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The impact of nesting cormorants on plant and arthropod diversity
Stockholm University, Faculty of Science, Department of Botany. (Plant ecology)
Stockholm University, Faculty of Science, Department of Botany. (Plant ecology)
Stockholm University, Faculty of Science, Department of Botany. (Plant ecology)
Stockholm University, Faculty of Science, Department of Botany. (Plant ecology)
(English)Article in journal (Refereed) Submitted
Abstract [en]

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).

Keyword [en]
seabirds, islands, species richness, species composition, heterogeneity, feeding groups
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
URN: urn:nbn:se:su:diva-43243OAI: oai:DiVA.org:su-43243DiVA: diva2:354911
Available from: 2010-10-06 Created: 2010-10-05 Last updated: 2010-10-06Bibliographically approved
In thesis
1. The impact of cormorant nesting colonies on plants and arthropods
Open this publication in new window or tab >>The impact of cormorant nesting colonies on plants and arthropods
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Seabirds concentrate large amounts of marine nutrients on their nesting islands. This nutrient input can have large consequences for island food webs and community structure. The high nutrient load may also cause runoff into surrounding waters and affect marine communities. In my thesis, I studied the effect of cormorant nesting colonies on the stoichiometry, abundance, species richness, and species composition of plants, algae, and invertebrates on land and in costal waters and investigated if differences in the elemental composition or homeostasis can explain differences in the numerical response among invertebrate groups. δ15N analysis indicated that ornithogenic nitrogen provided a significant nitrogen source for plants and arthropods on nesting islands and around high nest density islands also for brackish algae and invertebrates. Furthermore, nutrient runoff created a potential feed-back loop to spiders via chironomids. Cormorant nutrient input changed island vegetation and increased plant P and N content and epiphytic algae:Fucus ratio, but decreased plant species richness and vegetation cover. Invertebrates responded indirectly to these qualitative and quantitative changes in their food source and habitat, but also directly to cormorant subsidies. However not all taxonomic and feeding groups were affected and responses were both positive and negative. Differences in the numerical response among taxonomic groups could not be explained by differences in the level of homeostasis, since, generally, all invertebrates were strongly homeostatic. Similarly, consumer nutrient content was a poor predictor for displayed responses. I conclude that cormorant colonies have strong impacts on island vegetation and some consumer groups. However, even if they can decrease the species richness of some organism groups on their nesting islands, they increase the habitat heterogeneity in an archipelago and thus may increase the regional species diversity.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University, 2010. 80 p.
Keyword
seabirds, cormorants, islands, nitrogen, phosphorus, plants, arthropods, δ15N, Baltic Sea, ecological stoichiometry, species richness, species composition, numeric response
National Category
Biological Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-43165 (URN)978-91-7447-147-2 (ISBN)
Public defence
2010-11-05, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Accepted. Paper 4: Manuscript.Available from: 2010-10-14 Created: 2010-10-01 Last updated: 2010-10-18Bibliographically approved

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Kolb, Gundula S.Jerling, LennEssenberg, CarolineHambäck, Peter A.
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