Who Belongs Here?
Species are either native or non-native, and there's a clear way to delineate that distinction. Right? Right?
A month or two ago, a friend sent me this picture and asked what the cool-looking tree growing off the side of the road was.
I don’t think he was expecting the text storm he got back. That tree is Ailanthus altissima, the tree-of-heaven, and is one of the nastiest invasive tree species in North America. The roots of Ailanthus will grow straight through water pipes and foundations, crippling buildings as they set up a wide net to suppress any other trees from putting down roots. These networks will then send up dozens of shoots, creating dense patches of Ailanthus so heavily shaded that nothing can grow underneath them. The plants are almost impossible to remove — as any amount of root in the ground can lead to the plants reestablishing immediately — and a single shoot can cast out thousands of seeds each year to take over even more of the surrounding landscape. Plus, both the pollen and bark are toxic to humans, causing rashes, burns, and allergic reactions. This pest has established itself in most of the United States, spreading along the sides of highways and through rust belt cities alike to wreak its havoc.
Ailanthus altissima range. Map: USDA.
And that’s just the tree itself. Even worse are the insects that use the tree, following it along the sides of highways to expand their own ranges. Worst of these is the spotted lanternfly, perhaps the most charismatic invader in history:
Photo: USDA.
The spotted lanternfly is so charismatic that a friend in Pennsylvania’s Department of Conservation and Natural Resources told me that when they had citizens calling in saying they’d spotted an one — the sort of untrained report that is usually accurate less than 10% of the time — the caller was right 99% of the time. It’s a beautiful insect. It’s also one of the largest novel threats to American agriculture: the spotted lanternfly feeds on most crops we produce in America, producing a mold that can devastate harvests. When they aren’t able to feed on farms, these flies are more than happy to use Ailanthus, and have quickly leveraged tree-of-heaven’s extensive range to spread throughout much of the eastern seaboard.
This isn’t great. The situation as it stands is that one invasive species, which damages native ecosystems wherever it establishes, has lead to the explosive growth of another species detrimental to human-built systems. These are species that have been introduced to the US with clearly damaging outcomes, and which have been the target of millions of dollars of remediation programs over the years. But it’s worth looking into these species further, both to try and understand how the situation landed where it did today and also to shine a light on how we determine what species are allowed to exist where.
When you start looking into the natural history of invasive species, one trend becomes embarrassingly clear: almost all of the worst invasive species got established by a small number of people acting like idiots.
Ailanthus is no exception, first arriving stateside in 1784 when a gardener (William Hamilton) sent off for a plant that would survive in the poor Philadelphia soil he was growing in. The trees soon became popular, and by 1820 a nursery on Long Island established a business importing and distributing new Ailanthus to the far-flung reaches of the young nation. Before long, the tree was being used for ornamental plantings across the eastern half of the country, with some foresters even attempting to use it to forest the native grasslands of Kansas and to replace clear-cuts in Pennsylvania.
But Ailanthus would only stay popular for so long. The tree was a subject of a number of racist screeds following the Opium Wars of the mid-1800s, with some authors noting the tree as having “drawn away our attention from our own more noble native American trees.” The tree, at this point well known for escaping domestication and taking over whole swaths of the landscape, became a vessel for xenophobia as American settlers feared Chinese immigrants would have similar success in the new world.
So when cities and states began to ban the tree wholesale in the later 1800s they cited both the substantiated negative effects (remember, in addition to the ecological damage both the bark and pollen are actively poisonous) and the general sense that this species didn’t belong here, not in America.
Which brings up an interesting question: how can we judge what does belong?
At this point, I think it’s helpful to zoom out a little bit from just thinking about Ailanthus, and spend a minute talking about other places and times.
Great Britain is a really interesting place to be a forester, not least because you don’t have to know that much; there just aren’t that many tree species. Scotland, for instance, only has 31 native woody species, and the other countries within the UK have similar levels of species richness. Compare this to 950 unique native tree species in North America — and while the size comparison is maybe a little unfair, even Germany has 90 unique species. But Great Britain gets by just fine with its limited variety.
English elms lining a roadside. Photo: denisbin
Not included in its list of native species, however, is the English Elm. Once one of the most distinctive features of the British landscape (“its true value as a landscape tree may be best estimated by looking down from an eminence in almost any part of the valley of the Thames, or of the Severn below Worcester, during the latter half of November, when the bright golden colour of the lines of elms in the hedgerows is one of the most striking scenes that England can produce”), this tree — now devastated by an invasive disease of its own — isn’t counted as a native tree due to being introduced from Spain by the Romans sometime near 100 AD. Sure, it’s been in the landscape for thousands of years and is one of its most distinctive features, but we have evidence of humans actively guiding the migration. Therefore, it’s nonnative.
But drawing that line isn’t as easy as you might think. Take for instance wild horses: originally developing in North America, the species spread to cover most of Asia and Africa before dying out in their original range by about 12,000-10,000 BCE. In these cases, we generally think of horses and their descendent species as native to Asia and Africa — sure, they didn’t originate there, but they arrived under their own power as part of a bog-standard migration.
More confusing, however, are the wild horses since reintroduced into North America in 1519. There are genetic differences between these horses and their extinct ancestors, just as there are differences between the Asian and African lineages of horse, but the species is substantially similar to the one that originally evolved in the Americas. Yet the US still labels these wild horses non-native and attempts to exterminate them across most public lands in order to protect native species. These prodigal sons have been actively rejected upon their return.
Even less clear is the huge number of North American species who had their ranges extended due to planting by indigenous peoples. A large number of species moved outside of their original ranges due to human-assisted migrations, mixing existing ecosystems together and creating wholey new plant communities. These plant species might have been clearly native a few hundred miles away, but in these new reaches which they could only colonize with some help from humans, do they suddenly become non-native?
Or finally, consider palm trees, which spread to new islands by dropping buoyant fruits into the water and hoping they get washed ashore. In this way, they’re able to establish large populations on islands that have never so much as dreamed of a palm before, quickly becoming one of the dominant species. And in the modern era as the globe continues to warm, palm trees are spreading further and further north, taking over more land than had ever before been possible for the plant. There’s no humans involved in many of these migrations, but it certainly seems as though palms are set to expand aggressively. Are they simply new native species in these new worlds?
There’s not really clear answers for these questions, in part because it’s starting to become more obvious that the native/non-native dichotomy is the wrong lens to view the world through. Frequently these terms only reflect if a species has been spread by humans or not, which (as I’ve ranted about before) reflects a poisonous separation of the world into “natural” and “man-made”. The world is changing every day and no system can stay static for long without collapsing under external pressures, the environmental equivalent of bitrot. Viewing ecosystems as having one steady state with every new species as “non-native” misses the dynamic reshuffling that is constantly ongoing within every system; managing to preserve that steady state condemns the system to stasis and prevents it from adapting when made necessary by either large-scale global problems (such as climate change) or smaller incessant issues such as windstorms or new diseases.
Instead, we need to shift to viewing species introductions as being either helpful or harmful to the greater system. Some species — such as Ailanthus — weaken ecosystems by reducing species diversity, limiting the number of plant species that can survive at a site and the number of resources available for animal species. But other introductions — such as hybrid tortoise species in the Galapagos — can actually help shore up ecosystems, helping restore balance to species interactions that have been disrupted in the past few thousand years as humans themselves spread across the world.
This sort of calculation is more complicated than simply asking if the species has lived here before, but that’s more or less the point. The future of ecological management is going to require us to take these more complex considerations into account. We are as gods and might as well get good at it. So while some species — including Ailanthus — are obviously harmful under these considerations, it might be time for us to have a more open mind to some other accidentally (and possibly even purposefully) introduced species moving forward.
One last note on Ailanthus. There’s a really famous book, A Tree Grows in Brooklyn, that came out in 1943. The whole book focuses on the struggles of a poor family in New York city, constantly fighting new adversities and still somehow finding success and happiness in the difficult world they live in. Throughout the book, the family is compared to a tree growing in their apartment building’s yard, which somehow keeps reestablishing itself despite constant efforts to cut it down. The tree finds a way to persevere in the face of adversity, and so, in time, does the family.
The tree, of course, is a tree-of-heaven, an invasive species which — though obviously this doesn’t come up in the book — is being cut down to prevent any further damage to the surrounding community. The metaphor comes off awfully different if you know your natural history.
I don’t think there’s any particularly deep point to be made here. I just find this funny.
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