This has occasionally triggered situations, in which a locally increasing, protected species heightens the vulnerability of threatened co-occurring species, often due to anthropogenic-driven changes in the environment. We examined whether the recovery of a flagship species, the whooper swan (Cygnus cygnus), and the spreading of the non-native Canada goose (Branta canadensis), cause asymmetric competition with other sympatric waterbirds at their breeding sites in Finland, as there is a great controversy in the population trends of these large herbivorous waterfowl compared to smaller waterbirds. In Finland, pair numbers of the whooper swan increased from 15 in the 1940s to 9 000–12 000 in the 2010s, and the species’ range now covers the entire country. The population growth of the Canada goose in Finland was low until recent decades, when the population increased rapidly from 300–500 pairs in the late 1980s to 4 900–6 300 pairs in the 2010s, with population growth strongest in southern and central parts of the country.

We used data from the national Finnish waterbird surveys collected in the late 1980s and early 2020s at 942 sites, to assess the site-level effects of large herbivore occurrence on other waterbird species, while considering their trophic overlap. We hypothesised that there could be competitive effects of large herbivores on smaller species, especially those with similar foraging niches.

We however not only found that the overall abundances of all foraging guilds (surface feeders, invertivore diving ducks, piscivores) were positively associated with whooper swan presence at sites, but also that the numbers of surface feeders and diving ducks, were positively associated with whooper swan colonisation. Thus, their populations have decreased less at sites occupied by whooper swans since the 1980s.