I suspect this involves both evolutionary biology and genetics. Given the ongoing and escalating global spread of invasive species, I can’t help but think that early natural species expansions were likely driven by similar colonization of unfamiliar habitats. Over time, these ancestors might have evolved into distinct new species in their new environments, eventually becoming an integral part of the local ecosystem.
Since human activities have accelerated the dispersal process, will future events mirror history, leading to a new wave of species that have evolved from these introduced aliens? Is it possible for an alien species introduced to a new region to eventually evolve into a native species in the future? During its evolution, will it gradually integrate into the local food chain, and will its invasiveness continuously diminish?Are there any documented cases of this happening already?
If you allow allopolyploid speciation it can be quite quick. For example Tragopogon mirus and Tragopogon micellus evolved in North America from alien (European) species. At the other extreme I’d believe a figure of millions of years for tree species.
It depends how you define species, obviously in changing from A to B there’s going to be a point midway where half of researchers say it’s still A and half say it’s now B, and differences can occur as to whether they call it a species or subspecies.
The time it takes I’m sure will also depend on the organism, some changing their reproductive parts quickly, some slowly, after all you can adapt to a new environment without necessarily having to alter your reproductive details, whereas some plants might have to to normalise pollination.
I guess it will also depend how many initial organisms are there and how often fresh ones arrive and mix in, and for non-mammals whether there’s anything to hybridise with.
What I would suggest is that when for example a plant arrives it may be badly timed for the seasons etc, and so not seem like a spreader, but after a few generations it probably adjusts its flowering and seeding to maximise reproduction and would then spread more so.
I also think a lot of the plants people panic about will find they get insects and pathogens adapting to over time. I actually think humans could speed this up by finding a pathogen already present in the environment for a similar plant and gradually changing its “diet” to have the new alien plant in in an ever-increasing percent, so that the pathogen’s chemistry is adjusted, and then you could release it, which would be much safer than dropping in some exotic pathogen.
As far as I’m aware (and I got my PhD in a lab that thinks about these kinds of things) there is no useful definition of when things become native. We care about things being native primarily because they co-evolved with other species native to the same place, which often strongly influences their ecological interactions. But in addition to the population genetic questions and the microevolutionary processes they underpin, I don’t know of any standard for how co-evolved things have to be to be considered native. Nor am I aware of any published paper declaring that a particular species has become native in a particular place. Native is often defined roughly as “it was here before humans mixed everything up” and by that standard a species cannot become native, it had to be native at whatever (often unstated and undefined) past time is used as a baseline.
Now, I got my PhD close to 20 years ago, and have moved on to less academic fields, so there may be some newer consensus on all these questions. I would guess, however, that when closely examined, your question becomes as much about semantics as genetics.
I think that basically, on human timescales most organisms large enough we can see wouldn’t really be consider to have ‘naturalised’, since as dlevitis mentions, that suggests existing in some level of balance with other organisms in their local ecosystem. Meanwhile, organisms like the cane toad, reed canary grass, or nutria are rapidly upsetting prior equilibria in the environments where they have been introduced.
For the purpose of managing plants, a species that is introduced from another area is always non-native. It may become naturalized (reproducing well on its own and spreading) but it is not native. If a new, reproductively isolated species forms from the introduced species, that species is considered native. Thus, Tragopogon mirus is considered a native species but its parental species (T. dubius and T. porrifolius) are not.
The introduced species are not considered native in the areas where they are introduced and they will not be. This isn’t a question of ecological relationships or co-evolution but of ancestry. If the introduced species changed over time enough that it was considered a different species from the ones in its place of origin, that changed species would be considered native in the new area. However, the normal evolutionary process is slow and we don’t see speciation in North America in the mere 500 years since the first introduced plant species arrived here. So the introduced species are not native and will not be native any time soon. However, there are methods of sudden species (as in the Tragopogon example) where a descendant of introduced species is native in the new place.
Given the current political climate where I live, I feel I must state that what I have written here is not in any way the same as what happens as human populations move around the globe because we humans are all one species and intraspecific population genetics is much different from speciation.
A few additional points to all the above…
Although the original post uses the term ‘native’ I wouldn’t take that to mean ‘native’ in the conservational since, but in a more colloquial way of being present like other species and treated as a resident.
The other point is that whilst problem species get all the news, especially in some environments, it may well be that most species, certainly in certain types of environment, do not cause a problem and only a minority do. I’m sure someone will have researched when this might occur. For example, alien plants in Britain, of which there are a profusion, rarely cause a problem, only a small minority do, but actually the minority of thugs that do occur need to be put in context of native thugs like Brambles or Bracken.
And also the context of climate change… nature is not static at the best of times, but in these times of change, you are going to get alien thugs simply because they are more appropriate to a changing climate.