The topic of TNR and managed feral cat colonies is highly controversial. While many in rescue tout it as the self-sustaining solution to overpopulation, those on the opposing side are in constant defense of the devastating impact these cats have on native ecosystems, song birds, wildlife, etc.
A recent ecological study in 2010 by the University of Nebraska found that there are approximately 60 million feral cats living in the U.S. today. 
That does not include 60 to 88 million indoor/outdoor pet cats or owned barn cats. Worldwide the population is close to 100 millon, which means the U.S. accounts for 60% of the entire world feral cat population.
The exploding population of feral cats are directly responsible for the extinction of at least 33 known bird species worldwide, including the ‘Alalā (Hawaiian crow), native to the big island of Hawaii and the Stevens Island wren, native to New Zealand.[1.5]
Feral cats introduced to island ecosystems are responsible for at least 14% of all global island bird, mammal, and reptile extinctions, and are the primary threat to almost 8% of critically endangered birds, mammals, and reptiles. They cause destruction through both direct predation and introduction of parasites such as Toxoplasma gondii.
They are also wreaking havoc in Australia, so badly that their government has declared them as “pests” and will be culling 2 million of them by 2020. Australia has over 20 million feral cats, and they are directly responsible for or contributed to 28 of Australia’s 29 known native mammal extinctions over the past 200 years. They kill 75 million native animals every single day, which puts 120 native species at risk of extinction. 
“Australian marsupials are among the most susceptible hosts for Toxoplasma gondii and the parasite is known to cause both chronic and acute infection. Infection in marsupials is not always fatal and can result in long-term latent infection which may be reactivated during times of stress. T. gondii infection may make a marsupial more prone to predation by affecting its movement, coordination and sight. Not only is infection with T. gondii attributed to causing declines in marsupial populations in the wild, toxoplasmosis is associated with widespread pathology and death in several collections of captive marsupials.”
“T. gondii infection in kangaroos is also of public health significance due to the kangaroo meat trade.” 
It also isn’t just red and gray kangaroos that are affected by the spread of T. gondii. Lumholtz’s tree kangaroos are a threatened species that only exist in the tiny Atherton Tableland of Queensland, Australia. These rare marsupials are being blinded and killed by Toxoplasmosis, spread by outdoor and feral cats in the region.
“So far, the veterinarian with 30 years’ experience has found some evidence suggesting the cause of tree kangaroo blindness could be toxoplasmosis, a parasite found in cat droppings.
Figures from the Tableland Regional Council and Mareeba Shire Council show there were at least 939 registered cats in the region in 2014.
That figure is expected to be much higher since cats are not required to be registered within the local government areas due to changes in state legislation and the problem of feral cats.
“Toxoplasmosis causes a broad spectrum of disease, including blindness and death in a wide range of marsupials,’’ Ms Shima said.”
However, lung tissues of Lumholtz tree kangaroos are regularly infested by cysts of Toxoplasma gondii, which is transferred between animals though oocysts in cat faeces (George 1982) and can be lethal (Barker 1988). Tree kangaroos spend some of their time on the ground while foraging for food and are most likely to be infected there.
A study published in September 2016 by Deakin University has become the most comprehensive analysis to date of the destruction caused by invasive mammalian predators. The researchers compiled an extensive database using over 1,000 supporting references, in addition to current data from the IUCN Red List of Threatened Species, to “quantify the global impacts of invasive mammalian predators.” 
Invasive predators are implicated in 87 bird, 45 mammal, and 10 reptile species extinctions—58% of these groups’ extinctions worldwide over the past 500 years. These figures are likely underestimated because 23 critically endangered species that the study assessed are classed as “possibly extinct.” 
If that isn’t shocking enough, they also directly threaten 596 living species currently classed as “vulnerable” (217 species), “endangered” (223), or “critically endangered” (156), of which 23 are classed as “possibly extinct.” 
Cats were found to have directly attributed to 63 species extinctions alone; barely in second place behind non-native rats. 40 species of birds, 21 species of mammals, and 2 species of reptiles have perished due to feral cat predation. This accounts for 26% of all extinctions worldwide. Among the hardest-hit areas are of course Australia and New Zealand.
“In Australia, feral cats and foxes have caused the decline or extinction of two thirds of digging mammals – such as bilbies and bandicoots – over the past 200 years. Without these digging mammals, reduced disturbance to topsoil has led to impoverished landscapes with little organic matter, low rates of seed germination and higher bushfire risk.”
Dr. Tim Doherty
Centre for Integrative Ecology School of Life and Environmental Sciences
Here in the U.S., the Smithsonian Migratory Bird Center performed a joint analysis project with USFW and published the resulting paper in January 2013. Together, SMBC and USFW performed a rigorously thorough systematic review of every single U.S. study on domestic cat predation that exists in scientific literature.
Then researchers eliminated studies in which the sample size was too small or the results too extreme, and then extracted and standardized the findings from the 21 most rigorous studies.
Based on the resulting figures, they statistically quantified the total bird and small mammal mortality rate caused by domestic cats. The analysis revealed that feral and outdoor pet cats kill 1.4 to 3.7 BILLION birds annually. That’s just in ONE YEAR. On top of that, cats kill 6.9 to 20.7 billion small mammals, ranging from mice to rats to rabbits to squirrels to voles to shrews to chipmunks. Domestic cats also kill small reptiles and amphibians as part of their diet, but estimates are more difficult due to lack of a sufficient range of studies. However, using data taken from Europe, Australia and New Zealand and extrapolated to fit the parameters of the United States, SMBC and USFW estimate that cats kill 258 to 822 million reptiles and 95 to 299 million amphibians every single year.
The two agencies then conclude with the following:
“The magnitude of wildlife mortality caused by cats that we report here far exceeds all prior estimates. Available evidence suggests that mortality from cat predation is likely to be substantial in all parts of the world where free-ranging cats occur.
Our estimates should alert policy makers and the general public about the large magnitude of wildlife mortality caused by free-ranging cats.”
The island state of Hawaii is facing their own feral cat crisis. 300,000 ferals on Oahu are spreading FATAL Toxoplasmosis infections to several species, including critically endangered Monk seals which number only 1300 left in the wild:
Toxoplasma gondii is a microscopic protozoan parasite. The oocytes (immature egg cells) are found in cat feces which contaminate the ocean environment via terrestrial run-off. A seal may become infected either through direct contact or through ingestion of contaminated prey. T. gondii is potentially fatal and is thought to be a cause of mortality in the southern sea otter population along the central California coast. This pathogen was first identified in Hawaiian monk seals in 2004, and the deaths of several seals have been attributed to T. gondii infection. The impact of T. gondii and other protozoal parasites on monk seals continues to be examined and monitored.
Cats are the only definitive host of Toxoplasma gondii, meaning they are the only animal in which the parasite can reproduce. Most cats carry it asymptomatically, which means they do not suffer any symptoms but are still passing it on and infecting others.
The same thing is also happening to Southern sea otters. They are contracting Toxoplasma gondii from consuming contaminated prey items, which became carriers via water runoff that is contaminated with cat feces. Again, cats are the only animal that can shed the parasite and spread it. Scientists from UC Davis studied wild otters and found that 42% contained Toxoplasma gondii antibodies, which almost always indicates infection, and is definitive of exposure. 76% of otters that lived near freshwater runoff like storm drains and the mouths of rivers were infected with T. gondii antibodies. Necropsies on dead otters yielded death likely caused by encephalitis induced by T. gondii infection. They are finding the same cause of death in harbor seals and spinner dolphins. 
In addition, feral cats are spreading toxoplasmosis to critically endangered species in important zoo conservation programs. Here at the Chattanooga Zoo, our breeding male snow leopard, Czar, died after a sudden rapid decline in health. This was a year before I began my internship at the zoo in January 2016. Czar died of severe pneumonia caused by cysts embedded in his lung tissue. He was infected with T. gondii by local feral cats that enter the grounds and defecate in the soil, which is then spread by rodents, which get into the enclosure and the snow leopards catch them. Snow leopards are one of the most endangered cats on the planet and Chatt Zoo is one of the few with an AZA breeding permit. Czar had fathered 3 cubs, 2 of which are in Species Survival Plan breeding programs in Providence and Cincinnati, and his genetics were vital.
Zoo Miami has also lost FIVE animals in this manner — four spider monkeys and a red kangaroo. Countless other zoos have also lost animals to the parasite.
This parasite is also dangerous and can be fatal to children, elderly, and immunocompromised individuals (cancer patients and HIV patients are at huge risk), and causes birth defects and fatalities to the fetus in pregnant women. Feral cats spread the parasite when they defecate in gardens, sandboxes, playgrounds, etc. or track feces on their paws onto surfaces humans touch.
Where do TNR programs fit in?
TNR (Trap, Neuter, Release) means to trap a feral cat, neuter/spay it, vaccinate it (usually), and release it back into the home range it came from. The idea behind this is that it prevents the cat from reproducing, thus eventually reducing the population over a long period of time. It does indeed achieve that goal — however, the focus is on the cats, not the wildlife they decimate. These feral cats, while sterilized, still remain outdoors where they continue preying on wildlife and upsetting the ecological balance.
“It keeps the cats healthy. They really have great lives out there doing their feral cat thing.”
Executive Director, Nevada Humane Society
Organizations like Alley Cat Allies claim that 80% of Americans prefer humane TNR to eradication.
While this may be true and the desire not to see harm come to a beloved pet species is understandable, the vast majority of Americans that respond to such surveys likely do not understand the devastating impact that feral cats have on the native species they enjoy (and which are vital to Earth’s ecological stability). Even if made aware, the general population has yet to grasp the importance and priority of wild species and ecosystems over pet species and individual animals.
ACA also states:
“Cats have lived outdoors for thousands of years—in fact, keeping indoor-only cats only became possible in the mid-20th century. Outdoor cats are part of our natural landscape.”
This is a misleading statement. Cats may have begun domestication thousands of years ago, but they have not been part of the natural landscape of the U.S. that long. In fact, every single domestic cat can trace their DNA ancestry back to 15th century Europe. During Europeans’ discovery of new lands in the Americas, Africa and Asia (including islands), they introduced rabbit populations to ensure future food availability — which eventually grew out of control. Domestic cats were then introduced to help control the rabbit populations. They then developed their own colonies detached from their human connection, and spiraled out of control to the 60+ million we have here today, and the 12+ million devastating Australia.
Every day I see first-hand the ecological devastation caused by the hundreds of thousands of outdoor/stray/feral felines living in the South of the U.S. They are literally everywhere, and native species are suffering. Going out road cruising at night to look for snakes, I see 5-10 cats every single night, but rarely a snake, frog, gecko, etc. In the 3 years I’ve lived in Tennessee, I’ve seen exactly 2 snakes, yet easily 200+ cats. It speaks volumes as to the balance of wildlife.
Wildlife destruction aside, as a licensed LVT I also see the spread of diseases and parasites left and right. Feline Leukemia, FIV, Distemper, Calicivirus, Coronavirus, Herpes Virus, Toxoplasma gondii, and a host of internal and external parasites.
I have been a lifelong cat lover/owner literally since the day I was born, have spent the last 13 years as a private feline behaviorist, and have spent nearly my entire life rescuing and rehabilitating innumerable stray and feral cats. I have also been an avid reptile hobbyist and breeder for over a decade.
The bottom line is that the overpopulation of cats is a human-created problem that is wreaking irreversible havoc on native wildlife, and requires a human-created solution that does not continue to allow the decimation of native birds, mammals, and reptiles.