Trischen island is located in the core area of the Schleswig Holstein Wadden Sea National Park, north of the Elbe estuary, and holds one of the largest colonies of Herring Gull Larus argentatus (1,781 pairs in 2013) and Lesser Black-backed Gull L. fuscus (1,838 pairs in 2013) on the German west coast. Productivity has been monitored for both species since 2010, and was low throughout 2010-13, averaging
0.26 ± 0.12 SD fledged/nest for Herring Gull and 0.32 ± 0.14 SD fledged/nest for Lesser Black-backed Gull. Since 2011 excessive ant activity has been noted at some nest sites, causing distress for freshly hatched chicks. In 2013 a total of 83 gull nests (40 Herring Gull, 33 Lesser Black-backed Gull, 10 unspecified) were monitored at 2-4 day intervals and ant activity was recorded. Ten nest sites with chicks that were obviously suffering from attacks by the European Fire Ant Myrmica rubra were noted; all of these chicks (n = 25) died before the age of 4d (± 2d), reflecting a chick loss of 14.5% within the study colony. At the end of the breeding season, ant densities were compared between these ten nest sites where ant attacks had been observed and ten out of the 15 nests sites where at least one chick lived to fledging age. Results showed a 12-fold higher ant density at nests where ant attacks had been observed and a distinctive ant density pattern within the colony, suggesting that location of nesting sites affected chick survival.

Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus are among the most abundant breeding seabirds on the Dutch, German and Danish Wadden Sea islands. The island of Trischen holds the second largest population of Herring Gull and Lesser Black-backed Gull at the German coast, with 1,781 and 1,838 breeding pairs in 2013, respectively (Baer 2013). Trischen is also home to a large ant population, with especially dense colonies in dune and grassland Elymus athericus areas (pers. obs). In 2013 the ant species was identified as the European Fire Ant Myrmica rubra, a highly aggressive, polyphagous ant that may form supercolonies (Seifert 2007). Myrmica rubra is one of the most abundant native ant species in Europe, extending from Ireland and Portugal in the west across 8,000 km to central Asia and eastern Siberia (Wetterer & Radchenko 2011). The species is also currently increasing its range in North America, where it had been introduced in the early 20th century and is now considered a pest (Wetterer & Radchenko 2011).

Ants play a large role in terrestrial ecosystems. They act as predators, scavengers and herbivores, and constitute a great part of the animal biomass (Folgarait 1998; Holway et al. 2002). Ants have become inadvertently introduced to countless islands around the world, often causing widespread ecological damage (McGlynn 1999; Rabitsch 2011). There have been a number of cases in which invasive ant species are reported to have detrimental effects on ground-nesting birds, including seabirds. At Bird Island, Seychelles, Sooty Terns Sterna fuscata failed to nest in areas invaded by the Yellow Crazy Ant Anoplolepis gracilipes and White Terns Gygis alba suffered high chick mortality due to predation by the same ant species (Feare 1999). Suppression of the Red Imported Fire Ant Solenopsis invicta increased chick survival for the endangered Least Tern Sterna antillarum in Mississippi, USA (Lockley 1995). A similar problem was reported from the Hawaiian islands where booming populations of the Tropical Fire Ant Solenopsis geminata were expected to cause widespread damage to local seabird colonies; control of the ant populations resulted in temporary increase in the fledging success of Wedge-tailed Shearwaters Puffinus pacificus (Plentovich et al. 2009). Recently, Myrmica rubra has been reported to adversely affect Herring Gull reproduction on an island in the Gulf of Maine (DeFisher & Bonter 2013).

However, native ant species can also cause problems for nesting seabirds. Safina et al. (1994) reported that Roseate Tern chicks Sterna dougallii on Cedar Beach, New York, were more likely to die if the native ant species Lasius neoniger was found present on the chicks. Nisbet and Welton (1984) recorded ant predation on Common Tern chicks Sterna hirundo at a colony in Massachusetts between
1972 and 1981 and found that attacks by Lasius neoniger was one of the principal causes of egg and chick losses.

So far, most reported cases of ants affecting seabird reproduction have come from outside Europe. In Germany one incident was mentioned in 1938, where ants (species unknown) caused distress to seabird chicks on the island of Amrum (Emeis, in Schulz 1947). In 2011 and 2012 ants were observed attacking Herring Gull and Lesser Black-backed Gull chicks on Trischen (Philipps 2011; M. Mercker pers. comm.). During gull nest monitoring on Trischen in 2013, information on ant attacks and ant densities was collected in order to assess whether this had any effect on gull breeding success.

I would like to thank the Naturschutzbund Schleswig-Holstein (NABU) and the Trischen steering group for their project support. I am grateful to B. Seifert (Senckenberg Museum) and B. Hälterlein (Wadden Sea National Park Authority) for their help and advice throughout various stages of the study.

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