Identification of conservation priorities is a fundamental issue for successful nature conservation. This can be done by looking at population trends of species in different habitats. Boreal zone is the key breeding area for many Eurasian waterbirds. Wetlands of the boreal zone can be roughly classified into two different groups: nutrient‐poor oligotrophic and nutrient‐rich eutrophic water ecosystems. Earlier work has shown that eutrophic lake‐specialist species have declined compared with generalists and species preferring oligotrophic lakes. However, it is not known if species that live in both habitats show habitat‐specific population trends. We used monitoring data of breeding waterbirds in Finland from 1986 to 2013 to evaluate habitat‐specific population trends in five generalist duck species, which mostly overwinter in the coastal waters of West Europe. Population trend of tufted duck was highly significantly more positive in oligotrophic wetlands compared with populations in eutrophic wetlands, and two other species had similar but weaker effects. In addition, we established bird population state indicators for oligotrophic and eutrophic water ecosystems in Finland. The indicators revealed that the populations in eutrophic wetlands have on average halved since early 1990s. The populations in oligotrophic wetlands have remained stable, except a moderate decline since the late 2000s. Our findings support the idea that the waterbird populations of the eutrophic wetlands are in serious trouble. This is likely due to over‐eutrophication of these ecosystems. Conservation and management actions should be urgently taken to halt the loss of biodiversity in these globally threatened freshwater ecosystems.

Interactions and dependence between species can transmit the effects of species declines within and between trophic levels, resulting in secondary endangerments and, in some cases, extinctions. Many mixed-species avian breeding aggregations commonly have a protector species whose aggressive nest defense is used by other species to defend their nests. Disappearance of the protector species may have population demographic consequences on the dependent species. Aggressive nest defense behavior of small colonial gulls, such as the black-headed gull (Chroicocephalus ridibundus), is used by many waterbird species to gain protection against predators. We used data from 15 local waterbird communities in Finland to study long-term changes and dynamics of breeding numbers of other waterbirds as a response to long-term changes and dynamics of black-headed gull colonies. We found that breeding numbers of many species tracked long-term changes in the size of black-headed gull colonies. This was true even after controlling for a common trend in the size of the black-headed gull colony and the breeding numbers of the other species. The trend-controlled positive temporal association with blackheaded gull was relatively stronger in species that nest in similar habitats of a lake as the black-headed gull, and in species that have a more critical conservation status due to drastic population decline. Our results suggest that the overall decline of black-headed gull colonies has resulted in secondary endangerment of many other species in waterbird communities.

Density dependence, population regulation, and variability in population size are fundamental population processes, the manifestation and interrelationships of which are affected by environmental variability. However, there are surprisingly few empirical studies that distinguish the effect of environmental variability from the effects of population processes. We took advantage of a unique system, in which populations of the same duck species or close ecological counterparts live in highly variable (north American prairies) and in stable (north European lakes) environments, to distinguish the relative contributions of environmental variability (measured as between‐year fluctuations in wetland numbers) and intraspecific interactions (density dependence) in driving population dynamics. We tested whether populations living in stable environments (in northern Europe) were more strongly governed by density dependence than populations living in variable environments (in North America). We also addressed whether relative population dynamical responses to environmental variability versus density corresponded to differences in life history strategies between dabbling (relatively “fast species” and governed by environmental variability) and diving (relatively “slow species” and governed by density) ducks. As expected, the variance component of population fluctuations caused by changes in breeding environments was greater in North America than in Europe. Contrary to expectations, however, populations in more stable environments were not less variable nor clearly more strongly density dependent than populations in highly variable environments. Also, contrary to expectations, populations of diving ducks were neither more stable nor stronger density dependent than populations of dabbling ducks, and the effect of environmental variability on population dynamics was greater in diving than in dabbling ducks. In general, irrespective of continent and species life history, environmental variability contributed more to variation in species abundances than did density. Our findings underscore the need for more studies on populations of the same species in different environments to verify the generality of current explanations about population dynamics and its association with species life history.