The vacuum effect
                     The vacuum effect
 Quick facts:
• If cats are removed from their outdoor home, it creates a territorial openingor 
vacuumthat will not remain empty.
• Removing cats from an area may cause a temporary decrease in the cat 
population, but more cats WILL take their placeand it won’t take long.
• This phenomenon is known in conservation studies as the vacuum effect. The 
vacuum effect has been observed in many species, not just cats.
• Catching and removing (or killing) cats is therefore futile. It is an expensive, 
deadly cycle which yields no long-term benefits.
• Trap-Neuter-Return (TNR) is the only way to stabilize cat populations. It is the 
humane, effective approach to community cats and is sound public policy.
 You may have heard the expression “nature abhors a vacuum”. It refers to the 
phenomenon that when a space is emptied, nature will fill it. Once you 
understand this reality, you’ll know why killing cats (or otherwise removing them) 
from a given location is doomed to fail. The idea that removing cats will not lead 
to a decrease in cat populations across time may feel counter-intuitive, but it is 
grounded in a well-documented concept in biology known as the vacuum effect.
 Understanding the vacuum effect is vital to save lives. Animal control agencies, 
animal shelters, and municipalities must account for the vacuum effect in their laws 
and policies in order to govern effectively and create the best outcomes for every 
cat and kitten.
 For cats who live outdoors, it is a literal matter of life and death.
 So, what is the vacuum effect? Let’s start with the basics.
 What is the vacuum effect and what does it have to do with cats?
 The vacuum effect occurs when a portion of an animal population is permanently 
removed from their home range. These animals may have been killed or removed by 
people, a natural disaster, or any other means. The result is a temporary dip in 
population levels.
 To be clear, any such population dip will only be temporary. The initial population 
lived in that location because there were resources such as shelter, food, and water. 
Once emptied, this still resource-rich habitatthe vacuuminevitably attracts other 
members of the same species from neighboring areas. They move in to use the 
same resources that sustained the first group.
 Both the new individuals and any remaining members of the original population 
then reproduce. What’s more, they reproduce at higher rates to fill the habitat 
and take advantage of the available resources.
 Before long, the area fills back up to capacity again, as if the animals were never 
removed at all.
 The vacuum effect occurs across many species, including foxes, mice, coyotes, 
voles, possums, and badgers. Of course, it also occurs for cats.
 The vacuum effect makes or breaks public policy for cats
 The United States spends millions of dollars each year rounding up and killing 
cats through “catch and kill” schemes. The unfounded hope is that the killing will 
lead to reduced cat population levels.
 The vacuum effect ensures it will not.
 Scientific evidence proves that lethal cat population control schemes don’t work. 
A large body of research confirms what smart observers have long known: new 
cats will inevitably fill habitats emptied by cat removal schemes. In other words, 
the vacuum effect occurs, and it makes killing outdoor cats pointless.
 Not only does the cat population rebound, it rebounds fast. Before you know it, 
there are the same number of cats outdoors as there were before. The only result 
is that many cats are needlessly killed…often over and over again.
 What’s worse, lethal cat control schemes are as indiscriminate as they are cruel 
and ineffective. Countless cats, whether unowned or pet cats, are killed in the 
process. That is part of the reason rounding up and killing cats is massively 
unpopular with the public. Not only is it morally unsound, catch and kill can also 
put shelters and municipalities at legal risk.
 All of this for an inherently flawed policy that provides no long-term cat 
population control.
 We need to change the status quo for animal control and animal shelters
 For much of the past 100 years, animal control agencies and local governments 
have attempted to take an “easy route” to reduce or eliminate cat populations 
through catch and kill schemes.
 Yet as you’ve learned above, there is no way to make these lethal schemes work. 
The vacuum effect will always ensure a new group of cats will move into the 
emptied environment to take advantage of resources.
 Even the strictest feeding bans on cats won’t change anything. It is impossible to 
rid an area of food sources, especially for cats, who are naturally gifted scavengers. 
Just think of the abundant supply of insects and rodents in most outdoor spaces, 
plus food waste in and around dumpsters. It quickly becomes apparent how 
plentiful food is for cats in our towns and cities. (Mahlow)
 According to one scientific journal article, “…the presence of feral cats in a place 
indicates an ecological niche for approximately that number of cats.” 
(Zaunbrecher) Each time cats are removed, the population will rebound to fill that 
niche.
 All of these facts point to one simple conclusion: we have to change the status quo for 
animal control and animal shelters. Catch and kill does not and has never worked, and it 
only serves to take lives and drain taxpayer dollars.
 Animal control agencies and local governments need to shift their thinking and their 
approaches to cats outdoors in ways that are based in science, fact, and experience.
 Creating a better future with humane approaches
 Alongside the horrifying effects on cats, catch and kill has long impacted the mental and 
emotional wellbeing of animal control agency and shelter employees. Staff of 
institutions in which catch and kill is the go-to policy kill tens of thousands of cats each 
year with no signs of long-lasting reduction in community cat populations. The 
seemingly endless loss of life and lack of results is a crushing combination. Staff 
depression, burnout, and attrition rates climb.
 Science, experience, public sentiment, ethics, and common sense all line up together on 
this issue: catch and kill is a failed policy that needs to end. Understanding and 
respecting cat biology and their place in the natural world are the keys to finding 
effective approaches and real solutions. That’s why Alley Cat Allies educates advocates, 
animal control officers, shelter staff, government officials, and the public about the 
vacuum effect.
 It’s also why we advocate for programs like Trap-Neuter-Return (TNR), which 
stabilize cat populations by taking the vacuum effect into account.
 Jon Cicirelli, director of San Jose Animal Care & Services, 
which has a strong TNR program in place, sums it up nicely. “For the past 50 
years, we’ve killed umpteen million cats and we’re no better off,” he says. “That 
system clearly does not work. We have to try something new”.
 TNR programs, rather than removing cats and forcing a vacuum, ends the 
breeding cycle of the cats altogether. Through TNR, community cats are 
humanely trapped, spayed or neutered, vaccinated, eartipped for identification, 
and returned to their outdoor homes where they continue to lead full, healthy 
lives.
 According to a 2017 Harris Interactive poll, 84 percent of Americans prefer that 
their communities use tax dollars to adopt sterilization as its cat control policy 
instead of bringing cats found outdoors into shelters to be killed. And that is true 
even when they are unaware of the vacuum effect. People simply believe, 
correctly, that cats have intrinsic value to their lives and do not want them to 
suffer and be killed.
 Alley Cat Allies’ work and the growing shift in society’s mindset toward cats are 
resulting in amazing change. Over the past decades, a growing number of animal 
control agencies and shelters have learned that killing community cats has no benefits 
whatsoever. They are leaving lethal cat management control schemes where they 
belongin the past.
 Foiled by the vacuum effect a case study from Louisiana
 The Gillis W. Long Hansen’s Disease Center in Carville, Louisiana, once struggled to 
reduce cat populations. This was not for lack of trying.
 The center had continuously attempted to get rid of the community cats on its grounds 
by trapping and removing them. However, overall numbers were never reduced 
because of the vacuum effect. Any void created by the successful removal of cats was 
quickly filled back to capacity.
 Fed up with failure, the center’s staff decided to try a new approach. They conducted a 
three-year trial of non-lethal cat population management. The patients at the hospital 
were informed that instead of eradicating the cats, a Trap-Neuter-Return (TNR) program 
would be put in place. They were assured that the trapped cats would be returned to 
their outdoor home after they were spayed or neutered. They were grateful to hear it.
 The cats who were returned after being spayed and neutered were observed on a 
weekly basis for six months. A census was compiled after 18 months, and again 
after 36 months. At the end of the program, the center found that of the 40 cats 
“known to have been alive when the program was completed, 30 were located 
and identified 36 months later. New litters could not be located.”
 The overall health of the cats improved, and there was a reduction in 
reproductive or territorial behaviors as well as nocturnal vocalizing (AKA 
nighttime howling). Not only was the cat population successfully stabilized, but 
the concerns and complaints about nuisance behaviors also drastically 
decreased.
 TNR proved to have many benefits and gained great support from both the 
patients and the administration. It was all possible because TNR takes into 
account the vacuum effect. If the original population is allowed to remain in its 
territory, new individuals won’t arrive and reproduce. The original cats are unable 
to breed because of spay and neuter, so the area’s cat population naturally 
decreases. No killing required.
 More information:
• Video: All about the vacuum effect
• Just the facts: The vacuum effect
• Research: The vacuum effect
• Trap-neuter-return
• Community change