Conservation biologist Peter Marra talks with journalist Rene Ebersole about the threat of outdoor cats to wild animals and to human health. Marra is the co-author, with writer Chris Santella, of the book Cat Wars: The Devastating Consequences of a Cuddly Killer.
Steve Mirsky: Welcome to Scientific American "Science Talk" posted on April 24, 2017. I'm Steve Mirsky. On this episode …
Peter Marra: We've got somewhere between 60 and 100 million cats out there, outdoor cats. We're not going to get them off the landscape immediately, and we are not proposing mass euthanasia at all. We're saying that we need to take this seriously.
Mirsky: That's Peter Marra. He's the director of the Smithsonian Migratory Bird Center, and, with writer Chris Santella, he is the coauthor of the 2016 book, Cat Wars: The Devastating Consequences of a Cuddly Killer. Marra recently talked with Rene Ebersole. She is an independent journalist covering science, health, and the environment for such publications as Outside, Popular Science, National Geographic, and Audubon.
Rene Ebersole: What made you want to write this book?
Marra: I wrote the book because after years and years of studying a variety of the threats that biodiversity face, I felt like it doesn't matter how much science we continue to do. The answers are never really going to be completely satisfying or sufficient to a smaller majority of people that really value cats. I wasn't sure it was the science that really mattered anymore, so I wanted to bring it out in a popular way to try to explain to people what the consequences really are of cats, and how significant they are, not just from a biodiversity perspective, but also from a disease perspective and stick my neck out there, which is what I did to talk about what the choices are that we have to really make. These are choices that many of us deal with on a day-to-day basis as we continue to try to work to save biodiversity. That's why we wrote the book. I felt like this was really a huge issue. It is really a huge issue that most people don't recognize or don't understand.
Ebersole: How many birds are killed each year by cats?
Marra: In the United States, our best estimates are between 1.3 billion and 4 billion birds per year.
Ebersole: What about people who say, "My cat never kills anything when it goes outside"?
Marra: It's possible, and so there are some cats that don't kill anything, but we also know from several other studies that have been done, very good studies, that cats typically only bring in about a third of the animals that they kill. People actually put transmitters on cats and track them around, and they actually quantified and found everything they killed. They also put video cameras around the cats' necks, and they showed that they're only bringing a few of the animals home that they actually kill, so we adjusted our values for that, as well. But it is possible their cats don't kill, it is very possible, but that does not mean that it's okay to leave your cat outside, because there are lots of reasons not to let cats outside roaming around on their own.
Ebersole: Your numbers are pretty large, but a lot of people might have the attitude that if a cat does kill one bird here and there, "Really, what's the harm in that? It's not going to cause the extinction of a species."
Marra: Yeah, it's an interesting comment, and the fact of the matter is that cats have killed enough animals, mainly on islands, to cause the extinction of species. In fact, the paper that we use in the book by Medina, a publication that came out in Global Change Biology, attribute 33 species extinctions to cats. There was actually just a paper that came out that updated that figure and increased it to 63 species extinctions. I haven't had a chance to fully evaluate that paper yet, but that paper was published in the Proceedings of the National Academy of Sciences, which is a fairly reputable journal. It points to cats as being one of the most detrimental species on the planet. Now, the question that we get is, "Well, that's on islands, and cats aren't really predators on many of these islands, so these are species that are particularly vulnerable." That's correct, but then when you look at a variety of the studies that have been done on mainland areas at local scales, they do show that these local scales add up to population-level consequences, that there are particular vital rates that are impacted that certainly will influence population processes.
Does that mean that cats in mainland areas will impact or cause a species extinction? Not necessarily, but that doesn't mean that the mortality that they're imposing on particular species isn't important. That's where it gets difficult to really have these discussions, because scaling up from a local study to a species-wide range is something that's really tricky to do. It's almost impossible to do, in fact, because we just can't be everywhere at once, but these local-scale studies do show that there is really important mortality imposed on these populations and these species.
Ebersole: What about putting a bell on a cat's collar? That helps, right?
Marra: Actually, it probably doesn't help, in part because birds don't really know what a bell means. By the time a cat is in position and ready to pounce, that bell, it's too late. It's not going to do any good. Not only that, there are other reasons not to just let your cat outside. The idea of just kicking open your door and letting the cat roam is really not responsible pet ownership. We don't do that with dogs anymore. We sort of learned years ago that we can't just open our door and let dogs roam, because it's not good for the dogs. In this case, it's not good for the cats. Cats can get hit by cars, cats can get mauled by dogs, or worse, get eaten by coyotes. Not only that, there are disease consequences for letting cats outside. Letting cats outside, in general, is a practice that we argue really needs to stop.
Ebersole: How about those trap-neuter-and-return programs? Aren't those part of the solution?
Marra: There are several problems with trap-neuter-return. The first is that cats are still on the landscape killing wildlife. They're still out there. They're not stopping. They're also attracting more pet abandonment. These colonies rarely are stable, and they are rarely capturing enough cats so the population growth stops. You have to catch and neuter about 75 to 90 percent of the cats in these colonies to actually stop the growth of these colonies, and that rarely happens.
Ebersole: Are you saying we have to kill all feral cats?
Marra: No. No, I'm not saying that at all. That's not at all what I'm saying. What I'm saying is we need to do a better job of mapping out where all these colonies are. Where these colonies are providing or presenting the most risk for biodiversity, or the most disease risk in terms of spreading things like toxoplasmosis, those need to be our high-priority areas. There are some cat colonies in urban environments where they don't pose a significant threat, and I would say those are low-priority areas. Look, we've got somewhere between 60 and 100 million cats out there, outdoor cats, some of which are in TNR colonies, some of which are feral.
We're not going to get them off the landscape immediately, and we are not proposing mass euthanasia at all. We don't say that in the book. We're saying that we need to take this seriously, and so we need to figure out where the greatest risks are immediately for biodiversity, for endangered species, for threatened species, for species in decline. Those are the colonies we need to deal with immediately.
Ebersole: You mentioned toxoplasmosis. Could you tell us a little bit about what that is, and how it connects to people and cats?
Marra: Sure. Toxoplasmosis is a protozoan parasite that breeds sexually only in cats. Not just domestic cats, but also in pumas and a variety of felids. The problem is that domestic cats, by far, represent the largest population of cats on the landscape. Once this protozoan parasite breeds sexually, cats will go through about a week to three-week infectious state, once in their life, where they defecate out millions and millions of Toxoplasma oocysts that go out into the environment. These oocysts are some of the most resilient things on the planet. They can survive frozen soil, they can survive marine environments, they can survive freshwater environments. They persist for years. When people are exposed to these oocysts, or animals, wildlife are exposed to these oocysts, there are consequences.
Humans can pick them up either through fecal oral transmission ‒ cats like to defecate in gardens because it's loose soil, or in sandboxes ‒ if you happen to touch some infected fecal material, you can potentially get those oocysts in your mouth or some other way. Those oocysts then start to divide in your body, and they eventually make their way up to your brain. They go through several different stages. Those oocysts are there for good. They don't go away. They can actually go into your eye into something called ocular toxoplasmosis, which can eventually lead into glaucoma or blindness. People that do get _____, toxoplasmosis exposure immediately, or Toxoplasma exposure immediately, some people develop fevers, and sometimes worse than that. Many people don't develop any symptoms at all.
But, over the past 10 or 15 years, there have been some incredible research, quite disturbing research that has shown that people that are infected with Toxoplasma and develop toxoplasmosis also develop several different types of mental illnesses, including things like schizophrenia, bipolar disorder, depression, and suicide. Actually, there are some really alarming papers about suicide risk and toxoplasmosis. Toxoplasmosis is present globally. There are some countries that have infectious rates of about 50 percent. In the U.S., people think it might be between 11 and 22 percent infection rate in the U.S., so it is quite the significant disease, and I think we're just now really starting to understand what the implications are for mental health.
Ebersole: Are certain people more susceptible to toxoplasmosis?
Marra: Well, I think, in general, just like with any sort of infectious agent, if some individuals have compromised immune systems, like AIDS or other issues, those are the individuals that are more likely to develop symptoms. The long-term symptoms, I think it's probably safe to say that the mental illnesses that afflict folks over the long term are probably also compromised in terms of their immune system, but I think that's still trying to be understood. We still have a lot to learn there.
Ebersole: How are people reacting to your book?
Marra: It's mixed. It's sort of the way that I thought it would go. I mean we called it Cat Wars for a reason, because just like with climate change, just like with anti-vaccinations, just like with DDT, there is a proportion of people that are very vocal against certain things, whether it's anti-climate change, or anti-DDT. In this case, it's the folks that are the animal welfare activists, or the cat activists that are totally opposed to the book, in part because they are against any conversation if it involves euthanasia. They don't see that as ever being part of the conversation.
Ebersole: Are you getting hate mail?
Marra: Lots of hate mail, lots of bad reviews on Amazon. These are people who have not read the book. These are folks that are just very, very passionate about cats, and very, very passionate about the idea that cats should remain on the landscape regardless of what the consequences are.
Ebersole: Have you gotten any death threats?
Marra: I have. I have gotten a couple of death threats. It's a hard thing because, in my mind, while I understand the passion for cats, and I understand the passion for life, I really do, I understand that these folks feel like the life of the individual is what really means something. I really firmly believe that while the individual does matter, it matters to me ‒ I have pets ‒ I feel strongly about those individuals, I also see the long term and understand that those individuals make up populations, and the impacts can be disastrous. There can be devastating consequences on populations of natural animals, of animals that are out in nature. I wrote the book because it addresses these difficult questions that we have to deal with. If we were to ignore them, there are going to be long-term consequences that I'm not comfortable with.
Ebersole: What about the media? Do you feel like you're getting fair treatment from the media? Are you being labeled a cat killer? Are people being balanced? How is it going out there with the media?
Marra: Again, it's mixed. I feel like the media that is more objective has been very complimentary of the book, but I also feel like the media has gone for the headlines. You know, "Marra says keep cats inside or they die." We're not saying that. We're not saying that at all. I'm not saying anything about mass euthanasia, but the headlines are pushing that. It's hard, because we approached this from a strategic, thoughtful perspective, "Let's look at where the cats are. Let's weigh these situations." Yes, it is a state-by-state situation. Every state has areas that are really valuable from a biodiversity perspective, or have areas that are sensitive from a human health perspective.
In each of those areas, cats are having an impact and we may have to do some euthanasia, but the first approach is to get those cats off the landscape by capturing them, trying to adopt them out. If people want to put them in shelters, that's great. But, if we can't, then there is the harsh reality of what do we do with these cats? I'm not willing to suggest or to support the idea that we just leave them on the landscape where they continue to have an impact. I don't understand how that can be a solution. That's not a solution to me. I'm looking for solutions.
Ebersole: Is it a call to action?
Marra: It's completely a call to action. What I'm looking at is that over 30 to 40 percent of the birds in North America are declining significantly. Some, up to 90 percent, have disappeared. Now, I'm not suggesting that cats are causing all those declines. That's not what I'm suggesting at all, but what I am suggesting is that there are a variety of threats that are impacting biodiversity in birds, and it's hard to always pinpoint exactly what percentage of the population is going extinct because of cats, or declining because of cats. When I look at my data on the magnitude of mortality, cats are the top threat. I have looked at wind turbines, I've looked at all these things. Is habitat destruction the most important one? By far it's the most important one.
Is climate change going to be one of the most important factors driving populations extinct or to decline? Of course it is, but cats are a significant problem on the landscape, and we need to start addressing this problem. The number of TNR colonies is increasing. They're not decreasing. That should be an alarm call right there. I just see this as a problem that if we don't address it now, it's just going to get worse. As a conservation ecologist, as someone who really cares about our environment, I just can't do that.
Ebersole: Nothing is going to change over night. What are the first steps?
Marra: Well, I see the first steps here really as, again, getting a handle on where the high-priority areas are, places like in Hawaii. There is a group in Hawaii, in _____, that is trying to get a handle on the growing cat population there. They went to the table with the cat groups and the Humane Society to try to come up with a plan and a solution to get cats off the landscape. Unfortunately, there are some cat groups there that continue to create problems. I just see that as a continual issue. I would love to be able to see groups like the United States Human Society come to the table. Whether I'm at the table or not, it doesn't matter. Whoever it is, I would like to see them come to the table, but they have to come to the table where euthanasia is still a possibility. It's not a great thing, but TNR is not a solution, and so we've got to figure out what to do with the cats. Just like with deer, just like with numerous other animals whose populations have skyrocketed and continue to cause real problems, we've got to bring euthanasia to the table. It's just essential.
Ebersole: What about wildlife conservation groups? Can they play any part?
Marra: Yeah. Wildlife conservation groups would be tremendous. I think wildlife managers, in general, are a critical part of this. For those TNR colonies that do remain, I would argue that wildlife managers have to be part of those discussions, part of that management regime that's established for the TNR colonies that stay on the landscape. Again, I'm not suggesting that all TNR colonies be removed ‒ that's just going to be impossible ‒ but the ones that remain, state wildlife managers, county wildlife managers, animal shelters that have the proper training have to be part of that management of those predators that are on the landscape. All these people have to come to the table, but we have to have an honest conversation about whether or not TNR is successful, and TNR is just not successful. Over the long term, we can't keep creating TNR colonies, especially in places where there are sensitive wildlife.
Ebersole: One of the headlines that I've seen out there is the idea that you have to remove cats off the landscape by "any means". What did you mean by that?
Marra: That was a quote that was taken completely out of context, and it speaks to the war and the tactics that are used by the animal welfare activists. It's funny, because they'll use any means necessary to make their argument and to try to make the scientists look bad, and that's exactly what happened. In that case we say, "From a conservation ecology perspective, cats would be removed from the landscape by any means necessary." The very next sentence is, "But, with 60 to 100 million cats on the landscape," ‒ I forget the exact words ‒ "this is going to be a difficult thing to do, and then there is the difficult question of what to do with the cats once they're captured." That's exactly what we say, so it's clear we're not talking about doing mass euthanasia, but that's how they spun it in the press.
That's the line that is sticking, unfortunately, but that's not what the book says. We do go on to talk about euthanasia. We do go on to talk about the sad truth of what this means, and what people do in the field and how these things have been done to remove cats from the landscape. But that quote was taken completely out of context and spun in a way that just got people all riled up and incited a lot of people, and it was unfortunate.
Ebersole: What if nothing is done to deal with this problem? What do you think will happen then?
Marra: I think we'll continue to see the degradation of our environment. I think species will continue to decline, cat populations will continue to grow. It would be really unfortunate. Toxoplasmosis rates will continue to go up. We'll see other human health consequences. That's why I wrote the book, because I see this as a real problem. I really do see this as the Silent Spring of the 21st century. DDT didn't cause any extinctions because we stepped in. We let the science drive it. In this case, the science is really clear, and that's what we tried to argue in the book and to bring up all the papers, mostly papers that were not mine that I wasn't involved with. But we tried to bring the data forward and the science forward that made the case for the consequences of what cats are doing on the landscape. If we just step aside and let it continue, then we're just going to continue to see impacts on birds, impacts on mammals, impacts on reptiles, and human health consequences that eventually we may not be able to reverse them.
Ebersole: Do you think people will heed your warning? Do you feel optimistic based on the reactions to your book so far and where things stand? Do you think people will listen to what you're talking about and maybe take some real action?
Marra: I think it's going to be slow. I don't think this is going to happen over night, unfortunately. I choose to be optimistic. When I look at New York State and the battles that went on in New York State, I think it was just last year, where the TNR folks were trying to get state funding. The governor looked at the science, the governor looked at the evidence and said, "No." I think that it's going to be that sort of an uphill battle. I think there are going to be states that continue to push TNR, that continue to not enforce leash laws and cat responsibility as a general policy. But I think there will be states that will listen and will understand that we should start treating cats like dogs, which we did 40, 50 years ago, and take them off the landscape. I can only hope that it's going to take some time. I'm hoping it's not that much time, but I prefer to think optimistically. I'm not going to go away. I'm going to continue to push this topic, along with other topics that I work on, not just cats, and hopefully we'll make some progress.
Mirsky: That's it for this episode. Get your science news at our website, www.scientificamerican.com. We're also now bundling our daily 60-second science podcasts into weekly editions posted on YouTube, where you can enjoy them by subscribing to the Scientific American channel. Follow us on Twitter, where you'll get a tweet whenever a new item hits the website. Our Twitter name is @sciam. For Scientific American "Science Talk", I'm Steve Mirsky. Thanks for clicking on us.
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