Doug Tallamy’s Nature’s Best Hope denies the value of hybridization

In Nature’s Best Hope, Doug Tallamy concedes that there is no evidence of extinctions of native plants being caused by the introduction of non-native plants in the Continental US.  However, he accuses non-native plants of something more nefarious:  “There is one biological phenomenon associated with some plant invasions that is so pernicious, even continental scales are not protecting natives from invasive species.  I speak of…introgressive hybridization, where the invasive species hybridizes with a closely related native, and then through repeated backcrosses and directional gene flow, the gene pool moves closer and closer to that of the invader.” 

Jake Sigg calls this phenomenon, genetic pollution.  Both Tallamy and Sigg consider such hybridization a loss of the native species and, indeed, it can be the end of localized variants of a species.  However, hybridization is often instrumental in the creation of a new species, one that is often superior to its ancestors because it is better adapted to present environmental conditions.

In a recently published study of the evolution of oaks, scientists traced the 56 million year evolutionary history of roughly 435 species of oak across 5 continents where they are found today.  Oaks are wind-pollinated, leaving pollen fossil records of their presence where they may no longer live.  Using DNA analysis of fossil pollen, scientists tell us when and where oaks have lived.  Their presence or absence was determined by changes in climate that created or eliminated land bridges between continents enabling movement of plants and animals, as well as providing the climate conditions in which oaks can survive.

Hybridization was instrumental in the formation of oak species and the ability of oaks to survive in different climate conditions.  The article in Scientific American about the genetic study of oak species concludes:  “A firm grasp of when, where and how oaks came to be so diverse is crucial to understanding how oaks will resist and adapt to rapidly changing environments. Oaks migrated rapidly as continental glaciers receded starting around 20,000 years ago, and hybridization between species appears to have been key to their rapid response. The insights we can gain from elucidating the adaptive benefits of gene flow are critical to predicting how resilient oaks may be as climate change exposes them to fungal and insect diseases with which they did not evolve.”  

In fact, a recent study suggests that assisted species migration and intentional hybridization are necessary to prevent the extinction of plants in Arctic regions, where the climate is warming the fastest.  Intentionally planting species from warmer regions into colder regions in anticipation of climate warming is called assisted migration.  It is not a new concept.  The study acknowledges that intentional hybridization is a radical suggestion that contradicts conventional wisdom:  “Traditionally, hybridization is viewed as negative and leading to a loss of biodiversity, even though hybridization has increased biodiversity over geological times.  This study acknowledges the role that hybridization plays in increasing biodiversity.

In the Bay Area, we are surrounded by examples of hybridization, some intentional and tolerated and some natural, but not tolerated:

Sycamore. Selectree.
  • Sycamores are the most common street tree in the United States and we have many here in the Bay Area. They are a hybrid of London Plane Trees and our native Sycamore.  The California native was intentionally bred with the London Plane Tree to increase its drought tolerance.   Sycamore street trees are one of the most popular because they are extremely hardy and tolerant of challenging conditions in urban settings.  They are also the host trees of one of our native butterflies, Western Tiger Swallowtail.  The Tiger Swallowtail probably used our native Sycamore in the past, but made a seamless transition to the hybrid.

Update:  I learned about the hybrid origins of our local Sycamore street tree in an urban forestry class at UC Berkeley.  Peter Del Tredici has sent me this correction: “The london plane tree, Platanus x acerifolia is generally considered to be a hybrid between the european species, P. orientalis and the eastern species, P. occidentalis. the west coast species, P. racemosa is not part of the mix.”   

Western tiger swallowtail. Wikimedia
  • Spartina alterniflora is a marsh grass that is native on the East Coast. It grows taller and denser than our native marsh grass, Spartina foliosa that also dies back in winter, unlike the East Coast native that does not.  In other words, non-native spartina is superior protection from winter storm surges compared to native spartina.  Yet, non-native spartina is being eradicated using herbicides along the entire West Coast of the country because it hybridizes with the native spartina species.  The herbicide used for that purpose has been sprayed for about 15 years, which is probably why attempts to plant native spartina as a replacement have been unsuccessful.  The result of the eradication project has been bare mud that provides no protection from erosion caused by rising sea levels and more intense winter storms.  In other words, if non-native spartina were permitted to hybridize with native spartina on the West Coast, the result would be a new species that is better adapted to face the threats of rising sea levels and intense storm surges.
Doug Tallamy’s closing photo of his keynote speech to the California Native Plant Society Conference, 2018

Fear of hybridization is akin to fear of mongrelization–the mixing of races–by racists and xenophobes.  It is closely related to the fear of non-native plant and animal species, a short-step away from the fear of human immigrants.  Concern about racial purity is not far from fear of “genetic pollution.”  State laws in the US prohibiting interracial marriage were not repealed until 1967, when the US Supreme Court ruled in Loving v. Virginia that such laws were unconstitutional in the 16 states in which these laws still remained.  These are cultural fears, not grounded in biological science. 

In conclusion

Doug Tallamy’s intended audience is home gardeners.  Although he urges his readers to remove invasive species, he does not endorse the use of herbicides.  Unfortunately, his work is used by public land managers to justify their eradication projects that usually use herbicides.  If Tallamy’s work stayed in its home gardening lane, it would do less damage to the environment.

Conference of the California Native Plant Society

In January 2015, the California Native Plant Society (CNPS) celebrated its 50th anniversary by holding a gigantic conference.  About 700-1,000 people attended.  There were several hundred short presentations and many posters describing research and “restoration” projects.  The abstracts of these presentations are available on the CNPS website.  We are publishing a brief description of a few of the presentations sent to us by one of our readers who attended the conference.  We publish with permission but without attribution, on request.  We have added a few edits in brackets and italics as well as a few links to relevant articles on Million Trees.


I was very impressed with the quality of the presentations at the CNPS conference.  Some were given by academic scientists or their graduate students. Many were given by land managers and managers of “restoration” projects.  There were about 225 presentations in 5 simultaneous sessions, so it was possible to hear only about 45 of them. There were also many short “lightning” presentations and nearly 50 posters.  Please consider this an impression of the conference, rather than a comprehensive report.

Michael Soulé was the opening speaker.  You might recognize his name as one of the proponents of invasion biology who is angry about growing acceptance of “novel ecosystems” and the ecological functions they perform.  [Million Trees has posted articles about this debate among academic scientists.  Soulé is one of the invasion biologists who demanded that the Nature Conservancy abandon their support for novel ecosystems.]   His objection to any acceptance of non-native plants was the main focus of his presentation.  He closed by saying that he “cannot live” without wild nature.  Since his definition of “nature” seems to exclude non-native plants, one wonders how he will manage to survive.   Perhaps he lives in an alternate universe populated solely by native plants.

Trees in Paradise, by Jared Farmer
Trees in Paradise, by Jared Farmer

Jared Farmer was the speaker at the conference dinner.  His subject was the history of eucalyptus in California.  His presentation was similar to his treatment of the subject in his book, Trees in Paradise.  [Million Trees has posted articles about Farmer’s book.]  Like his book, his presentation was even-handed in its treatment of eucalyptus.  That enraged the audience, which booed every time he said something positive about eucalyptus.  One wonders why he was invited to speak to this audience.  Were the organizers of the conference interested in promoting a more balanced view of eucalyptus?  Or did they just want a provocative speaker to wake up a sleepy audience after hours of a fund-raising auction?

Many of the presentations were surprisingly frank about the difficulties experienced by “restoration” projects.  CNPS deserves credit for inviting speakers who described some stunning failures of their effort to “restore” native landscapes.  I’ll describe just a few of the themes of speakers I heard.

San Francisco’s Public Utilities Commission

I was surprised to learn that San Francisco’s Public Utilities Commission (PUC) is heavily engaged in native plant “restorations.”  The PUC is responsible for managing thousands of acres of open space in the watershed that supplies San Francisco’s drinking water.  Common sense suggests that the PUC’s top priority would be the purity and safety of the water supply.  The PUC presentations at the conference suggest otherwise.  The PUC’s commitment to native plant “restorations” seems to trump the goal of clean water.

The PUC attempted to “restore” 100 acres of wetland and riparian habitat in San Mateo, Alameda, and Santa Clara counties by planting over 500,000 native plants, obtained from several different nurseries.  They claim to have followed a strict protocol which theoretically should have prevented the introduction of diseased plants.  Their protocol obviously failed.  The fact that many of the plants were infected with Phytophthora was not discovered until they were planted in the ground.  Phytophthora is the pathogen that is causing Sudden Oak Death.  The PUC is now faced with the difficult—if not impossible—task of trying to contain the spread of a fatal pathogen for which there is no known cure.

This project was funded by a “mitigation” grant for capital projects elsewhere in San Francisco.  Environmental laws require the builders of new development to “mitigate” for the impact they have on the environment by funding projects elsewhere, which are considered beneficial to the environment.  This often looks like legalized extortion to me.  It also increases the cost of infrastructure improvements, which limits the number of improvements we can make.  In this case, there clearly was no benefit to the environment.  It was both money down the drain and a poke in the environment’s eye.

As pointless as that project seemed, the other project presented by the PUC seemed even more pointless.  They presented a poster describing an experiment intended to determine the most effective application method and type of herbicide to eradicate coyote brush.  They used several different methods and types of herbicide, including Garlon (triclopyr) [which is known to be very toxic to aquatic life] and Milestone (aminopyralid) [which is banned in the State of New York because it is persistent and very mobile in the soil].

Detail of poster about PUC project, CNPS Conference
Detail of poster about PUC project, CNPS Conference

As you know, coyote brush is a native plant, so one wonders why it was necessary to eradicate it.  According to PUC’s poster, it’s another example of trying to prevent natural succession from grassland to scrub.  You might ask why the PUC is obligated to maintain grassland?  You might also ask how the PUC can justify using toxic herbicides in our watershed?  I can’t answer those questions.  It doesn’t make sense to me.

San Bruno Mountain

Mission Blue butterfly. Wikimedia Commons
Mission Blue butterfly. Wikimedia Commons

There was also a discouraging presentation by the folks who have been engaged in the effort to “restore” San Bruno Mountain in order to preserve and maintain a population of several species of rare butterflies, including the endangered Mission Blue butterfly.  This project officially began 32 years ago when the Habitat Conservation Plan was created by federal environmental protection laws.  The goal was to restore native grassland required by the rare butterflies.  The speaker said this goal remains largely unfulfilled.  As for the butterflies, their current status is largely unknown because monitoring efforts are not sufficient to determine the size of the population.

While non-native plants considered “invasive” are a part of the problem in achieving the goal of this project, the biggest problem is, in fact, a native plant.  Once again, natural succession from grassland to native scrub, dominated by coyote brush, is the main reason why grassland continues to shrink on San Bruno Mountain:

“Although the last mapping effort in 2004 reported 1296 acres of grassland, we believe that many of these areas are in imminent threat of scrub encroachment and could be converted to scrub after a good coyote brush recruitment year. Large patches of contiguous grassland with less than 2% scrub cover are quickly vanishing…Baccharis pilularis (coyote brush) accounts for the majority of the scrub encroachment observed on San Bruno. It seems to follow the well documented pattern of episodic establishment in wet seasons when roots can more quickly tap into needed soil water. Once seedlings have survived the first critical year, mortality drops quickly and full establishment plays out over the next 5-7 years (Williams et al. 1987). During this process of establishment, grassland resources decline and eventually disappear. Soil changes such as increased nitrogen and allelopathic compounds often follow scrub encroachment (Zavaleta and Kettley 2006, Weidenhamer and Callaway 2010) reducing the ability of grasslands to successfully re-establish without an intermediate disturbance such as a fire or intensive browsing (Hobbs and Mooney 1986).” (1)

San Bruno Mountain from Daly City. Wikimedia Commons
San Bruno Mountain from Daly City. Wikimedia Commons

It’s seems almost comic that when all is said and done, the main threat to native grassland “restoration” is apparently a native plant that is just doing what comes naturally…”invading” grassland in the absence of fire or grazing.

Hybridization:  Friend or foe?

Dieteria canescens variety canescens, native to Wyoming and other western states. Photo by Stephen Perry.
Dieteria canescens variety canescens, native to Wyoming. Photo by Stephen Perry.

I also attended the presentation of a native plant advocate from Mammoth Lake, on the eastern side of the Sierras.  She is engaged in a futile crusade to prevent the hybridization of a new plant, which she considers non-native, with a closely related native plant.  When this new plant arrived in her neighborhood, she recognized that it was different, but she was unable to identify it.  It wasn’t easy to find someone who could identify it.  Eventually, she found a botanist in Wyoming (where it is native) who was able to tell her that the new plant is a variety of a plant that is native at Mammoth Lake.  These plants are in the aster family.  The native is Dieteria canescens.  The new plant considered a non-native invader is Dieteria canescens var. canescens.  In other words, they are the same species!

From a horticultural standpoint, the new plant is superior to the native in every way: it is a bigger plant with more flowers; the flowers are bigger with more rays; the flowers are a deeper color.  So, why must it be eradicated?  Because native plant advocates fear that it will hybridize with the native aster and “swamp” it genetically, i.e., wipe it out.  Would that be such a terrible thing?  That is a matter of opinion.

Dieteria canescens, native to Mammoth Lake. Photo by Steve Mason
Dieteria canescens, native to Mammoth Lake. Photo by Steve Mason

One person in the audience asked why the new plant was not being accepted as an adaptation to climate change that would probably increase the likelihood of the survival of the species.  The speaker’s answer was that she could not accept the loss of the variety she considers native.  Another person in the audience asked this rhetorical question:  “What is our narrative here?  How can we expect the public to understand that it is necessary to eradicate a plant that is the same species?” The speaker agreed that it is not an easy sell.  I was encouraged by these questions.  They seem to be a glimmer of common sense.  I hope they are prophetic of the future of the native plant movement.

On that happy note, I close with an invitation to visit the CNPS website to read the abstracts of the hundreds of posters and presentations at this excellent conference.


  1. “Assessment of the past 30 years of habitat management and covered species monitoring efforts associated with the San Bruno Mountain Habitat Conservation Plan (Draft),” Creekside Science, October 21, 2014.

A happy ending for the “Africanized” bee

Africanized honeybee  USDA
Africanized honeybee USDA

Bees were imported from Africa to Latin America in the 1950s by Brazilian researchers.  They planned to breed them with European honeybees to improve honey production because the African bees were believed to be hardier than their European cousins.  When the bees escaped from the laboratory, researchers learned that the African bees were also more aggressive than European honeybees.

When African bees began to spread throughout Latin America, they became one of the first media-promoted panics about “invasive” species.  The media reported that the bees were capable of swarming and killing people and animals and they predicted that the bees would eventually spread throughout the United States.

Like most of the media-promoted panics about “invasive” species, predictions about Africanized bees were eventually discredited.  The “invasion” stopped in Texas because cold winters prevented their movement further north.  And the extreme aggressiveness of the bees also proved to be an exaggeration, partially because interbreeding with the European honeybee moderated the behavior of the African bees.

The benefits of new species

Scientific American reports that after 60 years of interbreeding, bee researchers say the original goal of an improved bee species for Latin America has been achieved.  Hybridized bees have benefited from some of the characteristics of their African cousins.

  • Africanized bees are more resistant to parasites because they groom themselves more often than European bees.
  • Africanized bees are more aggressive foragers and are capable of finding nectar and pollen sources where European bees would not.

This interbreeding was accomplished by the bees themselves“…it is not even accurate to call them Africanized bees anymore.  After decades of a massive and uncontrollable continent-wide wild breeding experiment, the African-Italian hybrid has morphed into a totally new bee unlike either parent species.” (1)

Now bee researchers are trying to breed new varieties of bees that are tailor made for specific conditions.  For example, where humans are stealing honey, a more aggressive bee with more of the characteristics of the African bees may be best suited.  In places where mites are a problem, bee keepers will want a “bee that obsessively cleans itself.”

Personally, we prefer the earlier scenario, in which the bees sorted it out amongst themselves.  We are deeply suspicious of the claims of humans that we are capable of producing better results than nature can accomplish on its own. More often than not, the results of human interference are unintended consequences, if not disastrous.

Does this sound familiar?

This story is a familiar refrain for the readers of Million Trees:

  • New species should not be assumed to be “bad” species.
  • Problems caused by new species are often resolved without our interference.
  • New species often make positive contributions to ecosystems.
  • Methods used to eradicate new species are often futile as well as more harmful than the mere existence of new species.
  • Hybridization should not be viewed as a problem.  Particularly at a time of a rapidly changing climate, hybridization often facilitates natural selection, resulting in a new species which is better adapted to current conditions than its predecessors.

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(1) Erik Vance, “Bee Researchers Make Friends with a Killer,” Scientific American, December 11, 2013

Hybridization is an adaptive strategy for species survival

Large ground finch. Linda Hall Library

We introduced Darwin’s finches to our readers in our previous post.  We told you about the research of Rosemary and Peter Grant on the Galápagos Islands that documented the rapid adaptation of the finches to radical changes in their food sources resulting from extreme weather events.  In this post we will continue the story by telling you about another of the amazing discoveries of the scientists studying the finches over a period of nearly 30 years.

Natural selection resulted in the survival of finches with body sizes and shapes that were best suited to the availability and type of food.  Sexual selection enhanced those physical characteristics during periods in which females had more choice because they were greatly outnumbered by males.  In addition to these adaptations, the birds increased their cross-breeding with other species and the resulting hybrids actually had a survival and breeding advantage over their species “pure” parents.*

In the first five years of the research study, there was little evidence of different finch species interbreeding, known as hybridizing.  On those rare occasions when species interbred, the resulting generation was not as successful as their parents, with respect to finding a mate and raising another generation.

Such lack of success of hybrids is considered the norm in nature.  In fact, many hybrids are sterile, incapable of reproducing.  Think of the sure-footed but sterile mule—the offspring of a horse and a donkey—as the classic example of a hybrid.

After the severe drought of 1977 and the flood of 1983, the Grants began to notice an increasing number of cross-breeding birds.  It seemed that the resulting hybrids were having more breeding success than the pre-drought hybrids and the data confirmed their observation.

This counter-intuitive conclusion required some careful consideration and the conclusion is a valuable lesson in our rapidly changing environment.  The environment on the islands was radically transformed by the severe drought and subsequent flood.  The cactus was overwhelmed by a vine that smothered it.  The plants with big, hard seeds were attacked by a fungus that decimated the population.  The small seeded plants thrived and became the dominant food source.

The rapidly changing environment was causing more rapid evolution and the genetic variability of hybrids was giving them an advantage.  If the environment is changing rapidly in unpredictable ways, the birds could increase the odds of finding a winning strategy by increasing the variability of their genes, sometimes resulting in novel traits.

We cannot and should not, however, anthropomorphize the birds by imputing motives to the selection of a mate of another species.  The starving cactus finch probably observes that a male of another species—a seed-eating ground finch, for example—appears to be more fit than a male of her own species.  She is not thinking of the odds of increasing genetic variability.  Natural selection operates without the conscious effort of species.

The implications of hybridization

We are experiencing a period of rapid change because of the anthropogenic (caused by humans) impacts on the environment, most notably climate change, but surely many other impacts which we don’t necessarily understand.  These would seem the ideal conditions for the hybridization of species which speeds up evolution by increasing genetic variability. 

Unfortunately, one of many strategies of the native plant movement and nativism in the animal kingdom is to prevent hybridization because it is perceived as a threat to native plants and animals.  We have reported to our readers some examples of such attempts to prevent hybridization and there are many more in the literature:

The variety of California poppy being eradicated from the Presidio in San Francisco.

Are efforts to prevent hybridization depriving plant and animal species of opportunities to adapt to the rapidly changing environment?  We don’t know the answer to that question, but we find it a provocative line of inquiry.

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*This information is drawn from:   Jonathan Weiner, The Beak of the Finch, Vintage Books, 1994

Hybridization: “Genetic pollution” or a natural process?

 
Presidio variety of California poppy. NPS photo

The San Francisco Chronicle’s gossip columnist, Leah Garchik, recently published a story about California poppies in the Presidio.  Apparently, someone planted the “wrong” poppy, or it migrated there.  The poppy that is native to the Presidio is small and yellow.  This “alien” poppy is the large orange poppy that most of us consider the classic California poppy.  The historical record indicates that this classic poppy grew elsewhere in San Francisco, but since it didn’t grow in the Presidio it must be removed because the Presidio’s Vegetation Management Plan “contains the requirement to remove any plants that could jeopardize the integrity of the genetics of native plants in the Presidio.” 

The “wrong” poppy

This incident reminded us of an article published in the newsletter of the local chapter of the California Native Plant Society several years ago, entitled “Contaminating the Gene Pool.”  In this article Jake Sigg instructs gardeners to beware of planting the wrong variety of a native plant because it will cause “genetic pollution.”  The California poppy is one of the examples he gives of a variety of California native being planted in San Francisco that doesn’t belong here.  It isn’t sufficient in his opinion, to plant a California native if that specific variety of the species didn’t historically occur in San Francisco.  He asks gardeners to “think in terms of preserving the genetic integrity of the local landscape.”  And he speculates many negative consequences of selecting the wrong variety, such as “genetic swamping, you’ve got all these foreign genes that are going to overwhelm the native population.”  We were reminded of Mr. Sigg’s vocal opposition to human immigration. 

For the benefit of our readers who aren’t gardeners, we should explain what Jake Sigg and the Presidio are worried about.  In a word, they are worried about hybridization, defined as “to breed or cause the production of a hybrid,” which is defined as “the offspring of two animals or plants of different breeds, varieties, or species.”  Hybridization is as likely to occur between two native plants as two non-native plants, but native plant advocates are concerned about the possibility of a native and a non-native plant producing a hybrid variety that is distinct from the native plant.  Hybridization is not inevitable, but it does occur naturally as well as through human manipulation.  The “From the Thicket” blog recently told the fascinating story of the development of a valuable garden cultivar variety of a favorite California native, ceanothus or California Lilac. 

Aside from the unpleasant association with eugenics, Mr. Sigg’s advice raises several practical questions.  How is the gardener supposed to know exactly which variety of a native plant “belongs” in San Francisco or even in a specific neighborhood within San Francisco, such as the Presidio?  And, in the unlikely event that gardeners might have such esoteric knowledge, where would they get the seeds of this specific variety?  Jake Sigg acknowledges this practical obstacle, but advises gardeners to get their seeds and plants only from the annual plant sale of the local chapter of the California Native Plant Society.  One wonders how many gardeners will follow this rather restrictive advice.

However, the more important question is the scientific question.  Is hybridization an inherently harmful process that always reduces species diversity?    We turn to Mark Davis for a less gloomy view of hybridization.  Like most scientific questions, there is evidence of both positive and negative effects of hybridization on species diversity.  Since you’ve heard the negative view from Mr. Sigg, we’ll let Mark Davis speak for the positive view.  Professor Davis reports in his book Invasion Biology* that “the fossil record generally shows that following the invasion of new species, the number of species resulting from adaptive radiations and evolutionary diversification exceeds the number of extinctions.”   And he concludes his discussion of hybridization and evolution by saying, “…a fair appraisal must also acknowledge that species introductions can enhance diversity as well, through hybridization, and the creation of new genotypes.”

The native plant movement has a narrow view of nature, which we do not share.  Their ideology is based on dire predictions of ecological disaster if we don’t follow their restrictive advice.  And when the managers of public lands choose to follow their advice, the consequences are usually the destruction of plants and animals that we value, in this case a field of California poppies. 


* Mark Davis, Invasion Biology, Oxford University Press, 2009, page 78-82.