The closest I can come up with is an "ecosystem" but that's kind of general and doesn't say anything about density. I feel like I've heard a word for this before but I'm not sure. Any ideas?
"Flourishing ecosystem" "prolific and abounding" "teeming" "swarming" "multitudinous" "We were set upon by untold and innumerous crawling and flying things. George never was so happy."
According to Wikipedia, the term standing crop means the total biomass of the living organisms (all six kingdoms of living things: animals, plants, fungi, algae, bacteria and archaea) present in an environment. However, "biomass" excludes water (don't ask why), so you can't measure biomass while the organisms are alive!
"Fecund" is limited specifically to reproducing abundantly. Of course it also is used metaphorically for someone who is very talented, productive, etc.
It is also used metaphorically for Nature, when describing an ecosystem that is dense in population and variety of life.
Well, I suppose because the diversity depends on all the plants, insects, birds and animals engaging enthusiastically in their respective reproductive activities.
Not to mention, serving as food for the organisms at the higher levels. Please Register or Log in to view the hidden image!
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 general ones. Given the significance of biodiversity in a heating, choking world, it's probably time someone invented that specific adjective.
Wikipedia says that the definition has another very important dimension: the biodiversity is under threat of humans.
...and hey, what isn't. Please Register or Log in to view the hidden image! I've just remembered a (compound) adjective I have heard— 'species-rich'. Doesn't feel really... fecund enough, though, does it.