Identify a probable reason that the dark moths survived while the light moths did not.

This question has little context, but I understand what you are trying to say.

The reason why dark moths survived while light moths weren't able to survive is due to the dark moths having conditions that allowed them to camouflage better than the light moths. An environmental condition that would allow this would be the dark moths and the light moths living in the same dark environment.

not only is it colder, it also becomes darker, less salty, and more dense.