Humans. Humans are the direct causal link. There isn't going to be some single silver bullet explaining everything further than that, it is an ecosystem collap

Humans. Humans are the direct causal link. There isn't going to be some single silver bullet explaining everything further than that, it is an ecosystem collapse. Humans impact everything in small ways, these in turn impact everything else in its network. Sometimes positively but more usually negatively via various strains. It's not always pollution, sometimes it's disease, sometimes it's habitat sometimes it's climate, sometimes food chain, sometimes it's noise pollution, sometimes it's even more complicated than that for example you might knock out a predator but then create more disease or remove an ecological pressure that kept a species behaviour in line with its environment.