It’s late March and the road is dotted with toads. Small, dark lumps pull themselves across the asphalt, doing anything they can to reach their breeding ground near Helenesee, a lake in Brandenburg, Germany. It’s a journey the toads make through forest and over busy roads, and many don’t survive the encounter with oncoming traffic. They are drawn here by instinct: Each spring, the toads follow the same route back to the water where they were spawned. 

As winter comes to an end, signs of the changing season abound: Green leaves unfurl and migratory birds return. In fresh waters in the northern hemisphere, especially ponds and slow-flowing streams, much of spring’s magic lies just below the surface. Take a look in your backyard, local park or a protected nature reserve. Shrinkin...