In most UK circles those two Senecio (and a few other cases, like Erythranthe peregrina) are considered native - so, say, the BSBI’s Atlas (the Welsh groundsel here) describes them as native.
I think there is an invasive Spartina grass in Britain which is a new species, a hybrid between a native and a New World species.
Spartina anglica, from the native S. maritima and American S. alterniflora, has a habit of forming thick stands - though in recent years there’s been a huge dieback in some areas. It got planted quite widely to ‘stabilise’ but not much now.
I would consider this endemic, since the definition is clearly met (occurs nowhere else). The taxon itself is not introduced, but is rather the result of an introduced species interacting with a native one. The hybrid is as much native as it is introduced in my mind.
That said, I probably wouldn’t prioritize it for conservation if I were making that decision. An exception might be if one of the parent species were threatened with extinction. In that case, protecting the hybrids might be a way to protect what remains of the parent species’ genome.
Is Pluchea x fosberii reproductively isolated from its parents + fertile? or is it just a hybrid between two species? If its regarded as its own thing then I’ll add it to the project
I found O. oehlkersi which is a hybrid between a brazilian species and O. suaveolens. (although im not sure its valid- no genetic tests have been done to determine if its an alloploid. suaveolens confuses me because I can’t find much information on where it originated. I’m wondering if its a hybrid too? Currently trying to get papers on rubricaulis and chicaginensis.
This is an interesting philosophical question. To my mind, a new species that arose through hybridization at a particular location is native (and possibly considered endemic, if its range is limited), even if one or both parental species are non-native. Since speciation via hybridization seems to be fairly common in plants, the process isn’t that unusual. The circumstances (one or two introduced species contributing to the hybrid) is atypical since it is due to anthropogenic causes. But the result is akin to a natural process.
As for its conservation status, that is outside of iNat’s scope and for the relevant resource managers to decide.
Edit: Just noticed @deboas said basically the same thing.
I would be deeply dismayed if any conservation time/money/resources were directed towards protecting a hybrid Salsola in California. It would be pretty good evidence of semantic derangement.
As I once heard it put:
Some people can’t find their way out of a wet paper bag, but it takes an academic to talk themselves into one!