Semi-arid Kalahari Acacia-Baikiaea Woodland stretches across the center of southern Africa, from northern Namibia through Bostwana and just into the Tuli Block of South Africa. Surface water is scarce here and droughts occur roughly once every seven years. As a result, the human population is fairly low, especially on the sandveld that covers most of the ecoregion. The area supports a rich and diverse fauna, including a variety of ungulates and a number of threatened large animals such as white (Ceratotherium simum) and black rhinoceroses (Diceros bicornis), wild dogs (Lycaon pictus), and elephants (Loxodonta africana). However, human populations are increasing and the growing cattle industry has far-reaching effects on the environment and wildlife. The migrations of large herbivores which used to occur in the ecoregion are now hindered by veterinary control fences, resulting in mass die-offs in times of drought.