Editor’s Note: Call to Earth is a CNN editorial series committed to reporting on the environmental challenges facing our planet, together with the solutions. Rolex’s Perpetual Planet Initiative has partnered with CNN to drive awareness and education around key sustainability issues and to inspire positive action. CNN  —  Seven thousand head of cattle used to roam Ziwa ranch, a 27-square-mile (70-square-kilometer) expanse of grassland in central Uganda. Today, the cattle have gone and grazing in