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Journal of Ornithology

, Volume 159, Issue 2, pp 447–455 | Cite as

Recovering Whooper Swans do not cause a decline in Eurasian Wigeon via their grazing impact on habitat

  • Hannu Pöysä
  • Johan Elmberg
  • Gunnar Gunnarsson
  • Sari Holopainen
  • Petri Nummi
  • Kjell Sjöberg
Original Article

Abstract

The Whooper Swan (Cygnus cygnus) is a good example of successful conservation, with rapidly growing numbers in Fennoscandia in recent decades. To the contrary, Eurasian Wigeon (Mareca penelope) shows a strong negative trend in breeding numbers, which raises conservation concerns. Previous research suggests a causal link between recent population trajectories of the two species. Both preferentially breed on wetlands with abundant horsetail (Equisetum spp.), a plant providing food for Whooper Swan and crucial feeding microhabitat for Eurasian Wigeon broods. We here test predictions based on the hypothesis that grazing on Equisetum by Whooper Swan reduces breeding habitat or breeding habitat quality for Eurasian Wigeon. We use data from 60 lakes in which waterfowl were counted in 1990–1991 and 2016, and Equisetum was mapped in 1990–1991 and 2013–2014. Lakes colonized by Whooper Swan typically had more abundant Equisetum vegetation in the past than lakes not colonized. Lake-specific decrease of Equisetum was not associated with colonization by Whooper Swan. The number of lakes occupied by Eurasian Wigeon decreased, but the decrease was not stronger on lakes colonized by Whooper Swan than on those that were not. Contrary to our prediction, current Eurasian Wigeon abundance was positively associated with Whooper Swan abundance. Moreover, Eurasian Wigeon did not decrease more on lakes from which Equisetum disappeared than on lakes in which there was still Equisetum left. This study does not support the idea that Whooper Swan affects Eurasian Wigeon negatively by grazing on Equisetum.

Keywords

Colonization Grazing pressure Habitat change Lake-level extinction Species interaction Waterbird community 

Zusammenfassung

Wiedererstarken der Singschwäne-Populationen verursacht durch das Grasen im gleichen Habitat keinen Rückgang der Pfeifente

Mit seinen in Fennoskandinavien in den letzten Jahrzehnten rasch gewachsenen Populationen stellt der Singschwan (Cygnus cygnus) ein gutes Beispiel für gelungenen Artenschutz dar. Im Gegensatz dazu zeigt die Pfeifente (Mareca penelope) in der Anzahl an Brutpaaren einen stark rückläufigen Trend, was zu Besorgnis im Naturschutz führt. Frühere Untersuchungen legen einen ursächlichen Zusammenhang zwischen den Populationsentwicklungen beider Arten nahe. Beide brüten bevorzugt in Feuchtgebieten mit starkem Schachtelhalmbewuchs (Equisetum spp.); diese Pflanze ist eine wichtige Nahrung für Singschwäne, während Pfeifenten bevorzugt in den Schachtelhalm-Habitaten brüten. Wir testeten Vorhersagen basierend auf der Hypothese, dass das Abweiden von Schachtelhalm durch Singschwäne das Brutareal der Pfeifenten verkleinert, bzw. dessen Qualität verringert. Wir verwendeten Daten von 60 Seen, auf denen die Anzahl an Wasservögeln 1990–1991 und auch 2016 gezählt und der Schachtelhalm-Bestand kartographiert wurde (1990–1991 und 2013–2014). Seen mit Singschwänen hatten typischerweise früher mehr Schachtelhalmvegetation als solche ohne Singschwäne. Ein für bestimmte Seen spezifischer Rückgang des Schachtelhalms stand in keinem Zusammenhang mit einer Besiedlung durch Singschwäne. Die Anzahl der von Pfeifenten besiedelten Seen ging zurück, aber dieser Rückgang war auf Seen mit Singschwänen nicht stärker als auf solchen ohne Singschwäne. Entgegen unserer Vorhersage korrelierte das Vorkommen von Pfeifenten sogar positiv mit dem von Singschwänen. Außerdem ging der Bestand an Pfeifenten auf Seen mit rückläufiger Schachtelhalmvegetation nicht stärker zurück als auf Seen mit gleichbleibendem Schachtelhalmbestand. Unsere Untersuchung konnte die Idee, dass Singschwäne durch ihr Abweiden von Schachtelhalm einen negativen Einfluss auf Pfeifenten hätten, nicht unterstützen.

Notes

Acknowledgements

This study was supported by previous grants from the Swedish Environmental Protection Agency. We would like to thank Suomalais-ruotsalainen kulttuurirahasto (the Finnish-Swedish Cultural Foundation) for supporting a workshop in Sweden for work on this manuscript. A grant by Maj and Tor Nessling Foundation to S. H. is highly appreciated. A grant from the Letterstedtska Föreningen (67/16) facilitated J. E.’s participation in the project. We thank two anonymous reviewers for useful comments on the manuscript.

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2017

Authors and Affiliations

  1. 1.Management and Production of Renewable ResourcesNatural Resources Institute FinlandJoensuuFinland
  2. 2.Division of Natural SciencesKristianstad UniversityKristianstadSweden
  3. 3.Department of Forest SciencesUniversity of HelsinkiHelsinkiFinland
  4. 4.Department of Wildlife, Fish, and Environmental StudiesSwedish University of Agricultural SciencesUppsalaSweden

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