Aus Dem Forschungs- Und Technologiezentrum Westküste Büsum Der Christian-Albrechts-Universität Zu Kiel
Total Page：16
File Type：pdf, Size：1020Kb
Aus dem Forschungs- und Technologiezentrum Westküste Büsum Der Christian-Albrechts-Universität zu Kiel Habitat use of the coastal zone of the German North Sea by surface-feeding seabirds Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Philipp Schwemmer Kiel 2007 Referent: PD Dr. Stefan Garthe Korreferent: Prof. Dr. GB Hartl Tag der mündlichen Prüfung: 12.06.2007 Zum Druck genehmigt: Kiel, 17.04.2007 Der Dekan Table of contents Contents Summary.......................................................................................................................... 1 Zusammenfassung ......................................................................................................... 3 General Introduction....................................................................................................... 5 Thesis Outline ............................................................................................................... 11 Chapters I Regular habitat switch as an important feeding strategy of an opportunistic seabird species ........................................................................... 13 Submitted II At-sea distribution and behaviour of a surface-feeding seabird, the lesser black-backed gull (Larus fuscus), and its association with different prey......... 37 Published in Marine Ecology Progress Series 285: 245-258 (2005) III Spatial patterns in at-sea behaviour during spring migration by little gulls (Larus minutus) in the south-eastern North Sea ............................... 55 Published in Journal of Ornithology 147: 354-366 (2006) IV Area utilisation of gulls in a coastal farmland landscape: Habitat mosaic supports niche segregation of opportunistic species................ 71 Submitted V Influence of water flow velocity and water depth on distribution and foraging patterns of a surface-feeding seabird in the German Wadden Sea .................. 91 Prepared for submission General Discussion .................................................................................................... 111 Danksagung (Acknowledgements) ........................................................................... 121 References................................................................................................................... 123 Curriculum Vitae ......................................................................................................... 139 Erklärung ..................................................................................................................... 141 Summary Summary In seabird ecology much attention has been paid to explore the breeding biology of different species, whereas studies on the use of foraging habitats are far more scarce. However, it is necessary to reveal temporal and spatial patterns of foraging habitat choice to understand mechanisms of coexistence among closely related species, to shed light on the ecological function and quality of different habitat types for seabirds and to judge the overall role of seabirds in food web dynamics. Surface-feeding seabirds are expected to be particularly closely related to the spatial and temporal patterns of their main prey species as a consequence of their foraging technique which allows them to exploit only the uppermost part of the water column. The German North Sea coast is inhabited by various surface-feeding species. It provides a particularly complex habitat mosaic dominated by three major landscape types (i.e. the pelagic offshore zone, the Wadden Sea area and the coastal mainland). Using an integrative approach, this study aims to unravel important temporal and spatial patterns of habitat use by several surface- feeding seabirds (i.e. five gull and one tern species) in the coastal zone of the German North Sea by combining direct observations on the distribution with behavioural and dietary analyses. Furthermore, this approach uses seabirds as bioindicators to unravel the quality and ecological function of different habitat types of the coastal zone as well as historical changes of food web dynamics against the background of anthropogenic activities. As one of the most opportunistic species the black-headed gull (Larus ridibundus) showed a daily and seasonal routine of regular habitat switches between the major landscape types of the coastal zone as a consequence of varying prey availability and energy demands of growing chicks. A comparison with historical data clearly indicated a reduced utilisation of the marine foraging habitat and an increasing importance of terrestrial sites presumably related to major changes of foraging site quality. The predominantly pelagic lesser black-backed gull (L. fuscus) showed distinct spatial and temporal associations with its apparently most important natural prey, swimming crabs (Liocarcinus spec.). Moreover, at least during the post-breeding period, it was possible to detect a significant relationship between at-sea distribution and areas of intensive fishery that provide discards and offal as additional food. Historical changes of habitat utilisation as well as changes in food choice of this species indicated major alterations of food web dynamics of the pelagic offshore zone. The little gull (L. minutus) uses the German pelagic offshore zone as a staging site during its migration to and from its breeding grounds. Behavioural data on at-sea distribution clearly showed the existence of sites that were used primarily for foraging while others were used mainly for resting or as a migration corridor. The major foraging 1 Summary sites could be characterized by hydrographic phenomena that provided enhanced prey availability and prey quality. The coastal mainland holds a variety of different agricultural habitat types at a comparatively small spatial scale. It was possible to detect major differences in the utilisation of the terrestrial habitat mosaic among the four gull species black-headed, lesser black-backed, herring (L. argentatus) and common gull (L. canus). Notably black- headed gulls were highly associated with direct anthropogenic activities and with sites of intensive farming, while in contrast mainly common gulls depended on carefully managed pastures. The results demonstrate the high importance of the availability of the small scale habitat mosaic and of different intensities of anthropogenic activities for foraging niche segregation of the four closely related species. The predominantly piscivorous Arctic tern (Sterna paradisaea) was found to mainly forage within the tidal streams of the Wadden Sea. It could be shown that both water depth and the physical effect of tides (i.e. high water flow velocities) played a decisive role for the choice of foraging habitats. Highest foraging intensities were encountered within sites of low water depth and high water currents which indicates that prey availability might be significantly increased by turbulences caused by tidal flow. Furthermore, the location of breeding colonies could be related to the location of tide- induced optimal foraging sites. These finding indicate the important role of physical factors to influence the ecology of surface-feeding seabirds. In conclusion, this thesis revealed important temporal and spatial patterns of habitat use by surface-feeding seabirds (and its underlying causes) within the whole coastal zone. The integrative approach to merge observations of habitat use with analysis of behaviour and prey taken by seabirds provides an excellent opportunity to evaluate the ecological function and quality of different habitat types for seabirds as well as to monitor historical changes of habitat use and prey intake. This can be considered as a major step forward towards the identification of sensible sites for conservational management purposes within the whole coastal zone. 2 Zusammenfassung Zusammenfassung Die Seevogelökologie widmete sich bereits intensiv der Untersuchung der Brutbiologie verschiedener Arten, wohingegen die Nutzung von Nahrungshabitaten weitaus weniger gut untersucht ist. Das Aufdecken raum-zeitlicher Muster in der Wahl von Nahrungshabitaten ist jedoch besonders wichtig, um Mechanismen der Koexistenz zwischen nah verwandten Arten zu verstehen, um Licht auf die ökologische Funktion und Qualität verschiedener Habitattypen für Seevögel zu werfen und um die Rolle von Seevögeln in der Nahrungsnetzdynamik zu verstehen. Enge raum-zeitliche Beziehungen zu den wichtigsten Beuteorganismen lassen sich besonders bei an der Oberfläche fressenden Seevögeln vermuten, da ihre Fresstechnik ihnen nur erlaubt, den obersten Teil der Wassersäule zu nutzen. Verschiedene Arten von Oberflächenfressern kommen auch an der deutschen Nordseeküste vor. Diese ist durch ein besonders komplexes Habitatmosaik gekennzeichnet, welches hauptsächlich durch drei Landschaftstypen geprägt wird (der Seebereich, das Wattenmeer und das küstennahe Binnenland). Die vorliegende Studie bedient sich eines integrativen Ansatzes, um wichtige raum-zeitliche Muster der Habitatnutzung von fünf Möwen- und einer Seeschwalbenart in der Küstenzone der deutschen Nordsee aufzudecken. Die zugrunde liegende Methode verschneidet direkte Beobachtungen von Verbreitungsmustern mit Verhaltens- und Nahrungsanalysen. Des Weiteren werden Seevögel als Bioindikatoren genutzt, um die Qualität und die ökologische Funktion verschiedener Habitattypen der Küstenzone sowie historische Veränderungen der Nahrungsnetzdynamik vor dem Hintergrund