Carlsbad Caverns National Park
Seventeen species of bats live in the park. Many of the bats in the cave are Mexican free-tailed bats. People say that the number of Mexican free-tailed bats used to be several million, but has gone down a lot in modern times. The cause is unknown, but organochlorine pesticides (specifically DDT and dieldrin) are likely part of the reason. A 2009 study by a team from Boston University says that there may have never been millions of bats in the caves.