A three year Defra-funded study, published in 2012, registered 3,758 different non-native species. Of these, around 1,875 were regarded as ‘established’ and reliable records.
These Snow Geese are an obvious example, but of that 1,875 nearly 75% were plants, the vast majority of which were the product of our desire for exotic species for our gardens.
What is the correct response to such numbers? It does seem to depend on the appeal of the organism. While it’s easy to get support for eradicating Himalayan Balsalm, Japanese Knotweed, or the Signal Crayfish, it would be harder to build such consensus on the Muntjac. Talk negatively about Grey Squirrels and you risk ire, and the culling of the Ruddy Duck was highly controversial.
One argument is that we have to just accept that we live in a human-shaped world and that this means alongside the current wave of extinctions we must embrace a new pan-global existence for any species that can survive. After all, some noble but misguided notion of biogeographic purity could ultimately lead to a sterile landscape.
Species can also jump between categories. A few years back a Beaver would be non-native. Now, in Scotland at least, they are native.
I don’t know an answer, but when I sit and watch the Snow Geese I don’t feel an urge to remove them.