Someone I know built a pond about 6 weeks ago and did not add frogs. After algae appeared, suddenly tadpoles appeared and now they are frogs. I live in the desert, not near water. Has anyone ever heard of this?

You know what they say; build it and they will come.....

Yes, they will come. There are some species of small frogs and toads that live in arid regions that just wait for a pond to form. Because they are used to such ponds forming quickly and drying up very fast, they have a faster reproductive time from egg to adult. The tadpoles will grow fast and get out of there fast because such a natural pond would dry up quickly in a desert.