The impact of nesting cormorants on plant and arthropod diversity
(English)Article in journal (Refereed) Submitted
Seabirds can strongly affect major factors correlated with species diversity – primary productivity, heterogeneity, and disturbance – on their nesting islands through the concentration of marine nutrients and physical disturbing island vegetation. In this study, we investigated the effects of nesting cormorants on the abundance, species richness, and composition of plants and arthropods (Coleoptera, Heteroptera, Araneae, and Chironomidae) on islands in Stockholm archipelago, Sweden. We found that cormorants had a negative effect on plant species richness and vegetation cover and that they changed plant species composition. Plant biomass showed no linear correlation with nest density when considering all islands studied, but was negatively correlated with nest density when considering only cormorant islands. The effect of nesting cormorants on island arthropods varied across feeding groups and sampling methods. Coleopterans and cursorial spiders responded with shifts in species richness and composition, and several coleopteran feeding groups and chironomids also changed in abundance. The abundance and species richness of saprophagous and fungivorous coleopterans were higher on active cormorant islands than on reference islands, while the abundance and species richness of herbivorous coleopterans and the species richness of cursorial spiders were negatively correlated with nest density. In structural equation modeling we found that some feeding groups were directly affected by nest density, but that many of the effects of seabirds on island consumers were mediated by changes in vegetation. We conclude that nesting cormorants affect the diversity of their nesting islands and the archipelago as a whole. Although cormorant colonies can decrease the species diversity of plants and some invertebrate groups on their nesting islands, the total diversity in the archipelago may increase through regional increased habitat heterogeneity and by adding species which are favored by seabirds (e.g., saprophagous and fungivorous coleopterans).
seabirds, islands, species richness, species composition, heterogeneity, feeding groups
Research subject Plant Ecology
IdentifiersURN: urn:nbn:se:su:diva-43243OAI: oai:DiVA.org:su-43243DiVA: diva2:354911