From what I've guessed the Nugubechi are shape-shifters and can impersonate humans and presumably other species. I think the part where Yume is reading a book is one of the insects impersonating her, because right after being asked about the books, Yume says she didn't touch them.
Their modus operandi is probably whipping species into a murderous frenzy which causes them to attack others, which is what the page at the end is supposed to signify. I don't think they actually kill the people they're impersonating though, the scientist is still narrating at the end even after they get him.
The fungus at the beginning was trying to prevent the Nugubechi from maturing or leaving the park which is also why the park goers were so adamant about keeping the Nugubechi in the park.
Ohhh, thank you!!! This makes sense!!!
Can someone explain it to me, please?