Fact vs. Fiction: The real threats to native plants in California

The enduring fiction of the native plant movement is that the existence of non-native plants threatens the existence of native plants by “crowding out” native plants.  If that were true, we should expect to see some evidence of such a causal relationship after 250 years of steadily increasing numbers of non-native plant species.  But we don’t. 

Marcel Rejmanek (UC Davis) is the author of the most recent report on plant extinctions in California, published in 2017.  At that time there were 13 plant species and 17 sub-species native to California known to be globally extinct and another 30 species and sub-species extirpated in California but still found in other states.  Over half the globally extinct taxa were reported as extinct over 100 years ago.  Although grassland in California had been converted to Mediterranean annual grasses by grazing domesticated animals decades before then, most of the plants now designated as “invasive” in California were not widespread over 100 years ago.

Most of the globally extinct plant species had very small ranges and small populations.  The smaller the population, the greater the chances of extinction.  Most of the globally extinct plants were originally present in lowlands where most of the human population and habitat destruction are concentrated. Although there are many rare plants at higher altitudes, few are extinct.  Plants limited to special habitats, like wetlands, seem to be more vulnerable to extinction. The primary drivers of plant extinction in California are agriculture, urbanization and development in general.

Non-native plants are the innocent bystanders to disturbance

“Invasive species” are mentioned only once in the inventory of extinct plants published by California Native Plant Society and only in combination with several other factors. However, the identity of this “invasive species” is not clear.  Rejmanek suggests that the “invasive species” rating refers to animal “invasions” by predators and grazers.  He says, “Indeed, one needs quite a bit of imagination to predict that any native plant species may be driven to extinction by invasive plants per se.” (1)

Although climate change is not cited as the cause of any of the known plant extinctions in California, Rejmanek predicts that climate change is likely to be a factor in the future, not only because of the impact of drought and higher temperatures, but also because non-native plants may be better adapted to changed conditions.

There are over 1,000 naturalized non-native plant species in California.  Their presence is associated with human disturbance.  Naturalized non-native plants are a symptom of disturbance, not the cause.  The impact of non-native plants on native plants cannot be separated from other factors that created the conditions for success of non-native plants.

Specialized insects are exaggerated

Another popular fiction among native plant advocates who love to hate non-native plants is that specialized insects—especially pollinators—require specific native plant species. Again, the record of plant extinctions in California does not support that myth:  “…there is no indication that the loss of pollinators was an important factor in plant species extinctions in California. [For example, one of the native plant species extirpated in California] has many documented non‐specialized pollinators. There does not seem to be any particular dispersal mode associated with presumably extinct plants in California.” (1)

Putting plant extinctions into context

Mediterranean Climates are found in coastal temperate zones. Mediterranean climates are characterized by hot dry summers and mild wet winters.

Setting sub-species aside, there are 5,280 identified native plant species in California and 28 known extinctions of native plant species, including 15 plant species known to still exist in other states.  Only .53% of California native plants are known to be extinct in California, about one-half of one-percent.  Does that seem like a lot?  Rejmanek compared the extinction rate in California with other Mediterranean climates.  The extinction rate of native plants in California is similar to those in the European Mediterranean Basin, South Africa, and Australia, but a little greater than the rate in Chile, where there are fewer endemic plants that exist only in Chile.  Endemism is associated with small native ranges and small populations that are more vulnerable to extinction.

Why are there many endemic plants in California?

About 40% of native plant species in the California Floristic Province are endemic, found only in California and in most cases only in small areas within California, including our off-shore islands.  Their small populations in isolated geographic areas, sometimes within unique ecosystems, such as alkaline sinks, make them particularly vulnerable to extinction.

The evolutionary history of endemic plant species explains why there are so many in California.  Endemic plants are close relatives to plants that exist elsewhere and are sometimes plentiful where they came from.  For example manzanita is a genus of chaparral shrub that is plentiful in California, but there are also many rare endemic manzanita species that occur only in small areas and small populations.  There are several endangered manzanita species in the Bay Area (pallid, Raven’s, Franciscan).

Franciscan manzanita is one of 2 endangered manzanita species in San Francisco. There is one individual plant left of each of these two manzanita species. There are many endemic plants and insects in San Francisco and several are now extinct. San Francisco has a complex, diverse geology and topography and it is surrounded on 3 sides by water, creating many small, isolated microclimates in which many endemics have evolved.

The geography of California explains why the evolution of a plant species diverged from its plentiful ancestors to become an endemic species in a small geographic area.  Plants move around in a wide variety of ways, most natural, without the aid of humans.  Their seeds are dispersed by animals and birds that eat them or inadvertently carry them to another location.  Sometimes their seeds are carried on the wind or brought to islands by storms and currents.

When a plant arrives in a new location that is isolated from its original home and therefore cannot mate with its relatives, it begins its own, independent evolutionary history.  Each successive generation is reacting to its new environment, rewarding its fitness with its new home with a successful new generation.  Each generation rolls the genetic dice, its genome drifting away from its ancestors in a random way.  Occasionally a mutation will occur that alters the evolutionary trajectory.  Eventually, the plant in its new home is sufficiently genetically distinct that taxonomists are ready to call it a separate species.  Naming a new species is a judgment call, often questioned by some taxonomists, called “lumpers” as opposed to the “splitters” who are ready to name it a new species.

The factors that result in endemic species are many, but broadly speaking they are mobility and, ironically, isolation.  California is one of the most geographically diverse states in the country, with corridors for mobility, but many barriers that create isolation.  Gordon Leppig describes California’s geographic diversity in Beauty and the Beast:  California Wildflowers and Climate Change, published by California Native Plant Society:  “The state’s natural wonders include five deserts, the highest and lowest points in the continental United States, the third-longest state coastline (about one thousand miles), the most national parks (nine), the most federally designated wilderness areas (more than 140), the highest percentage of wilderness in the contiguous United States (14%), the most diverse conifer assemblage outside the Himalayas, the most federally listed species….”  The multitude of different ecosystems with unique microclimates produces one of the most diverse floras in the world.

Click on the picture to watch the movement of tectonic plates over one billion years. Watch California slowly emerge as the jigsaw puzzle takes shape. California is the edge of two tectonic plates that collide and grind past one another perpetually, uplifting and dropping the land into fractured geomorphic pieces.

Human activities penetrate the barriers that created genetic isolation in the past.  Our roads become corridors for the biological exchange that threatens small, isolated pockets of rare plants.  Trade and travel has ended the isolation of off-shore islands.  Our roads and dams also create new barriers for mobility.  In other words, we are altering pre-settlement corridors and creating new ones.  We should expect consequences for our ecosystems for the changes we have made.

Given the number of rare and endemic plants in California and the changes in the environment required to accommodate nearly 40 million human Californians, it seems that extinction of less than one-half of one percent of native plants is a surprisingly small loss. 

(1) Marcel Rejmanek, “Vascular plant extinctions in California: A critical assessment,” Diversity and Distributions, Journal of Conservation Biogeography, 2017

Anthropocene: The Sixth Extinction

There have been five major episodes of massive extinctions in the 4.5 billion years that our planet has existed.  All occurred within the past 500 million years because there was little known as “life” prior to that time.  We are now experiencing the sixth massive extinction episode which began approximately 50,000 years ago with the dispersal of humans around the world.  The causes of prehistoric extinctions are not fully known, unlike the current episode.  We know that we are the cause of the sixth extinction, but we seem to be incapable of preventing it.

Prehistoric extinctions

The fifth and most recent massive extinction event occurred about 65 million years ago. It brought the age of dinosaurs to an abrupt end.  There were no humans or even our primate ancestors at that time.  The cause of that extinction was only recently discovered in the 1980s and even more recently accepted by most scientists.  There is now general agreement that the entire environment of the planet was radically and suddenly altered by the impact of a huge asteroid that landed on what is now the Yucatan peninsula in Mexico.  The impact raised a huge dust cloud that engulfed the earth and precipitated the equivalent of a nuclear winter, killing most vegetation and the animals adapted to a much warmer climate.  As with all massive extinctions, it took many millions of years for the environment to recover from that event and for plants and animals to slowly evolve adaptations to the new environment.

Update:  There is an alternate theory about the cause of the fifth extinction.  Huge volcanic eruptions in India may have been the cause, or perhaps a contributing factor.  Explained HERE.

Scale of dinosaurs compared to human. Creative Commons - Share Alike
Scale of dinosaurs compared to human. Creative Commons – Share Alike

The third and biggest extinction event occurred about 250 million years ago at the end of the Permian geologic period.  Paleontologists tell us that about 90% of all living plant and animal species died as a result of that extinction event.   Like the fifth extinction, the End-Permian extinction was precipitated by a sudden and radical alteration in the climate.  However, less is known about what caused that change in the climate.  Like our current round of climate change, there was a massive release of carbon into the atmosphere with a related drop in oxygen.  These changes caused temperatures to soar and the chemistry of the oceans to acidify.  Although there is not yet consensus amongst scientists, current speculation in the scientific community is that the changes in atmospheric conditions were the result of huge volcanic eruptions in what is now Siberia that emitted carbon dioxide into the atmosphere. (2)

The first massive extinction occurred about 450 million years ago just 50 million years after the first land plants began to emerge on the planet.  In fact, the plants may have been a factor in the climate change that caused the extinction at the end of the Ordovician geologic period.  The cooling of the climate that caused the extinction was associated with a sharp drop in carbon dioxide levels which may have been partially the result of plants that convert CO₂ to oxygen.  The movement of the continents is also thought to have been a factor in the cooling because the breakup of the unified continent, Pangaea, changed the circulation of ocean currents which affect the climate on land.

All of the massive prehistoric extinctions were associated with sudden changes in climate, although human perception of time should not be imposed on the word “sudden.”  These events occurred over thousands of years and are only “sudden” when compared to the 4.5 billion years of the existence of our planet.

Extinctions of the Anthropocene

Genus Homo evolved into its only surviving species, Homo sapiens, about 200,000 years ago.  That’s us…humans.  However, we didn’t begin to extinguish plant and other animal species until our population grew and dispersed throughout the world.  And when we did, the first victims of our ability to hunt cooperatively with weapons were the megafauna, now largely gone from the world.

Megafauna are the huge animals now known primarily from their fossil remains that were so large they had no predators until humans brought their intelligence to the task of hunting which was previously limited by size and speed.  Megafauna reproduction wasn’t capable of keeping up with the pace of human hunting because they had long gestation periods, many years to sexual maturity, and small numbers of offspring.

Humans reached the Australian continent about 50,000 years ago.  When they arrived, Australia had its own megafauna:  giant kangaroos and other enormous herbivores.  Within 10,000 years the megafauna were gone and the landscape changed as grazing was significantly reduced:  “With no more large herbivores around to eat away at the forest, fuel built up, which led to more frequent and more intense fires.  This, in turn, pushed the vegetation toward fire-tolerant species.”  (1) Conversion to grassland savanna was also accelerated by the frequent fires intentionally set by humans to facilitate their hunting.

Eurasian Mammoth on left; American Mastodon on right. Creative Commons -dantheman9758
Eurasian Mammoth on left; American Mastodon on right. Creative Commons -dantheman9758

The same shift in vegetation occurred in North America when humans arrived about 13,000 years ago and American megafauna such as mastodons and giant sloths were hunted to extinction.  Grassland found in North America when Europeans arrived thousands of years later in the 16th century was therefore not adapted to heavy grazing and was largely destroyed by domesticated animals brought by early settlers.  Native Americans did not have domesticated animals. 

Similar scenarios played out around the world as humans arrived, most recently on the Pacific Islands where Polynesians arrived as recently as 1,500 years ago.  Huge flightless birds were found on New Zealand until they were hunted by humans just 500 years ago.

The second wave of extinctions caused by humans occurred during the age of exploration, beginning in the 16th century.  Humans wiped out many species of animals all over the world to feed their explorations and early settlements.  Huge turtles were brought on board ships to feed the crew on long voyages.  Passenger pigeons and American bison were killed by early settlers for food, leather, and sport.

As humans developed agriculture and domesticated animal-herding, hunting wild animals decreased.  In developed countries, extinctions today are largely by-products of western civilization, through mechanisms such as climate change and global exchange of diseases and pathogens…all equally deadly to other living things.

Modern Extinctions

There are no longer any physical barriers to the exchange of pathogens and pests.  Invasion biology is based on the fiction that such exchanges can be prevented or even reversed.  The most deadly invasions prove otherwise:

  • Amphibians, especially frogs, are being wiped out all over the world by a fungal disease that is traveling fast.  It is now known to exist in Central, South, and North America, as well as Australia.  The means of transmission is not yet known.
  • Bats are dropping dead by the tens of thousands primarily in New England as they succumb to a different fungal disease.  Nothing is known about how this disease is transmitted.  We should probably assume that it will also spread beyond its current range.
  • Insects, such as the emerald ash borer that is killing millions of ash trees in the United States, have been accidentally introduced as a result of global trade.

We should expect the loss of these species to reverberate throughout the food web, although little is known about the secondary effects of the loss of species.  For example, when bats are no longer available to eat insects, what will those insects eat?  And what will the animals that ate frogs eat when the frogs are gone?  These animals may be playing roles about which we know little and therefore cannot predict the consequences of their loss.

The spread of pathogens and insects that prey on plants could be related to climate change.  For example, the pine bark beetle is a native insect that has become a serious problem in the forests of North America because mild winters associated with global warming are not cold enough to cause an annual die-back of the insects.  The range of the pine bark beetle has expanded and is killing millions of acres of forests in North America.

Ecosystems are being fragmented by agricultural development.  Much of the Amazonian rainforest has been reduced to isolated fragments which are not large enough to support the diverse plants and animals that occupied intact ecosystems.

Climate change…the silent killer

When we look to the distant past, we can see how levels of carbon dioxide in the atmosphere have caused massive extinctions of plant and animal species.  Low levels of carbon dioxide have been associated with a cooling phase and high levels of carbon dioxide have caused temperatures to rise.  We are now in a period of a huge increase in carbon dioxide levels caused by the activities of humans, particularly emissions associated with the burning of fossil fuels and deforestation.  There is scientific consensus that the climate has changed and will continue to change as well as about the causes of those changes.  However, we still know little about the long-term consequences of climate change.

Coral reef. Creative Commons - Share Alike
Coral reef. Creative Commons – Share Alike

One consequence of increased levels of carbon dioxide is well known and that is the acidification of the oceans.  The laws of chemistry tell us that when carbon dioxide dissolves in water it forms carbonic acid.  Carbonic acid dissolves shells and coral.  Aquatic animals such as mussels, clams, oysters, crabs, and lobsters will be incapable of building the shells that protect their bodies when levels of carbonic acid increase.  Australian scientists report that coral cover of the Great Barrier Reef has decreased 50% in the past 30 years.  A paper published in 2008 predicted the imminent extinction of one-third of 800 reef-building species as a result of increased water temperature and acidity of the oceans.  An estimated one-half million to 9 million species “spend at least half their lives on coral reefs.” (1)

So why are we destroying trees?

As disturbing as it is to witness the death of plants and animals which are innocent by-standers to the choices made by humans, we have some sympathy and understanding for why our political system has been incapable of the fundamental changes needed to stop the process.  We burn the fossil fuels that emit carbon dioxide and other greenhouse gases to keep us warm in the winter and cool in the summer, to transport us to work and play, to power our industrial processes and many other vital functions.

But, we cannot understand why we continue to destroy millions of healthy trees (that we planted) essentially because they are out of fashion. These trees are storing tons of carbon that will be released into the atmosphere when the trees are destroyed and we will lose their ability to store carbon in the future.

We loved these trees as recently as 50 years ago.  Now many people have decided that they “don’t belong” because they aren’t native.  Eucalyptus is only one of many targets of this fad.  Norway maples are being destroyed in communities in eastern United States for the same reason.  And most of the trees being destroyed in the Midwest (because people wish to “restore” the prairie artificially maintained by Native American fires) are even native to the Midwest.

In the case of eucalyptus, the trees are expected to live in California for several hundred more years.  How will the climate have changed in 300 years?  Will any of the plants presently considered “native” even exist?  On our present climate trajectory, the answer to that question is clearly “no.”


Most information in this post is from these two sources:

(1)    Elizabeth Kolbert, The Sixth Extinction, An unnatural history, Henry Holt and Company, 2014

(2)    “Where have all the species gone?” University of California Museum of Paleontology, short course, March 1, 2014

Non-native species are NOT the “second greatest threat to species in peril”

One of many doom and gloom scenarios used by native plant advocates to frighten the public into accepting their destructive “restoration” projects is the claim that “non-native species are the second greatest threat to the survival of species in peril.”(1)  Although the statement originates with a scientific study published in 1998, the context in which it was originally reported has long since been lost as it has been cited more than 700 hundred times in scientific studies according to Mark Davis.(2)

The original 1998 article in BioScience by Wilcove et.al. clearly states that the claim is not based on any actual data:

“We emphasize at the outset that there are some important limitations to the data we used.  The attributes of a specific threat to a species is usually based on the judgment of an expert source, such as a USFWS employee who prepares a listing notice or a state Fish and Game employee who monitors endangered species in a given region.  Their evaluation of the threats facing that species may not be based on experimental evidence or even on quantitative data.  Indeed, such data often do not exist.”(3)

This caveat is rarely repeated when the claim is invoked by native plant advocates to justify their crusade against non-native plants and animals.  In fact, since the statement was originally made over a decade ago, it is now repeated without reference to the original source.  It has acquired the status of a mantra amongst native plant advocates that requires no citation to substantiate its “truthiness.” 

The Wilcove et.al. article in BioScience in which this statement was made was heavily influenced by selecting a geographic area which is not representative of the United States as a whole.  Although Hawaii is a part of the United States its rates of extinction are not typical of the contiguous states of the union.  Rates of extinction are substantially higher on islands because they contain many more endemic (unique) species that do not occur elsewhere. These endemic populations are small and vulnerable to the introduction of competing species.  Native populations on islands are not supplemented by immigrations as they are elsewhere.   

Coqui frog is being eradicated in Hawaii. USDA photo


If Hawaii is removed from the anecdotal information in the Wilcove article, the rates of extinction are comparable to those in Canada where introduced species are considered the least important of six categories of causes of extinction (habitat loss, over-exploitation, pollution, native species interactions, and natural causes such as storms) identified in a similar article in 2006(4).  This list doesn’t include climate change, which is now considered a serious threat for extinction.    Similar studies in the continental United States have reached similar conclusions.(5)

At the time the Wilcove et. al. article was published there was no evidence of a single extinction (or even local extirpation) of a native plant in the continental US resulting from competition from an introduced species of plant.  Clearly, the authors of this study were guilty of exaggeration.(6) 

Although native plant advocates have misused this publication by taking it out of context, the authors were complicit in its misuse by selecting a geographic area that is not representative of the United States.   Non-native species are NOT the second greatest threat to the survival of endangered native species.  In fact, they probably aren’t the third, fourth, or fifth greatest threat to native species. 

 We wish that native plant advocates would examine the origins of their assumptions more carefully.  We believe if they did so they would modify their destructive projects to reflect a more inclusive view of nature. 

(1) Wilcove, DS, Rothstein, D., Dubrow, J., Phillips, A., and Losos, E, “Quantifying threats to imperiled species in the United States,” BioScience, 48, 607-615, 1998. 

(2) Davis, Mark, Invasion Biology, Oxford University Press, 2009, page 181.

(3) Ibid.

(4) Venter, O, et. al., “Threats to endangered species in Canada,” BioScience, 56, 903-910, 2006.

(5) Ibid., page 182

(6) Ibid., page 183