Variation in population trends and spatial dynamics of waterbirds in a boreal lake complex

Abstract

Breeding waterbird populations were studied in a boreal lake system between 1986 and 2012 to find out whether there are differences in temporal trends and spatial variation between waterfowl, grebe, diver and gull species. The study was based on censusing all waterbirds in a lake complex of 45.75 km2 of water area and 290 km of shoreline length (scale 1:20,000), and the results were compared with the changes in water quality during the study period. The population density of species varied significantly between the water bodies, showing that waterbird species prefer certain water bodies within the lake system. Spatial variation of many waterfowl - gull species and diver - gull species pairs were similar probably due to heterospecific attraction or similarities in habitat preferences. The temporal density change of breeding ducks and grebes was high: seven of the eight most abundant species showed significant temporal density patterns, having either linear (negative) trend or non-linear (quadratic, first increase then decrease) population change. Pooled pair numbers of waterfowl species were first stable, but declined drastically by half between 2003/2004-2012. In contrast, two out of four gull species increased, and the Black-throated Diver did not show any significant trend. Water clarity in the lake complex had decreased during the study period, which may have been one reason causing the population changes of waterbirds, because the increase in turbidity largely explained the negative trend of declining ducks. Climate change may accelerate eutrophication, having, in turn a negative effect on waterbird populations.