That's it. That's the exact term. I recognise it instantly. If it wasn't a word, it is now.
...and hey, what isn't. :( I've just remembered a (compound) adjective I have heard— 'species-rich'. Doesn't feel really... fecund enough, though,...
I've heard the noun phrase "biodiversity hotspot", which really seems to nail it. But if you're after a specific adjective, I too can only think of...
Separate names with a comma.