Beavers are large, semi-aquatic rodents known as "ecosystem engineers" for their dam-building, which creates wetlands, benefiting biodiversity; they are social animals living in family colonies, have dense fur, distinctive flat tails, and use their powerful teeth to fell trees for food (bark, plants) and lodge construction, and can hold their breath for up to 15 minutes underwater. There are two species, North American and Eurasian, both vital for altering landscapes, controlling water flow, and supporting other wildlife, though they can conflict with human interests.