A landscape cut by roads or rivers into isolated patches resembles a shared apartment after partitions are installed: residents no longer bump into each other in the common hallway. Species that once competed for the same food or space now live apart. Competition weakens, and those that would have lost to pushier neighbors in a continuous landscape now survive. In this way, fragmentation accumulates diversity.
But remove the partitions and reconnect the patches, and the process reverses. Species with broad connections, those used to interacting with many neighbors at once, gain an advantage. They spread quickly through the restored landscape and outcompete the 'homebodies' that accumulated during isolation. Extinction hits the most sociable first.
If a landscape is alternately fragmented and reconnected, the ecosystem behaves like a ratchet. Each fragmentation adds a click — a few new species. Each reconnection tries to roll things back, but the rollback is incomplete: some accumulated diversity persists. Step by step, cycle by cycle, species numbers grow beyond what they would be if the landscape had remained continuous all along.
For living systems, this is an unexpected twist: it's usually assumed that connecting fragmented pieces of nature is always beneficial. But if the goal is to preserve maximum species, maintaining a permanently continuous landscape might be worse than alternating fragmentation and reconnection. Nature sometimes benefits from being torn apart — if the pieces are later allowed to fuse again.
The ratchet is an ancient mechanism: it was used in ancient ballistas to keep the bowstring from rolling back during cocking.