Ecosystems

, Volume 13, Issue 3, pp 353–366

The Impact of Cormorants on Plant–Arthropod Food Webs on Their Nesting Islands

Authors

    • Department of BotanyUniversity of Stockholm
  • Lenn Jerling
    • Department of BotanyUniversity of Stockholm
  • Peter A. Hambäck
    • Department of BotanyUniversity of Stockholm
Article

DOI: 10.1007/s10021-010-9323-8

Cite this article as:
Kolb, G.S., Jerling, L. & Hambäck, P.A. Ecosystems (2010) 13: 353. doi:10.1007/s10021-010-9323-8

Abstract

This study investigated the effects of cormorant colonies on plant–arthropod island food webs, the consequences of nutrient-rich runoff on marine communities, and feedback loops from marine to terrestrial ecosystems. Terrestrial plant responses were as expected, with the highest plant biomass on islands with low nest density and the highest nitrogen (N) content on islands with high nest density. In contrast to our hypothesis, we found no uniform density response across guilds. Among herbivores, the variable responses may depend on the relative importance of plant quality or quantity. As expected, nutrient-rich runoff entered water bodies surrounding cormorant nesting islands, but only at high nest density, and increased the density of emerging insects. This created a potential feed-back loop to spiders (major terrestrial predators), where stable isotope analyses suggested great use of chironomids. Contrary to our expectation, this potential feed-back did not result in the highest spider density on islands with a high cormorant nest density. Web spiders showed no changes in density on active cormorant islands, and lycosids were actually less abundant on active cormorant islands compared to reference islands. The variable response of spiders despite increased dipteran densities, and also in other consumer groups, may be due to direct negative effects of cormorants on soil chemistry, vegetation cover, and other density regulating forces (for example, top–down forces) not studied here. This study highlights the importance of including processes in the surrounding marine ecosystem to understand the impacts of seabirds on the food web structures of their nesting islands.

Key words

seabird islandsfood websvegetationchironomidsstable isotopesnitrogen

Supplementary material

10021_2010_9323_MOESM1_ESM.doc (161 kb)
Supplementary material 1 (DOC 161 kb)

Copyright information

© Springer Science+Business Media, LLC 2010