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

Alexander von Humboldt: “The Invention of Nature”

Portrait of Alexander von Humboldt by Joseph Stieler, 1843
Portrait of Alexander von Humboldt by Joseph Stieler, 1843

Alexander von Humboldt made so many contributions to science that we cannot do him justice here.  We will therefore focus on his study of nature and put his accomplishments in that area into the context of historical events and his personal views of those events.  We draw from Andrea Wulf’s The Invention of Nature in this article. (2)

Humboldt is considered the originator of the scientific concept of biogeography, “the study of the distribution of species and ecosystems in geographic space and through (geological) time.” (1)  In other words, biogeography attempts to explain why species are where they are.  It is therefore relevant to the mission of Million Trees, which is to make the case that species belong wherever they persist without human management and without regard to how they got there.

The journey begins

Alexander von Humboldt was born in 1769 in what was then Prussia and is now Germany.  Under the thumb of a domineering mother, he was required to spend most of his twenties as a mining inspector.  In that capacity, he mastered the science of geology that he put to good use throughout his long life.

At the age of 27, he was freed from his obligations to his mother by her death.  With the help of his inheritance he began his travels to satisfy his intense curiosity about the world.  He brought along on his journeys the tools of measurement that existed at that time.  He made detailed recordings of his measurements wherever he went and they were the basis of comparing the many places he visited.

His journey began in the mountains of Europe and they became the baseline of his comparisons throughout his journeys.  But his movements in Europe were hampered by the political situation.  Revolutionary France was expanding its empire with a series of wars in Europe that prevented him from visiting many places of interest to him.  After several failed attempts to join voyages to other places, the King of Spain granted Humboldt a passport to the Spanish colonies in South America to collect the flora and fauna of the New World.

Humboldt set sail from Spain in June 1799, along with “a great collection of the latest instruments, ranging from telescopes and microscopes to a large pendulum clock and compasses—forty-two instruments in all, individually packed with protective velvet-lined boxes—along with vials for storing seeds and soil samples, reams of paper, scales and countless tools.” (2)

Journey of Alexander von Humboldt in the New World, 1799-1804. Creative Commons - Share Alike
Journey of Alexander von Humboldt in the New World, 1799-1804. Creative Commons – Share Alike

Finding unity in nature

Alexander von Humboldt collecting plants at the foot of Chimborazo. Painting by Friedrich George Weitsch
Alexander von Humboldt collecting plants at the foot of Chimborazo. Painting by Friedrich George Weitsch

Humboldt and his companions climbed every mountain they encountered, taking precise measurements of altitude, temperature, and samples of soil and plants as they gained altitude.  As he accumulated data from each climb he began to see a pattern in what he encountered.  Humboldt’s new, revolutionary idea of nature fell into place for him as he climbed Chimborazo in Ecuador to an unprecedented height of 19,413 feet, about 1,000 feet shy of the summit:  “Nature, Humboldt realized, was a web of life and a global force…Humboldt was struck by this ‘resemblance which we trace in climates the most distant from each other.’ (2)

Humboldt's Naturegemälde
Humboldt’s Naturegemälde

Humboldt called the graphic depiction of his theory Naturegemälde which means roughly “painting of nature.”  His drawing shows that plants are distributed according to their altitudes and latitudes, from subterranean mushroom species to the lichens that grow below the snow line.  In similar latitudes, palms grow in the tropical zone at the foot of the mountain.  Oaks and shrubs prefer the temperate climate above the tropical zone.  Naturegemälde illustrated for the first time that “nature is a global force with corresponding climate zones across continents.” (2)

Addendum:  In 2012 – 210 years after Humboldt climbed Chimborazo – a scientific expedition surveyed the vegetation on Chimborazo and compared that survey to Humboldt’s survey in 1802.  They found that the vegetation has moved to higher altitudes on average 500 meters.   This movement of plants to higher elevations (and latitudes in other examples) is a response to climate change, which requires plant and animal species to move in order to survive.  It is one of many examples of why the concept of “native plants” is meaningless at a time of rapid climate change. (3)

Humboldt’s theory of the unity of nature, connected in a global web of life, required an interdisciplinary approach of which few of his contemporaries were capable.  He drew equally from his fund of knowledge about astronomy, botany, geology, and meteorology.  Sadly, as human knowledge has expanded exponentially in over 300 years, an interdisciplinary approach to science has become unattainable.  Scientific inquiry has become increasingly narrowly specialized, preventing the global view that informed Humboldt’s studies.  The “big picture” is lost in the compartmentalization of science into isolated, specialized scientific disciplines.

Socio-political context of Humboldt’s travels

We cannot fully appreciate Humboldt’s scientific accomplishments without mentioning his interest in and concern for the human realm.  Humboldt was 20 years old when the French Revolution occurred in 1789.  He was thrilled and delighted by the prospect of liberty and equality for the French people.  His admiration for the French made Paris his home for much of his life.  He believed that science would thrive in such an atmosphere of freedom of thought and action.

As Humboldt travelled in the New World, he brought this standard of freedom and liberty along with him.  New Spain was lacking in that regard.  He believed that the cultural accomplishments of the indigenous people were sadly devalued.  He was disgusted by the oppression of the indigenous people and the lack of basic human rights of the colonists.  He observed the consequences of this oppression in the colonial landscape.  Mineral riches were ravaged and forests were razed for the cash crops that impoverished the settlers.  The colonial government required them to grow monocultures such as indigo (blue dye), in lieu of the crops needed to feed the population.

Humboldt was one of the first scientists to observe the destructive consequences of deforestation.  Forests were cleared for agricultural fields.  Wood was the fuel of the time, providing heat and light.  Later forests would fuel ships, trains and industrial steam engines.  Where forests were cleared, the land quickly dried out because trees recycle water into the atmosphere.  Erosion and desertification are the eventual consequences of deforestation.

For the same reason that Humboldt admired the French Revolution, he also admired the new American republic that had only recently gained its independence from Britain and was founded on democratic principles.  He went far out of his way to visit the United States to consult with the young democracy.

Thomas Jefferson was the President at the time and they spent much valuable time together.  The Spanish jealously guarded all useful information about New Spain because the United States was clearly a rival in the New World.  Humboldt brought the Americans much useful information about their competitor because he not only wished to be helpful to their young enterprise, he firmly believed in universal access to information.

However, there was one horrible blot on the reputation of the United States that made Humboldt queasy.  He abhorred slavery in his own country as well as in the United States.  As much as he enjoyed the company of Thomas Jefferson and respected his intelligence, he was scandalized by the scale of enslavement on Jefferson’s Virginia plantation.

Humboldt’s concern for the welfare of humans extended to all races, ethnicities, and classes.  Just as he embraced all nature as worthy of his attention, his view of humanity was equally generous and inclusive.  We venture to guess that he would be as mystified by the pointless distinction between “native” and “non-native” plants and animals as we are.

Humboldt’s legacy

When Humboldt returned to Europe in 1804 at the age of 35, his reputation as an important scientist was already established by the correspondence in which he had engaged during the 5-year voyage.  He returned with the detailed record of his travels, including specimens:

“[He] returned with trunks filled with dozens of notebooks, hundreds of sketches and tens of thousands of astronomical, geological and meteorological observations.  He brought back some 60,000 plant specimens, 6,000 species of which almost 2,000 were new to European botanists—a staggering figure, considering that there were only 6,000 known species by the end of the eighteenth century.” (2)

He spent 21 years publishing over 30 volumes about his 5-year journey in the New World.  Not only were these books widely read, they influenced many of the greatest scientists and thinkers who succeeded him:

  • Charles Darwin’s correspondence with Humboldt and many references to Humboldt’s publications in his own work are evidence of Humboldt’s influence on Darwin’s work. Just as Humboldt’s journey inspired his theory of the unity of nature, Darwin’s journey around the globe was the inspiration for his theory of evolution.  Readers of Darwin’s early work observed the similarity of his writing style with that of Humboldt.
  • Humboldt was equally influential in the writings of Henry David Thoreau. Thoreau published an unsuccessful book shortly after his two-year reclusive experience on Walden Pond.  Then he learned from Humboldt’s books to observe and record the workings of nature, which eventually transformed his writings into the nature writing that made him famous.  The records he kept of the climate on Walden Pond are still used as a reference point for the climate change that has occurred in the past 150 years.
  • George Perkins Marsh is considered the first American environmentalist. His book Man and Nature was the first to express alarm in 1864 about American deforestation.  The writings of Humboldt were the first to alert Marsh to this issue and as they did for Humboldt this issue was brought into focus by extensive travels.  Marsh could see America’s future in Egypt where thousands of years of intensive agriculture left the land bare and unproductive.

Invention of NatureThis is but a short list of important scientists all over the world who were influenced and inspired by the writings of Alexander von Humboldt long after his death in 1859 at the age of 89.  We encourage our readers to read Andrea Wulf’s The Invention of Nature:  Alexander von Humboldt’s New World for the full story.  We have reported on other books by Andrea Wulf—The Brother Gardeners and The Founding GardenersMs. Wulf turns botanical history into a gripping story.

What is Humboldt’s message to Million Trees?

Alexander von Humboldt delivers a powerful message that fits well with our mission.  He saw the world and its inhabitants as fitting together in a harmonious and comprehensible manner.  He was always looking for the connections between seemingly disparate elements in nature and finding them wherever he looked.  His egalitarian hopes for humanity fit perfectly with his perception of nature as working together to make a web of life with humans as a part of that web.

We would like to think that Million Trees shares the Humboldtian viewpoint and we hope that our readers agree.  We hope that the New Year will bring more harmony to nature and those who live within it.


(1) Wikipedia https://en.wikipedia.org/wiki/Biogeography

(2) Andrea Wulf, The Invention of Nature:  Alexander von Humboldt’s New World, Alfred A. Knopf, New York, 2015

(3) Morueta-Holme N, Engemann K, Sandoval-Acuña P, Jonas JD, Segnitz RM, Svenning JC, “Strong upslope shifts in Chimborazo’s vegetation over two centuries since Humboldt,”  Proc Natl Acad Sci U S A. 2015 Oct 13;112(41):12741-5.  Available HERE.


Krakatoa: A case study of species dispersal

Islands are intensively studied by ecologists because they are hothouses for evolution.  Physical isolation results in the evolution of new species that are related to their mainland ancestors and the result is many endemic species of plants and animals which exist only on that island. 

Some islands originated when continents broke up into smaller pieces as a result of continental drift.  Madagascar and New Zealand are examples of islands that were originally attached to a continent.  These islands brought some of the inhabitants of the continent with them.  But many islands arose from the ocean as a result of volcanic activity and were therefore born bare as a newborn babe without vegetation or inhabitants.  All subsequent life on these volcanic islands arrived by dispersal from elsewhere via ocean currents, winds, or carried by traveling animals, most recently by humans.

Krakatoa map

Krakatoa is such a volcanic island in the Indonesian archipelago.  It has a long record of volcanic eruptions which both destroyed much of the island and created new islands.  Many of these eruptions occurred during prehistoric periods, but many have been recorded by human history.  These recent eruptions have created an evolutionary laboratory that enables us to answer the perplexing question of how quickly the dispersal of species occurs. 

The cataclysm of 1883

Krakatoa eruption, lithograph 1888
Krakatoa eruption, lithograph 1888

In August 1883 a series of volcanic eruptions on Krakatoa produced one of the most cataclysmic events of recorded human history.  The force of the blast was the equivalent of 13,000 times the nuclear bomb that devastated Hiroshima in 1945.  The blast could be heard as far as 3,000 miles away.  Shock waves from the blast reverberated around the planet 7 times.  The blast sent an ash cloud 50 miles into the atmosphere. The weather of the entire planet was altered by the ash cloud.  The temperature dropped by 1.2 Degrees Celsius in the year after the eruption and the climate did not return to normal until 1888.  The blast and subsequent tsunami killed over 36,000 people and destroyed two-thirds of the island.

 Scientists believe that nothing living survived the blast:  “no plant, no animal, no seed, no spore.” (1)  The first scientists visited Krakatoa nine months after the blast.  They reported finding nothing alive except a single spider.  Spiders are notoriously successful dispersers because the webs they weave can become sails on the wind.

Krakatoa is quickly repopulated

In 1886—three years after the eruption—the first botanical expedition arrived on Krakatoa.  They found mosses, algae, flowering plants and eleven species of fern.  They speculated that the arrival of algae enabled the spores of ferns to become established on the otherwise bare ground.  Amongst the plants there were two species of grasses.  Scientists assume that most of these plants arrived via wind, but some species could have arrived as seeds carried by the surf.

Further colonization of the barren island then began to accelerate.  By 1887, young trees were found as well as dense grassland and many ferns.  Butterflies, beetles, flies and a single monitor lizard were found in 1889.   The species of monitor lizard found in 1889 is known to be a good swimmer and is a “versatile opportunist” on land, which means it’s not a fussy eater and it can eat less often than other lizards.

By 1906, there were a hundred species of vascular plants, covering the summit in green and a grove of trees along the shore, including tamarisk and coconut palm.  The coconut is found on virtually any sunny beach in the Pacific because its seeds float in their large protective shell wherever the current carries them.

Fifty years after the eruptions of 1883, the island was home to 171 species of plants.  One botanist estimated that 40% of the plants came on the wind, 30% floated on the sea and most of the remainder were brought by animals.   The eruption of 1883 produced huge quantities of pumice–a lightweight, sponge-like volcanic glass—that floated on the ocean creating rafts that were observed for years after the eruption:  “…a ship’s captain…who encountered pumice on the Indian Ocean, lowered a boat for a closer look, ‘It was curious and interesting to note how it had been utilized by animals and low types of life as habitations and breeding places.’”  (1)

These early arrivals were effective dispersers, but they also had to be capable of surviving inhospitable conditions on arrival.  The order of arrival is therefore an important factor in determining successful establishment.  For example, animals won’t survive if they arrive before needed food resources.  The plants most likely to survive are capable of self-pollinating, that is they don’t require a partner to reproduce.

San Francisco is not an island

How does this experience on Krakatoa compare to our experience in the San Francisco Bay Area?  We’re so glad you asked!!

The many projects all over the Bay Area that destroy non-native vegetation are not isolated islands.  They are surrounded by more non-native vegetation which quickly re-populates the bare ground created by these projects.  Dispersal into small plots of land within San Francisco is much easier than onto isolated Krakatoa.  The majority of these projects do not have the resources to replant the areas in which non-native vegetation is eradicated.  The fiction is that native plants will magically reappear when non-natives are destroyed.  But we can see that the result is the return of the hardiest non-native weeds such as hemlock, star thistle, oxalis, and broom.  These hardy creatures don’t need to be planted.  Their seeds are carried by the wind or remain dormant in the ground to germinate when someone foolishly destroys the trees that provide shade and suppress germination of weeds.

California Academy of Sciences, April 2011
California Academy of Sciences, April 2011

Even when natives are planted, they are quickly out-competed by non-natives.  The best local example of that hard, cold fact is the living roof on the California Academic of Sciences.    When the California Academy of Sciences reopened in San Francisco in August 2008, its “living roof” was considered its most unique feature.  Thirty species of native plants were candidates for planting on the roof.  They were planted in test plots with conditions similar to the planned roof and monitored closely.  Only nine species of native plants were selected for planting on the roof because they were the only plants that were capable of self-sowing from one season to another, implying that they were “sustainable.”  A living demonstration of “sustainability” was said to be the purpose of the living roof.

Two of six of the predominant species on the roof after 2-1/2 years were native.  Four of six of the predominant species were mosses that are “cosmopolitan,” which means they are found everywhere.  They weren’t planted on the roof and were therefore “volunteers.”

The monitoring project divided the roof into four quadrants.  In February 2011, non-natives outnumbered natives in two of the quadrants.  Although natives outnumbered non-natives in the other two quadrants, non-natives were also growing in these quadrants.

The consultant who advised the Academy about what to plant on the roof would not be surprised by this monitoring report.  He advised the Academy to walk the streets of San Francisco and identify the plants growing from the cracks in the sidewalks.  These are the plants he advised the academy to plant because these are the plants that are adapted to current conditions in the city.  The Academy rejected this advice because they were committed to planting exclusively natives on the roof.

The many projects that are destroying non-native vegetation are not sustainable.  They are surrounded by non-native vegetation which is better adapted to current climate, soil, and atmospheric conditions.  Non-native vegetation will out-compete the natives that are not adapted to current conditions.  If these projects were merely futile, perhaps we could shrug and move on.  But we can’t turn a blind eye because these projects are harmful to the environment.  They use huge quantities of toxic herbicide and they are destroying healthy trees that are performing many valuable ecological functions.  These are not harmless experiments.

(1)    David Quammen, Song of the Dodo, Scribner, 1996.

(2) Some information for this post is from Wikipedia

The Monkey’s Voyage: How plants and animals are dispersed throughout our planet

The Monkey’s Voyage (1) is as much a history of the science of evolution and ecology as it is a report of the prevailing scientific opinion of the means by which plants and animals were dispersed around the world.  Just as life has evolved, so too has the science that studies it.

In the beginning….

The story begins with Charles Darwin, the author of the first publications that identified natural selection as the mechanism that drives the evolution of life on the Earth.  These ideas came to him as the result of a five-year voyage around the world in 1831-1836:  down the coast of Africa, across the Atlantic, down the coast of South America, around the horn, to the Pacific Ocean to many islands—most famously the Galapagos—to New Zealand, Australia, islands in the Indian Ocean, round the horn of Africa to home.

Voyage of the HMS Beagle, 1831-1836.  Creative Commons - Share Alide
Voyage of the HMS Beagle, 1831-1836. Creative Commons – Share Alike

He spent 3-1/2 of the 5 years on land, collecting plant and animal specimens, including many fossils.  The fossils suggested to him the existence of animals no longer occupying the land.  He also observed many similar plants and animals with slightly different forms around the world.  The classic example of closely related, but widely dispersed animals is a family of large, flightless birds:  the ratite family.

Family of ratite birds
Family of ratite birds

These similarities suggested a common ancestry to Darwin.  Yet, their dispersal across oceans was puzzling to him because at that time the continents were considered fixed in place both going back in time and going forward into the future.  Nothing was known at the time about the constant movement of continents, known as continental drift, because the movement was too slow to be observed by humans.

Darwin’s theory about the similarities he found in widely dispersed plants and animals was consistent with his perception of the fixed nature of the geography in which they were found.  He theorized that the common ancestors of the similar plants and animals had been dispersed by wind, ocean currents, carried by birds, or other means of transportation. 

He conducted experiments to determine how long seeds could survive in sea water to test his theory and he examined migrating birds for evidence of seeds and small animals in their feet and feathers.  What he found supported his theory that it was physically possible for plants and animals to be dispersed across oceans to new ranges where subsequent evolution in a different environment would eventually result in alterations of form.  When plants and animals are moved from their home ranges and are physically isolated, their genetic compositions diverge.  Over time they are sufficiently genetically and morphologically distinct to be considered different species. 

Continental Drift

Around the turn of the 20th century, scientists began to theorize that Africa and South America may have been merged at one time because maps revealed that they fit together like pieces of a puzzle.  Alfred Wegener is best known for his pursuit of this theory.  He visited both sides of the Atlantic and observed that seams of rock and sediments lined up on the two shores, suggesting their past connections.  Although Wegener’s theory gained considerable traction, he did not propose an equally compelling theory about the physical mechanism that would be capable of moving the continents apart.

The mechanism that moves the continents was identified about 50 years later when the ocean floor was studied as a result of developments in radar and sonar.  These analytical tools eventually identified seams running the length of the oceans that separate the tectonic plates on which continents ride.  Beneath the crust of the earth magma of molten material moves in a current, emerging through the seams of the Earth’s crust as volcanic activity.  As molten material emerges from this seam between the tectonic plates, it cools on the ocean floor to form new sea floor.  The expansion of the sea floor moves the plates away from the seams, which moves the continents.   This is the engine that drives continental drift.

Tectonic Plates - USGS
Tectonic Plates – USGS

By the late 1960s there was scientific consensus about plate tectonics and consequent continental drift. That knowledge led to an understanding of the history of the continental configuations.  About 300 million years ago, all continents were fused into one, called Pangaea.  Pangaea began to break up about 100 million years later.  However, South America, Africa, Madagascar, Australia, New Zealand, and Antarctica remained fused in a continent called Gondwana until about 100 million years ago.



“The history of life is the history of the earth.”

This new understanding of the history of the earth’s geology resulted in a paradigm shift in scientific theories regarding dispersal of life forms.  Very quickly, scientific consensus formed around the theory that life moved as a result of movements in the continents.  This theory was succinctly expressed as “The history of life is the history of the earth.”  That is, where life is found depends upon changes in the geology of the earth.  For example, scientists assumed that life found on Madagascar originated in Africa before Madagascar separated from the African continent.  Similarly, scientists assumed that life found in New Zealand originated in Australia before New Zealand separated from the Australian continent.  In other words, life migrated from the continent along with the land, like Noah’s ark carrying the animal kingdom.   Previous theories about trans-oceanic voyages of plants and animals were quickly abandoned in favor of this new, elegant theory which seemed so much more plausible than its predecessor.

DNA analysis trumps elegant theory

Although scientists were comfortable with their new theory of how life was dispersed, the inexorable forward movement of human knowledge intervened to disrupt their complacency.  The new analytical tool that overturned this theory was DNA analysis which enabled scientists to study the genetic composition of life forms. 

When there are two morphologically similar species in physically isolated locations, their common ancestry can now be determined by DNA analysis.  And the genetic distance between the species can help scientists determine when those species became physically separated.  When populations become separated their genetic pools become progressively more distant from one generation to another.  This rate of genetic change is called the “molecular clock” and it can be used to determine when the physical separation occurred if the rate of change is known.  Unfortunately, the molecular clock varies from one lineage to another, so first scientists must calibrate the clock and when they do they can estimate the arrival of a specific plant or animal in a new territory that is physically isolated from its former range and therefore its ancestors.

Genetic analysis has overturned former theories of how life was dispersed on the earth.  In most cases, plants and animals arrived in their present locations long after the continents separated into their present configuration. Plants are more likely to have been dispersed by wind and ocean currents than animals.  New ranges of plants are often on the receiving end of ocean currents and plumes from big rivers.

Also, new understanding (1980s) of the most recent mass extinction approximately 65 million years ago—when dinosaurs disappeared from the earth—would predict the same result.  The mass extinction at the end of the Cretaceous period occurred after the separation of the continents.  Therefore, most life forms that moved along with the separating continents were wiped out by the mass extinction about 65 million years ago.  Life forms found now are more likely to have arrived after present continental configurations formed and therefore are more likely to have arrived by long-distance dispersal. 

Evolutionary science comes full circle

Olive baboon, Old World monkey by Mohammad Mahdi Karim
Olive baboon, Old World Monkey by Mohammad Mahdi Karim

There are some die-hard scientists that have not made the transition from the “life-moves-with-the earth” theory.  However, the molecular evidence that life has dispersed across vast expanses of ocean is mounting and most scientists have accepted the reality of the evidence.  Science has come full circle, to return to Darwin’s original theory.  As improbable as it may seem, monkeys made the voyage from Africa to South America, across the Atlantic Ocean.


Brown spider monkey, New World monkey.  Creative Commons - Share Alike
Brown spider monkey, New World monkey. Creative Commons – Share Alike

But is that voyage really so improbable?  Within the past decade, we have witnessed two massive earthquakes that caused massive tsunamis.  In December 2004, a tsunami following an earthquake in Asia killed approximately 200,000 people.  A few survivors tell harrowing stories of clinging to rafts of debris at sea to arrive many days later on a foreign shore.  And less than 10 years later, in March 2011, an earthquake and tsunami in Japan killed tens of thousands of people.  Over a year later, huge rafts of debris washed ashore on the West Coast of America, encrusted with sea life that accumulated on that long trip.  They were called “invasive species” when they arrived.  But were they really?  After all they arrived as the result of a natural occurrence with no assistance from humans.

These may seem rare events to us because of our short time perspective.  Multiply those two catastrophic disasters by the millions of years of life on earth to arrive at the conclusion that these events are routine when put into the context of the lifespan of the earth rather than the lifespan of humans. 

Bringing it home

What we learn from The Monkey’s Voyage is relevant to the concerns of Million Trees:

  • Life is constantly in motion whether we are capable of perceiving it or not.  To choose some specific landscape that existed in the distant past as an ideal to be re-created is to deny the reality of nature.  The concept of “native plants” is meaningless.  Native to where?  Native to when?
  • Change in nature is random and therefore unpredictable.  Cataclysmic events render humans impotent to manipulate complex ecosystems.  Human attempts to “manage” nature are arrogant at best and harmful at worst.  For example, when we kill one animal based on a belief that it will benefit another animal, we haven’t sufficient knowledge to predict the outcome with certainty.
  • Science is constantly evolving, just as nature is evolving.  Invasion biology is stuck in a cul-de-sac that is contradicted by the reality of the dynamism and complexity of nature.  There is little scientific evidence that supports the assumptions of invasion biology.

(1)    Alan de Queiroz, The Monkey’s Voyage:  The improbable journeys that shared the history of life, Basic Books, New York, 2014