Activity traps and the corer: complementary methods for sampling aquatic invertebrates

We compared the picture of aquatic invertebrate fauna given by three types of sampling methods: the corer as well as bottom and mid-water activity traps. The corer mainly caught benthic animals (89%), the most numerous groups of which were chironomids and sphaerids. Nekton was more abundant in activity traps (77% in bottom and 63% in mid-water), the most numerous groups were Cladocera and water mites. The size distributions in catch of the corer and the activity traps differed but there was no difference between the two types of activity traps. In activity traps, individuals of the smallest size class were most abundant, wheras animals of the second smallest size class were most numerous in the corer. We suggest that activity traps and the corer could be used as complementary methods to sample aquatic invertebrate communities.     

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 disturbance caused by the beaver Castor canadensis in southern Finland during 1988–1996 Of the three species, the teal Anas crecca increased in beaver ponds during the first two years of beaver occupation compared 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 The mallard usually was more numerous than the teal before the beaver, but 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