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How is your garden changing the world?

“By transporting Asian species to North America, and North American species to Australia, and Australian species to Africa and European species to Antarctica, we are, in effect, reassembling the world into one enormous supercontinent — what biologists sometimes refer to as the New Pangaea.”

― Elizabeth Kolbert, “The Sixth Extinction: An Unnatural History,” 2014

I took a break from reading the garden-related literature on my 2022 reading list to delve into Pulitzer Prize-winning author Elizabeth Kolbert’s book, “The Sixth Extinction,” in which she laid out her argument that from the beginning humankind has had the innate propensity to move things around the world.

Scientific evidence shows that between 70,000 and 100,000 years ago, Homo sapiens began migrating from the African continent and populating parts of Europe and Asia. Not only did our earliest ancestors transport a completely new animal species to Eurasia — themselves — they most likely carried with them other life forms, such as plants and insects, that were not previously present in their new environment.

Homo sapiens interbred with Neanderthals in Europe and Denisovans in Asia, and their progeny continued to migrate around the world. Not only did early humans bring new lifeforms with them wherever they went, but scientific evidence also suggests they hunted some of the huge prehistoric ancestors of elephants, bears, antelopes and sloths to extinction.

Kolbert wrote, “Though it might be nice to imagine there once was a time when man lived in harmony with nature, it’s not clear that he ever really did.”

Our contemporary gardens, pastures and parks reveal examples of how humans have translocated animal and plant species, a proclivity with far-reaching effects. As Kolbert notes, “in effect, reassembling the world into one enormous supercontinent.”

Take the case of the European starling (Sturnidae vulgaris), a species that did not exist on the North American continent until a group of people released 100 birds in New York’s Central Park in the early 1890s. The group wanted America to have all the birds mentioned in Shakespeare’s works.

Today, more than 200 million European starlings range from Alaska to Mexico; they have been officially designated an alien invasive species throughout the U.S. Like all invasive species, S. vulgaris is aggressive, forcing out native bird species and damaging food crops.

Several weeks ago, I received an email from a Rogue Valley resident who was distressed about an invasive plant species called poisonous hemlock (Conium maculatum) that is taking over her pastureland. Poisonous hemlock is considered a noxious weed in all Oregon counties.

Lunette wrote: “I purchased a 3-acre farm five years ago. The hayfield surrounding the house and barn was in good condition. We had flood irrigation for the first two summers, and the field was surrounded by a large hedge of blackberries. We had no poisonous hemlock in the field. The (Almeda) fire burned the trees and blackberries on two sides of the hayfield. Since then, poisonous hemlock has crept into our field and the land surrounding us is one big hemlock plantation.”

Lunette is particularly concerned about poisonous hemlock, native to Europe, North Africa and Asia, because it’s very toxic to livestock as well as humans. She pointed out the rampant growth of C. maculatum on roadsides and beside irrigation ditches throughout the Rogue Valley. “Once you know what it looks like, you will be able to spot it as you drive I-5 or Highway 99,” Lunette said.

C. maculatum is an herbaceous biennial in the carrot family. Its lacy, white flowers are pretty, which is why the plant was intentionally introduced to North America in the 1800s as an ornamental garden plant. However, all parts of the plant are highly poisonous; ingesting six to eight leaves is enough to kill an adult human. Legend has it that poisonous hemlock killed Socrates.

The plant grows up to 7 feet tall with hollow stems and deep taproots. Like a carrot’s top foliage, poisonous hemlock has dark green, feathery leaves. As a biennial, it grows a rosette of leaves the first year, then dies back over the winter. The following spring, it grows to its mature height and blooms in June/July. The mature plant’s stems have purple markings, which helps distinguish poisonous hemlock from wild carrot.

Around this time of year, C. maculatum sets seeds, which — like carrot seeds — are tiny and, thus, easily dispersed by wind and birds. One flower of a poisonous hemlock plant can produce up to 30,000 seeds. The seeds can germinate almost immediately after contact with soil, or they can remain in the ground for several years before germinating. It’s a “pioneer” plant, meaning it quickly colonizes disturbed sites, which is why it infested Lunette’s pastureland.

I have spotted several sprawling stands of poisonous hemlock along Foothill Road, where they compete successfully with Himalayan blackberry bushes (Rubus armeniacus), another invasive plant species. R. armeniacus was introduced to North America in 1885 by famed California horticulturalist Luther Burbank, who thought they should be grown as a commercial crop.

Of course, the more poisonous hemlock and Himalayan blackberry spread across the valley, the more they displace native species of riparian plants and the native wildlife that depends on them. Over time, biodiversity is lost, and what’s left are a lot of the most aggressive plant and animal species: poisonous hemlock, European starlings — us.

Do humans care that we are “reassembling the world into one enormous supercontinent” with vastly less biodiversity?

In the spring, before the poisonous hemlock grew tall and dominated the landscape along Foothill Road, the hayfields were a sea of cheerful yellow from blooming mustard (Brassica nigra). I watched people climb over the fence onto private property to take pictures of themselves and their friends with a mass of pretty yellow flowers and the mountains in the background.

The scene made a great (if technically illegal) photo op, and I’m pretty certain the folks smiling for the camera were unconcerned that Brassica nigra is yet another nonnative, highly invasive plant species. According to some plant historians, black mustard was introduced to California in 1769 by traveling Catholic missionaries who threw out mustard seeds to mark their way.

I think Kolbert was right when she said that it misses the point to argue that the current extinction event could be averted if only people cared more, or knew more, about biodiversity. “It doesn’t much matter whether people care or don’t care,” she wrote. “What matters is that people change the world.”

How are you and your garden changing the world?

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. See literarygardener.com or email her at Rnowak39@gmail.com.