Despite the valuable role that mangroves and seagrass play in the Florida Keys ecosystem, these plants are facing one particularly large threat: Us. In the Florida Keys, human development is the greatest threat to mangrove communities. Since the 1950s, more than 60 percent of mangroves in Monroe County have been destroyed for development. Activities such as dredging, using herbicides, and increasing waste water runoff have destroyed thousands of acres of mangrove habitat.