POPULATION AND COMMUNITY-LEVEL RESPONSES IN ANAS-SPECIES TO PATCH DISTURBANCE CAUSED BY AN ECOSYSTEM ENGINEER, THE BEAVER

We studied the response of a guild of three Anas-species to patch dist urbance caused by the beaver Castor canadensis in southern Finland dur ing 1988-1996. Of the three species, the teal Anas crecca increased in beaver ponds Juring tile first two years of beaver occupation compare d to the situation before the beaver. The mallard, Anas platyrhynchos and wigeon Anas penelope did not respond to beaver flooding during the first two years. Also dominance relations changed upon inundation. Th e mallard usually was more numerous than the teal before the beaver. b ut the reverse was the case during beaver occupation. The study shows that some animal guilds may benefit from disturbance although species dominance relations may change. And, that this change in dominance can be caused by an ecosystem engineer, the beaver.