Expecting to validate the use of lethal culling, researchers find just the opposite
HOBART, Tasmania, Australia––Perhaps the strongest scientific support yet for one of the key presumptions behind neuter/return feral cat control has emerged from a 13-month study by Tasmanian Department of Primary Industries biologist Billie Lazenby and two colleagues.
Expecting to validate the use of lethal culling, Lazenby and fellow researchers N.J. Mooney, and C.R. Dickman instead found that culling tends to markedly increase the numbers of feral cats hunting in favorable wildlife habitat.
Their study, entitled “Effects of low-level culling of feral cats in open populations: a case study from the forests of southern Tasmania,” appeared in a recent edition of the journal Wildlife Research, 2015. The findings, soon amplified by the Australian Broadcasting Corporation, Discovery Science, and other media, directly contradict the beliefs underlying Tasmanian and Australian national efforts to exterminate feral cats.
Tasmanian sheep farmers blame feral cats for recent outbreaks of the bacterial disease toxoplasmosis among their flocks, while Australian environment minister Greg Hunt, appointed in 2013, has called for the “effective” eradication of feral cats by 2023.
Summarized Vox Felina blogger Peter Wolf, “Using remote trail cameras, Lazenby et al estimated the number of cats at two southern Tasmania study sites before and after ‘a 13-month pulse of low-level culling’ intended to ‘simulate the resource-effort that typically might be available to and expended by natural resource managers.’”
However, observed Wolf, “The effort expended was anything but low-level. Over the course of 13 months, the researchers managed to squeeze in 2,764 trap-nights—an average of seven traps set for every day of the culling period. Each of the 26 cats trapped was, after being left in the trap for up to 12 hours or more, ‘euthanized by a single shot to the head from a 0.22 rifle using hollow point ammunition.’”
75% to 211% more cats
“Contrary to our prior expectations,” reported Lazenby, Mooney, and Dickman, the “minimum number of feral cats known to be alive” rose, by an average of 75% at one of the two test sites, and by 211% at the other. Further, “Cat numbers fell, and were comparable with those in the pre-culling period, when culling ceased.”
Lazenby et al postulated that culling killed the boldest and most dominant cats. This “allowed greater access to resources by remaining cats, thus promoting an increase in juvenile survival.”
But the increase in kitten survival “could have provided only a marginal boost” to the feral cat population, Lazenby, Mooney, and Dickman wrote. “This is because the reproductive potential of female feral cats within and around the study sites is unlikely to have been large enough over a 13-month period to produce the rapid changes in numbers that we observed.”
Instead, the researchers concluded, “The culling sites experienced influxes of new [adult] individuals after dominant resident cats were removed.”
“More damage than good”
When culling feral cats, Lazenby told Australian Broadcasting Corporation science reporter Anna Salleh, “You may be inadvertently doing more damage than good.”
Wrote Salleh, “Lazenby said that the reason culling has been so widely used in Australia is that studies have shown how effective it can be. But the problem is these studies have really only been done on islands, rather than mainland areas,” such as the Australian continent, “where as soon as one cat is removed, another will readily take its place from an eternally self-replenishing population.”
Said Lazenby, “What we really should be focusing on when we talk about managing introduced species like feral cats is reducing their impact, But it is really important that we keep in mind that you don’t always reduce impact by reducing numbers, as one individual might cause 90% of the damage.”
Cats become trap-wary
But Lazenby and colleagues also turned up strong evidence that cats rapidly become trap-wary.
“Because the number of cats being trapped decreased over time, it appeared the lethal efforts were actually effective,” summarized Wolf, even though the trail cameras showed that the cat population had significantly increased.
Concluded Wolf, “Don’t expect a press release from the American Bird Conservancy, U.S. Fish and Wildlife Service, PETA, or any of the other organizations that continue to promote the senseless killing of outdoor cats.”
Cats vs. rabbits
The Lazenby, Mooney, and Dickman study appeared about a month after Edith Cowan University researcher Tim Doherty published findings showing that feral cats increase predation on native marsupials when rabbit control programs achieve temporary reductions in the rabbit population.
“If rabbit abundance declines,” Doherty told Calla Wallquist of Guardian Australia, “then a primary food source for cats would decline, so cats would then turn to native animals and prey more heavily on them. This is something that should be considered when doing broad-scale rabbit eradication.”
Summarized Wahlquist, “The study looked at 49 published and unpublished datasets of the scats and stomach contents of feral cats around Australia and found they dined on about 400 Australian species, including 28 listed [protected] species. Reptiles were the most commonly eaten animals, making up 157 species on the list, followed by birds (123), marsupials (58), rodents (27) and frogs (21). Rodents were most likely to be on the menu in northern Australia, while larger possums and bandicoots were among the most eaten in southeastern Australia.”
While Doherty documented some cat predation on Australian native species, his findings as a whole reinforce older research indicating that non-native predators in Australia chiefly help to control introduced prey species including rabbits, mice, and rats.
Charles Darwin University conservation biologist John Woinarski published in the February 2015 edition of Proceedings of the National Academy of Sciences a paper reviewing extinctions among 273 Australian land mammal species since the first European settlers reached Australia in 1788.
Thirty of the 273 species (11%) are now extinct, an extinction rate about 10 times the global rate. Another 60 Australian land mammals (21%) are considered threatened.
“By comparison, just one North American native land mammal became extinct since European settlement––the sea mink, which was overhunted,” wrote Kristen Gelineau of Associated Press.
Woinarski and team attributed the exceptionally high Australian loss of species to the early 19th century introduction of cats and red foxes.
Fewer extinctions in presence of cats
But biologist Emily Hanna of the Australian National University in Canberra in July 2013 reported contradictory findings to the International Congress for Conservation Biology and in the journal Global Ecology and Biogeography, after creating a database covering “934 living and extinct populations of 107 mammal species on 323 Australian islands between the early 1800s and today,” summarized Maryland science writer Gabriel Popkin.
“For each island,” Popkin wrote, Hanna and colleagues “recorded the presence or absence of various native mammals, and of rats, cats, foxes, and wild dogs known as dingoes, which some scientists believe help control invasive predators. The researchers also included other factors that might affect extinction risk, such as the size of the island and distance from the mainland. Hanna then analyzed these data to find which factors most often correlated with native mammal extinctions.
“The study yielded some surprising results: Native mammals were most likely to die off on islands that had rats, but not cats, foxes, or dingoes,” Popkin concluded. “Extinction rates on such islands ranged from 15% to 30%, but when cats, foxes, or dingoes were present, the rates plummeted to just over 10%—not much higher than on islands without any introduced predators.”
“Curiosity” meant to kill cats
The flurry of new studies of feral cats in Australia coincided with the introduction of a cat-specific poison based on sodium monofluroacetate, better known as Compound 1080.
Curiosity was registered for use in West Australia in February 2015, but “is not suitable for use in the eastern states, where native animals lack the natural tolerance to the toxin that West Australian species have,” reported Wahlquist of Guardian Australia.
The Registry of Toxic Effects of Chemical Substances, published by the U.S. Environmental Protection Agency and National Institute of Occupational Safety & Health, lists results from LD-50 tests of sodium monofluroacetate performed on chickens, ducks, quail, and unspecified other bird species between 1947 and 1971.
All were killed with exposure to no more than five milligrams per kilogram of body weight: the same dose ratio that killed human beings in Nazi experiments during World War II.
Developed in Germany in 1942, ostensibly as a rodenticide, Compound 1080 is odorless, has no taste, and is lethal if ingested, inhaled, or absorbed through skin contact.
Apart from the Nazi experiments, at least 13 humans have been killed by Compound 1080 accidents and use as a murder weapon. Three children were killed in an Oklahoma City incident in which Compound 1080 was spread on vanilla wafers.
Banned in U.S.
Manufactured in the U.S. since 1956 by Tull Chemical of Oxford, Alabama, Compound 1080 was commonly used to kill coyotes until 1972, when use was prohibited by order of then-U.S. president Richard Nixon to protect wildlife. Nixon did not, however, ban manufacturing Compound 1080 for export to nations including Australia, Israel, Mexico, New Zealand, and South Korea.
In 1985 former U.S. President Ronald Reagan amended Nixon’s order to allow the use of Compound 1080-coated sheep collars, meant to kill coyotes as they bite the throats of the sheep.