I have a question about Establishment Means for insect species whose host plants are considered adventive in a region. Should such insects be listed as “Introduced” if they arrived to the region on their own, if the presence of the non-native plant is what allows them to persist there. I can think of two variations on this:
A plant’s range has expanded greatly due to human plantings, though the native range and adventive range are contiguous. Now the insects that specialize on that plant have expanded their range along with the plant. Black Locust and Honey Locust in New England come to mind. Should all Black Locust specialists be considered Introduced in New England?
A newly introduced exotic plant catches on as an alternative host for an insect, allowing its range to expand dramatically by switching hosts. The Wild Indigo Duskywing spreading as it began using Purple Crown Vetch and the Ailanthus Webworm spreading as it began using Ailanthus come to mind. (Despite its common name, the Ailanthus Webworm naturally fed on Simarouba, not Ailanthus, and was restricted to the tropics, but it spread northward in the 1800s when Ailanthus was brought to the USA and Canada as an ornamental) Should Ailanthus Webworm be listed as Introduced throughout the temperate USA, and should Wild Indigo Duskywing be listed as Introduced in counties where no host besides Crown Vetch is available?
I ask this because someone recently made a change like this to a particular insect (a case similar to #1 above), and it got me wondering whether this was considered a proper practice. I personally think “it’s using a non-native host here” is insufficient reason to call a species Introduced. But then again, if there truly was no host plant present in the region before human intervention, I can see making an argument against it being “Native” to the region also. Does anyone have guidance on what ought to be done in these cases? Currently it’s a bit of a hodgepodge- for example, Canada has the Ailanthus webworm listed as “Introduced” in each of its provinces, but the USA hasn’t listed it as “Introduced” in most of its states, despite the species’ status/situation being similar in the temperate USA and Canada.
I would consider Black Locust non-native and invasive here in Ontario, but my assumption is that Locust Borer (Megacyllene robiniae) spread naturally from the native range of the locust and wouldn’t consider the beetle to be “introduced”. Calling them “native” is iffier but if it’s binary then sure (seems kind of like arguing whether Western Cattle Egret is native if we know it spread recently from Africa). If it were documented that the earliest records of the beetle were of grubs transported in lumber or something then I’d change my mind, but documentation of something like that seems unlikely.
I have nothing to contribute to Paul’s question, but I WILL contribute something for anyone reading along, like me, who is wondering what “adventive” means:
If the species naturally expanded its range on their own without direct human assistance, that seems native to me, even if it was caused by indirect human changes to the environment. At least, assuming we only have two options to work with (“native” or “introduced”).
Lots of species expand their range for unclear reasons, which are generally assumed to be somehow anthropogenic (Great-tailed Grackle, for example), but pretty much anyone would call those native. Even if we know the cause, insects with non-native hosts would seem to be a similar situation.
It seems to me to be entering dangerous territory to consider naturally dispursed species as introduced just because humans have interfered with the distribution of their host or prey. How would this situation be really any different from a species that expands or alters its range due to human-induced climate change? There must be thousands of knock-on effects of human behaviours that can influence species’ ranges. In most cases of a change of distrubution, it may not even be possible to categorially untangle the human factors from the natural ones.
Yes, I wouldn’t call this introduced. The plain meaning of introduced seems pretty straightforward, and a species that expands under its own power doesn’t really meet that. I probably wouldn’t call an organism native in its expanded range either. I think “adventive” is a better term for the scenario laid out. The most accurate choice might be to not set any establishment means. But if the choice is just between introduced and native, I would select native as introduced is clearly incorrect in my opinion.
For instance, we are seeing all kinds of range shifts in organisms due to human-induced global climate change (species moving up mountains/changes in altitude, latitudinal shifts, changes in depth in oceans, changes in ocean currents/winds/storms moving organisms to new places). By the same logic for listing the species using a human-introduced host as introduced, we’d also need to list all those organisms as introduced in the places where they previously did not occur prior to climate change. That’s not very productive/useful in my opinion.
“Where a species evolved” seems to be setting a very high threshold for nativeness. And if you could know the place where a species evolved, how wide a boundary does one allow around the place of evolution? Is it considered native anywhere in the same country, which is usually an artificial geographic unit? Or the same continent? Or do you consider each case on its own merits? You would also need to know when it evolved, in order to work out what the state of the tectonic plates was at the time.