What is a "Native" Plant in a Changing World?

This common term is not as easy to define as you might think

California Fan Palms in Anza-Borrego State Park in California (Photo by author)

(originally written May 27, 2020)

The term “native plant” has become a common one, and many people probably assume that the definition is clear cut. However, like many other seemingly simple designations, that’s not the case.

Whether a given plant is considered “native” where it is found growing is dependent on the interpretation of the interrelation of three factors: time, place and human involvement.

So, in the United States, a plant is generally considered native only if it grew here before European colonization. On the East Coast, that’s the 1500s and in California, that’s 1769. Plants introduced since then, whether deliberately or by accident, are labeled “non-native,” “introduced,” “exotic,” or in some cases, “invasive.”

In the UK, some would set the date for ~8000 years ago, when rising sea levels made those landmasses islands.

The checkered history of “nativeness”

The UK was where the concept of “nativeness” was first proposed, in the mid-19th century, by Hewett Coltrell Wallace. Coltrell also included “naturalized” species, which were species that humans had introduced but which had come to live without them unaided.

The correlation of native=good and non-native=bad was popular with the Nazis. In reference to an introduced Asian plant species, Impatiens parviflora, a team of biologists there wrote: “As with the fight against Bolshevism, in which our entire Occidental culture is at stake, so with the fight against this Mongolian invader, in which the beauty of our home forest is at stake.”

A US American took up the cause next. In 1958, a man named Charles Elton published a book entitled, “The Ecology of Invasions by Animals and Plants” which was not so much a scientific treatise as a polemic laden with war jargon. He set a bellicose tone against “invasives” that is still with us today.

In the late 1990’s, the National Invasive Species Council was promulgated by an executive order of Bill Clinton, with Monsanto and other chemical giants among the parties who helped design it (in order to encourage the use of the herbicides they manufacture).  State and local governments took their lead from this council, quickly spreading its viewpoints and methodologies nationwide.

Plants on the move

Returning to the world of empirical science, we know that, historically, plant ranges have always been in flux, often in response to climatic shifts. Fossils and phylogenetics are two things that can show us where plants used to live and where they came from. Such information, though, raises questions even as it illuminates.

For example, when Spanish colonists arrived in California in the 18th Century, Coast Redwoods (Sequoia sempervirens) grew in a strip nearly 500 miles long and 5-47 miles wide from what is now Monterrey County in the south to Curry County (Oregon) in the north. But only 10,000 years ago, they grew as far south as Los Angeles, and five million years ago, they were found in Europe and Asia. Today (after over 95% of the Coast Redwoods in California were heartlessly cut) the species has been spread around the world by humans, including to New Zealand, where a 15 acre grove has been growing for over a century. Due to favorable differences in soil and rainfall there, the trees happen to grow faster than on the US West Coast.

The grievous sin of destroying so many Redwoods in California is compounded by the fact that much of their former habitat is now so altered by factors such as erosion from logging and that it won’t be home to these grand trees for the foreseeable future.

We can ask, then, whether such degraded places still comprise part of the current “native range” of the species. Further, we could also ask why that range cannot include places like the grove in New Zealand where the tree is thriving? Or whether a Redwood grown in a European location within its historic range 5,000,000 years ago is truly “exotic” or if it’s just coming home?

For many, the issue is “human interference” as opposed to “natural dispersal.”

In this way of thinking, the Creosote Bush (Larrea tridentata), an emblematic plant of the US Southwest’s Mojave Desert is native even though it’s from South America, because its means of conveyance over those many thousands of miles was non-human; possibly in the tail feathers of migrating plovers.

But this way of thinking also tends to ignore an important element: the influence of indigenous humans over history, which definitely impacted the “native ranges” of many plants and animals.

Indigenous Land Management Practices

Controlled burns by Indians on the Great Plains expanded prairies at the expense of forests, which led to the spread of Buffalo.

Similar techniques on the West Coast maintained Oak Savannah and suppressed the growth of Firs and Hemlocks.

Seeds, bulbs, corms and other plant material for propagation were collected, transplanted and traded far and wide among tribes in North America. Some species (such as certain Mariposa Lilies in the genus Calochortus) may have dwindled in number to the point of being endangered these days in part because they are no longer actively tended by humans.

The case of the California Fan Palm is particularly intriguing. For years, it was believed that the iconic species was a millions-of-years-old relict, left over from when its current desert home in southern California was much moister. However, phylogenetic analysis proved that the species emerged quite recently, since the last glaciation period 11,000 years ago.

It’s long been known that Indians made use of Fan Palms and their groves for food, craft material, and as places to live. They planted trees and they also set fire to them to clear away the dead leaves so they would be easier to climb to collect the dates. (Fan Palms are fire tolerant.) However, it also appears that they might have been responsible for introducing them to the majority of their “natural range” beyond the small area in Baja California where they originated. (See my Did Native Americans introduce Fan Palms to California?)

If this is the case, then the groves that remain are not the result of “natural dispersal” as that term is usually understood and are more akin to abandoned agricultural sites than to “wilderness.” What, then, is the best way to treat them? I mean, if we’re not going to allow tribes to maintain and use them as they did which is obviously the right answer? Burning is prohibited, as is harvesting and planting the fruits when the trees are on public land. One current policy aims to protect (which is understandable) but perhaps the actual result is neglect.

California Fan Palms are not the only trees that humans have moved around. In Asia, the “native range” of the Carpathian Walnut coincides with the route of the Silk Road. In eastern North America, the ranges of Black Walnut, Pawpaw, Persimmon, Chestnut, and Shellbark Hickory also seem to be the result of indigenous human influence. (h/t to Zach Elfers for this info.)

So, what we consider to be “natural” or “wild” is in many instances human-made or human-impacted. Some would go so far as to say that the very concept of “wilderness” is tantamount to indigenous erasure.

That settler-colonialists, mostly of European descent, have wreaked havoc on the ecosystems of the Americas is all-too-clear. To conclude from this that all the introduced plants who live here now “don’t belong” is a step too far, and the idea that they should be eradicated is not merely misguided, but dangerous. Fortunately, the conversation does not need to be so limited.

“Novel Ecosystems” & Ecological Succession

Often, native plants are valorized and non-natives villainized in a reflexive manner that belies the facts on-the-ground. How well an introduced plant has integrated into its new setting is rarely considered. Or the question of whether plants can become “native.”

In California, 1/3 of native butterfly species now use non-native plants as food sources and as egg-laying sites. The range of some of these butterflies has expanded as a result. (See: “Exotics as host plants of the California butterfly fauna“) This has been fortunate for the butterflies, since so much of the habitat that previously provided for them has been destroyed by human activity since 1769, through activities including agriculture, ranching, deforestation, mining, urban sprawl and—most recently—industrial-sized “green” energy installations.

Saltcedar/Tamarisk (Tamarix sp. and Russian Olive/Oleaster (Elaeagnus angustifolia) are oft-maligned as “invasive plants” that should be eradicated. But, as I co-wrote with Nicole Patrice Hill in 2019:

“In the western United States, these two trees are now the third and fourth most frequently occurring woody riparian plants, and the second and fifth most abundant species along rivers. To eradicate them would entail destroying a significant amount of healthy vegetation (with no little amount of collateral damage to other flora) and would incur a hefty cost.”

These two species are accused of pushing out native flora such as Cottonwoods and Willows, not providing food for native fauna, and of monopolizing water.

However, the success of these trees has resulted not from stealing space or moisture from native plants, but of destructive changes to watercourses by industrial development. Dams significantly change the flow, temperature and cycles of rivers. Water tables are drawn down by agricultural irrigation. Tamarisk and Russian Olive happen to be better adapted to these harsher circumstances than the native Cottonwoods and Willows. They have filled gaps that opened, rather than forced their way in.

The result is what is called a “novel ecosystem.” Fifty kinds of birds nest in Tamarisk, including the Southwestern Willow Flycatcher, which is endangered because of habitat loss. At least 44 kinds of birds, as well as various native mammals, eat Russian Olives as winter hardy food. Given the prevalence the introduced trees now, and the dearth of the natives, many animals are now dependent on them. Spraying the trees with herbicides has not, and will not, change the fact the dams are responsible for the altered landscape, not the trees themselves.

At some point, do we recognize that the Tamarisk and the Russian Olive are de facto “native” even if they’re not de jure? For what it’s worth, all those birds have already cast their vote.

Additionally, “novel” aspects might be temporary after the process of “succession” advances. “Succession” is a common ecological process in which the dominant flora of a landscape changes over time due to the ways that landscape is changed by the flora itself. So, after a disturbance—such as a landslide or the building of a road—the first wave of plants (which are sometimes called “pioneer species”) are often annuals that quickly fill the space. They will produce a profusion of flower that attract pollinators and seeds that feed animals. Such pioneers can be thorny, which is nature’s way of saying, “Keep out while I fix this!” A hallmark of this stage is the rebuilding fertility in the soil.

The annuals might be followed by shrubs, including berry bushes, which attract yet more animals, including birds. The scat left by these animals enriches the soil more. The bushes provide shelter for trees to germinate, and in time, the trees shade out the berries.

There are cases where disturbed landscapes that have been “invaded” by non-native plants have been left untouched, and the exotics have ended up doing nothing more than fulfilling the role of pioneer species, and the area has returned to “natives” over time. So, when “invasives” are constantly beaten back in a given location, it’s possible that this interference is holding back the natural processes of healing and recovery—“succession”—and ironically working against the intended goal of bringing back natives.

Novel ecosystems can demonstrate nature’s inherent resilience. What we need to do is see it. As time goes on, we’ll certainly have more opportunities.

Climate Change

According to National Geographic, “Half of All Species Are on the Move.” This is because, as the climate changes, so do ecosystems. With temperatures rising, species are moving further north or higher in elevation. As time goes on, this means that more and more species will migrate “outside their natural range” thereby becoming “non-native” or even—to some—“invasive.”

Those that can migrate, that is. Many plants will become, as wildtending guru Finisia Medrano used to say, “refugees without legs,” unable to flee fast enough and far enough to find safe haven. If that’s the case, then we must help them, Finisia repeatedly counseled.

The biologists call this “assisted migration” and it’s a topic that coming up more frequently as time goes on. Some of the strongest arguments against it come from the anti-“invasive” crowd, who are not—I will stress—in total overlap with the crowd that loves native plants; however, many spaces where native plants are discussed, especially online, have become dominated by anti-“invasive” rhetoric, with other views prohibited. It’s sad to see for those of us who just love plants.

Why Does It Matter?

The term “native” can have utility; it tells you that a plant was well-adapted to a given place in a given time period, which can be helpful for understanding a species or an ecosystem.

But as the basis of an us vs. them ideology, it becomes a weapon. Some people say that the proliferation of non-native plants is a manifestation of colonialism, but I would counter that that the mindset which insists on declaring them irredeemably bad is far more so.

That mindset leads to herbicides and other forms of mass killing that inevitably cause damage to “non-target” species. That mindset is all about imposing an old (and possibly imagined) order, rather than appreciating a new succession. That mindset seeks control not cooperation. We cannot afford to approach the world that way anymore (as if we ever could).