Every year, the tropical rain forest has a rainy season. The forest floor floods, covering once dry areas with water. Some organisms are unable to tolerate the increased water, so they have to move to other locations to survive.

Which word can best be used to describe this scenario?

The word that can best be used to describe this scenario is "adaptation". Organisms that are unable to tolerate the increased water have to adapt to the changing environment by moving to other locations to survive.