Conservation by Adrian Warren, Producer Trials in the Lives of Animals In order to provide visitors with convenient and easy access to some areas of Etosha National Park, 2,165 kilometers (1,345 miles) of gravel roads were constructed. While these did not inflict great physical impact on the environment, they did play a crucial role in the spread of anthrax (Bacillus anthracis). In order to obtain fill for the bad surfaces, large gravel pits were dug close to the intended routes. These subsequently filled with water, which became alkaline due to the mineral salts present. The anthrax bacterium thrives under alkaline conditions, and many of the herbivores are extremely susceptible to the disease. As a result, the population of wildebeest crashed by 90 percent (from 25,000 to 2,500) in only 30 years, while the zebra population declined from 16,000 to 5,000 in an even shorter period. With so many weak and dying animals, the population of some 500 lions increased dramatically, and had to be reduced.