overgrazing increases soil erosion & topsoil is a kind of soil ->  overgrazing can increase topsoil erosion  if something causes something else then that something results in that something else & soil erosion can cause desertification of the land ->  soil erosion can result in desertification of the land  soil erosion can result in desertification of the land & topsoil is a kind of soil ->  topsoil erosion can result in desertification of the land  overgrazing can increase topsoil erosion & topsoil erosion can result in desertification of the land ->  overgrazing can result in desertification by increasing topsoil erosion  increasing a process can mean increasing the rate at which that process is performed & erosion is a kind of process ->  increasing erosion can mean increasing the rate at which something is eroded  overgrazing can result in desertification by increasing topsoil erosion & increasing erosion can mean increasing the rate at which something is eroded -> overgrazing can result in desertification by increasing the rate at which topsoil is eroded