they next brought in dogs to flush out the remaining cats. Between 1986 and 1991, the last cats were hunted with guns and, when that also failed, trapped and poisoned.1
It took 19 years and ruthless methods to clear Marion Island of cats. That’s nearly two decades to kill all of the cats in an environment where no new cats could enter. Even in this extremely isolated environment, scientists noted “[T]he recolonization of preferred habitats, cleared of cats, from neighboring suboptimal areas…”2 In other words, they still observed the vacuum effect.
The outrageously inhumane methods used to kill the cats of Marion Island were unacceptable 20 years ago and they remain unacceptable today—not only because of their horrific cruelty, but also because they are impossible to replicate in populated areas like cities and towns. Although often held up by opponents of TNR as an example of successful feral cat control, all the Marion Island example proves is the existence of the vacuum effect and the futility of attempting to permanently clear an area of cats by killing them.
 Bester, M. N., et al. "A Review of the Successful Eradication of Feral Cats from Sub-Antarctic Marion Island, Southern Indian Ocean." South African Journal of Wildlife 32, no. 1 (April 2002): 65-73.