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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Predator-prey interactions as possible explanation for recent declines of seabird breeding populations - the example of the Wadden Sea island Trischen
Auteurs :	Moritz Mercker, Auteur ; Julia Baer, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (134(2) 2013)
Article en page(s) :	115-127
Résumé :	Trischen Island is located between the estuaries of river Eider and Elbe in the German Bight. The island is part of the Wadden Sea National Park and is strictly protected. Systematic surveys of breeding birds have taken place on the island for the past 100 years. These records have shown several population collapses over the past century. They have been attributed primarily to anthropogenic influences, e.g. direct pursuit of birds or biocide contaminations of food and water.Successful conservation efforts led to a significant elimination (or at least reduction) of these influences during the last decades. Initially, this caused a rapid increase in the total number of breeding pairs (bp) at the end of the last century on Trischen, leading to a maximum number of about 20,000 bp in the 1990s. After having reached this maximum, the amount of breeding pairs on Trischen decreased again rapidly: the current breeding population comprises about 5,000 bp. This time, underlying reasons are widely unknown. This negative trend mainly affects gulls, terns and waders; only few bird species show positive trends.The present study documents and debates these recent population trends on Trischen. Population models based on simple predator-prey interactions between gulls and prey species strongly reflect the observed population dynamics on Trischen during the last decades. This suggests that natural population interactions may play a crucial role in the reduction of breeding bird numbers on the island. To gain further insights in predator-prey relationships a study on Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus diet was conducted during the 2013 breeding season, which revealed that 3.7% of pellets (overall number sampled: n = 351) contained chicks or eggs during the chick rearing phase. This translates into a potential > 1 tonne of chicks/eggs taken on the island during the chick rearing phase and highlights the possible effect a large gull colony can have on the breeding success on a small island such as Trischen.Interference in population dynamics on the island by active management is not a long-term sustainable solution and is also not in accordance with the national park ethos. However, since gull predation pressure is largely linked to the availability of natural marine food sources, the protection of key foraging sites would be an important conservation measures, particularly in the light of the planned reduction of fisheries discards in the near future.

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