Fen/fen/ Definition 1 of 3Pro Tip 1/3Wetland Variation Fen is a specific wetland, usually less acidic than a bog. Unlike the murky waters of a bog, this fen was clear and full of life. Pro Tip 2/3Biodiversity Hotspot Fens can be ecological treasures with diverse plants and animals. The fen was a birdwatcher's paradise, with species rarely seen elsewhere. Pro Tip 3/3Environmental Concern Due to their ecological importance, fens are often protected areas. The new wildlife preserve will protect the fen from industrial development.