dispersal of species – Conservation Sense and Nonsense

Gardening with the help of nature

Juliet Stromberg is a plant ecologist who specialized in wetland and riparian ecosystems of the American Southwest. Her friends call her Julie and I will presume to do the same. She has retired from her position at Arizona State University, but her husband, Matt Chew, is still teaching ecology from a historical perspective at ASU. He is very much her partner in their 20-year project to restore 4-acres of dead citrus grove and an 80-year old Spanish colonial house, long abandoned and derelict. The property came with water rights, without which their project would not have been possible.

In her recently published book, Bringing Home the Wild: A Riparian Garden in a Southwest City, Julie tells us how she and her partner transformed—with the help of natural processes–this dead patch of land in South Phoenix, Arizona into the oasis that it is today. The first step was to restore the irrigation system, which immediately brought much of the dormant seed bank back to life.

Julie & Matt’s garden is in the center of this aerial view

Using the riparian vegetation of the Salt River—the source of their water—as her reference, she chose a half-dozen tree species as the foundation of their garden, such as Fremont cottonwood, Gooding’s willow, and velvet mesquite. Twenty years later, there are now 300 trees, sheltering a community of plants and animals. How did they get there?

The seeds of some trees such as blue elderberry and mulberry were brought from neighboring gardens by birds and small animals. Julie and Matt have seen 157 species of birds in their garden, so we can assume birds have done some of the planting. The seeds of some plants are aerodynamically shaped and were blown in by the wind, adding to the diversity of the garden.

*Tropical milkweed seeds ready to be launched by the wind from a neighbor’s front yard. Conservation Sense and Nonsense, Oakland, CA, October 2023*

Many of the trees are American in origin, but others are not. Regardless of the method of dispersal, most introductions are welcome in Julie’s garden. She spares her readers the tedious recitation of which plants are considered native and which are not. The Southwestern desert is not an ecosystem with which I am familiar. I was glad to have a tour of Julie’s garden without irrelevant information about the nationality of every plant. For the same reason, I like to travel in distant places where I can’t distinguish natives from non-natives. Everything looks great to me and nothing brings me down more than a guide who wants to inform us of what “belongs” and what doesn’t.

Julie and Matt also planted a fruit orchard and a vegetable garden that bring more birds, insects, and animals to the garden as well as providing food for their table. Eating the fruits of our labors in the garden deepens our respect for what plants do for us and establishes our working relationship with the land.

Managing a wild garden

In keeping with Julie’s opinion that ecological restoration is a form of “glorified gardening,” she actively manages her garden. A few plants that annoy members of her community of plants and animals—such as puncture vine and tumbleweed—are not welcome.

When the delicate balance between predator and prey becomes unbalanced, some protective measures are necessary. If coyotes and dogs can’t keep up with the rabbit population, it’s sometimes necessary to put vulnerable plants into cages to protect them. The root balls of some plants are covered in wire mesh to protect them from hungry gophers.

Plants also assist in their own defense. Where mesquite is grazed by cattle, the tree responds by growing longer thorns to repel the cattle. When plants are attacked by plant-eating insects, some emit a toxin to render themselves inedible. The scent of the chemical wafts to neighboring plants, alerting them to the arrival of predators. These natural defenses are an important line of scientific inquiry that has potential to substitute nature-based solutions for synthetic chemicals.

The population of roof rats in Julie’s home is kept in check with liquid birth control, lest they chew on electrical wires or build nests in car engines.

Gardening with the help of friends

Julie’s is not a manicured garden, but it requires constant pruning to keep trails clear and provide light and space for plants to thrive. The annual scouring of the flood plain by spring floods is one of the natural processes that Julie and Matt could not use to restore their land because irrigation water is channelized and confined by concrete. Julie has come to appreciate the flies and other insects who are the decomposing crew, helping to reduce the accumulation of debris in the absence of annual scouring floods. Sixty-six species of flies assist with decomposition as well as pollination in Julie’s garden.

Julie is happy to have coyotes in her garden, but her dogs disagree. Violent and fatal confrontations between these closely related species required building a wall that confines dogs close to the house at night, while coyotes safely roam most of the garden.

Dogs are an important part of Julie and Matt’s life. Early in the book’s introduction Julie warns readers that they should put her book down “NOW!” if they don’t want to hear dog stories. Julie has walked thousands of dogs in a nearby animal shelter. In addition to her own 4 dogs, there are also occasional foster dogs who need to recover from traumatic experiences to be adoptable. In Julie’s refuge, these traumatized dogs learn to trust again.

Peaceful co-existence

Julie is a recovering academic scientist. Before she retired, she felt that her focus on the accumulation of data needed for scientific analysis was causing her to lose track of the big picture. She needed to stop and smell the flowers, so to speak.

She received her graduate education during the heyday of invasion biology. Julie slowly shifted away from native purism based on her experiences in the field. She has rejected that doctrine, and regrets teaching her students to fear “those who came from somewhere else.”

Julie has a vivid memory of the first step she took on that journey to her gardening ethic of peaceful coexistence. She had been instructed to pull tree tobacco from land along the Salt River that was being restored. The nicotine in the plant was making her feel sick, which seemed to bring her to her senses. She began to wonder what she was doing, “following orders to kill creatures she barely knew.”

Fly on desert tobacco. Photo courtesy Juliet Stromberg

Part of Julie’s skepticism about such eradication projects is based on her understanding of how little we know. She realizes that the harm done by non-native species is exaggerated and their benefits are underestimated. Given the limits of our knowledge, we should be obligated to give introduced plants the benefit of the doubt before killing them. She now appreciates the beauty of tree tobacco, which also feeds birds, fixes carbon, and stabilizes the soil. Its seeds were naturally dispersed to Julie’s garden and tree tobacco is welcome there.

Imperatives imposed by climate change

Julie says, “The preoccupation with provenance diverts conservationists and gardeners from critical issues,” such as climate change, food security, and extinction (which, studies show, are not caused by introduced plants). Living in the Southwest, Julie has a front row seat on climate change. It’s always (within the context of our lifetime) been hot there, but now it is blisteringly hot during summer months. She watches hummingbirds in her garden seek shelter in the shade, close to the irrigation drip. She watches dogs panting, birds gasping for breath and plants wither and die in the heat. And she knows that both native and non-native plants store carbon that would otherwise contribute to greenhouse gases causing climate change. Carbon storage varies according to certain plant characteristics, but those characteristics are unrelated to the nationality of plants.

Those who insist on replicating the landscape that existed 200-400 years ago in America are depriving nature of the evolutionary opportunities that will enable survival. We don’t know what life will be capable of living in the climate of the near-future. Nature needs as many alternatives as possible to find the species that can survive. Plants and animals are blameless in this struggle of survival of the fittest. The least we can do is to get out of their way as natural selection finds the life that is adapted to the current and future climate.

Showing respect for nature

Julie does not use any pesticides in her garden….no herbicides, fungicide, or insecticide. She is concerned about the pesticides used by her neighbor across the road who grows cotton. She notices the blue cotton seeds scattered on the ground and surmises that they were coated in insecticide or herbicide that will infuse pesticide into the plant as it grows. The poisoned seed can kill seed-eating birds and other animals and the plant itself will be poisonous as it grows. The dust from the cotton field blows into her property when the field is plowed and after the cotton is harvested because no cover crops are grown to tamp down the dust and prevent the loss of carbon stored in the soil. Julie can see firsthand the damage caused by industrial agriculture and is confirmed in her commitment to avoid using pesticides.

Julie shows her respect for everything living in her garden by her choice of pronouns to describe them: “who” not “what,” “she/her” not “it.” She asks her readers to show the same respect for plants and animals, regardless of their nationality. Avoiding the use of pesticides in our gardens is another way to show our respect for the plants and animals on which we depend, with the added benefit of not poisoning ourselves.

Thank you, Juliet Stromberg, for telling us about your garden and congratulations for what you have accomplished and learned from the experience of nurturing it back to life with the help of nature.

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

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

In the beginning….

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

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

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

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

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

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

Continental Drift

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

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

Tectonic Plates - USGS — Tectonic Plates – USGS

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

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

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

DNA analysis trumps elegant theory

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

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

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

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

Evolutionary science comes full circle

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

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

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

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

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

Bringing it home

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

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

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