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.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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)
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.
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.
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.
David Quammen, The Tangled Tree: A radical new history of life, Simon & Schuster, 2018
Specific examples of these HGTs are available HERE.
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.
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.
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.
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!
Update: Professor Thomas gave a presentation to the Long Now Foundation in San Francisco on June 19, 2018. HERE is a video of his presentation. If you haven’t read his book, his presentation is a good summary of the issues he covers in his book. MT
Arthur M. Shapiro, “Exotics as host plants of the California butterfly fauna,” Biological Conservation,110, 413-433, 2003
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.
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.
“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)
“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:
The methods used to eradicate non-native species are harmful to the environment: pesticides, prescribed burns, destruction of species that are performing valuable ecological functions such as healthy trees.
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)
Jason Fridley and Dov Sax, “The imbalance of nature: revisiting a Darwinian framework for invasion biology,” Global Ecology and Biogeography, 23, 1157-1166, 2014
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.
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.
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.
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)
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.
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?
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:
“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?
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?
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
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 introduction. Since 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 herbicideare 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.
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
Maria Dornelas, et. al., “Assemblage times series reveal biodiversity change but not systematic loss,” Science, April 18, 2014
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 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
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.
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.
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.
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.
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.
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 expansionof the sea floor moves the plates away from the seams, which moves the continents. This is the engine that drives continental drift.
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
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.
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