Can TNR control the number of stray cats?

TNR - also known as "trap, de-sex and release" - is used to control the number of stray cats in a certain area and ultimately achieve "achieving zero stray cats" in a certain area.

 

Can TNR control the number of stray cats?

 

It looks good, but it is useless

 

TNR, known as "Trap, Neuter, Release", controls the fertility of cats through de-sexing, thus reducing the number of stray cats, and has attracted much attention in recent years.

 

It sounds like a proven method - won't the number be reduced by not giving birth?

 

However, in reality, there is no definite conclusion as to whether TNR is really effective. A number of scholars have done research in this area.

 

Researchers at the University of Central Florida (UCF) began TNR on campus in 1991, and if a cat was found to be seriously ill, it would be euthanized directly. By 1996, the team had completed a "cat population survey" of 68 cats on campus, and all but one of the 67 cats were de-sexed. All new cats were also de-sexed afterwards.

 

By the end of the 2002 count, the team had recorded a total of 155 traffic cats, with 23 left on campus.

 

This seems to be working, doesn't it?

 

If you read the paper carefully, you'll see the flaw in it.

 

First of all, the UCF campus is not small - the main campus is nearly six square kilometers. Counting the number of cats in such a large open area is not an easy task. In their paper, the researchers do not explain in detail how the 1996 "cat population census" was conducted, but simply mention one such thing.

 

UCF Campus Map

 

Even if there is no problem with these data, the significant decrease in the number of stray cats on campus is not due to TNR. Of the 132 reduced stray cats, 73 were adopted, 17 were euthanized due to major diseases, 10 died accidentally due to car accidents and other reasons, 9 disappeared into the forest next to the school, 23 are unaccounted for, and 23 are still active on campus.

 

Fifty percent were adopted, nearly 20 percent died, and about 20 percent disappeared - such data really makes it hard to believe that the decline in the number of stray cats at UCF is due to TNR.

 

And that's just one university. What if we extend the scope to a city?

 

In 2006, North Carolina State University researcher Felicia Nutter published a study that provided a more detailed estimate of the effects of TNR.

 

As a veterinarian, Nutter was in a unique position to do this study, selecting nine local colonies of stray cats and de-sexing 98 percent of them. Seven years later, one colony had completely disappeared and the remaining colonies had fewer than five cats left each.

 

According to her projection, when 75-80% of stray cats in an area are de-sexed and new members are also de-sexed, the stray cats in this area will be extinct in about 12.8 years.

 

What is this concept?

 

A TNR research team in San Diego, California, caught more than 14,000 stray cats in the 11 years from 1992 to 2003, of which only 5% were originally de-sexed.

 

You know, San Diego is not a small city, a little bigger than Guangzhou and Shenzhen, China combined. Imagine what a huge project it would be to de-sex 70% of the stray cats in such a large area.

 

There are similar cases of practice in China.

 

Scholars at Shandong University counted the number of stray cats on campus in the three years from 2017-2019 and concluded that "the number is basically flat.

 

Data source: @knowledgeable public WeChat

 

However, in reality, they did not implement TNR on a large scale and had to adopt an average of 29 cats per year. This is also relatively consistent with the findings of the UCF study mentioned earlier, and it can be argued that the number has remained the same mainly by adoption, not TNR.

 

But then again, if stray cats are just lying there in the sun, harmless, there is no need to kill them, right? Unfortunately, the reality may not be so rosy either.

 

Cats - Natural Born Killers ......?

 

In 2013, ornithologist Scott Ross published a paper estimating that native U.S. cats are capable of killing approximately 1.3-4 billion (median 2.4 billion) birds and 6.3-22.3 billion (median 12.3 billion) mammals a year. Moreover, the paper repeatedly emphasized that this was a 'conservative estimate'.

 

Since then, researchers around the world have built on this to estimate the number of wild animals killed by local cats and the ecological impact. in December 2020, Li Zhongqiu of Nanjing University published a paper estimating that stray cats in mainland China kill a total of tens of billions of wild animals, including birds, mammals, and insects, every year.

 

Li Zhongqiu estimated the number of wildlife killed by stray cats in China each year, source: Estimates of wildlife killed by free-ranging cats in China

 

But these estimates are not rigorous. For example, the total number of feral cats has never been accurately counted in the U.S., studies on the food return rate of free-ranging cats are very few, and the large variation in wildlife population abundance at the continental scale also makes it difficult to estimate prey mortality, all of which lead to the possibility of very large differences between the final estimation results and the real situation.

 

In Ross's paper, for example, according to the 2013 estimate by the bird conservation group Partners in Flight (PIF), there are only 2.78 billion birds in the 48 states of the U.S., a figure revised to 3.44 billion in 2020. That means that at least a third of all birds in the U.S. die each year in the mouths of cats.

 

If we calculate the maximum of 4 billion in the paper, it would take less than a year for stray and free-ranging cats in the US to hunt all birds. This clearly defies common sense.

 

In fact, several other studies, including Li's team, have similar problems, and the final estimates are hardly accurate.

 

Of course, Mr. Li did not stop here, and his team is currently collecting more data on stray cats. In addition, although these estimates may differ greatly from the real value, note the unit behind - "billion". Therefore, no matter how to calculate this account, the number of small animals that die under the mouth of stray cats is not a small number.

 

Another crime of stray cats is that they are "ecological killers".

 

The International Union for Conservation of Nature (IUCN) Species Survival Commission's Invasive Species Specialist Group (ISSG) has long listed the cat as one of the world's top 100 invasive alien species, ranking it at number 38.

 

A review paper published in 2011 analyzed the impact of cats on island vertebrates and concluded that feral cats on islands are responsible for the extinction of at least 14% of birds, mammals and reptiles worldwide, and are a major threat to nearly 8% of critically endangered species, with at least 120 island vertebrate populations affected by feral cats worldwide.

 

"Thirty-three species of birds, mammals and reptiles on the IUCN Red List are extinct because of cats.

 

A well known case is the Stephen Island wren, which once lived on Stephen Island, New Zealand, and is one of the few flightless birds in the world. It is said that when a lighthouse was being built on this island, a pregnant female cat followed the workers to the island and later escaped into the forest, where she was free to hunt and breed. About a year later, it was discovered that the island's Stephen's Island Wren had completely disappeared.

 

Stephen's Island Wren, image source: Wikipedia

 

But I don't think it's all the cat's fault.

 

Stephen Island is only about 1.5 square kilometers, less than one-fifth the size of the community of Tiantongyuan, and researchers have found many fossils of this bird in New Zealand. In other words, the Stephen's Island Wren was found all over New Zealand during its vast history, but lacked the ability to defend itself and escape, and was slowly predated out of existence, only the isolation of Stephen's Island and the lack of natural predators allowed the bird to survive in this paradise.

Stephen Island, New Zealand, photo credit: Google Earth

 

Top predators like cats may only be one factor accelerating its demise. In other words, even if there had never been a cat on Stephens Island, this species might not have escaped extinction.

 

This is also the case for most of the other 32 species. The studies are only for very small islands, and for continents like Australia, which have been isolated and uninhabited for a long time. Many of the creatures that once thrived in these places have been deprived of natural predators for so long that their defenses have deteriorated and they have been forced to face alien predators like cats and foxes.

 

But places as vast and populous as China, Europe, and the United States are completely different from the special environment of Stephen's Island. For thousands of years, cats have been integrated into the local ecosystem and may be an integral part of it.

 

The entry for "Top 100 Invasive Alien Species in the World" cats mentioned earlier clearly mentions that it is difficult to find cats that can cause prey populations to decline, except on islands. In other words, the stray cat you feed in your neighborhood cannot be said to be a destroyer of the local ecosystem and biodiversity.

 

How to control the number of stray cats in a scientific way?

 

More cats are still annoying.

 

Stray cats living in the city can also be a nuisance to people's lives if they overpopulate. Cats barking in heat may disturb your sleep, carry and spread rabies virus, scratch humans, bite domestic pet birds, etc.

 

Therefore, controlling and reducing the number of stray cats is also an important issue in urban governance. The TNR mentioned at the beginning emerged in a similar context at the beginning. But if TNR is useless, can we trap and kill all stray cats like the student from Tongji University in the news?

 

Regardless of how much manpower and resources it takes to trap and treat these stray cats, doing so may also seriously damage the ecosystem.

 

This brings us to the "intermediate predator release hypothesis" in biology textbooks when we were in high school.

 

The so-called "intermediate predator" refers to animals in the middle trophic level of the food chain, such as rats, raccoons, snakes, etc., which both prey on other animals and are eaten by other animals. Simply put, the hypothesis is that when predators at the top of the food chain in the environment decrease or disappear, the number of intermediate predators will increase, leading to a decrease in the number of their prey.

 

In a paper published in 2007, researchers actually observed changes in the population of Cook's Petrels on Little Barrier Island, New Zealand. All cats and rats on the island were killed in 1980 and 2004, making Little Barrier Island an excellent place to observe the effects of cats and rats on bird survival.

 

Little Baril Island, New Zealand, 28 square kilometers, image credit: Google Earth

 

It was found that after the complete removal of cats from the island in 1980, the Cook's Petrel population as a whole gradually declined, mainly due to the lack of cat restriction, the island's "intermediate predator" rats began to breed like crazy, stealing chicks and eggs, resulting in a decline in the breeding rate of Cook's Petrels, which in turn led to a decrease in the population size.

 

Therefore, the New Zealand government had no choice but to spend a lot of manpower and resources to eliminate the rats on the island by feeding them poisonous bait, which gradually restored the population of Cook's Petrel. Therefore, even on a small island off the beaten track, it is not easy to protect birds and other animals by controlling the number of cats, and the predation relationship between multiple animals in the food chain needs to be considered.

 

It may be difficult to estimate the ecological impact of trying to remove stray cats in big cities by simply trapping and killing them.

 

In short, it is not yet known how much impact stray cats will have on the ecology. What we can do is to "not abandon" and reduce the production of stray cats from the source. When you decide to accept a cat as a new family member, please live your life with it responsibly.