One thing I've been wondering about, is when we get new species arriving in the UK, seemingly under their own steam with no human intervention, when do we class them as natives to Britain? Take the Collared Dove for example, it had a massive expansion to it's range last century, we class it as part of our native fauna now don't we?
The question occured to me when reading about butterflies such as the Queen of Spain Fritillary. There have been a fair number of sightings in the past few years and even evidence that a breeding population may be gaining a foothold in the south east. However there is some doubt as to whether they are natural vagrants, even though there are nearby populations across the channel. We don't really know and probably never will, so (and this I suppose is a slightly different question) at what point do we decide when it is likely a natural expansiona and part of our native fauna?
Sorry if some of that doesn't make sense, it's been a long day
