What is a specific scenario, aside from the climate change scenario we all know about, that could lead to collapse that's discussed in the paper?
Frog A goes extinct, frog A just happens to eat beetle B.
Without (as much) predation pressure, beetle B goes completely fucking nuts and eats everything in sight, including most of the forests in the area, crops, etc.
Everything in the area starves, the ecosystem collapses, and you're left with scrub, beetles and widespread erosion until something else can colonise the area and start a new ecosystem.