All these patches of natural habitat are disconnected and this is particularly damaging for wildlife that is used to migrating along the rivers of the valley. Agriculture, the creation of grazing land, and the draining of lakes and rivers have radically altered the habitats of the Valley. Most of the grassland has been overtaken by new species, most of the vernal pools have been destroyed with only those on the higher slopes remaining, the marshland has been drained, and the riverbank woodlands have nearly all been affected.