Complex sentence: Rainforest collapse refers to the actual past and theoretical future destruction of rainforests through a process of habitat fragmentation to the point where little rainforest biome is left, only to survive in isolated refugia.
Split into simpler sentences: Rainforest collapse refers to the actual past and theoretical future ecological collapse of rainforests. It may involve habitat fragmentation to the point where little rainforest biome is left, and rainforest species only survive in isolated refugia.