An invasive gull displaces native waterbirds to breeding habitats more exposed to native predators

Abstract

The effect of invasive opportunistic predators may include population changes in both native prey and native predators as well as alteration of predator-prey interactions. We analyzed the activity of native magpie Pica pica and changes in population, nest sites and nesting success probability of native waterbirds (namely: grebes, ducks, rails and native gulls) in response to the population growth of the invasive Caspian gull Larus cachinnans. The study was carried out at a reservoir in southern Poland and at a similar control reservoir where the Caspian gull was absent. Both the invasive gulls and the native magpie are opportunistic predators of nests of native waterbirds. The population increase of the invasive gull led to a decline in the population of native black-headed gulls Larus ridibundus only. However, the invasive gull displaced all the native species from the breeding islets located in the central part of the reservoir to islets located close to the shoreline. The latter were frequently visited by magpies, which depredated on nests along the shores, leading to an up to threefold decrease in nesting success as compared with nests located in the central area of the invaded reservoir. Predation by Caspian gulls was rarely observed. Thus, the invasion of Caspian gull caused complex direct and indirect effects on the waterbird community that included competition for breeding sites, changes in the spatial distribution of nests and alteration of predation rate by native predators. Moreover, the effects of invasion may not be reflected by changes in population size of native species.

Implications of spring water levels on the production of American white pelicans nesting at marsh lake, Minnesota

“…Our observations from nest cameras and nest survival rates support a similar hypothesis for American white pelican nesting at Marsh Lake. In other colony‐nesting birds, the benefits of island nesting (Koczur et al , Anteau et al ) or nesting farther from mainland areas (Skorka et al ) are consistent with our findings for American white pelicans at Marsh Lake.…”

Coping with nest predation risk in a species-rich bird community inhabiting a Siberian wetland

“…This might suggest that predators (in particular terrestrial mammals) penetrating into the swamp from adjacent forests very easily reached nests situated closer to marshland edges. The risks arising from this possible edge effect (Skórka et al 2014) points up the importance of size in designing protected areas to effectively support breeding species inhabiting internal habitats such as open wetlands (Arnold et al 2007). To some extent, however, the latitudinal effect may be locally conditioned, as it was associated with the positions of large colonies of gulls, situated more to the south within the study site, and predation risk might therefore be lower there than in more northern sections with generally lower numbers of active nest defenders.…”

“…The impact of such great gulls as the Mongolian gull on the rest of the breeding bird community is certainly more complex than is seen solely in nest predation risk. It includes also competition for nest sites (Skórka et al 2014), alterations in nesting habitat due to changes in plant composition around nests (reviewed by Vidal et al 1998), and perhaps also higher predation pressure on hatchlings than on eggs (e.g. Dwernychuk & Boag 1972a, Chytil & Macháček 2000, Bowman et al 2004.…”

“…Dwernychuk & Boag 1972a, Chytil & Macháček 2000, Bowman et al 2004. Therefore, Skórka et al (2014) suggest that expansions of great gulls can have cascading and multilevel effects on populations of native species and may strongly alter the structure of the original communities. In any case, "great gulls" play an active role in shaping interspecific relationships, predation patterns, and population dynamics within wetland bird communities.…”

Shifting individual habitat specialization of a successful predator living in anthropogenic landscapes

“…They efficiently adapt to exploiting a wide suite of novel resources that provide direct biotic and abiotic benefits, including food, shelter and refuge from predation (Ramírez et al 2012, Alonso et al 2015, Osterback et al 2015. These winning seabirds are often perceived as pests because of their impacts on urban areas, water reservoirs and threatened species (Vidal et al 1998, Skórka et al 2014, Rock et al 2016. As a consequence, abundant research has been conducted on these opportunistic predators, mainly focused on their population dynamics, feeding ecology, pollution levels and pathogen load (e.g.…”