Evolutionary advantage of introduced species

We have often wondered why so many plants and animals introduced to North America become invasive, compared to species introduced to Europe.  In California, there are about 200 plants on the inventory of “invasive” plants.  In Britain, there are only about a dozen plants considered “invasive.”  In past articles, we have speculated that Americans are using different standards to determine invasiveness and that may be a factor.  But now scientists, Jason Fridley and Dov Sax have recently reported the empirical evidence that suggests some regions are more vulnerable to invasion than others because of competitive advantages of species from regions with longer evolutionary histories.  In fact, Charles Darwin is the original author of this theory:

“Darwin (1859) observed that because ‘natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates, such that, we need feel no surprise at the inhabitants of any one country…being beaten and supplanted by naturalized productions from another land.’  Darwin’s view, one of the earliest on biological invasions, presents invasion as an expectation of natural selection – a view largely absent from modern invasion biology.  Darwin further suggested that species from larger regions, represented by more individuals, has ‘consequently been advanced through natural selection and competition to a higher stage of perfection of dominating power’ and therefore be expected to beat ‘less powerful’ forms found in other regions.” (1)


Based on Darwin’s speculation, Fridley and Sax formulated the evolutionary imbalance hypothesis, based on three postulates:

  • Evolution is essentially an infinite series of experiments as each generation is tested by the conditions they encounter. The more tests the species passes by surviving and reproducing, the more fit the species is to face the next test.
  • The number of such experiments vary by region that differ in size and biotic history, which influences the intensity of competition each species encounters.
  • “Similar sets of ecological conditions exist around the world” thereby facilitating the movement of species from their native ranges to new ranges.

It follows from these postulates that when species from previously isolated habitats are mixed, some species will be more fit than others for any given set of conditions.  In other words, they have an evolutionary advantage by virtue of having faced more competition for a longer period of time.   These are the environmental conditions that are likely to confer such an evolutionary advantage:

  • Larger regions with large expanses of habitat usually have larger populations of species. Larger populations have more genetic variation, which provides more opportunities for natural selection to choose a “winning” genetic combination.
  • Also, more stable environments enable lineages to survive for longer periods of time. The longer the opportunity for natural selection to operate, the more fit the surviving lineage.
  • The greater the competition each species experiences, the more fit the surviving species is likely to be. Therefore, species occupying diverse habitats are likely to be more fit than species in less diverse habitats.

The authors of this new study tested these hypotheses in three geographic areas that have well-documented non-native floras, including Eastern North American, the Czech Republic, and New Zealand.  For example, the climate of the Northeast of America is similar to East Asia.  Some of the most destructive invasive species in the Northeast are from East Asia, such as the emerald ash borer.  Yet species from North America do not become invasive when introduced to East Asia.  Species from East Asia have a much longer evolutionary history than species native to the Northeast because much of the United States was buried in glaciers during the Ice Ages, while East Asia was not.  (2)  The longer evolutionary history of East Asia makes East Asian species “fitter” and more likely to be successful in North America, while North American species are less successful in East Asia.

Kudzu evolved in Japan.  USDA
Kudzu evolved in Japan. USDA

Failure of the competing theory

Invasion biology is the competing theory of why introduced species become invasive when introduced outside their native ranges.  It is a theory that turns its back on evolutionary theory by assuming that plants and animals are incapable of adapting to changed conditions.  Invasion biology assumes that introduced plants become invasive because they leave their predators behind.  This is the predator release theory which also implies that introduced plants are not useful to native animals.

The problem with the predator release theory is that there is no empirical evidence that supports it.  For example, equal numbers of insects are consistently found in native and non-native habitats.  And when empirical studies claim to have found evidence of predator release, sampling errors have discredited those studies:

“For example, one study found fewer parasitic worms in introduced starlings in North America than in the entire native range of Europe and Asia.  But once allowance was made for the actual local source of the starlings, the difference disappears:  various evidence suggests starlings arrived in North America via Liverpool, and American starlings have most of the parasites of Liverpool starlings, plus quite a few others, either American natives or European parasites introduced with other birds.  In fact, American starlings have more parasites than are found in the likely source population.”  (3)

Starling in breeding plumage.  Creative Commons - Share Alike
Starling in breeding plumage. Creative Commons – Share Alike

“Resistance is futile”

And so we add the evolutionary imbalance hypothesis to the long list of reasons why we are opposed to fruitless attempts to eradicate well established non-native species of plants and animals:

And now we know that many invasive species have evolutionary advantages over the native species they have displaced:  “The evolutionary imbalance hypothesis…could have a grim implication for conservation biologists trying to preserve native species:  They may be fighting millions of years of evolution.  If that’s true, the phrase ‘Resistance is futile’ comes to mind.” (2)


  1. Jason Fridley and Dov Sax, “The imbalance of nature: revisiting a Darwinian framework for invasion biology,” Global Ecology and Biogeography, 23, 1157-1166, 2014
  2. Carl Zimmer, “Turning to Darwin to Solve the Mystery of Invasive Species,” New York Times, October 9, 2014
  3. Ken Thompson, Where do camels belong?, Greystone Books, 2014

Climate change requires plants and animals move to survive

Our readers know that we consider climate change the most critical environmental issue of our time.  We also believe that the native plant ideology is antithetical to our concern about climate change for two reasons:

  • The changing climate requires that plants and animals move in order to survive. Therefore, the demand that historical ranges of native plants and animals be restored and maintained is both unrealistic and harmful.  It is unrealistic because the environment has changed in the past 250 years since the arrival of Europeans on the West Coast and it will continue to change.  Therefore, we cannot assume that the native plants that existed here in 1769 are still capable of surviving here.  It is harmful because animals can and do move as the climate changes.  Therefore, eradicating the plants they need for survival is harmful to them.
  • The eradication of non-native plants and trees is exacerbating climate change by releasing their stored carbon into the atmosphere, thereby contributing to the greenhouse gases that cause climate change. When prescribed burns are used to eradicate non-native plants or prevent natural succession the release of carbon into the atmosphere by the plants that are burned is immediate.  When large, mature trees are destroyed, the carbon they have stored as they grew is released into the atmosphere as the wood decays.  Furthermore, their ability to store carbon in the future is lost to us going forward.  Since carbon storage is directly proportional to biomass, whatever we plant in their place is incapable of storing as much carbon as the mature trees.
The umber skipper has adapted to Bermuda grass in lawns in the East Bay.  Creative Commons
The umber skipper has adapted to Bermuda grass in lawns in the East Bay. Creative Commons

There is an important caveat that we must add to our first bullet point.  Changing location is not the only mechanism that can ensure species survival in a changing climate.  Many species are probably “pre-adapted” to the changed climate.  That is, they may be capable of surviving changes in the climate.  Secondly, species can adapt and/or evolve in response to changes in the environment, which is another mechanism that facilitates species survival.  We invite our readers to visit our post about the rapid evolution of finches in the Galopagos Islands in response to extreme weather conditions that caused selection events.

Today we will inform our readers of the scientific record regarding the need for plants and animals to move as the climate changes.  We will use the recently released fifth report of the Intergovernmental Panel on Climate Change as our source.

Intergovernmental Panel on Climate Change

First we will establish the credibility of the Intergovernmental Panel on Climate Change (IPCC).  The IPCC was formed in 1988 by the United Nations.  It is composed of thousands of scientists from all over the world, representing the 190 member nations of the UN.  The IPCC does not conduct original research.  Rather it compiles thousands of peer-reviewed scientific studies into reports that represent a consensus viewpoint of the global scientific community.  Typically, scientists from 120 countries participate in marathon sessions in which consensus must be reached before reports can be published.  The IPCC has published 5 reports since 1988, the most recent earlier in 2014.

How the climate has changed and how it will continue to change

The IPCC compiled several different sources of data to report how the climate has changed from 1900 to the present.  Then they modeled the multitude of variables that influence climate to predict different trajectories for the climate going forward to 2100.  The many variables that influence climate interact in complex ways that are not entirely predictable.  There is therefore some uncertainty in those predictions, as there is in any prediction of the future.  Therefore, future temperature is depicted by the following graph as “bands” of probability.  The bands become wider as the graph depicts further into the future, as we would expect; that is, the distant future is less predictable than the near future.

Observed and projected temperature change, IPCC 2014
Observed and projected temperature change, IPCC 2014

Here’s what we learn from this graph:

  • The graph reports that the average global temperature has increased by 1° Celsius (1.8° Fahrenheit) from 1900 to the present.  Graphs depicting the more distant past indicate that the climate began to warm around the time of the industrial revolution, about 1850.  Therefore the total increase in temperature is greater than that depicted by this graph.  However, the rate of increase has accelerated greatly in the past 50 years.
  • The upper range of projected temperature increases on the graph is labeled RCP8.5 (Representative Concentration Pathway 8.5). That pathway is based on the assumption that present levels of greenhouse gas emissions will continue to increase at the same rate as they have in the recent past.  The mean prediction of that pathway is a global temperature increase from the present to the end of the century of 3.7° Celsius (4.6° Fahrenheit).
  • The lower range of the projected temperature increases on the graph is labeled RCP2.6 (Representative Concentration Pathway 2.6). The mean prediction of that pathway is a temperature increase to the end of the century of 1° Celsius (1.8° Fahrenheit).  That pathway is based on the assumption that greenhouse gas emissions are radically reduced, beginning immediately, as represented by the following graph from The Guardian.  This graph also depicts two intermediate emission scenarios between the present trajectory(RCP 8.5) and the maximum predicted reductions in emissions (RCP 2.6)
Projected energy use
Projected energy use

Movements needed for survival in a changing climate

The world has done little to reduce greenhouse gas emissions and America has done even less.  According to a recent Gallup Poll, only 39% of Americans are “concerned believers” in climate change.  Another 36% of Americans believe the climate is changing, but don’t believe it will affect them.  Twenty-five percent (25%) of Americans do not believe the climate is changing.  Therefore, for the time being, it seems extremely unlikely that our polarized politics in America will be capable of responding effectively to the grim reality of climate change.  Within that context, we inform you of the final graph from the IPCC report about the need for plants and animals to move from their present ranges in response to climate change and their variable ability to do so.

Adaptation to Climate Change.  IPCC
Adaptation to Climate Change. IPCC

On the vertical axis, the graph depicts the ability of plants and animals to move, measured in kilometers per decade.  The horizontal lines depict the need of plants and animals to move in response to various scenarios of climate change as we described earlier.  The bars depict the ability of plants and animals to move and the height of each bar informs us of the variable ability of plants and animals to move.  Trees are the least able to move, unless we have the wisdom to plant them outside their native ranges—at higher latitudes or elevations–where they are more likely to survive in the future. 

For example, if we radically reduce greenhouse gas emissions immediately (RCP2.6), most species of trees and plants will be sustainable at their present latitudes and elevations.  But if greenhouse gas emissions continue on their current trajectory (RCP8.5), most species of trees and plants will not be capable of moving far enough, fast enough to survive as the climate warms.  Although trees and plants are capable of moving only very slowly, most animals are capable of moving more rapidly.  Will they have the plants they need to survive in their new ranges?

 Putting our heads in the sand

Surely there aren’t many native plant advocates in the San Francisco Bay Area who don’t believe in the reality of climate change.  The Gallup Poll reports that most people who don’t believe in climate change are Republicans and in the San Francisco Bay Area Republicans are a small minority.  And so we ask native plant advocates this question:  How do you reconcile the reality of climate change with your demand that native plants be restored and maintained where they existed 250 years ago in a very different climate?


Global increases in biodiversity resulting from new species

Great horned owl in eucalyptus.  Courtesy urbanwildness.org
Great horned owl in eucalyptus. Courtesy urbanwildness.org

One of the most popular justifications for eradicating non-native plants is the claim that they will out-compete native plants, ultimately causing their extinction.  Innumerable studies have found no evidence to support that claim, but the belief persists amongst those who demand the eradication of non-native plants.

Islands have been considered particularly vulnerable to extinctions because they contain many endemic species (found only on that island) that have evolved in physical isolation from their ancestors from other places and become unique species.  And there were many animal extinctions–particularly of flightless birds–with the arrival of humans who were both their predators and brought predators with them.

However, despite the conventional wisdom that the introduction of new species of plants to islands would result in extinction of their predecessors, there is no evidence that this is indeed the case with introduced plants.  In 2008, Dov Sax and Steven Gaines published a study of species diversity on islands.  This is what they found:

Honeybee on wild mustard.  Courtesy urbanwildness.org
Honeybee on wild mustard. Courtesy urbanwildness.org

Predation by exotic species has caused the extinction of many native animal species on islands, whereas competition from exotic plants has caused few native plant extinctions…By analyzing historical records, we show that the number of naturalized plant species has increased linearly over time on many individual islands. Further, the mean ratio of naturalized to native plant species across islands has changed steadily for nearly two centuries. These patterns suggest that many more species will become naturalized on islands in the future.” (1)

In other words, the introduction of new plants to islands has not resulted in extinctions of the plants that preceded them.  Therefore, the result of plant introductions has been greater plant diversity on islands.

But what about the continents?

Painted lady butterfly on Weigela.  Courtesy urbanwildness.org
Painted lady butterfly on Weigela. Courtesy urbanwildness.org

Recently a new study was published that asked the same question on a global scale:  Has the introduction of new plants and animals resulted in the extinction of their predecessors?  The answer is a resounding NO!  (2)

The study was conducted on a huge scale by an international team of scientists:

  • “6.1 million species occurrence records from 100 individual time scales”
  • “35,613 species were represented…including mammals, birds, fish, invertebrates, and plants”
  • “The geographical distribution of study location is global, and includes marine, freshwater, and terrestrial biomes, extending from the polar regions to the tropics in both hemispheres.”
  • “The collective time interval represented by these data is from 1874 to the present, although most data series are concentrated in the past 40 years.”

Like most scientists who expect to find evidence of decline, this team of researchers was surprised to find little evidence of loss.  Here are some of their key findings:

  • “Surprisingly, we did not detect a consistent negative trend in species richness or in any of the other metrics of α diversity.”
  • “There is no evidence of consistent loss of biodiversity among terrestrial plants.”
  • “Time series for terrestrial plants exhibit, on average, a positive slope for species richness.”
  • “Collectively, these analyses reveal local variation in temporal α diversity but no evidence for a consistent or even an average negative trend.”  (Alpha diversity is species richness at the local level.)
  • “An analysis of slopes by climate regions reveals that temperate time series have a significantly positive trend…”

In other words, new plants result in more plants, particularly where we live, in the temperate zone.  There is no empirical evidence that new plants have resulted in the loss of the plants that were there before they arrived.

So what’s the beef?

Song sparrow in wild radish.  Courtesy urbanwildness.org
Song sparrow in wild radish. Courtesy urbanwildness.org

You might think that this huge new study would put the controversy to rest.  You would be wrong.  For every answer we find, there is a new question from nativists.  The response of native plant advocates to the good news that the plants they prefer will not disappear if new plants are allowed to live in their company is that the plant world is being “homogenized.”  They say that if new plants are permitted to remain, all landscapes will become the same, resulting in the loss of unique landscapes that existed in the past.

They are, of course, mistaken.  Their dire prediction will not come to pass because the biotic and abiotic conditions of every landscape are unique.  The climates are different.  The soils are different.  The atmosphere is different.  The plants and animals that are there when they arrive are different.  If the new plant survives in its new home, it will be capable of adapting to these local conditions and over time it will change, ultimately becoming a unique species.  When the first family of monkeys made the voyage from Africa to South America, they were the same species as those they left behind.  Now they are unique species as a result of genetic drift and genetic divergence.

The process of adaptation and evolution is often more rapid than we expect.  Sometimes such changes have occurred within the lifetimes of scientists who were able to witness these changes.  More often, the changes occur more slowly and are only visible in museum collections or fossil records.

Consider the consequences

Garter snake in eucalyptus leaf litter.  Courtesy urbanwildness.org
Garter snake in euclypatus leaf litter

It is physically impossible to prevent the arrival of new species.  Even when they are not intentionally introduced they find a way to piggy back on the daily activities of humans.  They arrive on our airplanes and cargo ships.  We aren’t going to stop importing or exporting our products all over the world.  Nor are we going to quit traveling.  We must accept the consequences of the way we live and quit blaming plants and animals for their passive participation in our movements.

Aside from the question of whether or not it is physically possible to stop the arrival of new plants and animals, let’s acknowledge that at least in the case of plants no great harm has come from their introductionSince we now enjoy more plants than were here when they arrived, just what is it that we’re griping about?  We seem to be griping about change.  Change will occur whether we like it or not.  We can’t prevent change, so we must quit fighting against something that we are powerless to prevent.  That is the definition of wisdom.

Finally, we must consider the consequences of trying to eradicate non-native plants that are firmly entrenched in our landscapes.  Huge amounts of herbicide are being used in the futile attempt to eradicate them.  Fires that pollute the air and endanger our homes are set for the same purpose.  Trees that are performing valuable ecological functions are being destroyed.  The animals that use these plants and trees for food and cover are being deprived of their homes and their food.  We are doing more harm than good.


  1. Dov Sax and Steven Gaines, “Species invasions and extinctions: The future of native biodiversity on islands,” Proceedings of the National Academy of Sciences, August 12, 2008
  2. Maria Dornelas, et. al., “Assemblage times series reveal biodiversity change but not systematic loss,” Science, April 18, 2014

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

A Book Review: The Signature of All Things

Organisms classified as mosses.  72nd plate from Ernst Haeckel's "Kunstformen der Natur" (1904, public domain)
Organisms classified as mosses. 72nd plate from Ernst Haeckel’s “Kunstformen der Natur” (1904, public domain)

We have read little fiction in the past few years, as we struggle to keep pace with the scientific literature that is revising conservation biology.  Happily, we were recently given the opportunity to read a charming work of fiction that is firmly in the center of our interest in botanical issues.

The Signature of All Things was written by Elizabeth Gilbert.  Its title refers to a botanical myth about which we have published an article that is available here.  The Doctrine of Signatures seemed a logical botanical belief at a time when plants were one of man’s few medicinal tools and religion was a powerful influence in human society.  The Doctrine of Signatures, which was actively promoted by the church in 17th century Europe, was based on a belief that God had “signed” plants with certain suggestive shapes and colors to inform humans of their medicinal properties.  For example, a heart-shaped leaf was considered God’s message to us that a particular plant would be beneficial to the human heart and this message was strengthened by a flesh-colored flower. Every plant was believed to be useful in some way if man could only discern its purpose.  Else why would they have been created, since the Garden of Eden was created for the benefit of man?  The church encouraged man’s study of plants as a way to worship God’s creation.

After reading a rave review by one of our favorite authors, Barbara Kingsolver, we were unable to resist the diversion to this story that is inspired by botanical history.  Kingsolver concludes, “The Signature of All Things is a bracing homage to the many natures of genius and the inevitable progress of ideas, in a world that reveals its best truths to the uncommonly patient minds.”

Signature begins in Kew Garden in London during the 18th Century reign of one of our great horticultural heroes, Joseph Banks.  We featured Banks in an article about the English garden.  He began his career as an intrepid collector of exotic plants when he joined one of Captain Cook’s voyages into the Pacific.  He returned with thousands of plants from all over the world and they became the core of Kew Gardens, one of the greatest horticultural collections in the world.

The hero of Signature is sent by Banks on expeditions to collect valuable plants and his adventures are an historical account of early explorations of the New World.  We learned from Richard Henry Dana’s Two Years Before the Mast that the physical hardships of these voyages are not exaggerated by Signature’s fictional account.  The hero of Signature eventually makes his home in Pennsylvania and his extensive garden there is reminiscent of the garden of John Bartram, the Early American collector of plants about whom we have also written.

So you see, Signature covers familiar ground for us and we enjoyed revisiting it in the company of an extraordinary heroine, Alma Whittaker.  She is gifted with a remarkable mind and her equally intelligent parents provided her with the education and tools needed to make life-long good use of her talents.  She “discovered” her own version of evolutionary theory based on a deep understanding of mosses, which model the mechanics of natural selection.

We don’t wish to give away too much of the plot because we hope you will be intrigued to read it.  Readers will have the privilege of eavesdropping on a fascinating (fictional) conversation with Alfred Russel Wallace, a contemporary of Darwin.  Although Darwin and Wallace shared a belief in evolution, they diverged on a variety of other topics.  Wallace’s busy mind strayed into spiritualism, hypnotism, and mesmerism as well as left-wing politics.  Wallace was as eccentric as Darwin was sensible and cautious.

Alfred Russel Wallace
Alfred Russel Wallace

Our heroine, Alma, confides to Wallace that despite a tortuous path in life, she considers herself lucky: “I am fortunate because I have been able to spend my life in study of the world…This life is a mystery, yes, and it is often a trial, but if one can find some facts within it, one should always do so—for knowledge is the most precious of all commodities.” 

Alma’s confession was a welcome reminder of why we persist in our effort to inform the public of the destruction of our public lands by native plant “restorations.”  Although we make little visible progress, we have learned a great deal about nature.  That is our reward.  Thank you, Alma, for the reminder of our mission to understand and inform and to Elizabeth Gilbert for the very pleasant entertainment of The Signature of All Things.

The Endangered Species Act is based on outdated science

We have reported to our readers many times about the changes in scientific opinion regarding invasion biology in the past fifty years, since the inception of the theories that originally supported that discipline.  Now we see an acknowledgement of the changed scientific viewpoint in a critique of the Endangered Species Act by the legal profession.

Holly Doremus is Professor of Law at Boalt Law School at the University of California, Berkeley.  Her critique of the Endangered Species Act (ESA) was published by the Journal of Law & Policy in 2010.  She previews her theme in her introduction:

“I am interested in why the ESA came to assume an unrealistically static vision of nature.  First, the Act’s static structure is typical of law in general, which has traditionally embodied the human search for stability.  Second, the Act is inevitably, a product of the political times in which it was drafted and of a rapid and chaotic legislative process, which did not encourage thoughtful examination of the complex contours of the conservation problem.  Third, it followed in part from incorrect but widely shared assumptions about the nature of the problem and potential solutions.  Fourth, scientific understanding was itself in transition as the law was being crafted, moving from a focus on the tendency of ecological systems to approach equilibrium to one on the ongoing dynamics of many systems.” (1)

We will focus on the fourth issue, i.e., how the ESA is in conflict with the reality of constantly changing ecosystems. 

A Static Vision of Nature

The ESA is based on assumptions about nature that were the conventional wisdom at the time the law was passed in 1973:

  • Evolution was considered an historical process that was no longer actively changing plant and animal species.  Theoretically evolution does not end, but at the time the ESA was passed in 1973, it was not believed to occur within a time frame that would be observable by man.  Plant and animal species were therefore viewed as being distinct and unchanging.
  • This view of evolution was consistent with the prevailing public opinion in the United States, which does not believe in evolution.  Many Americans believe that species have not changed since they were created by God.
  • Nature was perceived as reaching an “equilibrium state” that was stable over long periods of time.
  • Early conservation efforts were therefore based on the assumption that once achieved, an equilibrium state could be sustained if left undisturbed in nature preserves.

    Darwin's finches are an example of rapid evolution
    Darwin’s finches are an example of rapid evolution

We now know that these assumptions were mistaken.  Evolution can occur very rapidly, particularly amongst plants and animals with short life spans and frequent generations.   And ecosystems are constantly changing, particularly at a time of a rapidly changing climate and associated environmental conditions such as atmospheric conditions. 

Professor Doremus tells us that ecological scientists played no role in the writing of the ESA and took little notice of the law when it was passed.  The press also ignored the new law, which may have been a factor in its being unnoticed by the scientists who may have been in a position to raise the questions that should have been asked.  “It seems that conservation scientists, like the general-interest press, and most legislators, did not consider the ESA groundbreaking, or even particularly important.” (1) In any case, the problems that have arisen in the implementation of the law were not foreseen by the politicians who passed it, nearly unanimously in 1973.

How does the ESA define “species?”

As its name implies, the heart of the law is how “species” are defined.  In fact, if the law had stopped at providing legal protection for “species,” we would not be experiencing nearly as much difficulty with the implementation of the law.  Unfortunately, the ESA’s “…definition of ‘species’ [is] broad, but not a model of clarity, ‘The term “species” includes any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature.”  (1)

Splitting species into sub-species and “distinct population segments” has proved very problematic because taxonomy (the classification of organisms) has always been inherently subjective and will probably continue to be.  The taxonomic system that was popular at the time the ESA was passed was Mayr’s biological species concept which identifies as a species any group that interbreeds within the group but not with outsiders.  This definition is not useful for a species that hybridizes freely, such as the manzanitas of which six species have been designated as endangered.  Professor Doremus tells us that US Fish & Wildlife Service now evaluates the legal consequences of hybridization on a case-by-case basis. 

Since the ESA was passed, many competing definitions of “species” have been proposed by scientistsThere were 22 different definitions of species in the modern literature as recently as ten years ago.  These competing definitions reflect disagreement about appropriate criteria for identifying species—morphology, interbreeding, or genetic divergence, as well as the degree of difference needed to define the boundary between species.   We see these scientific controversies played out repeatedly in the law suits that are interpreting the ESA. 

The identification of “distinct population segments” amongst vertebrates has proved to be even more problematic.  Legal challenges to the determination of distinct population segments have reversed the rulings of the US Fish & Wildlife Service for many species that were considered genetically identical such as the sage grouse (eastern vs, western?) and the Preble’s meadow jumping mouse (found in different meadows in the Rocky Mountains).  In some cases, these rulings were reversed several times, and perhaps will be again!  These reversals reflect the ambiguity of the law, as well as the science of taxonomy.  The fact that the ESA specifically allows “citizen suits” has pushed the regulating agencies to implement the law more aggressively than  politics alone would have predicted.

Species can and do move

In addition to considering species immutable and unchanging, the ESA also takes a static view of where they live.  The concept of “distinct population segments” depends somewhat on the assumption that species of animals don’t radically alter their ranges in the short-term.  The assumption is also consistent with the underlying conservation policies that tend to preserve specific places in order to protect rare species within those places.

We now understand that some ecosystems are internally dynamic.  We recently told our readers of the need for the sand dunes near Antioch, California to move freely in the wind to sustain that fragile ecosystem.  Professor Doremus also tells us about the constantly changing courses of braided rivers in Nebraska that are essential to the sustainability of that unique ecosystem. 

Platte River in Nebraska is a braided river.  Creative Commons
Platte River in Nebraska is a braided river. Creative Commons

In a rapidly changing climate, the preservation of a species may require changing ranges.  If the climate becomes too cold, too hot, too wet, or too dry for a species of plant or animal, its immediate survival may require that it move to higher or lower altitudes or latitudes.  Moving may be a more effective strategy than the adaptation that may be slower than necessary to survive.  Freezing species into their historic ranges does not ensure their survival at a time of rapidly changing climate.  In some cases, a species has become plentiful in the new territory it has freely chosen to inhabit and simultaneously rare in its historic range where it has been designated as an endangered “distinct population.”  Draconian measures have been taken to restore a species in its historic range, where it is no longer adapted to current conditions.  

We leave you with Professor Doremus’ observation about the ESA:  “The ESA’s static view of species, landscapes, and conservation obligations, while entirely understandable, has become a hindrance to effective conservation.  The ESA’s lofty goals of conserving species and the ecosystems upon which they depend cannot be achieved without a more realistic vision of the dynamic qualities of nature and the ability to respond to the changes that are inevitable in dynamic systems.”


(1)    Holly Doremus, “The Endangered Species Act:  Static Law Meets Dynamic World,” Journal of Law & Policy, Vol. 32: 175-235, 2010.

Wily weeds win the war

Farmers have been battling with weeds since the advent of agriculture, about 6,000 years ago.  Most of the weapons used against weeds were mechanical until the last century or so when herbicides became the primary weapon.  At the same time that the weapon became more lethal, farming techniques changed to give weeds the advantage.  Farms became huge monocultures and crop rotations were abandoned in favor of the most profitable crop. 


When weeds evolved defenses against the herbicides, farmers responded by increasing doses and manufacturers created new products to which weeds hadn’t yet evolved resistance.  Finally, the use of herbicides skyrocketed when crop seeds were invented that weren’t killed by the herbicides, so that huge amounts of herbicides can be used without killing the crop. 

When Roundup with the active ingredient glyphosate went on the market in the 1970s, its manufacturer, Monsanto, claimed that weeds would not be able to evolve resistance to it.  And apparently that was initially true until Roundup was used on a huge scale when herbicide-resistant seeds were put on the market in the 1980s.   Norman Ellstrand of UC Riverside explains why:  “He argues that the reason was that farmers applied glyphosate to relatively little farmland.  As they applied it to more and more acreage, they raised the evolutionary reward for mutations that allowed weeds to resist glyphosate.  ‘That ups the selection pressure tremendously,’ he said.” *

There are now 24 species of weeds that are resistant to glyphosate and they are rapidly expanding their range in agricultural areas all over the world.  In 2012, an agricultural consulting firm reported that 34% of farms in the US had glyphosate-resistant weeds.  In the first half of 2013, half of all farms in the US are reporting glyphosate-resistant weeds.

Let’s take another approach

Obviously, we are losing the war against weeds.  So, let’s examine the strategy we have been using and try another approach.  Weed ecologists are now studying the strategies that weeds have used to cope with the weapons we have been using against them.  Here are a few of those strategies:

  • Some weeds have changed color so that they are indistinguishable from the crop they are hiding in.
  • Weeds that grew in dry ground, evolved to thrive in wet ground in rice fields that are flooded much of the crop season.
  • Some weeds became shorter to escape the mowing and harvesting of the agricultural crop.
  • Some weeds drop their seeds and go dormant before the crop is harvested and create seed banks that can sprout when conditions are more favorable for them.
  • Parasitic weeds wrap around their host and steal nutrients from them.

Some weed ecologists believe that a better understanding of the mechanisms used by weeds to foil our attempts to control them will enable us to devise better weapons against them.  They believe that developing new herbicides and/or using more of them will always be a short-term solution. 


For example, David Mortensen of Penn State is “investigating controlling weeds by planting crops like winter rye that can kill weeds by blocking sunlight and releasing toxins.  ‘You want to spread the selection pressure across a number of things that you’re doing so that the selection pressure is not riding on one tactic,’ he said.”*

Regardless of what method is used to control weeds in the future, let’s consider the toxicity of the method the most important criterion for judging their effectiveness.  Even if it kills fewer weeds, the least toxic alternative is the best alternative in our opinion.


*Carl Zimmer, “Looking for Ways to Beat the Weeds,” New York Times, July 15, 2013.

Professor Arthur Shapiro’s Review of Emma Marris’ Rambunctious Garden

Rambunctious GardenProfessor Arthur Shapiro is Distinguished Professor of Evolution and Ecology at University of California Davis and a renowned expert on the butterflies of California.  His public comment on the Draft Environmental Impact Report for the Natural Areas Program is one of the most popular articles on Million Trees.

Professor Shapiro has written a review of Emma Marris’ Rambunctious Garden and given us permission to reprint it here.  We share his high opinion of Ms. Marris’ book and we urge you to give it the careful read it deserves.


Rambunctious Garden: Saving Nature in a Post-Wild World by Emma Marris

Review by: Arthur M. Shapiro

The Quarterly Review of Biology, Vol. 88, No. 1 (March 2013), p. 45

Published by: The University of Chicago Press

Stable URL: http://www.jstor.org/stable/10.1086/669328 .

“Several years ago, I attended a seminar on the psychology of the animal-liberation movement. The speaker observed that although very few animal-lib activists were actually religious, most such people scored very highly on the “religiosity” scale in personality inventories. He suggested that animal liberation served the same functions for such people as religion did for many more: it gave life meaning and conferred a group identity centered on shared moral superiority over others. After years of interacting with “weed warriors”—people who spend their free time trying to eradicate “invasive species” from parks and public lands—I would advance the same hypothesis about most of them. They tend to be absolutely convinced of the righteousness of their cause and highly resistant to any suggestion that naturalized exotics might not be all bad. They also tend to be oblivious to the disconcerting degree to which their rhetoric converges to that of racists and xenophobes, and highly defensive if you point that out to them. After all, they are on the “green” side, right?  

In the face of such popular enthusiasm for the alarmist viewpoint on exotics, Emma Marris, a professional science writer, has produced an eminently reasonable, well-researched, and engagingly written defense of the notion that human beings have changed the world and the most sensible way to deal with that is to manage it for the greatest good. She demonstrates very convincingly that communities and ecosystems have always been in flux as the physical world changes around them. The idea of freezing them at some arbitrary moment in time is as wrongheaded as it is impractical. Some naturalized exotics present serious threats to human beings or their support systems: we call them pests, pathogens, and vectors, and they are not what is at issue. Some are such radical ecological gamechangers that they need to be assessed with an eye to the full scope of their impacts (think cheatgrass in the desert and its impact on fire ecology). Most, however, are trivial, and in a world of limiting resources where we must assign priorities to our actions, they do not merit serious attention. But it is not merely a matter of using our management resources effectively. Much of our “invasive species” discourse simply ignores the evolutionary creativity consequent on community reorganization.

Yet we know both in theory and from the fossil record that precisely such creativity is essential for long-term survival in a changing physical context. Ecotypes or ecological races arise in response to novel challenges, both biotic and abiotic. The future of endangered species is likely to depend on such processes. Failure to appreciate this is the single biggest flaw in the “climatic envelope” or “niche modeling” approach to conservation biology. Much of California’s lowland butterfly fauna is now dependent on nonnative larval host plants. When I tell garden clubs—or public land managers—that successful eradication of invasive “weeds” would drive their beloved backyard butterflies to extinction, they stare at me in disbelief. But it is true and emblematic of the larger problem explored very well in this volume.

Shortly after Marris’s book appeared there was a flurry of articles in the professional literature advancing precisely the same ideas. Among the best are by Carroll (2011. Evolutionary Applications 4:184–199) and Thomas (2011. Trends in Ecology and Evolution 26:216 –221). But Marris got there first, and with luck her wise words will be read and acted upon far and wide.”

Arthur M. Shapiro, Center for Population Biology, University of California, Davis, California

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.


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