I want to tell a story about two sentient species inhabiting the same planet, but never meeting. Essentially, some kind of random event (every couple of days up to every couple of months) causes species A to disappear temporarily (until the next event) and species B to appear. On the next event they switch. It could be some kind of process where their bodies actually disappear or temporarily dissolve, but a scenario where the affected species retreat, e.g. to the bottom of lakes or into caves, would also be possible. I don't want a scenario where one species just stays in their houses and sleeps through the event, as that would mean that the other species would find their bodies. If the result is a process that causes the species to hide (instead of just disappearing), ideally the process would work in such a way that it would be hard to adapt to both extremes. One species would adapt to one end and the other one to the other end. Otherwise both species would just be present all the time.
So far, I've mostly thought about the air rapidly and radically changing its chemical composition. Both species adapted to one type of atmosphere and developed some kind of hibernation in the course of their evolution. One could retreat to the bottom of lakes, while the other one goes underground. During their "absence" they don't breathe and are not conscious. Which natural process on Earth (or a similar planet) could explain this? By "similar planet" I mean that I would like a setting that feels as close to earth as possible (similar climate zones, day-night cycle, seasons). Differences are okay, but a pure water or lava planet wouldn't be.
You can also give other answers as long as they satisfy these conditions:
- Causes one species to be absent while the other one is there, either because their bodies actually disappear, or because they voluntarily retreat to an inaccessible place
- It must be possible for plants to have adapted to it. The idea is not that the surface of the earth gets regularly sterilized by gamma rays. When one species returns, they still recognize the face of the earth, roads are still there etc
- Both species share more or less the same environment (it's okay for places to exist where only one species can live), without actually meeting and only having indirect hints about the other one's existence