A Cornerstone of Invasion Biology Crumbles

Over 20 years ago a study was published about the economic costs of introduced species. The study by David Pimentel et.al. (1) claimed that the economic costs of introduced species in the United States are $137 billion per year.   Despite many critiques of that study by academic scientists, the study remains a cornerstone of invasion biology and the “restoration” industry it spawned. 

The study has been cited by other academic scientists over 4,500 times and an update of the study published in 2005 has been cited over 5,800 times.  In addition to being influential with academic scientists, most media articles about “invasive species” begin with reference to that study and comments from native plant advocates on Conservation Sense and Nonsense often begin by quoting that study.  In other words, the bloated estimate of the economic costs of introduced species in the US is a powerful tool that continues to fuel attempts to eradicate non-native plants and animals in the US.  Therefore, a new study by Demetrio Boltovskoy et. al. (2) that critiques this estimate is of interest to us and we report that study to you today. 

The abstract of the new study (2) outlines the critique of Pimentel’s study by an international (Argentina, Canada, Switzerland, and US) team of scientists:

“The economic costs of non-indigenous species (NIS) are a key factor for the allocation of efforts and resources to eradicate or control baneful invasions. Their assessments are challenging, but most suffer from major flaws. Among the most important are the following:

  • the inclusion of actual damage costs together with various ancillary expenditures which may or may not be indicative of the real economic damage due to NIS;
  • the inclusion of the costs of unnecessary or counterproductive control initiatives;
  • the inclusion of controversial NIS-related costs whose economic impacts are questionable;
  • the assessment of the negative impacts only, ignoring the positive ones that most NIS have on the economy, either directly or through their ecosystem services. Such estimates necessarily arrive at negative and often highly inflated values, do not reflect the net damage and economic losses due to NIS, and can significantly misguide management and resource allocation decisions.”

The Pimental study misrepresents the economic impact of introduced plant and animal species in the US.  The most significant flaw in the evaluation of costs is that it does not take into consideration the benefits of introduced species.  Pimentel’s formula for evaluating economic impacts of introduced species is simplistic:

Losses & Damages + Control Costs = Total Costs

Source:  David Pimentel, et.al., “Environmental and Economic Costs of Nonindigenous Species in the United States” (1)

We need look no further than Pimentel’s study to see how the absence of economic benefits of introduced species in that equation distorts the evaluation of the economic impact of introduced species.  Pimentel has included the pests of agricultural crops and livestock in his calculation of total economic costs of $137 billion per year.  He calculates the cost of agricultural weeds, insect pests and pathogens as well as livestock diseases as $77.3 billion per year, which is 57% of his estimate of total costs of introduced species. 

In the same study, Pimentel says that 98% of the “US food system” are introduced species (corn, wheat, rice, cattle, and poultry) and he reported that the value of those products was $800 billion per year at the time of his study in 2000.  In other words, if the benefits of agricultural products had been included in Pimentel’s formula, the net benefit to the American economy of introduced species would be $663.4 billion per year ($800 billion of benefits – $136.6 billion of costs = $663.4 net benefit). (3) Similar calculations for most items on Pimentel’s hit list of introduced species would be required to accurately assess the economic costs of introduced species:

Purple loosestrife. GNU Free
  • The Boltovskoy study considers purple loosestrife an “innocuous species.” Studies have shown that purple loosestrife thrives where nutrient pollution feeds it and its presence reduces nutrient pollution, which is a benefit to the ecosystem in which it thrives.  Poisoning loosestrife to control it increases pollutants in the ecosystem.  Controlling the sources of nutrient pollution, such as leaky septic tanks and agricultural runoff, is the only long-term method to control purple loosestrife.
  • Millions of starlings are killed in the US every year because they eat crops, but they also eat insect predators of crops.  When the economic benefits of insect control by starlings are subtracted from the costs of crop predation, European countries choose not to kill starlings.
  • Zebra and Quagga mussels are on Pimentel’s list of troublemakers because they clog the water intake pipes of industrial, water, and power plants.  But there are substantial economic benefits of these mussels“these invasive bivalves significantly clarify the water of lentic waterbodies, which can mitigate phytoplankton blooms, including toxic Cyanobacteria…lessening the costs of [purifying drinking] water, and enhancing recreational activities…” (2) They are also a major source of food for waterfowl and have contributed to significant increases in waterfowl populations.  There are mechanical methods of preventing mussels from clogging water intake pipes.
  • Cats are often the target of eradication efforts, and they also appear on Pimentel’s list.  A fair assessment of the economic costs of cats should include their benefit as predators of rats and rabbits.  Cats are a non-toxic method of rodent control.  In their absence, rodenticides are used to kill rodents and rodenticides are known killers of birds.  Do rodenticides kill as many birds as cats?  Maybe. 
  • Many introduced plants are providing valuable food and habitat for animals, including native animals.  Eucalyptus that provide nectar during winter months, when little else is blooming, is essential to hummingbirds, bees, and other animals.  Eucalyptus are the also the winter homes of migrating monarch butterflies in California.  Yet, they are being destroyed by many public land managers because they are introduced.  Likewise, many berry-producing plants that are important food sources for birds and other animals are being eradicated by native plant advocates.
Monarchs roosting in eucalyptus, Pismo Beach, November 2021

Although the costs of control methods are included in Pimentel’s calculation of the economic costs of introduced species, the collateral damage of control methods are not.  Here are a few examples of the collateral costs associated with methods used to control introduced species.

Lanphere Dunes and Mad River Slough

In summary, a more accurate cost/benefit analysis of introduced species would look something like this:

(Losses & Damages + Control Costs) – Benefits – Damage of Control Methods = Total Cost or Benefit

In the absence of such an accurate assessment, scarce public resources will continue to be wasted on eradication projects that do more harm than good.  “Admittedly, [such an accurate assessment] requires much more knowledge of the effectives of nonindigenous species, yet it does not justify using [Pimentel’s] numbers for weighting the risks and harms involved, let alone using them for engaging in potentially feckless and wasteful eradication and control initiatives.” (2)

  1. David Pimentel, et.al., “Environmental and Economic Costs of Nonindigenous Species in the United States,” BioScience, January 2000
  2. Demetrio Boltovskoy, et.al., “Misleading estimates of economic impacts of biological invasions:  Including the costs but not the benefits,” Ambio, 2022
  3. It seems likely that Pimentel’s estimate of the value of agricultural products is the net value after costs of controlling agricultural pests are subtracted from gross value.  In other words, a more accurate calculation of the economic benefit of agricultural products in Pimentel’s formula is probably $800 billion + $77.3 billion (pest control costs). 

11 thoughts on “A Cornerstone of Invasion Biology Crumbles”

  1. Hopefully this will drill some sense into those who are so enthusiastic a bout getting rid of
    non natives, and so called invasives, and have no qualms about using pesticides.

  2. “Do rodenticides kill as many birds as cats? Maybe. ”
    Nope. Not even close. Rodenticides are certainly harmful, not for the sheer numbers of birds killed but because they tend to kill raptors, owls, and scavengers, including threatened species like Northern Spotted Owls. There aren’t very many of those left, so the losses from rodenticide poisoning are significant even though the numbers aren’t large.

    Cats, by contrast, kill very large numbers of songbirds. Estimates range into the hundreds of millions per year, just in North America. Mostly they kill the abundant species, but sometimes they kill rare birds as well, and local effects can be significant. The sheer numbers of cats, and the very high population densities in many areas, make them vastly more deadly to birds.

    Cats do kill a lot of rodents of course, but they aren’t really as effective at reducing the populations or mitigating the problems caused by rodents as people often think. We see them hunting and catching mice every day and think they are doing a good job, but the mice are so fecund that the population is sustained in spite of predation. Rodent control requires more work than just letting the cat out.

    1. The claim that cats kill 3.7 billion birds annually is grossly exaggerated, as is Pimentel’s estimate of the economic impacts of non-native plants and animals. A meta analysis of 8 studies of nest predators of song birds in North America (“Factors affecting nest predation on forest songbirds in North America.” Frank R. Thompson III, 07 May 2007, International Journal of Avian Science) used video cameras to identify the predators of 242 depredation events, that is, nests in which eggs were destroyed or nestlings killed. These studies were conducted all over North America in different vegetation types, such as forests, shrublands, and grasslands. There studies report that the predators of the nests were:
      • 88 mammals
      • 86 snakes
      • 52 birds
      • 16 insects
      Only one of the 88 mammals was a domestic cat. The detailed list of all 242 predators is fascinating reading, which we recommend to you.

      We also turn to A. Starker Leopold’s book about California quail (The California Quail, 1977) 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.”

      Attempts to eradicate feral cats sometimes have devastating consequences. On Macquarie Island, the rabbit population exploded when feral cats were eradicated and vegetation was decimated. On the Farallon Islands the mouse population exploded after feral cats were eradicated. Choose your scapegoats carefully.

      Sorry for the delay in posting your comment. I was traveling. I don’t post comments without addressing the claims they make.

      1. Thanks for the reference to the 2008 study, I will check that out. (The 1951 data are pretty much irrelevant.) While I agree that the claims about actual numbers of birds killed by cats are sometimes overstated, that doesn’t change my argument. In your article you implied that the numbers of birds killed by rodenticides might be comparable to those killed by feral cats, and that just isn’t even remotely possible. (I would venture to speculate that rodenticides likely kill more cats than birds.) It weakens your otherwise strong argument when you toss in such unsupportable remarks. Let’s be careful not to stretch the arguments too far – in any direction.

        1. p.s. As for the effects on islands: Invasion biology works differently in island (and island-type) ecosystems, which are much less resilient than terrestrial systems. Nativists often cite the effects of invasions on islands to support their arguments about the necessity of eradicating invasive species everywhere. This has been refuted by people like Matt Chew and Mark Davis but continues to resonate with a public that does not understand the distinctions. Let’s not do that here.

  3. Pimentel, et al 2000 is one of a cluster of early invasion biology manifestoes that were at best naïve overreaches, and at worst, “bullshit” in the sense described by philosopher Harry G. Frankfurt in his eponymous little 2005 book (“On Bullshit,” Princeton University Press, ISBN 9780691122946). In Frankfurt’s estimation, the important difference between bullshitting and lying is that lying entails a direct reaction to knowing a truth, whereas bullshitting need have no relationship to either truth or knowledge. Bullshitting is merely convenient. Pimentel’s assertions were adopted and promoted as gospel by people who neither fully read nor critically engaged with them. They were suitably framed to enhance the decor of eco-nativism, much as occurred with Wilcove, et al, 1998 (“Quantifying Threats…” see my analysis via https://www.environmentandsociety.org/mml/ecologists-environmentalists-experts-and-invasion-second-greatest-threat).

    There was a great deal of eye-rolling when Pimentel, et al, first appeared. Perhaps the main reason nobody attempted a comprehensive refutation is that (as usual) truth and bullshit are mismatched opponents. Producing a true account requires orders of magnitude more earnest effort than bullshitting. In 2000, few serious, critically-minded ecologists took Pimentel, et al, seriously enough to worry that it could become a “cornerstone” of anything.

    Maybe we’ll never be adept at recognizing potential bullshit bombs when they’re first lobbed into the conservation literature. There are so many! The power of their visceral appeal is a lesson we seem doomed to never quite appreciate, except in hindsight. “Life can only be understood backwards; but it must be lived forwards” (or words to that effect). — Soren Kierkegaard

  4. I love this post, like so much here, but hesitate to share because of the cat section. Naming the numbers doesn’t take into account the reality for some of us of having seen wildlife decimated by cats. We almost no small animals in our yard because of cats, which we can’t keep out. They have killed most of the birds, all the amphibians and reptiles, small mammals… The birds can return but the others haven’t, over decades. Just our next door neighbor getting a cat adds to the multitudes. He doesn’t want to clean up after her, so he lets her go into our yard. (There are SO many cats in the Bay Area.) He would never accept a dog coming into his yard to kill his cat, but cats are allowed everywhere. He basically just told me today, he won’t keep her in, even though she is in danger in this neighborhood, from humans, as well as dogs, raptors, etc.) He also doesn’t consider the effect on us with nightly hearing a man in our yard calling out, in this incredibly high crime neighborhood.

    Toxoplasmosis from the cat feces is also a worry, as is never knowing if we will step in feces when we want to go into our yard. In San Francisco, a friend live-trapped 27 cats in one month in her garden.

    Cats are also being deliberately fed as “colonies” right next to bird refuges in Richmond. I’ll never forget the spring when there were so many adorable golden baby geese, and how quickly they were all killed by cats, so a week later, the geese parents were fighting over the remaining baby, who also didn’t last long.

  5. Imagine that you wanted to rank your blog entries by benefit/usefulness. An easy way to do so would be to give everybody the opportunity to vote for their favorite ones. Alternatively we could rank your entries via donations. The more money that I donated to a conservation organization (of your choice) the more influence that I’d have on the rankings.

    How differently would these two systems rank your entries? This is the most important economic question, given that economics is a matter of prioritizing how resources are used.

    Pimentel’s study was ranked by citations, which are essentially votes. Scholars use this system because it ranks things better than donations would? Show me the study.

    Striking the root of the problem depends on understanding that no two systems of prioritizing can be equally effective at ranking things by usefulness.

    Honey bees are invasive here in the Americas. Contrary to popular belief, hives aren’t democracies. Bees don’t rank flower patches simply by sitting on a couch and pressing a “Like” button. They do so by dancing. The more beneficial a bee perceives a patch to be, the more intensely she will dance, the more of her precious calories she will sacrifice. As a result, the hive will quickly and correctly reallocate its resources, which it wouldn’t do if it was a democracy.

    1. You say, “Show me the study.” A link is provided to the Pimentel study in the first sentence of the article on which you are commenting. Here it is again: https://academic.oup.com/bioscience/article/50/1/53/231855?login=false

      Beyond that, I don’t understand the point you are trying to make. I am not trying nor do I expect to win a popularity contest because I know that mine is a contrarian view of invasion biology.

      I assume you are a proponent of the nativist viewpoint because of your statement about honeybees. There is no evidence that honeybees are “invasive.” They are called “invasive” by those who assume all non-native plants and animals are “invasive.” Honeybees were introduced to America by colonists over 400 years ago. They are now responsible for pollinating about 1/3rd of America’s food supply. They are uniquely useful in that regard because unlike most bees, they are social rather than solitary bees that nest in the ground and hibernate during winter months. They can therefore be transported to the orchards where they are needed at specific times of the year. This article compares the effectiveness of honeybees to native bees as pollinators: https://wp.me/pT04m-zf There is no evidence that honeybees have a negative impact on native bees: https://wp.me/pT04m-1md

      1. The point that I’m trying to make is that popularity contests should be replaced with value contests.

        Popularity contest = voting, cheap talk
        Value contest = spending, sacrifice

        Let’s say you give your readers the opportunity to rank fruits. First you let them vote for their favorite fruits. Personally I’d vote for achacha.

        After the popularity contest was finished, then we’d have the value contest. This would involve sacrifice on the part of the participants. For example, I’d donate $40 (to the conservation organization of your choice) to improve the ranking of achacha.

        Once we finished the value contest we would compare the results of the two contests. One list would show us fruit ranked by popularity. The other list would show us fruit ranked by value. I can virtually guarantee that achachas would be ranked much higher on the value list.

        You’re not going to win any popularity contests? Well yeah, neither would achachas. That’s my point. Popularity contests are never going to elevate the most valuable things. The most valuable things can only be elevated by personal sacrifice.

        The cornerstone of invasion biology was not elevated by personal sacrifice. It was elevated by cheap talk (aka citations/votes). This makes it the scholarly equivalent of PewDiePie.

        Does my point make sense? Conservation will never be anywhere close to optimal when it’s based on a garbage system.

        1. Sorry, your approach doesn’t make sense to me. If that method were used to evaluate invasion biology, it would “win” such a competition hands down.

          According to a study published by the Boston Federal Reserve Bank, “All included, we estimate that the restoration economy generates approximately 221,000 jobs and $24.86 billion in economic output annually. https://www.bostonfed.org/publications/communities-and-banking/2016/summer/the-economic-impacts-of-the-us-ecological-restoration-sector.aspx

          There is also an army of non-profit organizations supported by private citizens who contribute to and volunteer in “restoration” projects that kill non-native plants and animals. To mention a few locally: California Native Plant Society, California Invasive Plant Council, Xerces Society, Sierra Club, Audubon Society, etc. With the exception of organizations that are opposed to the use of pesticides, there are no organizations that are opposed to destructive “restorations.”

          Your approach reminds me of the Citizen’s United decision of the US Supreme Court, which legalized unlimited political contributions of corporations. The case was won on the grounds that money is a form of free speech. Personally, I believe Citizen’s United was the beginning of the end of democracy in America. The “best” political candidate is rarely identified by how much he/she is able to raise. The amount of money the candidate is able to raise is what predicts which economic interests he/she is beholden to and the strength of that commitment.

          It is an approach that is based on the fact that everything in American society is evaluated based on money. Conservation does not lend itself to such an evaluation because it is partially a moral judgment. Frankly, I can’t see any benefit to monetizing conservation.

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