More evidence that eucalypts are not invasive

Eucalyptus, Mountain View Cemetery, Oakland

We have provided our readers with photographic evidence that eucalypts are not invasive in the San Francisco Bay Area (click here and here).  Now we are going to tell you about more confirmation of this fact from a reputable source that will be difficult for native plant advocates to ignore:  Encyclopedia of Biological Invasions.

The Encyclopedia was edited by Daniel Simberloff (U of Tennessee) and Marcel Rejmanek (UC Davis) and published by UC Berkeley Press in 2011.  Many of our readers will recognize Simberloff as a prominent scientist in invasion biology.  He is responsible for the “invasional meltdown” hypothesis which is central to invasion biology.  A recent survey of empirical tests of the hypotheses of invasion biology found that there is considerable support for the “invasional meltdown” hypothesis, but that support is declining. 

Professor Simberloff has aggressively defended the assumptions of invasion biology against scientists who think that a revision of those assumptions is required by recent empirical evidence.  When Professor Mark Davis and 18 of his colleagues in ecology signed a comment in the Nature journal entitled, “Don’t judge species on their origins,” Professor Simberloff promptly recruited 140 of his colleagues to publish a rebuttal. 

We establish Professor Simberloff’s credentials for our readers as a scientist who firmly believes that non-native species are a serious threat to biodiversity so that native plant advocates will consider him a credible source of information regarding eucalyptus.   

“Eucalypts” according to the Encyclopedia of Biological Invasions

According to the Encyclopedia of Biological Invasions, eucalypts are “some of the most important solid timber and paper pulp forestry trees in the world.”  There are about 40 million acres of eucalypts planted in tropical, sub-tropical, and temperate countries.  The predominant species of eucalyptus in the Bay Area, Blue Gum (E. globulus), is grown in 13 countries in addition to the US and Australia.  About 70 species of eucalypts are naturalized outside their native ranges. “However, given the extent of cultivation, eucalypts are markedly less invasive than many other widely cultivated trees and shrubs…they have been orders of magnitude less successful as invaders than pines and several other widely planted trees…Where eucalypts have invaded, they have very seldom spread considerable distances from planting sites, and their regeneration is frequently sporadic. “ (1)

Although the Encyclopedia admits to being puzzled by why eucalypts aren’t invasive, it offers “three major reasons for the limited invasiveness of eucalypts:”

Reason One:  Seed dispersal of eucalypts is limited

The seeds of eucalypts have no natural means of dispersal, such as fleshy tissue which can function as wings on the wind.  Tests have shown that the seeds “are dispersed over quite short distances.”  (1) “Seed dispersal is mainly by wind or gravity and is virtually limited to twice the tree height.” (2) 

The seeds of the Blue Gum are encapsulated in a woody pod which makes them inedible to birds and mammals.  So, the seeds of the Blue Gum are not dispersed by animals.

Reason Two:  High mortality of eucalyptus seedlings

Eucalyptus seedlings die quickly if they don’t establish roots in moist soil quickly.  If the soil is too moist they are susceptible to destruction by fungus.  If there is too much leaf litter or there is an understory, they are unlikely to find the quick access to the soil they need to survive.  There is a narrow range of conditions needed to successfully establish eucalyptus seedlings.

Reason Three:  Lack of compatible mycorrhizal fungi

Mycorrhizal fungi exist in the soil and sometimes form a symbiotic relationship with the roots of plants. They are often essential to the health of the plant because they facilitate the absorption of water and nutrients by the plant.  Some biologists speculate that the specific species of mycorrhizal fungi needed for successful seedling development have not been exported with the eucalypts to foreign soils. 

A balanced discussion of the pros and cons of eucalypts

Given the strong commitment of the authors of the Encyclopedia of Biological Invasions to invasion biology, we are impressed with its even-handed discussion of the ecological pros and cons of eucalypts as well as its recognition of the lack of hard data to support a particular conclusion:  “Conclusions about positive or negative environmental and economic impacts of eucalypts are often anecdotal, highly controversial and context dependent.”   

The authors suggest that eucalypts not be planted near streams as the moving water is a means of seed dispersal.  On the other hand, when planted on degraded soil, the eucalypts have provided a fuel source which reduces pressure on remnants of native forests.  Eucalypts have been a valuable source of nectar for honey production all over the world.  More birds are said to be found in native forests than eucalyptus forests in California.  However, three times as many salamanders are found in the eucalyptus forests compared to native forests in California. 

Eucalyptus and honeybee. Painting by Brian Stewart

The Encyclopedia also addresses the controversial question of whether or not eucalypts are allelopathic, which means chemicals in their roots or leaves suppress the germination of the seeds of other plants.  It reports that there is no conclusive evidence on this question.  However, the accumulation of leaf litter is probably a physical barrier to the germination of seeds in its understory, which is not an allelopathic method of suppressing competition.  This is clearly true of other trees as well.  For example, the tannins present in both oak and eucalyptus leaves prevent the rapid break down of the leaf litter which accumulates and creates a physical barrier to competing vegetation.  This is one of many examples of the characteristics that both native and non-native plants have in common.

The Encyclopedia attributes the flammability of the eucalyptus forest to leaf litter which is exacerbated in California by rare deep freezes.  These deep freezes cause die-back of eucalypts, contributing to fuel loads.   It makes no mention of the oiliness of leaves as a factor in flammability. 

There has not been such a deep freeze in the East Bay in over 20 years and 20-year intervals of such weather events have been historically typical.  These deep freezes do not occur on the San Francisco peninsula because its climate is moderated by the ocean and bay surrounding it.  Its climate is therefore warmer in winter and cooler in summer.  Therefore, this caveat about the flammability of eucalypts does not apply in San Francisco.

The myth lives on…..

Despite the fact that there is no evidence—scientific or experiential—that eucalypts are invasive, the myth lives on amongst the community of native plants advocates.  We will continue to provide the evidence that eucalypts are not invasive.  We hope that eventually the public will be sufficiently informed that they will become resistant to this claim of native plant advocates which is one of many myths used to justify the needless destruction of eucalypts.


(1)    Marcel Rejmanek and David Richardson, “Eucalypts,” in Encyclopedia of Biological Invasions, eds, Daniel Simberloff and Marcel Rejmanek, University of California Berkeley Press, 2011.

(2)    Craig Hardner, et. al., “The Relationship between Cross Success and Spatial Proximity of Eucalypts Globulus ssp. Globulus Parents,”  in Evolution, 212, 1998, 614-618.

Invasion Biology: The way forward

We’re following up on our previous post in which we reported that empirical studies do not support the hypotheses of invasion biology.  In that case, six hypotheses of invasion biology were tested by empirical studies and largely failed.  Furthermore, more recent studies are less supportive than older studies, indicating declining support for the assumptions of invasion biology.

Now we are going to tell you about a new publication by another team of scientists who challenged other assumptions about invasive plants and also conducted their own original research of one of the most basic assumptions of invasion biology:  that invasions are facilitated by disturbance.

Wildfire, Bitterroot National Park, 2000. Wildfires are a type of disturbance that has increased with global warming and drought.

We introduced our readers to the leader of this research team, Professor Angela Moles, in a recent post about the mounting evidence that attempts to eradicate non-native species are futile.  Professor Moles (University of New South Wales, Australia ) gave a TED (Technology, Entertainment, Design) presentation in which she reported that introduced species have changed significantly since their introduction and that if they weren’t yet new species, they soon would be.  She proposed that non-native plants in Australia be granted citizenship.

Professor Moles collaborated with 21 scientists all over the world (Uganda, Indonesia, Mexico, USA, Australia, New Zealand, Japan, Argentina, Estonia, New Zealand) in the study that resulted in a recently published article entitled, “Invasions:  The trail behind, the path ahead, and a test of a disturbing idea.”  *

The trail of invasion biology

As the title suggests, the article begins by reporting that after 30 years and 10,000 publications, invasion biology has tested many assumptions and found inconsistent evidence to support them:

  • The search for traits of introduced plants that predict invasiveness has been a dead end:  “…it is not currently possible and will probably never be possible to predict which species are likely to become problem invaders on the basis of traits alone.  We therefore suggest that this is one area of invasion biology that merits less attention in the future.”
  • Invasion biology predicted that lack of genetic variability would hinder evolutionary adaptation in introduced species.  This assumption has not been supported by empirical studies:  “…rapid evolution has been repeatedly demonstrated in introduced populations, and the predicted reduction in genetic variance has not been observed.” 
  • Rapid evolution of introduced species has been well established by empirical studies:  “We have reached the point where additional case studies demonstrating rapid evolutionary change in introduced species are unlikely to have a major impact on our understanding of invasions.”  New research questions are needed.
  • There is little evidence to support the assumption that introduced plant species will cause extinction In native communities:  “…there are astonishingly few documented cases of native plants being driven to extinction by competition from introduced plants.  There is no evidence for any native species in the United States being driven to extinction even within a state, by competition from an introduced plant species.”

The way forward in invasion biology

Professor Moles and her team then tell us why invasion biology has not been able to prove the assumptions on which the theory is based.  The theory of invasion biology was based on untested assumptions that have been accepted as true although there is no empirical evidence to support them.  The goal for the future of invasion biology should be to identify these assumptions that have been accepted as dogma, test them, and abandon those that are not consistent with empirical facts. 

The authors of this study also, “…join a growing chorus, suggesting that our approach to invasion biology has been too simplistic.”  Studies have tended to focus on the features of introduced plants in isolation.  A more fruitful line of inquiry will consider the complex interactions between newly introduced species and their new environment:

“Rather than focusing on one factor at a time, we need to find ways (including multivariate analysis) to synthesize information about the recipient habitats/ communities, the characteristics of both resident species and the invaders, demographic processes, propagule pressure [measure of the number of species released into a region in which they are not native], the differences between current conditions and those with which the resident species evolved, evolutionary change to both native and introduced species, plasticity and feedbacks and interactions between different species and processes.”

You might say, “Phew! That sounds like a daunting task.”  And so it is, but this team of scientists takes it on with an elaborate and complex study of one of the most basic assumptions of invasion biology:  that disturbance facilitates plant invasions.

Does disturbance facilitate plant invasions?

“Disturbance is thought to facilitate invasion by simultaneously opening new ground for colonization, decreasing the competition from resident native species and releasing pulses of resources.”  The definition of “disturbance” has varied in different studies, but generally includes fire, grazing, agriculture, erosion, wind, and flood.  Empirical tests of this theory have produced mixed results.  Even when the results have been positive, they have not persisted over the long-term.

Because disturbance is a natural feature of all ecosystems, native species have adaptive features that enable them to respond to natural disturbances.  Therefore, the research team theorized that it is not disturbance per se which creates opportunity for invasions by introduced species, but rather changes in the disturbance regime.  Their research study was therefore designed to distinguish between the level of disturbance and changes in the level of disturbance.

Given the international composition of their research team, they were able to select 200 sites in eight countries.  They selected only those sites for which the natural patterns of disturbance were known.  Their research methods were statistically complex and a detailed description of them is beyond our comprehension and probably many of our readers, but we encourage those with the necessary scientific knowledge to read the article which is available on the internet.

Their analysis of these 200 sites led them to the conclusion that the change in disturbance regimes was far more predictive of the success of invasions than the level of disturbance but that both variables explained only 7% of the variation in the percent of cover or species richness contributed by introduced species.

In other words, one of the most basic assumptions of invasion biology did not pass an empirical test of its validity.  Invasions by introduced plants are largely unexplained by disturbance.

Post Katrina New Orleans. Floods are another type of disturbance that is likely to increase with climate change.

The future of invasion biology

Science is rapidly revising the assumptions of invasion biology.  We strongly believe that it is just a matter of time before science informs us that introduced species are here to stay and that this is not the terrible news we have been led to believe.  It is inevitable that this information will filter slowly from the scientific community to the community of native plant advocates.  We hope that they hear and accept this good news before our non-native trees are destroyed.


*Moles, Angela, et. al., “Invasions:  The trail behind, the path ahead, and a test of a disturbing idea,” Journal of Ecology, British Ecological Society, 2012, 100, 116-`127.  All quotes are from this article

Integrating new species into the food web

Zebra mussels, open underwater with siphons out. Creative Commons

We have been reading panic-stricken news reports about zebra mussels for over 10 years, but we weren’t paying much attention until a recent news report that they have arrived in California.  We decided it was time to educate ourselves about this “invasive species.”

Zebra mussels and their close relative, the quagga mussel, arrived in the Great Lakes Region of the United States in 1988, probably in the ballast water of big ships.  Although they are native to southern Russia and Ukraine, they are now found throughout Europe and England.

The negative side of the ledger

What the mussels lack in size, they make up for in numbers.  Though they are tiny—about the size of a dime–they are prolific breeders capable of creating big colonies rapidly.  They are a fresh-water mussel which means they exist where there are often water treatment facilities that supply our drinking water.  Their larvae are microscopic so they can enter water treatment facilities through the intake pipes and clog the system. 

They filter huge quantities of water, consuming plankton (microscopic plants and organisms) depriving other animals of nutrition.  This filtering of the water also increases water clarity and light penetration, changing the entire ecosystem in complex and unpredictable ways.

The positive side of the ledger

Where the mussels have gained a foothold, they have quickly entered the food web.  A monitoring program was started soon after mussels were found at Long Point Bay in Lake Erie.  The first sampling done in 1991 found mussels in 27% of the sampling stations, an estimated 1,189 tons of mussels.  By 1992, mussels were found in 80% of the sites, an estimated 4,536 tons of mussels.  (1)

In 1992, the monitoring program also started conducting stomach analysis of ducks killed at Long Point Bay.  Three species of duck (Greater and Lesser Scaup and Bufflehead) were found to be feeding heavily on the mussels.  Between 1993 and 1995 the population of mussels declined significantly from the highpoint of 4,536 tons to only 758 tons in 1995.  The population of the duck predators increased correspondingly during the same period of time. (1)

The authors of this study speculate that the mussels were also depleting their food source at the peak of their population and that they had exhausted available attachment sites, but the scientists believe duck predation was the primary reason for the declining population of mussels.  As always, there are many variables operating simultaneously in the ecosystem, and it isn’t possible to isolate one from the others.  (2)

Ducks aren’t the only predators of the mussels.  Crayfish are apparently capable of consuming large quantities of the mussels.  And some fish eat the mussels.  One study found that yellow perch didn’t eat the mussels in 1994, but a later study in 2004 reported that the perch were eating the mussels.  Plankton waste from the mussels settles on the lake bottom and the bottom feeders benefit from that fall out.

There is a downside to this story, however.  Remember that the mussels filter the water as they eat.  In addition to filtering plankton, they also filter pollutants and contaminants.  Researchers assume that the predators of the mussels are consuming those pollutants which then become a part of the food chain.  The mussel-consuming ducks at Long Point Bay apparently had elevated levels of contaminants in their tissue compared to ducks that consume only aquatic plants. (2)

What should we do?

According to the news story about the mussels in a local paper, the California legislature is considering increasing the registration fee for boats which would raise about $5 to $8 million dollars.  Although the news story isn’t clear about how this money would be used, let’s assume for the sake of argument that it would be used to prevent the spread of these mussels beyond the 25 lakes in California where they are now found.  That would apparently involve more inspection of boats being put into the water where the mussels don’t presently exist.  If that’s the plan, we enthusiastically endorse it.  Prevention is the best medicine, as they say.

But once the mussels have arrived, all scientists agree that eradicating them is not a realistic option.  Therefore, dousing them with chemicals—which is one of the recommended treatments—will undoubtedly do more harm than good. 

New species quickly become a part of the landscape.  Our initial reaction to them tends to be negative because we are suspicious of change.  In fact, there may be benefits that aren’t immediately evident and even if there isn’t an immediate benefit, they are often integrated into the environment over time.  Their populations often stabilize once they have exhausted available resources.  We should be patient because nature is resilient and our time frame is much shorter than nature’s time frame.    

Are we learning this lesson?

Broom, Redwood Park, Oakland, California

The California Invasive Plant Council (Cal-IPC) is dedicated to the eradication of non-native plants.  Scotch broom is one of their favorite targets for eradication.  Little progress has been made in this effort (see “Broom:  ‘I’ll be back’” and “Broom:  ‘I’m ba-ack’”) and recently Cal-IPC acknowledged this in their newsletter.  However, they urged their supporters not to lose heart because they reported that broom is now being browsed by herbivores.  So, what native plant advocates could not accomplish with manual labor and chemical warfare, the animals may accomplish by incorporating broom into their diets.  One hopes the animals aren’t eating broom doused with herbicides.

Cal-IPC also acknowledges in this article that broom does not grow in shade:  “Broom cannot tolerate heavy shade.  It usually established following logging or other activities that remove tree canopy.”  Could it be that they have finally noticed that the result of clear-cutting non-native trees in the East Bay hills is more broom, not more native plants?  We can only hope so.

There are pros and cons to every decision we make.  We don’t always know in advance what they are.  So, it pays to be cautious.  If we are patient, maybe nature will sort it out without our interference.  Particularly when our interference damages nature, we should exercise restraint.  We should give nature more credit for healing itself.  It has a much better track record than we do.


(1)    Cox, George W., Alien Species and Evolution, Island Press, 2004

(2)    Petrie, Scott A., Knapton, Richard H., “Rapid Increase and Subsequent Decline of Zebra and Quagga Mussels in Long Point Bay, Lake Erie:  Possible Influence of Waterfowl Predation,” J. Great Lakes Research, 25(4) 772-782

The Sparrow Wars: America’s first “invasive species”

The public’s mania about “invasive species” often seems new to us.  It’s not.  In Peter Coates’ provocative book, American Perceptions of Immigrant and Invasive Species, we learn about one of the first episodes of public concern about an introduced species in American history, known as the “sparrow wars.”

English sparrow. US Fish & Wildlife photo

Like many introductions of non-native species of plants and animals, the English sparrow (AKA house sparrow) was introduced to perform a practical function.  Elm trees on the East Coast were being defoliated by a voracious native caterpillar.  In 1852, The English sparrow was brought to America to rescue the trees from the caterpillars.  The sparrows thrived and were soon reviled by ornithologists who considered them alien invaders.

The debate between ornithologists and those with a more cosmopolitan view of nature is reported at length by Coates.  Long story short, the debate is reminiscent of what we hear today from nativists:

  • They feared that the English sparrow would compete with native species for food and habitat and that native species would lose this competition.
  • They considered native birds superior to the English sparrow which was considered dirty and a promiscuous breeder.
  • The English sparrows were city dwellers and were considered the bird equivalent of ghettoized immigrants.
  • The English sparrows were criticized for not eating enough of the caterpillars they were imported to eat.  They weren’t doing the job they were hired to do!

This debate raged on amongst birders for decades according to the historical record reported by Coates.  However, we no longer hear birders complain about the English sparrow, although we hear them complain about many other birds.

Update:  This post requires an update.  The New York Times published an op-ed in which a woman describes in horrific detail the monomaniacal attempts of her mother to exterminate all house sparrows in their neighborhood based on her belief that their eradication would benefit blue birds.  It is a blood-curdling story that contradicts my naïve belief that after nearly 200 years, the house sparrow has been accepted in America. 

Modern equivalents of the “sparrow wars”

Cherry-headed conure. Attribution: Share Alike

Birders in San Francisco are currently complaining about the cherry-headed conures, more commonly known as the parrots of Telegraph Hill.  They believe the parrots are depriving native birds of food and nesting places.  They object to their presence in a place where they “don’t belong.”

We were introduced to this mindset by an ominous encounter with a birder in Florida who is typical of the nativist viewpoint of the avian world.  The sound of gunfire drew us to a man with a shot gun on the lawn of our motel.  Starlings were falling around him, where he quickly finished them off with a vigorous stomp of his booted foot.  We were unfamiliar with the hatred of non-native species at that time and asked him why he was killing the birds.  He seemed stunned to be questioned.  He explained, as though speaking to retarded children, that the starlings were “trash birds” that must be killed.  Following a basic rule of survival, we walked away from a person wielding a gun.

Why was the English sparrow redeemed?

Returning to the English sparrow, why are they no longer the target of hostility from  birders?  We speculate that one reason may be that they have been here for a long time, nearly 200 years.  Just as human immigrants are often the target of prejudice and discrimination when they first arrive, they eventually become a routine part of our world.  We rarely think of the Irish or other Europeans as immigrants in America.

Another reason is that the population of English sparrows is actually declining:  “Since 1966 its North American population has declined by 2.5 percent annually.” (1) However, there is still an estimated population of 150 million in North America.

Ironically, the population of English sparrows is declining significantly in Britain, its ancestral home, where only 13 million are estimated to remain.  In 2000 the British press was full of stories about the sudden decline of their iconic bird, “Responding to the strong sense that an essential part of the nation’s natural heritage…was disappearing…”

The lessons of the sparrow wars

These are familiar themes to the readers of the Million Trees blog:

  • Some people fear newcomers to their world, whether those newcomers are people, animals or plants and that fear can result in destructive hatred.
  • Newcomers usually fit in eventually.  What is initially perceived as a threatening “invasion” rarely turns out to be a problem in the long run.
  • Because nature is dynamic, the new home of an introduced species sometimes becomes the only home of that species.  The movement of species is another way to ensure their survival.  In fact, there is a new movement amongst citizen “scientists” to move rare species which are threatened by changed climate conditions into new locations.  This is called “assisted migration.” (2)


(1) Peter Coates, American Perceptions of Immigrants and Invasive Species, UC Press, 2007.  All quotes are from this book.

(2) Emma Marris, Rambunctious Garden, Bloomsbury, 2011.

Rush Limbaugh on “invasive species”

We have noted in previous posts the common ancestry of native plant and animal advocacy and anti-immigration sentiment.  The relationship between these sentiments goes all the way back to 1930s Germany when there was a concerted effort to rid Germany of non-native plants, as well as people perceived as alien.  We have also reported that some native plant advocates—though not all–in the San Francisco Bay Area are also strongly opposed to immigration. 

Rush Limbaugh. Creative Commons Attribution

Today we will provide another example of the connection between these two apparently related opinions.  Rush Limbaugh, the right-wing talk show host, has a track record of calling immigrants an “invasive species.”  The following Limbaugh quotes are provided by Media Matters for America, a web-based non-profit dedicated to “comprehensive monitoring, analyzing, and correcting conservative misinformation in the U.S. media.”

On April 1, 2005, Limbaugh described undocumented immigrants as an “invasive species,” saying:

LIMBAUGH: So invasive species like mollusks and spermatozoa are not good, and we’ve got a federal judge say, “You can’t bring it in here,” but invasive species in the form of illegal immigration is fine and dandy — bring ’em on, as many as possible, legalize them wherever we can, wherever they go, no matter what they clog up. So we’re going to break the bank; we’re going to bend over backwards. The federal judiciary is going to do everything it can to stop spermatozoa and mollusks from coming in, but other invasive species? We’re supposed to bend over and grab the ankles and say, “Deal with it.”

On August 15, 2011, Limbaugh said:

“[S]ome people would say we’re already under attack by aliens — not space aliens, but illegal aliens.”

Rush Limbaugh has been much in the news recently for his verbal attacks on a 30-year old female law student at Georgetown University who would like to have access to birth control.  In Limbaugh’s opinion she is a “slut,” and a “prostitute” who has “so much sex that she can barely walk.”  Women are not Limbaugh’s only target for abuse.  He routinely says equally nasty things about gay people, poor people, ethnic and racial minorities, and labor unions.

Native plant advocates who are also opposed to immigration might give some thought to the implications of their ideology.  Do they want to be associated with the likes of Rush Limbaugh?  If not, how do they explain the difference between the crusade against non-native plants and animals and the crusade against immigrants?

Some “alien invasions” are a bust!

The Argentine ant is one of a gazillion non-native species that have been labeled “invasive species.”  Like most non-native species, they are considered aggressive competitors of native ants.  The usual tools are employed to eradicate them, e.g., pesticides. 

Argentine ant. UC Davis

But wait!  Now scientists are suddenly noticing that the Argentine ant is disappearing from some of their colonial haunts.  Scientists in New Zealand have recently reported the disappearance of the Argentine ant from 40% of sites they populated in the past and their populations have shrunk significantly where they are still found.  Native ants have “reinvaded” the areas vacated by the Argentine ant.  The scientists reporting this finding “concluded the species naturally collapses after 10 to 20 years.”

The scientists in New Zealand don’t claim to know why the populations of Argentine ant have disappeared.  They speculate that a virus is to blame.  They don’t claim “pest control” deserves credit for the disappearance which is estimated to have cost $53 million (NZ$68) per year since the Argentine ant was originally found there in 1990. That’s right, New Zealand spent approximately $53 million per year trying to eradicate the Argentine ant, which apparently is disappearing on its own.

This is apparently not an isolated phenomenon.  An entomologist at UC Davis reports that the Argentine ant has been declining in California as well.  How much pesticide was poured on Argentine ants before they showed the good grace to just disappear?

The Africanized Bee:  Another scary story that didn’t materialize

Another example of an “invasion” that seems to have resolved itself is the Africanized bee.  Do you remember about 15 years ago when the media created a panic about the Africanized bee?  We were told that it was spreading rapidly from Latin America, headed our way and that it was so dangerous that it was capable of swarming people and animals and stinging them to death! 

Africanized honeybee USDA

What happened to that particular “alien invasion” story?  Professor Gordon Frankie (UC Berkeley), our local expert about bees, was asked that question in a lecture he was giving to Cal Alumni in October 2011.  He said that the Africanized bee didn’t turn out to be as aggressive as it was originally thought to be and that it didn’t spread as far or as fast as predicted.

Does “Invasion Biology” make more problems than it solves?

Some months ago we created a Google alert for “invasive species.”  Now we are treated to a daily barrage of scary “alien invasion” stories from all over the world.  We wonder how many of these “invasions” will eventually prove to be benign.  We wonder how much money will be spent, how many animals will be killed, how much pesticide will be poured on our public lands, before we figure out that we need not be afraid of everything that is new in the environment.  We wonder how many people are making their living on these eradication efforts and what role they play in frightening the public into funding their projects and tolerating the destruction they inflict on the environment.

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.