Estimates of economic impact of “invasive species” fail smell test

Native plant advocates use a variety of strategies to motivate public policy makers to invest in their “restorations.”  One of their rhetorical tools is the claim that “invasive alien species” cause economic harm.  They refer to a controversial study (1) that claims the economic impact of alien plant and animal species in the US is over $120 billion per year.  Since this figure always struck us as rather fantastic we weren’t surprised by this critique of it in a recent scientific publication:  “The study has been roundly criticized for ignoring major economic benefits [of non-native plants and animals] and for including the cost of controlling species that may not need controlling, as well as factoring in events of questionable relevance, such as bird deaths caused by domestic cats.”(2)  Since most of what is being done in native plant “restorations” seems unnecessary to us, we have always assumed these cost estimates are more a reflection of money wasted than a report of actual economic harm.  For example, if tons of herbicide are used to kill plants just because they aren’t native, the harm is more in the herbicide use, than in the money wasted on it, in our view.  In any case, the waste of money is not being caused by the non-native plants, but rather by the ideologues who choose to destroy them.

Wikimedia Commons, photo by Sage Ross

We were inspired to drill down into these estimates of alleged economic harm by non-native plants and animals by a recent “study” about feral cats by the University of Nebraska Extension which claims that feral cats cause $17 billion of economic damage every year.  This guesstimate is based on these assumptions:

  • Feral cats kill an estimated 480 million birds per year,  based on an assumption that there are 60 million feral cats and that each cat is estimated to kill 8 birds per year.
  •  The “value” of each bird is $30, based on an assumption that each bird is worth $.40 to a bird watcher, $216 to a hunter, and $800 to someone who raises birds.

[Addendum:  One of our readers has alerted us to the fact that the estimate of economic impact doesn’t compute.  See below*]

The estimate of the number of birds each feral cat kills is based on one study (3) done in Australia in 1996.  As native plant advocates are quick to tell you when they are advocating for the destruction of eucalyptus (which are native to Australia), Australia is a very different place.  Many questions would have to be asked and answered before we could assume that feral cats kill the same number of birds in Australia and the US.  For example:  (1) Is the ratio of birds to cats the same in Australia and the US?   (2) Are there the same percentages of ground-dwelling and nesting birds in both countries?  (3) Are there similar quantities of alternate food sources available to cats in both countries?  Etc.  In fact, since the answers to these questions also vary within the US, we don’t think it is justifiable to use the same “bird-kill-rates” for all locations within the US, let alone from another country.

We also turn to A. Starker Leopold’s book about the California quail (4) for a more benign view of the feral cat:

  • “Hubbs (1951) analyzed the stomach contents of 219 feral cats taken in the Sacramento Valley and recorded one California quail.  Feral cats, like bob-cats, prey mostly on rodents.” Page 142
  • “Feline pets that are fed regularly are not dependent upon catching birds for a living, but rather they hunt for pleasure and avocation.  They can afford to spend many happy hours stalking…birds around the yard, and hence they are much more dangerous predators than truly feral cats that must hunt for a living and therefore seek small mammals almost exclusively (wild living cats rarely catch birds).”  Page 212

The method used to assign a $30 “value” to each theoretical bird killed by a feral cat seems fanciful to us:

  • In what sense does it cost a birder $.40 for each bird that is theoretically missing?  The birder is unaware that a bird is absent.  Is the birder’s experience materially different whether he sees 25 birds or 24 birds on a walk in the forest?  It seems a philosophical question akin to “If a tree falls in the forest and there is no one there to hear it, does it make a sound?”
  • It seems even more absurd to assign a “value” of $216 to a hunter for each bird killed by a cat.  Since the hunter plans to kill the bird, how does it save $216 to prevent the bird from being killed by a cat?  A dead bird is a dead bird.
  • As for a bird breeder who spends $800 raising each bird, one must ask how a feral cat could gain access to birds which we assume are kept in cages.

In other words, valuing birds theoretically killed by feral cats seems a rhetorical, not a scientific undertaking; that is, a method of advocating for the extermination of feral cats.  And, as we would expect, that is exactly what the “study” published by the University of Nebraska Extension does.  It advocates for a variety of methods of eradicating feral cats, including shooting them from a distance with a rifle or trapping them in a trap that kills the animal instantly.

This publication makes the usual meaningless distinction between feral cats and cats that are pets.  It is a meaningless distinction because when cats are roaming free it is impossible to determine which it is.  The Nebraska project suggests protecting the pet cat by having it micro chipped for identification.  Even in the unlikely event that all owners of cats would have them micro chipped, one wonders how someone shooting a cat from a distance would be in a position to determine that the cat is micro chipped.  Nor would an “instant-death” trap be capable of identifying a micro chipped cat before it enters the death chamber.

And as with all eradication efforts of both plants and animals, there are unintended consequences of exterminating feral cats. 

  • “Only once conservationists had eliminated feral cats from Macquarie Island in the south-west Pacific did they realize that these non-native predators had become a vital link in the local food web.  Since the last cat was killed in 2000, exploding rabbit populations have eaten much of the island’s unique flora bare.”(5)


Brown (Norway) Rat, Wikimedia Commons


  • Cats are well-known predators of rats.   The University of Nebraska publication acknowledges this and proposes that increased use of rodenticides will compensate for the loss of cats and consequent increases in rat populations.  Ironically, rodenticides are known to kill birds of prey.    The East Bay Regional Park District used 1,509 pounds of rodenticide in 2008, so this is not an insignificant problem.  From the standpoint of the bird, or the birder, or the hunter, does it matter if the bird is killed by a cat or by rodenticide?   Another philosophical conundrum.

Finally, we must evaluate the credentials of the authors of the publication of the University of Nebraska.  The publication credits 22 undergraduate students of the University of Nebraska for “providing the preliminary information, photos, and resources used in developing this Neb-Guide.”  And the authors of the publication describe themselves as “technicians, coordinators, or specialists.”  Although the publication claims to be “peer reviewed,” if the peers were people with similar credentials, we can’t consider this a scientific study.  Rather it is typical of the hobbyist credentials of most native plant advocates.  A spokesman for the Veterinary Information Net said the report “…almost looks like a senior level wildlife and fishery sciences or ag science book report.”  When we drill down into the hype, we often find that information is manufactured by native plant advocates and their allies to support their mission, in this case exterminating feral cats.  In particular, we conclude that:

  • Estimates of the “economic damage” caused by  feral cats are propaganda not science.
  • Although we would not support extermination efforts in any case,  the unintended consequences of eradicating feral cats should be scientifically evaluated before any policy decision regarding feral cats could be considered.

*60 million cats times 8 birds per year equals 480 million birds killed.  However, 480 million birds times $30 per bird equals $14.4 billion NOT $17 billion.  Jeez, they can’t even do the math and we’re embarrassed to admit that we didn’t catch this.  Thanks to our readers for keeping their eyes on the ball.  

(1) Pimentel, David, “Update on the environmental and economic costs associated with alien-invasive species in the United States,” Ecological Economics, 52:273-288, 2005

(2) Hamilton, Garry, The New Scientist, January 20, 2011.  N.B. The article actually says that economic impact is estimated at $137 billion/year, but we are using the lower figure for which we can provide a reference.

(3) McKay, G.M., “Feral cats:  origins and impacts:  Unwanted Aliens?” Nature Conservation Council of New South Wales, Australia, 1996

(4) Leopold, A. Starker, The California Quail, University of California Press, 1977

(5) Hamilton, Garry, ibid.

14 thoughts on “Estimates of economic impact of “invasive species” fail smell test”

  1. The claim that feral cats cause $17 billion of economic damage every year is ridiculous propaganda obviously spread by people who believe that feral cats should be killed. I read the University of Nebrasks Extension study and I think it is totally unscientific. There is no evidence given for many of the statements, such as the “usual weight” and length. How many cats were in the study? How were they observed? Although I have not studied feral cats, I know it is false to state that “family units break up once the kittens reach 7 months of age.” They break up much earlier than that, usually after the mother stops nursing the kittens. That is just one small detail, but it is typical of the false statements made in this article.

    The Issues and Impacts section of the Nebraska study seem almost like a joke. There is no way to quantify the value of a bird, and, as Hubbs points out, cats, even feral ones, prefer catching small rodents, not birds. The number of birds killed by feral cats is wildly exaggerated. As for ferals living 3-5 years, I have heard that before, and do not believe it. I am sure how long a feral cat lives depends on its environment, food sources, climate, etc.

    I believe that the extermination of feral cats can never be justified; I believe that no species should be exterminated, even pests like fleas (they should be controlled, not exterminated), and certainly no species that are disliked just because they are nonnative.

    I guess I’d make an exception about bedbugs. I’ve never seen a bedbug, and never hope to see one, but I can’t see any harm to doing away with them, and with things like harmful bacteria.

    The world is filled with all kinds of creatures that prey upon each other, eliminating the weakest and least able to survive. Feral cats provide a “service” by helping to keep the small rodent population from exploding. But even if they didn’t provide a service, cats are intelligent and beautiful animals. Domesticated cats enrich the lives of the people who make them part of their households; feral cats should, in my opinion, be trapped, neutered and then released to be protected (and fed) away from the human enemies who would harm them.

  2. In response to the value of dead birds vs. feral cats:
    1. My cat, and feral cats, eat rodents and convert them into nitrogenous waste products which they then deposit outside. There, this waste promotes the growth of seed bearing plants that help feed birds, plants that provide shelter for birds, worms that feed birds, and other animals such as insects that feed a variety of wildlife including birds. Since most of this comes from non-bird sources I think I should bill the birds in my yard $0.40 each time I feed my cat. (This assumes that the birds’ credit cards are valid.)
    2. If $216 is the value of the pleasure derived mutilating each bird it should be added to the cost of each hunting license. Feral cats should not be forced to subsidize hunters. Their credit cards are no more likely to be valid than the birds’.
    3. $8oo to a breeder for each bird? A program like this would be another example of government waste in a time of huge deficits. Let the birds and bees breed by themselves. They enjoy doing so enough to sing while doing it and this will keep US jobs here rather than letting them be shipped overseas. What next, government subsidies to puppy mills?
    Webmaster: Thanks, BB, for bringing a chuckle to this otherwise joyless topic. And thanks to your cat and her feral friends for helping to feed the birds and the bees.

  3. Thank you for this! Interestingly, the work on which the $17 billion number the UNL authors based their number on is also replete with errors. (Pimentel et al. 2005). One would expect more from Cornell. As it turns out, the $30 per bird number IS subjective.

    If you would like an even more detailed review of why the $17 billion “impact” of cats is just bad science, please take a look at the report, “17 reasons the economic impact of the cat as a non-native species in the U.S. does not cost $17 billion,” available at

  4. Why TNR and Cat Advocates Even Exist …

    It has been found that Toxoplasma gondii parasite is capable of changing the brains of whatever organism it infests. In mice, they lose the fear of cats and are even attracted to cat-urine. Making the asexual portion of the Toxoplasma gondii life-cycle faster to complete in order to replicate more quickly into its sexual reproduction phase in all host cats. This loss of fear and apprehension manifesting itself in humans in a similar manner, even when common-sense tells them they should depend on that sense of fear or doubt for their own survival.

    Here are other ways that this parasite have been known to alter the thinking patterns of humans:

    I strongly suspect that it might even be responsible for all cat-lovers’ wholly contradictory behavior of putting cats, all other animals, and even all humans in harm’s-way through their adamant insistence of promoting TNR programs, just to ensure the survival and spread of more Toxoplasma gondii parasites throughout the food-chain and in more humans. They are, in effect, being controlled against all reason and common-sense by the very parasite that is reproducing in their cats.

    The stuff that sci-fi used to be made of come to reality. Real-life “pod-people”. They can’t think nor reason beyond the need of ensuring the survival and proliferation of Toxoplasma gondii. It won’t let them.
    Webmstaer: We have not tried to verify the statements made in this comment. Our posting of this comment should not be viewed as an endorsement of this viewpoint. We invite readers who may be knowledgeable on this subject to respond to the comment.

  5. re: “We have not tried to verify the statements made in this comment. Our posting of this comment should not be viewed as an endorsement of this viewpoint. We invite readers who may be knowledgeable on this subject to respond to the comment.”

    But of course you won’t try to verify the facts about T. gondii controlling your thoughts and behavior. The parasite’s cysts in your mind are doing exactly what it’s supposed to be doing. Preventing you from taking ANY action that is detrimental to the survival of this parasite in your mind. Scary how that works. Isn’t it.

    Thanks for proving it so succinctly.

    Get tested for T. gondii if you are defending these destructive, disease-ridden, invasive-species cats’ lives. You’re actually obeying cats’ parasites in your brain. You no longer think and reason like a human anymore, ignoring all common-sense.
    Webmaster: I don’t have a cat. I’ve never had a cat. I don’t plan to have a cat in the future.

    I have now done a little reading about this toxin. There seems to be enough evidence to support at least some of what you claim.

    However, your leap to a claim that cat lovers are essentially zombies under the control of this toxin seems to be an extreme over-interpretation of available evidence from reputable sources. And it is, of course, insulting both to the person and the cat to suggest that it is not possible to love a cat just because a cat is loveable.

    1. Here’s some more information for those who are still able to fight against the cats’ parasites in their minds controlling them (unlike what we’ve just witnessed).

      Toxoplasmosis and psychology: A game of cat and mouse|hig|06-03-2010|editors_highlights

      Webmaster: We have received many comments from readers about the parasite (Toxoplasma gondii) that infects rats and is passed to cats. We have not been willing to post all of those comments.
      The New York Times published an article in their Science Section on August 17, 2011 about Toxoplasma:

      We recommend this article to anyone who may be concerned about this issue. Here is a quote from this article:
      “The parasite can also infect all sorts of other animals, including humans, to which it causes toxoplasmosis—one of many good reasons to avoid contact with cat droppings. But outside of cats, its reproductive circle cannot be completed…Toxoplasma infection in humans almost certainly would not affect human behavior as it does the rats…At least two billion people worldwide are infected by the protozoan, many from eating infected meat. Initial symptoms are mild flu, after which the parasite forms cysts that lodge in the brain. There they remain for decades and are thought to have little or no effect in adults, except in people with compromised immune systems. Infection does, however, pose serious dangers for a fetus, so pregnant women are advised to take special care to avoid infection.”

  6. Trying to argue of the numbers of wildlife killed, or their perceived monetary value is ludicrous. Because …


    If only one waterway in N. America is clogged by Zebra Mussels, should we let them be because, after-all, it’s only ONE waterway?

    If only one lake is destroyed by Eurasian Water-Milfoil should we let it be and allow it to spread everywhere? Because, after-all, it is only ONE lake. Right?

    If only 1/8th acre of property in N. America is overtaken by the Brazilian Pepper-Plant we shouldn’t care at all. Because, after-all, the value of that 1/8th acre of land isn’t worth very much. Right?


  7. “End of argument” ?? There has been no argument yet. Jeff hasn’t said anything of a factual nature, only that he wants all cats destroyed because they are not native. “There is no other solution.” Solution to what? Jeff presents no evidence that there is a problem, except that he doesn’t like cats.

    Jeff provides readers with a concise summary of the nativist position. Nonnative species must be destroyed simply because nativists don’t like them. Arguments and evidence are beside the point; arguments and evidence have no impact on such deeply held prejudices.

  8. I am an avid bird watcher and I’ve spent thousands of dollars on equipment and travel. The idea that bird watchers spend $.40 cents per bird observed is an utterly meaningless statistic. We don’t spend money on a per bird basis any more than movie goers pay by the minute or baseball fans pay for the number of pitches thrown in a game.

    The $30 estimate of every individual bird’s economic value is actually from a 1992 study on pesticide use which cites out of context survey data from 1985, which the author clearly made no effort to understand. Birds have tremendous ecological value and they have significant economic value, not as individuals, but as populations of species. For instance, Colima Warblers attract many travelers each year to Big Bend National Park in Texas, the only reliable spot to see them in North America. However, some of these warblers die every year and there is no economic impact unless their numbers decline to a point that bird watchers no longer believe that making the trip to Big Bend is worth the expense.

    Regardless of how one feels about feral cats, we ought to have more respect for the truth than what organizations like American Bird Conservancy and The Wildlife Society have shown in regurgitating the absurd, unscientific figure. Feral cats are certainly not native and they are certainly not good for wildlife. How best to remove them is the challenge. The American Bird Conservancy for many years was instructing communities to address the problem by building sanctuaries and used Chico, CA as their prototype. Of course, the sanctuary filled up and Chico quickly had a bad feral cat problem again. Trap-euthanize programs face the same limitations as TNR but they also don’t attract the same level of volunteers and donors. In settings where professional staff can sustain a trap-euthanize program over many years, such a program may out-perform TNR, but that is not the case in many urban or residential communities.

    As for the gentleman suggesting that cat advocates are controlled by parasites, I suggest getting checked for rabies as you seem to be frothing at the mouth as you write your posts.
    Webmaster: Thank you for your visit and for your informative comment. We were shocked by some of the rabid comments we received on this article. In fact, we were unable to publish some of them, as they were beyond the pale. I don’t have a cat, but I am still unable to understand the intense hatred they seem to generate in some people. As you have, we have birded all over the world and are very fond of birds, but as you are unable, we are unable to generate hatred on their behalf. Our interest in this subject was in the deeply flawed “science” used to justify the crusade against non-natives, which I understand is also your point. Our intention was to critique bad science rather than to defend cats.

  9. Excellent site! I thought I was the only one who was more than a little skeptical about all these specious “non-native eradication programs”. The real question is: why are such programs even being floated? Money. Always follow the money trail. I’d pay good money myself to see some of the financial linkages between the parks directors, pesticide companies, industry lobbyists, and gov’t officials.

  10. read the study, it cites 14 billion as the cost to bird. never saw 17 billion, but i’d guess that was for a total.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: