Humans - rightly viewed as animals - arrived in Britain entirely under their own steam (no pyramidal-spaceships involved, I'm reasonably sure) several times over, the earliest at least a million years ago. Only major ice-ages temporarily eliminated them from areas of permanent ice cover. We are very much a native species - for better or worse...
Now, were you to argue that present-day humanity should be culled for ecological (and other) reasons, you'd have my wholehearted agreement, if perhaps not my co-operation...
And again!
Humans have introduced ecologically harmful species worldwide (and infuriatingly, continue to), and only humans can extirpate them in an attempt to put-right our mistakes. There are many arguments for this, but the most important is to preserve ecological diversity, and thus the resilience of ecosystems. Without resilient ecosystems, we run the risk of catastrophic losses as humans continue to heap diverse pressures upon them. This is why (e.g.) we are gradually re-introducing species driven to extinction in the UK hundreds of years ago.
It's an important characteristic of xenospecies that they are often in a position to outcompete their near-equivalents in any given ecosystem. In the case of predators or parasites, potential prey or hosts are often unable to properly recognise them as a threat, while in the case of fungi and microbes of various kinds, organisms may have no immunity to infection. The result is often extinction, sometimes of many species in a given area.
Sentimentality cannot contrive a case against this, no matter how convolutedly it's argued. Freedom-of-choice, is quite another matter, and of course you and your tree-rats may do as you please!