The little brown bat has been hit hard by the white nose syndrome, and are among the species being killed en masse by wind turbines, but in addition to that, recent research has shown that as summer droughts worsen due to climate change, bat populations decline. The reason is fairly simple - when there's a drought, the insect population drops, and so bats have to put much more effort into finding food, and less into reproduction - the end result is fewer new bats every year that's low on water.
The impacts of a reduction in bat populations are many. The most obvious is that they eat mosquitoes, which are notorious disease vectors, carrying such classics as malaria, equine encephalitis, West Nile virus, and more. On top of that, however, is the recent revelation that bats eat enough agricultural pests that if they were to go extinct (which is likely for the little brown bat), it would cost the agricultural industry billions of dollars, perhaps tens of billions.