(Reuters) - Reducing the tens of thousands of wild horses and burros on public lands to a level that is better for both the animals and their habitat will cost U.S. taxpayers $5 billion and take 15 years, a Trump administration official said on Wednesday.

On a conference call with reporters, U.S. Bureau of Land Management (BLM) Acting Director William Perry Pendley said the 88,000 wild horses and burros on federal lands were destroying rangelands, encroaching on the habitats of other species and costing taxpayers $81 million a year to manage.

“These aren’t the only animals in town,” he said.

The BLM aims to reduce the Western herd to 27,000, a number that its experts say is sustainable, according to Pendley.

In 2018, the bureau removed nearly 11,500 wild horses and burros from public lands, in part through adoptions and sales, a major increase from the 4,200 removed in 2017.

Some animals removed from public rangelands are held in off-range corrals and pastures at a cost of $50 million a year.

Wild horses and burros have few predators, so their numbers can swell by 20 percent per year, the bureau says. Its data indicates that there were just 50,000 wild horses and burros on public lands in 2014.

Just 700 animals were given vaccines to control their fertility last year, according to the data. Administering the treatment is a costly and logistical challenge, Pendley said.

He called on the scientific community to develop more effective methods to control the herd size.

“I believe in mankind’s ability to solve problems,” he said.