The Dawn and Dusk of the Age of Mammals

In Beasts Before Us, paleontologist, Elsa Panciroli, traces the evolutionary history of the mammal class of the animal kingdom, of which humans are members, to its origins about 300 million years ago.  It’s a tedious recitation of multitudes of now extinct species from their earliest ancestors up to the dawn of the age of mammals that began 66 million years ago after the abrupt end of the age of dinosaurs. But it’s also a rewarding read because it reminds us of our close relationships with other animals as well as the ways in which we are different.  Those differences predict which mammals will survive the forthcoming sixth great extinction that humans have inflicted on life on Earth.

Mammals living today have in common only one characteristic that distinguishes them from other classes of close relatives.  The subdivisions of mammals alive today have mammary glands that produce milk to feed their young.  The three subdivisions of mammals are monotremes, marsupials, and placentals.  Monotreme species alive today are platypus and echidna whose young are hatched from eggs, but are milk fed by their mothers.  Marsupials are born at an undeveloped stage and carried to term in their mothers’ pouch.  By far the largest group, placentals carry their developing offspring inside the mothers’ abdomen until birth. 

The earliest ancestors of mammals were four-legged vertebrates called amniotes. Amniotes were named for the membrane that lined the hard shells of their eggs, protecting the embryo.  The development of the amniotic membrane provided protection needed to lay and hatch eggs on land rather than the ocean where earlier forms of life lived.  This evolutionary development was associated with the transition of life from the ocean to the land.  The earliest amniotes diverged to take two different evolutionary paths, one as reptiles and dinosaurs (sauropsids) and the other as mammals (synapsids).  Pause here briefly to contemplate our close relationships with other animals. 

The Science of Paleontology

Beasts Before Us is also interesting as a history of paleontology, the branch of science that studies fossils of plants and animals to determine the evolutionary history of life.  Beasts focuses on advances in modern paleontology, but this article takes readers further back in time to appreciate how recently we learned about the scale of past extinctions that predict future extinctions.

Prior to the 19th century, an understanding of extinction was inconsistent with prevailing Western belief that the world was created by God as complete, perfect, and unchangeable.  In the late 17th century fossils of extinct animals were discovered that appeared to be unlike any living species.  Inquiring minds began the search for an explanation for what happened to these unknown species. 

George Cuvier is credited with establishing the modern concept of extinction in a lecture to the French Institute in 1796.  Cuvier is sometimes called the “founding father of paleontology.”  He rejected the theories of evolution, believing instead that extinctions could be explained by “cyclical creations” and catastrophic natural events such as floods. 

The fossil record is limited in what it can tell us about life in deep time because it does not preserve the remains of extinct species with equal reliability.  Bones survive to tell the tale with greater accuracy than soft tissues and plants.  Paleontology is developing techniques to compensate for gaps in the fossil record, drawing from other scientific disciplines, such as botany, biochemistry, mathematics, and engineering. 

Since more than 99% of all species that ever lived on Earth—more than five billion species–are now extinct, we can only imagine the difficulty of the task of piecing together the complete phylogenetic tree of life.  Beasts Before Us gives us a current view of what has been accomplished to date.  Clearly it is not the end of the story and much of the story is still speculative. 

Divergent Evolution

The 300-million year journey from the first ancestors of mammals to modern mammals of today is a story of divergent evolution, the accumulation of differences between closely related populations within species that lead to new species. Tracing that long process was until recently dependent upon the fossil record and was therefore focused on changes in bone structure, particularly teeth, jaws, and skulls for which the fossil record is more intact. 

Evolutionary tree of mammals. Wikimedia Commons

These bone structures are important clues about the diet of animals. The teeth of herbivores, insectivores, and carnivores are different.  “Mammal fossils can be distinguished and named based on their teeth alone.” (1) Nearly half of all mammal species are rodents, a name that comes from the Latin word for gnaw.  Their long front teeth grow continuously as they are ground down by gnawing on tough plant material such as tree bark in the case of beavers or the wooden shingles on my home in the case of squirrels.

The digestive systems of mammals also diverged to accommodate their different diets (or vice versa).  Carnivores typically have a short intestinal track where digestion is accomplished with enzymes and resident microbial communities.  Herbivores have a longer digestive system in which plant material is fermented in a series of separate chambers in the case of ruminants (cows, sheep, deer, etc.). 

Divergent evolution creates diverse species with diverse abilities to exploit different ecological niches while reducing competition between species.  Shortly after the divergence of mammal and reptile lineages, the characteristic most consequential to the fate of those lineages was endothermy (warm-bloodedness) in mammals and ectothermy (cold-bloodedness) in reptiles. The divergence of this characteristic occurred about 250 million years ago, shortly after the divergence of mammal and reptile lineages. 

Only mammals and birds are generally capable of generating their body heat internally.  Over millions of years they also evolved insulation that conserves body heat with fur, feathers, and blubber in the case of marine mammals. A diet high in sugar and fat also helps to maintain body heat. Cold blooded animals depend on external heat sources such as sunlight to be active.  These crucial differences in mammals and reptiles relegate them to different ecological niches to which they are suited, for example:

  • Mammals and birds can survive in colder climates than reptiles.
  • Mammals and birds can be more active at night when it is cooler.
  • Mammals and birds can be more active for longer periods of time than reptiles.
  • Mammals and birds can live below ground where it is colder in summer and warmer in winter than above-ground temperatures.
  • On the other hand, mammals and birds must eat more and more frequently than reptiles. 

These significant differences are partly responsible for the sudden transition from the age of dinosaurs to the age of mammals 66 million years ago.  During the age of dinosaurs, mammals were small, lived below ground, and ate primarily insects.  This lifestyle avoided competition with huge dinosaurs that dominated the land. 

Scale diagram comparing a human and the largest-known dinosaurs of five major clades Creative Commons Attribution-Share Alike 4.0 International license.

When the asteroid hit the Earth 66 million years ago, the climate was suddenly and drastically transformed from a tropical climate to a cool, partly sunless climate.  Vegetation adapted to a tropical climate quickly died, depriving dinosaurs of their food if they weren’t killed outright by the impact. 

Beasts paints a vivid and dire picture of the cataclysmic event that ended the age of dinosaurs.  The asteroid created a crater almost 100 miles in diameter and 12 miles deep.  “An earthquake larger than any recorded in human history would have made the Earth reverberate like a bell.  The thermal shockwave would have flash-fried all life for hundreds of miles.  The blast of air probably flattened forests as much as 1,000 kilometers away…[the impact] created a mega-tsunami at least 330 feet in height…[that] mounted the coasts of North American and barreled inland like a liquid steam-roller…The dust in the atmosphere swirled its way around the planet until it enclosed all life in its smothering grip.  The sun rose, but as little as half of its light could penetrate the dust in the atmosphere.  The sulphur in the dust combined with water droplets to rain sulphuric acid on the land, burning away the green vegetation…Few animals bigger than a Labrador dog survived the extinction event.”  (1; not verbatim)

The fifth extinction predicts the consequences of the sixth extinction

Small mammals were safely below ground and they didn’t require the great quantities of plant food required by dinosaurs.  Mammals inherited the Earth and over millions of years they evolved into some 5,500 mammal species today of which 90% are still small bodied, most of them rodents.

The final chapter of Beasts uses the consequences of the fifth extinction that ended the age of dinosaurs to predict the consequences of the anticipated sixth extinction because “Humans are replicating many of the conditions of previous mass extinctions.” (1)

  • Animals are likely to become more active at night, when temperatures are cooler.
  • Animals are likely to find some respite by living below ground where temperatures are more moderate in winter and summer. 
  • Animals will move to more temperate regions if they can.

The animals that are most likely to survive will be small generalists, who need less food, are not fussy about what they eat, and are more capable of tolerating heat. Think rats. Beasts advises, “If I were you, I’d say goodbye to any wild animal bigger than a pig—zoos are likely to be the only refuges for them in the future we are creating.”

Birds were the only descendants of dinosaurs to survive the fifth great extinction.  They are expected to fare better in the sixth extinction for much the same reason:  they can be active at night; they eat insects as well as plants; they are more mobile than most classes of animals.  We often hear dire predictions of the fate of birds, but in fact they are less threatened than other classes of animals.  A recent study reported that 21% of reptiles are threatened with extinction, a higher risk than birds (of which about 13 percent of species are threatened with extinction) and slightly less than mammals (25 percent). Amphibian species are at highest risk with about 40 percent of species in danger of extinction.  We hear more about birds because their popularity motivates greater media coverage about them.

I will also presume to give my readers some advice. 

  • Quibbling about whether native plants are superior to non-native plants is like arguing about the color of the lifeboat. It really doesn’t matter.  Soon enough we will be glad to have ANY vegetation that is capable of living in the climate we have created. The universe is indifferent to the survival of any specific species of life.
  • You can do more for the environment and the animals that live in it by stopping the use of pesticides than by planting native plants. 
  • Be humble about what you think you know.  Many important scientific concepts such as evolution and extinction are less than 200 years old and the cause of the extinction of dinosaurs was discovered less than 50 years ago.  What you learned 50 years ago may need to be reconsidered and revised.  A rapidly changing situation requires that we keep an open mind to new information.
  • Set meaningful prioritiesClimate change is an existential threat to all life on Earth.  Ask yourself how we can justify the destruction of healthy trees that sequester the carbon that contributes to climate change? 

  1. Elsa Panciroli, Beasts Before Us: The Untold Story of Mammal Origins and Evolution, Bloomsbury Sigma, 2021

History of Earth predicts its future

My interest in the native plant movement began about 25 years ago when my neighborhood park was designated as a “natural area” by San Francisco’s Recreation and Park Department.  My park was only one of 33 parks in San Francisco that were designated as a “natural area.” 

What did it mean to be a “natural area?”  As I studied the plans, my reaction was primarily to the proposed destruction of non-native plants and trees.  Later I realized that the eradication of non-native plants and trees would be accomplished with herbicides. 

Stern Grove Park in San Francisco was my neighborhood park where I began my long journey to understand why anyone would want to destroy trees in a treeless neighborhood. 

How could the creation of native plant gardens justify the destruction of our urban forest using herbicides?  I have spent the last 25 years trying to answer that question.  There are many useful lines of inquiry in the search for the answer, but the approach that has been most helpful to my understanding of the futility of the undertaking has been the study of the physical and biological forces that created Earth and its inhabitants.  Today, I will take you on an abbreviated journey of the past 4.6 billion years of events on Earth that have resulted in present-day nature, drawing from A Brief History of Earth by Andrew Knoll, Professor of Natural History at Harvard University. (1)

Gravity “created” the Earth

“Gravity is the architect of our universe.”  Gravity is the attraction of objects to one another in proportion to their mass and proximity that over billions of years accumulated the elements dispersed in Earth’s universe.  As these dispersed objects coalesced into stars, planets, moons, and asteroids, Earth was formed about 4.6 billion years ago.

Cross-section of Earth. Source: USGS

“Earth is a rocky ball.”  Its inner core is solid, composed mostly of iron.  Earth’s molten outer core moves by convection as hotter, denser material near the base rises and cooler, less dense matter toward the top sinks.  This circular motion generates electrical current that creates the Earth’s magnetic field.  The mantle is composed of the molten magma that emerges on the surface crust of Earth where tectonic plates are separating and when volcanoes erupt where tectonic plates submerge into the mantle. The crust of Earth that is visible to us is only 1% of Earth’s mass.

Physical Earth

Simplified map of Earth’s principal tectonic plates, which were mapped in the second half of the 20th century (red arrows indicate direction of movement at plate boundaries).  Source:  USGS

The crust of Earth is composed of plates that are moved on the surface of the Earth by the convection current of the mantle.  Some of the plates are moving away from one another where they meet.  As the plates separate, molten magma from the mantle is pushed through the crust, forming new crust.  The North American and Eurasian plates are moving apart in the middle of the Atlantic Ocean at the rate of about 1 inch per year. 

Since the Earth is not getting bigger, the expanding crust collides with adjacent plates.  In some places, the collision of the plates pushes up the crust into mountain ranges.  The Himalayan mountain range is the result of the collision of the Indo-Australian and the Eurasian Plates, a process that continues today.

Map of subducted slabs, contoured by depth, for most active subduction zones around the globe. Source:  USGS

In other places, the expanding crust is pushed below the adjacent plate in subduction zones, where the crust dives below the crust into the mantle.  Earthquakes are common in subduction zones and the subducting plate triggers volcanic eruptions in the overriding plate.  Earthquakes are also common where adjacent plates are grinding against one another in opposite directions, as is the case on the coast of California.

Pangea super-continent

The movement of tectonic plates has assembled and reassembled the Earth’s continents many times. The entire history of the configuration of continents is not known to us because of the cycle of the crust emerging from the mantle only to return to the mantle about 180 million years later.  We know that all continents were fused into a single continent, named Pangea, about 350 million years ago and began to break up 200 million years ago.  Much of life as we know it evolved on Earth while the continents were fused, which is one of the reasons why all life on Earth is related.  Geographic isolation of species results in more biodiversity as genetic drift and different environments result in greater speciation.  Geologists believe such continental mergers are likely in the distant future.  

Earth’s oceans and atmosphere were formed within the first 100 million years of its birth.  Continents were visible above oceans, but small compared to their present size.  The absence of oxygen in the air at that early stage was the most significant difference between present and early Earth.

Biological Earth

Life, as presently defined, requires growth and reproduction, metabolism, and evolution. (I say, “presently defined” because debate continues about defining viruses as life since they do not meet all criteria.)  The chemical components required to perform the functions of life and the natural processes to combine them (such as heat and lightning) were available on Earth for millions of years before they combined to perform the functions of life.  Precisely how and when that happened on Earth is studied intensely, but not conclusively known, although Professor Knoll describes theoretical possibilities. 

The geological record suggests that “Earth has been a biological planet for most of its long history.” Microbes may have been living on Earth 4 billion years ago.  Climate on Earth was warm at that time for the same reason the climate is warming today.  The atmosphere was composed primarily of carbon dioxide (the greenhouse gas that traps heat on the surface of the Earth) and nitrogen:  “…life emerged on an Earth barely recognizable to the modern eye—lots of water and not much land, lots of carbon dioxide but little or no oxygen…”

Oxygen Earth

Phylogenetic tree of life based on Carl Woese et al. rRNA analysis. The vertical line at bottom represents the last universal common ancestor.

Two of the three domains of life were capable of living without oxygen:  archaea and bacteria.  Archaea are single-cells without nuclei.  We are all too familiar with bacteria, as they are as much a part of our bodies as our own cells.  Oxygen was the prerequisite for the evolution of the third domain of life, eukarya.  The kingdoms of eukarya most familiar to us are plants, animals, and fungi. 

Oxygen arrived on Earth when early life forms evolved the ability to photosynthesize, the process by which plants (and some other organisms) use sunlight to synthesize food from carbon dioxide and water, generating oxygen as a byproduct.  This transition occurred about 2.4 billion years ago, as measured by the absence of iron on the seafloor after that time. 

Photosynthesis alone could not have accomplished the transformation of Earth’s atmosphere to the balance of carbon dioxide and oxygen needed to support complex life on our planet because photosynthesis also requires nutrients as well as sunlight and water.  Phosphorous weathers from rocks, a process that was initially limited by the small amount of land above sea level.  As the planet matured, more land emerged from the sea, making more phosphorous available to photosynthesizing organisms.  Photosynthesis was also enhanced when some bacteria and archaea evolved the ability to convert nitrogen gas into biologically usable molecules, a process called nitrogen-fixing.  Many plants in the legume family are capable of nitrogen-fixing today.

Extinctions of the past predict extinctions of the future

There have been five major extinction events in the past 500 million years that changed the course of evolution of life on Earth and at least 20 mass extinctions in total (2).  The first representatives of all modern animal phyla (a taxonomic classification between kingdom and class) evolved during the Cambrian Period (541-486 million years ago).  All extinction events were associated with radical changes in the climate.  Many of the changes in the climate were caused by changes in the balance of carbon dioxide and oxygen in the atmosphere.  All these catastrophic events were natural events, not caused by the activities of humans because they all occurred long before the advent of human evolution. 

The third and biggest extinction event occurred 252 million years ago at the end of the Permian geologic period, when more than 90% of marine animals and 70% of terrestrial species disappeared.  At that time, continents were fused into the single supercontinent of Pangea.  The extinction of most life on Earth was caused by the sudden and catastrophic change in the atmosphere–and therefore the climate–by an episode of volcanism in Siberia “a million times greater than any volcanism ever witnessed by humans” or our primate ancestors.  Gases emitted by volcanism at the end of the Permian period rapidly increased the carbon dioxide content of the atmosphere and oceans by several times greater than before that event.  “It would take 10 million years for life to reassemble into something approaching the complexity of the ecosystems that preceded it. The world that emerged from the volcanic dust was unlike anything that came before.” (2) The current increase of carbon dioxide in the atmosphere caused by the burning of fossil fuels by human activities is comparable to this event and is expected to cause the sixth great extinction on Earth.  

The fifth and most recent massive extinction event occurred 66 million years ago, bringing 170 million years of dinosaur evolution to an abrupt end. The entire environment of the planet was radically and suddenly altered by the impact of an asteroid 7 miles in diameter that landed on what is now the Yucatan peninsula in Mexico.  The impact engulfed Earth in a dust cloud that precipitated the equivalent of a nuclear winterkilling most vegetation and animals adapted to a much warmer climate.  As with all massive extinctions, it took millions of years for plants and animals to slowly evolve adaptations to the new environment.  Dinosaurs did not evolve again, a reminder that evolution does not necessarily repeat itself (although birds evolved from dinosaurs).  Although there were small mammals during the dinosaur age, the disappearance of dinosaurs and corresponding changes in the climate introduced the age of mammals, including the human lineage about 300,000 years ago.  When multiple animal groups disappear it creates opportunities by reducing competition between groups.

What can we learn from the history of Earth?

If a native plant advocate were reading this abbreviated history of Earth, these are the lessons I would hope they might learn from it:


  1. Andrew H. Knoll, A Brief History of Earth, 2021.  All quotes in this article are from this excellent book unless otherwise indicated.
  2. Elsa Panciroli, Beasts Before Us: The Untold Story of Mammal Origins and Evolution, Bloomsbury Sigma, 2021.

A Natural History of the Future

“The way out of the depression and grief and guilt of the carbon cul-de-sac we have driven down is to contemplate the world without us. To know that the Earth, that life, will continue its evolutionary journey in all its mystery and wonder.” Ben Rawlence in The Treeline

Using what he calls the laws of biological nature, academic ecologist Rob Dunn predicts the future of life on Earth. (1)  His book is based on the premise that by 2080, climate change will require that hundreds of millions of plant and animal species—in fact, most species–will need to migrate to new regions and even new continents to survive.  In the past, conservation biologists were focused on conserving species in particular places.  Now they are focused on getting species from where they are now to where they need to go to survive.

In Dunn’s description of ecology in the future, the native plant movement is irrelevant, even an anachronism.  Instead of trying to restore native plants to places where they haven’t existed for over 100 years, we are creating wildlife corridors to bypass the obstacles humans have created that confine plants and animals to their historical ranges considered “native.” 

The past is the best predictor of the future. Therefore, Dunn starts his story with a quick review of the history of the science that has framed our understanding of ecology.  Carl Linnaeus was the first to create a widely accepted method of classifying plants and animals in the 18th century.  Ironically, he lived in Sweden, one of the places on the planet with the least plant diversity.  Colombia, near the equator, is twice the size of Sweden but has roughly 20 times the number of plant species because biodiversity is greatest where it is hot and wet.

Global Diversity of Vascular Plants. Source: Wilhelm Barthlott, et. al., “Global Centers of Vascular Plant Diversity,” Nova Acta Leopoldina, 2005

 

Humans always have paid more attention to the plants that surround us and the animals most like us.  Dunn calls this the law of anthropocentrism.  We are the center of our own human universe.  Consequently, our awareness of the population of insects that vastly outnumber us came late to our attention in the 20th century.  In the 21st century we learned that all other forms of life are outnumbered by the microbial life of bacteria, viruses, and fungi that preceded us by many millions of years.  Our knowledge of that vast realm of life remains limited although it is far more important to the future of the planet than we realize because those forms of life will outlast our species and many others like us.

Tropical regions are expanding into temperate regions

The diversity and abundance of life in hot and wet tropical climates give us important clues about the future of our warming climate.  We tend to think of diversity as a positive feature of ecosystems, but we should not overlook that tropical regions are also the home of many diseases, such as malaria, dengue fever, zika, and yellow fever that are carried by insects that prey on animal hosts, including humans.  In the past, the range of these disease-carrying insects was restricted to tropical regions, but the warming climate will enable them to move into temperate regions as they warm. The warming climate will also enable the movement of insects that are predators of our crops and our forests into temperate regions.  For example, over 180 million native conifers in California have been killed in the past 10 years by a combination of drought and native bark beetles that were killed during cold winters in the past, but no longer are.  Ticks are plaguing wild animals and spreading disease to humans in the Northeast where they did not live in the cooler past. 

Human populations are densest in temperate regions“The ‘ideal’ average annual temperature for ancient human populations, at least from the perspective of density, appears to have been about 55.4⁰F, roughly the mean annual temperature of San Francisco…” (1) This is where humans are most comfortable, free of tropical diseases, and where our food crops grow best.  As tropical regions expand into temperate regions, humans will experience these issues or they will migrate to cooler climates if they can.

Our ability to cope with the warming climate is greatly complicated by the extreme variability of the climate that is an equally important feature of climate change.  It’s not just a question of staying cool.  We must also be prepared for episodic extreme cold and floods alternating with droughts. Animals stressed by warmer temperatures are more easily wiped out by the whiplash of sudden floods or drought.

Diversity results in resiliency

Diversity can be insurance against such variability.  If one type of crop is vulnerable to an insect predator, but another is not, growing both crops simultaneously increases resiliency.  That principle applies equally to crops that are sensitive to heat, cold, drought, or floods. 

Agricultural biodiversity. Source: Number of harvested crops per hectar combining 175 different crops. Source: Monfreda et al. 2008. “Farming the planet: Geographic distribution of crop areas, yields, physiological types, and net primary production in the year 2000”. Global Biogeochemical Cycles, Vol. 22.

Historically, cultures that grew diverse crops were less likely to experience famine than those that cultivated monocultures.  The Irish potato famine of the mid-19th century is a case in point.  The Irish were dependent upon potatoes partly because other crops were exported to Britain by land owners. When the potato crop was killed by blight, more than one million people died in Ireland and another million left Ireland.  The population dropped about 20-25% due to death and emigration.  The diversity of crops in the United States (where corn is the commodity crop) and Brazil (where soy is the commodity crop) is very low, compared to other countries.  This lack of diversity makes us more vulnerable to crop failure and famine, particularly in an unpredictable climate.

Change in total use of herbicides, antibiotics, transgenic pesticide producing crops, glyphosate, and insecticides globally since 1990. Source: A Natural History of the Future

Instead of increasing crop diversity, we have elected to conduct chemical warfare on the predators of our crops by using biocides, such as pesticides for agricultural weeds and insects and antibiotics for domesticated animals.  The scale of our chemical warfare has increased in response to growing threats to our food supply.  This is a losing strategy because as we increase the use of biocides we accelerate evolution that creates resistance to our biocides. We are breeding superweeds, insects, and bacteria that cannot be killed by our chemicals.  This strategy is ultimately a dead end.

Evolution determines winners and losers

Inevitably, evolution will separate the survivors of climate change from its victims. Dunn reminds us that “The average longevity of animal species appears to be around two million years…” for extinct taxonomic groups that have been studied.  In the short run, Dunn bets on the animals that are most adaptable, just as Darwin did 160 years ago.  The animals most capable of inventing new strategies to cope with change and unpredictability will be more capable of surviving.  In the bird world, that’s corvids (crows, ravens, jays, etc.) and parrots.  In the animal world that’s humans and coyotes.  We aren’t helping adaptable animals survive because we are killing abundant animals based on a belief it will benefit rare animals.  Even in our urban setting, the East Bay Regional Park District contracts Federal Wildlife Services to kill animals it considers “over-abundant,” including gulls, coyotes, free-roaming cats, non-native foxes, and other urban wildlife throughout the Park District.  We are betting on evolutionary losers.

 

If and when humans create the conditions that cause our extinction, many of our predators are likely to disappear with us.  Bed bugs and thousands of other human parasites are unlikely to survive without us.  Many domestic animals will go extinct too, including our dogs.  On the bright side, Dunn predicts that cats and goats are capable of surviving without us.             

Timeline of the evolution of life. Source: CK-12 Foundation

However, in the long run Dunn bets on microbial life to outlast humans and the plants and animals with which we have shared Earth.  Humans are late to the game, having evolved from earlier hominoids only 300,000 years ago, or so.  The plants and animals that would be recognizable to us preceded us by some 500 million years, or so.  But microbial life that is largely invisible to us goes back much further in time and will undoubtedly outlast us.  Dunn says microbial life will give a big, metaphorical sigh of relief to see us gone and our environmental pollutants with us.  Then microbial life will begin again the long process of rebuilding more complex life with their genetic building blocks and the tools of evolution. 

Some may consider it a sad story.  I consider it a hopeful story, because it tells me that no matter what we do to our planet, we cannot kill it.  For the moment, it seems clear that even if we are not capable of saving ourselves at least we can’t kill all life on Earth.  New life will evolve, but its features are unfathomable because evolution moves only forward, not back and it does not necessarily repeat itself. 


  1. A Natural History of the Future, Rob Dunn, Basic Books, 2021

Science meets the “restoration” industry

I was encouraged to hear a presentation by an academic scientist at the recent Beyond Pesticides Forum that was another indication of the paradigm shift in invasive species management toward a less destructive approach.  Dr. Bernd Blossey is a Professor at Cornell University, where he directs the Ecology and Management of Invasive Plants Program in the Department of Natural Resources.  His many years of studying invasive plants, such as purple loosestrife, garlic mustard, water chestnut, Japanese knotweed, and phragmites have convinced him that there are often “multiple stressors” that contribute to such invasions.  Some factors such as the presence of earthworms and deer can be more important factors in the Northeast than the non-native plants themselves. 

Based on his research experience, Dr. Blossey delivered wise advice to land managers at the Beyond Pesticide Forum.  The featured photo at the top of this article was his introductory slide. 

Before a restoration project begins, these questions should be asked and answered:

Source: Dr. Blossey’s presentation to Beyond Pesticides Forum on June 8, 2021

If the project seems worthwhile after such analysis is done, this is Dr. Blossey’s advice about monitoring the project and measuring its success:

Source: Dr. Blossey’s presentation to Beyond Pesticides Forum on June 8, 2021

Practicing what he preaches

Dr. Blossey used these principles in his study of garlic mustard in the forests of the northeast. (1) Over a period of more than 10 years, Dr. Blossey and his collaborators measured the abundance of garlic mustard in 16 plots from New Jersey to Illinois where no attempt had been made to control or eradicate it.  They found that growth rates initially increased, but decreased over time and eventually the population started to decline.  Dr. Blossey explained their findings in a recent webinar that is available HERE:

Garlic mustard was first recorded in North America in 1868 on Long Island, New York.  It spread west from there and is now found from southern Canada to Georgia and from New York and Quebec to Oregon, British Columbia and Alaska.  Because land managers believed that garlic mustard suppresses populations of native plants, they have been trying to eradicate garlic mustard in northern forests for decades, with little long term success.  Dr. Blossey addressed that concern in his webinar. 

Source: Dr. Blossey’s webinar about garlic mustard

Earthworms are the prerequisite for garlic mustard invasion.  Earthworms in northern forests are also considered alien invaders because they were killed, along with forests, by advancing glaciers during the Ice Age.  When forests returned after the Ice Age over 10,000 years ago, they evolved without earthworms that were reintroduced by European settlers less than 500 years ago. 

When deer are excluded from areas by fencing plots with garlic mustard populations, abundance of native vegetation does not decline.  Deer have a strong preference for native vegetation.  Absent deer, garlic mustard does not seem to suppress the growth of native plants in northern forests.

In other words, garlic mustard is not guilty as charged.  Dr. Blossey explains the disadvantages of attempting to eradicate it.  The decline of garlic mustard abundance over time is attributed to negative soil feedback that builds over time as the soil microbial community responds to the new plant. Removing garlic mustard episodically prolongs the process of building that negative soil feedback.  When groups of well-meaning young people are sent into the forest to pull garlic mustard, they trample the very native plants they are trying to save. 

Are there lessons for land managers in the Bay Area?

Because garlic mustard doesn’t exist in California and our native earthworms are considered beneficial to soil health, you might wonder if this study is relevant here.  California was not glaciated during the Ice Age.  Our earthworms survived the Ice Age and they evolved with our forests. 

So, what can we learn from this study?  The pattern of initial growth and eventual decline of populations of introduced plants is not unique to garlic mustard“A phenomenon that has received increased attention is whether introduced species go through boom and bust cycles, ultimately becoming non-threatening members of local communities.” (1)  One recently published study was based on nearly 5,000 vegetation inventories collected in 49 National Parks in the eastern United States.  It reported that non-native plants appeared to decline after 100-200 years: 

Residence time appears a core part of invasion that interacts with other mechanisms, such as climate matching, propagule pressure and empty niche. Initially, time appears to benefit non-native species as they establish in a novel range. They likely face low enemy loads, and any successful dispersal increases their populations and invaded range. As they spread, initial barriers, such as distance or suboptimal habitat, were overcome, as was resistance from native relatives. However, their biggest challenge appeared to be time, as they all declined after ~1 to 2 centuries, suggesting that pathogens and herbivores caught up with them.” (2)

The message for land managers everywhere is that patience is needed to judge the impact of introduced species.  Most will fit into ecosystems eventually and attempts to speed up that process often do more harm than good.  We can’t judge changes in nature by the short-term perspective of human lifetimes because the evolution of nature is a continual process that began long before humans existed and is likely to persist long after we are gone. 

Applying Dr. Blossey’s “Core Knowledge” to local projects

What if Dr. Blossey’s “Core Knowledge” had been applied to projects in the San Francisco Bay Area?  Here are examples of local eradication projects that might have benefitted:

  • San Francisco has been trying to eradicate oxalis in its parks for over 20 years by spraying a selective herbicide (Garlon).  There seems to be more oxalis now than there was 20 years ago.  Oxalis is visible only about 2 months of the year.  When it dies back in the spring it leaves behind the native plants with which it co-exists.  If a control plot had been set aside before they started eradicating oxalis perhaps we would know the answer to these important questions:  Does oxalis suppress the growth of native plants?  Does attempting to eradicate oxalis produce more or less oxalis?
  • California, Oregon, and Washington have been trying to eradicate non-native spartina marsh grass along the entire West Coast for over 20 years.  Here in the Bay Area, non-native species of spartina have been 99% eradicated, but a hybrid of the native and the non-native remains and is poisoned with imazapyr annually.  According to a recent presentation by the Invasive Spartina Project, the hybrid is visually indistinguishable from the native and it occupies the same elevation of the marsh.  Over 500 genetic tests are needed every year to distinguish the hybrid from the native in order to poison the hybrid.  Dr. Blossey’s approach might ask these important questions:  What harm is hybrid spartina doing?  Do more or fewer animals live in hybrid spartina?  What effect has 20 years of spraying imazapyr had on the soil and the microbes that live in it?  Is the eradication project doing more harm than good? 
oxalis bloom, February 2021

We don’t know the answers to these important questions because projects were initiated and implemented without the analysis and monitoring metrics needed to answer the questions.  The projects continue without being accountable for the damage they are doing.  Public money is funding these projects without requiring the projects to be accountable for the consequences. 

California has made a commitment to spend billions of dollars on “nature based solutions” and achieving “biodiversity goals.”  This is an opportunity to start new projects off on the right foot by:

  • Requiring the analysis needed to determine the impacts and causes of perceived problems in the environment.
  • Requiring control plots so that the effects of the project can be compared with the option of not doing the project.
  • Requiring that projects be monitored, using established metrics so that the success of the project can be measured.

  1. Bernd Blossey, et. al., “Residence time determines invasiveness and performance of garlic mustard (Alliaria petiolota) in North America, Ecology Letters, February 2021.  
  2. Robert Warren, et. al., “Multiple mechanisms in woodland plant species invasion,”  Plant Ecology, April 2019.

Hybridization, a post script

We recently published an article in defense of hybridization, inter-breeding of two different species.  Conservation Sense and Nonsense defends hybridization because it is under fire from the native plant movement.  Many projects that needlessly destroy non-native plants (or one locally perceived as such) do so to prevent them from hybridizing with a native plant, which has the potential to cause a localized loss of a variant of a  native plant species.

This classic California poppy is eradicated in the Presidio in San Francisco because of fear it could hybridize with a sub-species of poppy that is considered “native” to the Presidio.

We are revisiting the topic because The Economist magazine recently published a comprehensive article about recent discoveries of the prevalence of hybrids among both plants and animals.  Until the advent of DNA analysis in the 1970s, the extent to which plant and animal species were the result of inter-breeding was largely unknown.  Also, conventional wisdom was that such inter-breeding was usually an evolutionary dead-end because offspring were often sterile, as exemplified by mules, the offspring of horses and donkeys.  In general, the consequences of hybridization were assumed to be negative.

Recent advances in DNA analysis have largely disproved these assumptions.  Hybridization is not only common, it can result in the creation of new species more rapidly than other forces of evolution, such as mutation and natural selection:  “Hybridisation also offers shortcuts on the long march to speciation that do not depend on natural selection at all.” (1)

Both the positive and negative effects of hybridization are real. In plants, the effects of hybridization are often beneficial because of plants’ unusually flexible genetics.  Plants, for instance, are frequently polyploid—meaning that each nucleus contains genomic copies in greater multiples than those of animals.  Polyploidy provides spare copies of genes for natural selection to work on, providing additional possibilities for selection.

Polyploidy confers another advantage. It creates a barrier to breeding with either parent species. That gives a new, emerging species a chance to establish itself without being reabsorbed into one of the parental populations. Recent evidence suggests that hybridization between two plant species in the distant past, followed by a simple doubling of the number of chromosomes in their offspring, may be responsible for much of the diversity in flowering plants that is seen today.

Plants seem to benefit from hybridization more often than animals. “For many animals, however—and for mammals in particular—extra chromosomes serve not to enhance things, but to disrupt them. Why, is not completely clear. Cell division in animals seems more easily confounded by superfluous chromosomes than it is in plants, so this may be a factor. Plants also have simpler cells, which are more able to accommodate extra chromosomes. Whatever the details, animal hybrids appear to feel the effects of genetic incompatibility far more acutely than do plants.” (1)

The Economist provides many important examples of hybridization among animal species, most notably the history of hybridization of our species, Homo sapiens.  We are now the sole surviving species of genus Homo.  Our genome contains the relicts of the genes of other members of our genus that are now extinct, which indicates hybridization with other hominoid species.  The modern human genome contains 1-4% of Neanderthal genes. 

The Economist article concludes, “This is a more complex conception of evolutionary history, but also a richer one. Few things in life are simple—why should life itself be?”   Keep your eyes and your mind open to new scientific knowledge that improves our understanding of life.

The bottom line

Biodiversity is the mantra of the native plant movement.  Native plant advocates claim that the primary purpose of saving native plants is preserving biodiversity.  But is it?  When non-native plants are eradicated, aren’t we depriving native plants of the opportunity to breed with a hardy new comer?  Are we preventing the creation of a new species by eliminating potential mates?  Are we dooming the native plant that is not adapted to survive the changing climate by depriving it of the opportunity to improve its survivability?


  1. The Economist, “Match and mix, hybrids and evolution,” October 3-9, 2020, page 67-70.  Available here:  Economist – hybridization and evolution

 

Nativism in the Natural World

Invasion biology is the scientific discipline that spawned the native plant movement.   Charles Elton published a book in 1958 that is considered the origin of the modern version of invasion biology, although there are precursors centuries earlier.  These are the basic tenets of modern invasion biology:

  • Plants and animals that are “native” to a specific location are considered members of an ideal ecosystem that have co-evolved over thousands of years so that members of the community are dependent upon one another.
  • Plants and animals introduced to an ecosystem by humans are assumed to disrupt the equilibrium balance of the community and threaten its existence because introduced plants and animals do not have predators that would control their spread.  All introduced plants and animals are therefore considered potentially invasive.
  • Animals are believed to be dependent upon the plants with which they evolved—and only these plants–and these mutually exclusive relationships are disturbed by the introduction of new plants and animals. 
  • Adaptation and evolution of introduced plants and animals is believed to be too slow for introduced plants and animals to successfully enter the food web.
  • Native members of the ecosystem are presumed to be inherently superior to introduced plants and animals.  Invasion biology does not acknowledge that introduced plants and animals are often functional members of the ecological community.
  • Native ecosystems are said to be in “balance” and introduced species are presumed to cause “imbalance.”  Introduced species must be eradicated to restore balance to the ecosystem, presumed to be the ideal for a particular location.

Hundreds of empirical studies have been conducted since the 1960s to test these assumptions.  Little scientific evidence has been found to support them. Current knowledge of ecology explains why the assumptions of invasion biology are mistaken. 

What is native?

The native plant movement defines native as the plant species that lived in a specific location prior to the arrival of Europeans.    In the San Francisco Bay Area, “native” is defined by native plant advocates as the plants and animals that lived here prior to 1769 when Europeans first laid eyes on San Francisco Bay.  When Europeans arrived, the San Francisco Bay Area was already occupied by indigenous people who had arrived approximately 10,000 years earlier. 

The arbitrary selection of the pre-European settlement period to define the ideal landscape was based on the mistaken assumption that the indigenous human population had not radically altered the land. Anthropological and paleontological research informs us that the landscape was essentially gardened by the indigenous population to provide food and cultural implements. 

Pomo gathering seeds, 1924. Smithsonian photo archive

The landscape found by Europeans at the end of the 18th century was not “natural.”  It was altered by humans to serve humans who lived as hunters and gatherers.  Since modern society no longer hunts and gathers for its food and shelter, the landscape that served that lifestyle cannot be maintained without mimicking the land management practices of native people such as frequent burning of the landscape and grazing by animals.  Indigenous people in California did not have domesticated animals (except dogs), but the grassland was grazed by wild deer, elk, and antelope. 

Plants and animals have migrated around the world without the assistance of humans since life began.  The seeds of plants are carried in the stomachs of migrating birds and on the winds of storms.  Animals, including humans, move to wherever they can find what they need to survive.  Migration is natural and often necessary for survival.  Making a distinction between species moved by humans and those moved by natural forces is pointless and usually impossible to distinguish. 

Climate change renders the concept of “native plants” meaningless because when the climate changes, the vegetation changes.  The plants that live in tropical climates will not survive in arctic cold and vice versa.  Introduced plants are often better adapted to current climate conditions than their native predecessors because the climate has changed and it will continue to change. 

Mistaken assumptions about evolution

Animals rarely depend upon a single plant species for survival.  Such mutually exclusive relationships rarely exist in nature because they are evolutionary dead-ends. Animals can, and often do, adapt quickly to changes in the environment.  Transitions from native to introduced plants are routinely made by animals, including humans.  Indigenous hunter/gatherers quickly incorporated plants introduced by European settlers into their diets.  Plants in the same family and genus are often chemically similar, making the transition more likely. 

Native plant advocates assume that evolution only occurs slowly, over thousands of years, but evolution can be faster than they assume.  Rapid environmental change accelerates the speed of evolution because extreme weather events caused by climate change increase the speed of natural selection, the primary tool of evolution.  When cataclysmic events such as hurricanes, droughts, floods, extreme temperatures kill many members of a species population, these are selection events in which the fittest members survive to breed and the next generation inherits the genetic traits that helped their parents survive.  The classic example of this principle is the finches in the Galapagos Islands who died if they didn’t have big enough beaks to eat the seeds of the only plant that survived extreme drought.  The next generation of finches had bigger beaks. 

Darwin’s finches are an example of rapid evolution

Evolution occurs when genetic changes enable future generations to inherit the genetic change.  Adaptation occurs when animals respond to environmental challenges by changing behaviors that aren’t necessarily inherited by the next generation.  Adaptation to changed environmental conditions is even more rapid than evolution and equally effective to ensure survival. Genetic changes are not required for an insect to make the transition from a native host plant to a chemically similar introduced plant.   Extreme temperatures require that plants and animals move to more temperate climates.  “Native” ranges must change to survive changes in the environment.  A plant or animal that cannot survive extreme heat will migrate (if it can) into regions where temperatures are not as warm.  They should not be prevented from doing so. 

Adaptation to Climate Change. IPCC

Plant and animal species with large populations and short lives, such as insects, evolve more quickly.  This more rapid pace of evolution enables a more rapid transition from native host plants to closely related introduced plants.

soapberry bug made transition from native to non-native balloon vine in 20-50 years. Scott Carroll, UC Davis

Nativism and the native plant movement

The native plant movement is based on the belief that native plants are superior to introduced plants, that native plants are somehow “better” than immigrant plants.  That assumption of superiority is the definition of nativism.  It is as specious an assumption in the natural world as it is in human society and it is equally dangerous. 

There are pros and cons to everything living in the natural world and there is no right answer to the question of which species is “best.” When evaluating introduced plants, nativists consider only the negative aspects. They refuse to acknowledge that there are also advantages and a death verdict should take both into consideration.  For example, native plant advocates want all eucalyptus trees in California cut down because they were planted here after European settlement.  This negative judgment of eucalyptus does not take into consideration that 75% of monarch butterflies who spend the winter in California use eucalyptus trees for their safe haven. Also, eucalyptus blooms in California from November to May, providing nectar to butterflies, hummingbirds, and bees at a time of year when native plants are not blooming.  Eucalyptus trees are also nesting homes of owls and other raptors.  Cutting down eucalyptus trees simply because they are not native in California ignores the many benefits they provide to wildlife. 

Monarch butterflies over-winter in California’s eucalyptus groves

Confusing cause and effect

The native plant movement mistakenly assumes that the mere existence of introduced plants threatens the existence of native plants.  They believe that native plants will magically emerge if introduced plants are eradicated.  They have spent 25 years eradicating non-native plants and do not seem to have noticed that native plants have not returned.  They make this mistake because they do not acknowledge the changes in the environment that make non-native species better adapted to current environmental conditions. 

Many of the changes in the environment that are inhospitable to native species are caused by structural changes made to accommodate human activities, not by introduced species.  For example, all the major rivers in California have been dammed to prevent floods and store water for use during the dry season.  These dams have fundamentally altered the ecology of our rivers.  There are no longer cleansing spring floods that clear rivers of accumulated mud and vegetation.  Channeled rivers are deeper and warmer.  Salmon can no longer get to their spawning grounds past the dams.  The altered structural conditions are more hospitable to bass than to trout.  Aquatic plants from tropical regions become invasive in warmer water.  None of these conditions are reversed by spraying aquatic plants with herbicide or killing introduced bass.

Butterfly bush (buddleia) is now being eradicated by nativists.. butterflybush.com

Wherever “invasions” are observed, no thought is given to why.  Instead, a convenient plant or animal scapegoat is found and poisoned.  That death sentence doesn’t reverse the underlying reason for the invasion.  Therefore, the invasion persists.  Society is unwilling to make the sacrifices, even inconveniences, needed to address the underlying cause of the “invasion.”  We have done little to address the causes of climate change.  We are unwilling to destroy the dams and the system of supplying water to serve agriculture needs.  Invasions are the symptom, not the cause of the changes in nature.

All life on Earth is related

Today we will take a deep dive into evolutionary history to talk about the origins of life on Earth.  Drawing from David Quammen’s new book, The Tangled Tree, we will tell you about “a radical new history of life,” as promised by the subtitle of his book. (1)

Categorizing Nature

Throughout written history, humans have demonstrated a compelling need to name and categorize everything in our world, including nature.  Naming and categorizing passes for understanding and enables us to talk about issues using commonly understood definitions.

Linnaean taxonomy was one of the first and most influential attempts to classify the natural world into three kingdoms:  plants, animals, and minerals.  Since Systema naturae was published by Carl Linnaeus in the 18th century, many other classification systems have been proposed by subsequent generations of scientists.

The conventional wisdom about classifying nature changed radically after the discovery of the molecular structure of DNA in the 1950s and the molecular analysis that it enabled in the 1960s.  Genetic analysis revealed the evolutionary relationships between organisms, enabling the development of phylogenetic “trees” depicting those relationships.

Haeckel’s Tree of Life, 1879

The revolutionary work of Charles Darwin was instrumental in initiating such speculation about evolutionary history.  Such theories about the history of life on Earth were often depicted as “trees of life,” showing the progression of evolution.  One of the earliest of such “trees” was published in the 1870s, shortly after the publication of Darwin’s Origin of Species in 1859.

In 1977, using molecular analysis, Carl Woese published his hypothesis of a new kingdom of life, Archaea.  He proposed a new categorization of all life on Earth, which he called domains:  Bacteria, Archaea, Eucarya.  Bacteria and Archaea are one-celled organisms without a nucleus.  Eucarya are every other living organism, including plants, animals, and fungi. Kingdoms of life were relegated to the second level of taxonomy (the classification of organisms).

The hypothesis of Woese was challenged, often contentiously, for decades, but is now conventional wisdom among scientists of phylogenetics, as the genetics of evolution is called.  However, as tidy as these new categories might appear, they aren’t.  As human intellectual constructs often are, many species of life defy neat categorization.  Around the edges of every domain there are many species of life that don’t entirely fit the criteria.  Likewise, around the edges of every genus and species, there are many gray areas.  Just as the distinction between “native” and “non-native” is often ambiguous, so is the categorization of many organisms.  This is a reminder that we must use such definitions with humility, always being prepared to consider a new hypothesis that improves our understanding.

Domains of life

Revising the mechanisms of evolution

Molecular analysis has also radically altered our understanding of how evolution proceeds.  Charles Darwin’s hypothesis about evolution was that change in organisms occurs through genetic variation from one generation to the next.  Occasional genetic mutations from one generation to the next was later added to what is called “vertical evolution.”  Each subsequent generation of a species is tested by the environment and that test is called natural selection.  The individual member of a species that is best adapted to the environment survives to reproduce, while less well-adapted individuals do not survive to reproduce.

Scientists have more recently observed that species in one domain of life also exchange genetic material with another domain of life, as well as exchanges between different species within domains.  This is called “horizontal gene transfer.”  The discovery of horizontal gene transfer (HGT) has revolutionized how we think about evolution.  Natural selection remains as the mechanism that confers success or failure on such changes in genes from one generation to another.

Significance of horizontal gene transfer

Horizontal gene transfers occurred in deep time, but are known to be a significant issue at the present time.  Horizontal gene transfer is the primary mechanism for the spread of antibiotic resistance in bacteria and plays an important role in the evolution of bacteria that can degrade synthetic compounds such as pesticides. Antibiotic resistance in one species of bacteria can be transferred to another species of bacteria, multiplying the incidence of antibiotic resistance. (2)

The introduction of chloroplasts into plant cells roughly 3.5 billion years ago was one of the most significant events in the evolution of life on Earth.  The introduction of chloroplasts into plant cells was an example of a horizontal gene transfer from a bacteria cell into eucarya cells.  Chloroplasts are the organelles (specialized structures inside eucarya cells that perform specific functions) that perform photosynthesis in plant cells.  Photosynthesis enables plants to convert the energy of the sun into carbohydrates that feed the plant and emit oxygen as its waste product.  Photosynthesis converts carbon dioxide into oxygen.  This neat trick of photosynthesis radically altered the atmosphere by reducing carbon dioxide and increasing oxygen.  Just as increased carbon dioxide in the atmosphere is now increasing temperatures on Earth, lower carbon dioxide levels in the atmosphere reduced temperatures.  This so-called “Great Oxidization Event” was the probable cause of one of the five great extinctions hundreds of millions of years ago. (3)

The horizontal gene transfer of mitochondria from bacteria cells to eucarya cells was equally significant to the evolution of life on Earth.  Mitochondria are organelles in eucarya cells that perform respiration and energy production functions in most eucarya species of both plants and animals. (2)

The list of such horizontal gene transfers is long.  Here are some examples to help you understand that HGT is an extremely important evolutionary mechanism, perhaps even more important than vertical evolution (2):

• From bacteria to fungi
• From bacteria to plants
• From organelle to organelle
• From plant to plant
• Fungi to insects
• From bacteria to insects
• From viruses to plants
• From bacteria to animals
• From plants to animals
• From plant to fungus

Implications of horizontal gene transfer

Our bodies contain more microbes, such as bacteria, than they do human cells.  Those microbes are interacting with our own cells.  Sometimes the microbes cause problems and sometimes they solve problems.  The microbes in our bodies cannot be called enemies or friends.  Sometimes their interactions with our cells permanently alter our genes and are inherited by our offspring.  Such permanent alterations of our genes are called horizontal gene transfer.  Such interactions between microbes and cells occurs in all life forms, altering plants, animals, etc. 

What are the implications of these interactions?

  • All life forms on Earth are related. No life form on Earth can be considered “alien.”  Every organism on Earth is constantly undergoing change, as it interacts with other organisms.  No “species” is immutable in the long term.
  • Critics of genetic engineering say it is “unnatural” and risky because it introduces genes into organisms in which they did not evolve naturally. But horizontal gene transfer does exactly the same thing and it is a “natural” process.  Genetic engineering is risky, just as HGT is, but it is mimicking a natural process.
  • Many pesticides are known to kill bacteria. Since bacteria are resident in our bodies in huge numbers and are known to sometimes be beneficial, it seems unnecessarily risky to kill them with pesticides.   As with genetic engineering, the risk should be weighed against potential benefits.  Are the risks worth taking?
  • Epidemiological studies report correlation between increased pesticide applications and increased birth defects in humans. Laboratory studies on rats report birth defects in rats exposed to low doses of glyphosate as well as birth defects in subsequent generations of the exposed rats: “A 2018 study of pregnant rats exposed to low doses of glyphosate-based herbicides revealed that the rats had difficulties in getting pregnant and surviving the pregnancy. The second generation offspring suffered from being smaller than normal. They were also afflicted with abnormalities developed before birth. This means the glyphosate-based weed killers inoculate their victims with monstrosities.” (4) These studies suggest that genes may have been altered by pesticide exposure.

A Milestone for Million Trees

As the Million Trees blog approaches the anniversary of its eighth year, we are celebrating a milestone. Yesterday, Million Trees reached a total of 250,000 individual views of posts on Million Trees.  We now have over 300 subscribers and we are averaging about 150 views per day.  About 25% of our readers are outside the United States.  Since nativism in the natural world is an international fad, we are gratified that Million Trees is being read by people in other countries.  Million Trees is also proud and grateful for the participation of several academic scientists who have written informative guest posts for Million Trees in the past year.  Thank you, Dr. Matt Chew, Professors Mark Davis and Art Shapiro, and Dr. Jacques Tassin for your help!

Our most popular posts have each been visited by over 10,000 readers.  They are, in the order of their popularity:

  • “Darwin’s Finches: An opportunity to observe evolution in action.”  This article about the speed with which adaptation and evolution occur in a rapidly changing environment is the bedrock of the Million Trees blog.  Nativists mistakenly believe that evolution is much slower than it is.  Therefore, nativists believe plant and animal species are nearly immutable and that they are locked into mutually exclusive relationships, which are, in fact, extremely rare in nature.
  • “Nearly a HALF MILLION trees will be destroyed in the East Bay if these projects are approved.” The Million Trees blog was created to inform the public that nativism is destroying our urban forest in the San Francisco Bay Area.  Our urban forest is composed of predominantly non-native trees.  If they are destroyed, we will not have an urban forest because native trees will not survive in our changed and rapidly changing environment.  Non-native trees were planted here because people wanted trees and native trees existed only in riparian corridors where they were sheltered from the wind and there was sufficient water.
  • “Falling from Grace: The history of eucalyptus in California.”  Because people wanted trees, they planted non-native trees that were capable of surviving in the San Francisco Bay Area.  Non-native trees were valued for nearly one hundred years until nativism got a death grip on our public lands. This article on Million Trees tells the history of why eucalypts were planted and why they “fell from grace.”

In the past year, one of the most popular posts on Million Trees was “Krakatoa:  A case study for species dispersal.”  This post has been viewed by over 7,000 readers.  Understanding how plants and animals were dispersed around the world by natural means–such as by birds, wind, and ocean currents—is another way to realize that the concept of “native vs. non-native” is an artificial construct with little practical meaning.  Plants and animals have always moved and they will continue to move.  In fact, as the climate changes, they MUST move if they are to find the environmental conditions in which they can survive.

Million Trees Commitment

Million Trees will continue to advocate for the preservation of our urban forest in the San Francisco Bay Area.  Our strategy is to inform the public of the many projects that are destroying our forests and to describe the damage that is being done by those projects.  We are particularly concerned about the use of pesticides to eradicate non-native plants and trees.  We are equally committed to providing our readers the latest scientific discoveries that relegate invasion biology to a scientific back-water.  We are hopeful that the gap between public policy and the scientific knowledge discrediting invasion biology will eventually be bridged and bring an end to this destructive fad.

Stevie Nicks, Naturalized Species, and the future of the biosphere

Professor Arthur M. Shapiro, at work, UC Davis

Art Shapiro is no stranger to the long-time readers of Million Trees.  Professor Shapiro is Distinguished Professor of Ecology and Evolution at UC Davis, and a renowned expert on the butterflies of California.  He is the author of a seminal, frequently cited study of California butterflies that reported the results of 30 years of observing butterflies in his research transects. (1)  He summarized this study in his Field Guide to the Butterflies of the San Francisco and Sacramento Valley Regions:

“California butterflies, for better or worse are heavily invested in the anthropic landscape [altered by humans].  About a third of all California butterfly species have been recorded either ovipositing [laying eggs] or feeding on nonnative plants.  Roughly half of the Central Valley and inland Bay Area fauna is now using nonnative host plants heavily or even exclusively.  Our urban and suburban multivoltine [multiple generations in one year] butterfly fauna is basically dependent on ‘weeds.’  We have one species, the Gulf Fritillary that can exist here only on introduced hosts.  Perhaps the commonest urban butterfly in San Francisco and the East Bay, the Red Admiral is overwhelmingly dependent on an exotic host, pellitory. And that’s the way it is.”

 Professor Shapiro has given us permission to reprint his Amazon review of the most recently published critique of invasion biology, Inheritors of the Earth, by Professor Chris Thomas (University of York, United Kingdom).  We recommend Professor Thomas’s book to our readers.  Although it is learned, it is accessible to the general public.  This book is another step forward in the long march to acceptance of the reality of existing landscapes that are adapted to present climate conditions.

 Million Trees


2011 Chris Thomas published a paper in the journal “Trends in Ecology and Evolution” entitled “Translocation of species, climate change, and the end of trying to recreate past ecological communities.” I immediately e-mailed him (April 11, 2011): “I have been delivering the same message in my advanced courses in Community Ecology and Biogeography for years, and have found the students by-and-large highly receptive, especially when they have internalized the overwhelming evidence for wild fluctuations in climate and vegetation since the end of the Ice Age 10-20,000 years ago. But over and over I have been told ‘but of course that is not the Party line…restoration ecology,’ blah, blah….Thank you for giving me a respectable citation, since merely citing one’s self can never do.” He e-mailed back: “…the conservation community in Britain seems mainly to be treating me with bewildered patience! I think that it will take time for everyone to become re-programmed to accept change as a reality.”

But of course change is not only a reality, it is the norm in ecology. Belief in equilibrium states and a “balance of nature” has been a dogma without a rationale beyond sentimentalism for many decades. There are coevolved segments of communities that are intimately synchronized and interdependent (say, figs and fig wasps or yuccas and their moth pollinators), but a great deal of any community is the product not of coevolution but of what Dan Janzen calls “ecological fitting,” whereby things haphazardly thrown together by the vicissitudes of geology, climate or commerce just happen to click. We are surrounded all over the globe by functioning communities and ecosystems with little to no history in geologic time. For about 40 years I have asked my students on their final exam how one might go about telling the difference between coevolved communities and “communities” assembled by chance. It is an exceedingly difficult question.

So this book is an expansion of the TREE [Trends in Ecology and Evolution] paper, and its message is vital. Resources for conservation are limited, and one must prioritize. The vast majority of naturalized alien species are harmless and many may be potentially beneficial. The ones that are genuinely harmful should be fought tooth and nail, but of course we do that anyway–we call it “pest management” and “public health.” The blanket indictment of “invasive species” makes no more sense than the blanket condemnation of human immigrants. Of course, when we say this, Thomas and I and Fred Pearce and “that Marris woman!” are immediately called out as shills for the extractive industries or the nursery industry or the Bilderbergers or the Zelosophists (conspiracy theory villains!!) or some despicable cartel of nature-haters. Pure poppycock. Truth-tellers attract trolls. That’s just the way it is.

Quite a few years ago a group of us took a prominent visiting British ecologist (not Thomas) on a field trip to the Sierra Nevada. We had half a dozen grad students and a few faculty crammed in a van. On the way up, one of the students sort-of apologized for the predominance of naturalized alien plant species in the lower foothill landscape. Our guest demurred forcefully: “Why must you consider this some kind of tragedy? Why don’t you see it as an opportunity for all kinds of evolutionary novelty to arise?” Indeed.
Thomas asks (p. 104): “How long will it be before the environmental police force of ecologists and conservationists is prepared to step back and decriminalize introduced species that have had the temerity to be successful?” An excellent question.

Stevie Nicks got over her fear of change: “Time makes you bolder…children get older…I’m getting older too.” Maybe conservationists can mature after all.

Arthur Shapiro


Professor Thomas’s book is very much in the mainstream.  The Economist magazine included it in their list of important books published in 2017.  It is one of only a few books in the category of “Science and technology” and it is at the top of the list.  The Economist says of the book, “Humans have consigned species to extinction at an alarming rate.  But hybridization and speciation is happening quickly too.  An ecologist at the University of York shows how humans are bringing about a great new age of biological diversity.  Extinctions ain’t what they used to be.”

The New York Times published a review of “Inheritors…” on New Year’s Eve.  The reviewer summarizes Thomas’s main argument: “He argues that new species are arriving and evolving faster than old species are dying out globally…Instead of the sixth extinction, it’s a sixth genesis.”  The reviewer faults Thomas for not portraying the “wonder of nature” and for giving oceans short shrift.  But, the reviewer concludes with this observation about the unhelpful role that humans often play in conservation efforts: “It is human concerns that determine everything here on Earth now.  An animal that arrived in a particular location hundreds or thousands of years ago is fine with us, while a more recent immigrant, like garlic mustard, is cause for alarm and extensive campaigns to extirpate the interloper.  Nostalgia is deadly, as people kill to preserve or restore some ill-remembered but more natural past, and we disdain new species as weeds.”  That observation about human attempts to control nature says it all.  Plants and animals are not to blame for the damage we are doing to satisfy our ideological commitment to the distant past. They are symptoms of change, not the cause of change.

Happy New Year!

Million Trees

Update:  Professor Thomas gave a presentation to the Long Now Foundation in San Francisco on June 19, 2018.  HERE is a video of the introduction to his presentation.
And HERE is a presentation at the National Academy of Sciences, “Moving Times for the World’s Biodiversity.”
If you haven’t read his book, his presentation is a good summary of the issues he covers in his book.  MT

Anise Swallowtail butterfly in non-native fennel. Courtesy urbanwildness.org


  1. Arthur M. Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation,110, 413-433, 2003

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)

Darwin2

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