Boy, Drake's kind of a git, isn't he?
It's a nice little moment in a series that, like The Sarah Jane Adventures, has chosen to view aliens with a sense of wonder rather than dread. There's not necessarily a deeper meaning than that; the fact that a large number of species of trees have gone extinct in England is treated more as background details for the future we're in, rather than as a warning or a major problem -- it's not exactly Star Trek IV: The Voyage Home. But because "The Last Oak Tree" is about finding beauty in different guises and about helping people and creatures in distress, Drake's presence really sticks out. He's only there to supply an external threat and that's it.
"Black Hunger", at least, uses Drake in a more reasonable context, as he's engaged in another covert operation that secretly uses alien technology to forward his agenda. Well, sort of; he's made this special life-eating microbe supercharged so that he can use it to help steal more alien tech. Or something. It's not exactly clear. What is clear is that Drake is supremely confident that nothing will go wrong with Operation Black Hunger. Of course, something does go wrong; Gryffen calculates that the Hunger could wipe out all life on Earth in a week if it gets loose, and so naturally it gets loose, causing an apocalyptic crisis that's only averted because K-9 vacuums up all the microbes inside himself, ready to be discharged the next time he visits Atrios, as there's no carbon-based life there.218
But the damage is done; once again Drake has operated without official permission and this time he nearly destroyed the world. He's gone too far, and thus this, it seems, is Drake's final appearance on K-9, as he's replaced by Inspector Thorne (who you might remember was the prison governor in "Liberation"). I would like to say it's sad to see him go, but that wouldn't be true. This appears to have been one of Connor Van Vuuren's first roles, and while he appears to have done what they asked of him, which sometimes seemed to essentially be a future Snidely Whiplash, he wasn't able to do much more than that. Van Vuuren's certainly done better for himself since this (albeit more as a stuntman and in a behind-the-scenes capacity), so it does look like it was a lack of experience rather than talent informing his performance in K-9. But as far as villainy goes, I guess we'll have to see how Thorne does in Drake's place.
218 The last time we saw Atrios was in The Armaggedon Factor (which K-9 co-creator Bob Baker would know, as he cowrote that story), when it sure looked like there was carbon-based life there. So either that took place in the long-distant future, or possibly in the long-distant past -- in which case, sounds like tough times ahead for Princess Astra and company.