Much of the destruction of the rainforest has occurred through slash and burn farming, which allows for little to no fallow time and creates soil erosion. It also depletes what little nutrition there is in the soil, which is then also polluted by the use of fertilizers and pesticides. As of the first decade of the 21st century, it is estimated about two thirds of the Lacandon outside the main biosphere reserve has been converted into pasture or cropland. Once this land has been cleared and used, it is very difficult to revert it to rainforest, even after it has been abandoned. Grass for pasture is particularly problematic because after it takes hold, does not allow natural vegetation to compete with it. In addition, the soil becomes compacted by the trampling of livestock. This degradation causes streams to dry up as evaporation rates rise from the lack of shade.