The history brief by NPS shows a population of about 40 horses in 1975 and 155 in 1986.
Plugging those numbers into the growth rate spreadsheet shows the herd was expanding at an average rate of 13.1% per year.
Western Horse Watchers Association
Exposing the Hypocrisy, Lies and Incompetence of the Wild Horse Advocates
Western Horse Watchers Association 