Tavy works well and does not permanently destroy the forest as long as field sizes are small and farmers leave adequate time for re-growth. However, if you do this too much, too quickly soil starts to leak nutrients. It is hard to stop tavy farming because farmers in Madagascar are needing the nutrients for their crops; because they don't have enough money for fertilizers. So on without doing it, they could starve, but they are taking down trees and animal homes to do so, leaving a scar on the environment.