Forest disturbance by an ecosystem engineer : beaver in boreal forest landscapes

Natural disturbances are important for forest ecosystem dynamics and maintenance of biodiversity.In the boreal forest, large-scale disturbances such as wildfires and windstorms have been emphasized, while disturbance agents acting at smaller scales have received less attention.Especially in Europe beavers have long been neglected as forest disturbance agents because they were extirpated from most of their range centuries ago.However, now they are returning to many parts of their former distribution range.As a disturbance agent, beaver plays two roles: of an ecosystem engineer and of a herbivore.The engineering impact is realized through dam construction resulting in a transformation of an originally terrestrial ecosystem into an aquatic one.As herbivores, beavers affect stand structure and tree species composition by preferring deciduous trees over coniferous ones.After abandonment, a beaver pond gradually turns into a terrestrial habitat again.At well-drained sites, forest will return in due course, first dominated by deciduous trees.At poorly drained sites, moistness of beaver patches may result in fen development.We conclude that beaver has an important impact on ecosystem processes and biodiversity in boreal forest ecosystems because it creates and maintains a spatio-temporal mosaic of successional habitats and associated species communities that would otherwise not exist in the landscape.