The effect of cormorants on the plant-arthropod food web on their nestinf islands

Background/Question/Methods Seabirds have profound effects on plants and animals on their nesting islands by depositing large amounts of nitrogen and phosphorus rich guano and providing their living and dead bodies. Previous studies have shown responses by specific taxa to seabird colonies, but our study is the first to include the entire aboveground ecosystem, plants and all major arthropod groups. We investigated the effects of cormorants on the food web of their nesting islands in the archipelago of Stockholm, Sweden. On these islands, we examined three questions: 1) How do cormorants affect the aboveground plant biomass and the nitrogen content? 2) What are the main prey sources for different predator groups, terrestrial arthropods or emerging insects with aquatic larvae? This question was examined by using the different carbon isotope signals from terrestrial plants and marine algae, and the strong nitrogen isotope signal from guano to identify major pathways in the island food web. 3) Do cormorants affect the density of herbivorous, detritivorous and predatory arthropods respectively?

Results/Conclusions Seabirds have profound effects on plants and animals on their nesting islands by depositing large amounts of nitrogen and phosphorus rich guano and providing their living and dead bodies. Previous studies have shown responses by specific taxa to seabird colonies, but our study is the first to include the entire aboveground ecosystem, plants and all major arthropod groups. We investigated the effects of cormorants on the food web of their nesting islands in the archipelago of Stockholm, Sweden. On these islands, we examined three questions: 1) How do cormorants affect the aboveground plant biomass and the nitrogen content? 2) What are the main prey sources for different predator groups, terrestrial arthropods or emerging insects with aquatic larvae? This question was examined by using the different carbon isotope signals from terrestrial plants and marine algae, and the strong nitrogen isotope signal from guano to identify major pathways in the island food web. 3) Do cormorants affect the density of herbivorous, detritivorous and predatory arthropods respectively?