What’s the difference between native and exotic plants? And is one better than the other?
Paul Cappiello Ph.D.
Yew Dell Botanical Gardens Executive Director
Ansel Adams has long been my favorite nature photographer. Immediately recognizable, his black and white images are not just brilliant because he happened to set up his tripod on the Kodak Picture Spot at Walt Disney World at the perfect time of day. Rather it is his manipulation of the image, the coaxing of the perfect contrast of dark and light that sets his images apart.
Even calling the images “black and white” seems a gargantuan insult. Like most things in life, his images are made up of a million shades of gray. Indeed, even an ultimate definition of “black” still eludes us today.
In the world of plant ecology, the term “native” seems to occupy a similar place as the word “black.” We all feel like we have an instinctive understanding of the term. We apply value to that knee-jerk definition — native plants are better than exotics, using natives is more environmentally responsible.
Yet the closer we look, the more perplexing it can seem.
The words “native” and “exotic” are terms of art in the world of plant ecology. They refer to the geographic origin of a species. And that seems pretty straightforward. A plant (or a fish or a bird or any living thing) is either from here or it isn’t. It’s like the 10 years I lived in Maine. I could have lived there a thousand years and the locals would still have considered me “from away.”
Pretty easy, right?
Well, if the world was all truly black and white, it would be just that easy. But those darned shades of gray.
Let’s start with a mythical earth where humans never evolved. It’s just a bunch of other animals, plants and microorganisms, swimming, galloping and sliming around the ancient planetary soup. Pick a point in time and map out where a particular plant species is found to grow — from the farthest west to the east, north and south — and you can easily create a little map of what we’ll call that plant’s “range.”
Now let’s talk about our mythical plant species that on our chosen date has a range stretching from Birmingham, Alabama to Chicago, Illinois and east to Pittsburgh, Pennsylvania. It’s growing all over within that range without any impact from humans. Then along comes an ice sheet that covers the ground a mile thick from the North Pole to Indianapolis. Certainly, none of our little plants are left growing between Indy and the Windy City now.
Has the range of our plant changed? Of course it has.
The same would be the case if instead of a northern glacier, we found ourselves with a dramatic increase in the grazing buffalo population between Indy and Birmingham. If over a number of years or decades or centuries, the buffalo grazed out all of our little plants in the southern part of its range, the actual range of our little plant might vary considerably from the original map.
The same goes for a flood that carries some seed beyond the existing range of a species. If those seeds sprout and establish a successful population, they’ve expanded the range of the species.
So the range of a species is not a static thing. It shifts with climate, competition and based on its overall competitiveness in a particular place on the planet.
The next term that is essential to a good understanding of the native/exotic question is the term, “Provenance.”
Provenance refers to the specific geographic origin, not of a whole species, but of a subset of the species — a single plant, even a bag of collected seed. Thinking back to our little plant, it’s easy to recognize that even if it grows from Chicago to Birmingham to Pittsburgh, the growing conditions throughout that range are not uniform. Winters are way colder in Chicago and Birmingham summers are a whole lot hotter. It’s not much of a stretch to think that competition from other species might differ across a plant’s range as well.
So the provenance of a particular plant specimen or small group of plants defines geographically based variations in adaptability or other characteristics (some differences immediately visible and others not so much) across the range of a plant species.
A quick example of provenance variation is a red maple (Acer rubrum) that has a range that stretches from northeastern Canada to south Florida. Southern provenances are far less cold tolerant and also tend to have darker red spring flowers compared to northern provenances of the same species. And because they evolved across a wide north/south range, red maples of different provenance also react differently to things like changing day length across the seasons. They all belong to the same species. They’re just a little different — sort of like New Yorkers and Mainers.
"Native"
So now we get to the star of the show — the term “native.” And this is where things get complicated.
It gets complicated because in order for the term to mean anything, we have to, one, introduce humans into the equation and two, agree on the term’s definition.
And I don’t know about you, but if my wife and I can’t even have a civilized conversation about whether a particular paint color we’re considering for our bedroom is, in fact, blue or gray — or whether that one color is spelled “gray” or “grey” — you can imagine the difficulties encountered with charged terms like “native.”
Generally, most people consider a plant’s native range as the range that existed (in North America, anyway) in 1491 — pre-Columbus. But of course, that Eurocentric version of the definition completely misses the significant impact pre-Columbian North Americans had on plant species distributions. For a good overview of this topic I highly recommend Charles C. Mann’s, “1491: New Revelations of the Americas Before Columbus.”
Ok, you might say. Let’s go back to before any humans inhabited North America. Fine. Do you want to go back pre or post glaciation, and if so, which glacier.
Do you see where I’m going here?
This exercise becomes death by a thousand cuts. We can tweak it all day long but we will always end up in the same place. And that place is — no matter what definition we use for the term “native,” it will be arbitrary!
That’s not necessarily a bad thing. It’s just a really, really hard thing to get billions of people to agree on.
Finally, there’s the hardest part of this discussion and that is, how do we incorporate human impacts into any definition of “native” or “native range”? Fifty million years ago, Dawn redwood (Metasequoia glyptostroboides) was one of the most widespread tree species in the northern hemisphere. But its range had so shrunk over the eons that until the 1940s, it was considered to have been extinct for 20 million years before a small population was discovered in China. Clearly, its range shrunk due to non-human forces.
This article was originally published in the Courier Journal on February 4, 2022.
About the Author
Blurb about Paul here.
