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 PracticesControlled 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 SuccessionOften, 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.
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